Transition Metal Oxides

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. _ ^ ^ „ OCT 14 1975 175118 '-cy NSRDS-NBS 49

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1 Phase Transition Iq-i Crystal Chemistry, and Related Aspects

C. N. R. Rao and G. V. Subba Rao

Department of Chemistry Indian Institute of Technology Kanpur-16, India

t> N$RD6-KB5 »o. ^

NSRDS

U.S. DEPARTMENT OF COMMERCE, Frederick B. Dent, Secretary .OVAJTIO/u

W.*5. NATIONAL BUREAU OF STANDARDS, Richard W. Roberts, Director

» * % Issued June 1974 Library of Congress Catalog Number: 73-600267

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iii Preface

As part of our program in presenting critical reviews of phase transitions of inorganic solids,* we have surveyed the phase equilibria, crystal chemistry, and phase transitions of transition metal oxides in detail. Transition metal oxides form an interesting series of materials for the study of a variety of solid state phenomena, many of which arise from the presence of different d-electron configurations. Crystal structure transformations in several of the transition metal oxides are accompanied by significant changes in electrical, magnetic, and other properties and we have, therefore, briefly presented the recent data on the various properties of these oxides. We trust that this monograph will not only serve as a source of valuable information, but also provide a fascinating case history of an important class of solid state materials.

In this paper, we have covered the binary oxides of 3d-, 4d-, and 5d-transition metals with important references to the literature up to 1973. The authors acknowledge the support by the National Bureau of Standards through their Special International Programs. The authors’ thanks are due to G. Rama Rao for his assistance in the preparation of the manuscript.

* The first monograph of this series is on binary halides by C. N. R. Rao and M. Natarajan, NSRDS-NBS-41, 1972. (Supported by NBS Project G-77).

IV Contents

Page

Foreword iii Preface iv Symbols and abbreviations 1 Introduction 1

I. Oxides of 3d transition elements

1. Scandium oxide 7 2. Titanium oxides 9 3. Vanadium oxides 31 10. 4. Chromium oxides 50

5. Manganese oxides 54 6. Iron oxides 60 7. Cobalt oxides 69 8. Nickel oxides 75 9. Copper oxides 79 Zinc oxides 81 II. Oxides of 4d transition elements

10.1. Yttrium oxides 83 2. Zirconium oxides 85

3. Niobium oxides 92 4. Molybdenum oxides 99

5. Technitium oxides 102

6. Ruthenium oxides 103

7. Rhodium oxides 104 8. Palladium oxide 105

10.9. Silver oxides 106 Cadmium oxides 107 III. Oxides of 5d transition elements

1. Lanthanum oxides 109 2. Hafnium oxides Ill

3. Tantalum oxides 113 4. Tungsten oxides 117

5. Rhenium oxides 123 6. Osmium oxides 125 7. Iridium oxides 127 8. Platinum oxides 127 9. Gold oxides 128 Mercury oxides 129 IV. Some recent studies 130

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Transition Metal Oxides

Crystal Chemistry, Phase Transitions and Related Aspects

C. N. R. Rao and G. V. Subba Rao

A survey is made of the data describing the thermodynamics of phase equilibria, crystal chemistry and phase transformations of binary oxides of 3d, id, and 5d transition metals. Changes in electrical, magnetic, and other properties which accompany phase transitions are discussed- Nearly complete coverage of the literature is provided up to 1973.

Key words: Crystal structure transformations; critical data, transition metal oxides; electronic properties; phase equilibria; phase transitions; magnetic properties.

Symbols and Abbreviations Tsc—Superconducting transition temperature Tm— ransition temperature (Morin transition) AFMR—Anti -Ferromagnetic Resonance T t Transition temperature APW—Augmented Plane Wave — t—Temperature in °C (degree Celsius) B—Crystal field Racah parameter TEC Thermal Expansion Coefficient bp—boiling point — TGA Thermo Gravimetric Analysis C —Heat capacity at constant pressure — p A V Volume change eal—Thermoehemical calorie = 4.1840 Joules — cl— ongitudinal sound-wave velocity C v—Heat capacity at constant volume Shear sound-wave CS—Crystallographic shear vs— velocity UHF Ultra High Frequency DTA—Differential Thermal Analvsis — Watt d—Density W— a—Seebeck coefficient; angle E ft—Energy of activation of* rohlich polaron coupling constant Ef— ermi level — a td—Thermal diffusivitv (x/Cpd) E g—Band gap /3 —Angle e—electronic charge Angle EMF—Electro Motive Force 7— 7g Griineisen constant ESCA—Electron Spectroscopy for Chemical Analysis e—Dielectric constant Debye temperature ESR—Electron Spin Resonance do— k Thermal conductivity AG—Gibbs free energy change — H—Enthalpy Mb— ohr magneton AH—Enthalpy change Md— rift mobility h—hours Mh— all mobility p Resistivity k—Knight shift — Ap Jump in resistivity k—Boltzmann constant — a Conductivity LCAO—Linear Combination of Atomic Orbitals — X—Magnetic susceptibility M 0—Spontaneous magnetization Xm—Molar magnetic susceptibility M s—Saturation magnetization min—minutes Z—Number of molecules per unit cell m—free electron mass m*—Effective electron mass Introduction mp—melting point m * Effective polaron mass p — Transition metal oxides probably form one of the NMR—Nuclear Magnetic Resonance most interesting class of solids, exhibiting a variety n—Refractive index of novel properties [1-5]. 1 These properties un- n c—number of charge carriers P—Pressure: 1 atm = 101325 Nm~2 = doubtedly arise from the outer d electrons of the -2 1013250 dyn cm transition metal ions. These d electrons can neither 02—Partial pressure of oxygen P be described by a collective electron model (as in Rh—Hall constant AS—Entropy change the case of s and p electrons) nor by a localized T—Temperature in degrees Kelvin electron model (as in the case of / electrons which Tc—Curie temperature 1 Figures in brackets indicate the literature references on page that appear Tn—Neel temperature at the end of each section.

1 are tightly bound to the nuclei). Outer d electrons erally, when compounds deviate from stoichiometry, are not screened from the neighboring atoms by the an increase in density indicates interstitials and a outer core electrons, and the intermediate character decrease denotes vacancies. Electrical conductivity of these electrons shows itself in terms of localized of oxides is markedly affected by deviations from electron behavior in some oxides and collective elec- stoichiometry. Similarly, foreign metal ions, present tron behavior in some others; in a few instances, as impurities or doped intentionally, would also both kinds of d electrons can exist simultaneously. cause marked changes in the properties of metal In recent years, the field of transition metal oxides oxides. There are many instances where foreign im- has been a subject of intensive study by chemists purities stabilize a particular phase. Thus, sulfate +3 and others interested in solid state materials. A ion stabilizes the anatase phase of Ti0 2 while Fe study of the properties of these oxides not only stabilizes the high temperature pseudobrookite phase

serves as one of the best introductions to solid state of Ti 30 6 . chemistry and materials science, but also provides a Transition metal oxides exhibit a wide range of wealthy source of fascinating research problems for magnetic properties: TiCh, ZrC>2, V2O6, and M0O3 experimental and theoretical investigations. are diamagnetic; metallic TiO and Re03 are Pauli-

Transition metal oxides of the general formulae paramagnetic; VO2, Nb02, and Ti 2 03 are paramag- are MO, M2O3, M0 2 , M2O5, MO3, M„02 n-1, MrAn+i netic; MnO, CoO, NiO, V2O3 and O2O3 are anti- known to exist. The nature of bonding in these ferromagnetic with well-defined Neel temperatures;

oxides varies from purely ionic (e.g., NiO, CoO) to Mns0 4 and Fes0 4 are ferrimagnetic; OO2 is

purely covalent (e.g., 0 s0 4 , Ru0 4 ); metallic bonding ferromagnetic. The magnetic properties of binary

is seen in oxides like TiO, NbO, and Re03. The oxides have been reviewed by Goodenough and transition metal oxides varies crystal structure of others [ 1 , 3 , 4 , 8]. from cubic to triclinic symmetry. Simple binary Transition metal oxides show a spectacular range oxides of the composition, MO, generally possess of values of electrical conductivity. At one extreme

the rock-salt structure, while the dioxides, M0 2 , we have the insulator behavior as typified by MnO -15 _1 -1 possess fluorite, rutile, distorted rutile or even more (

sess the corundum structure. Transition metals form metallic conductivities (fig. 1 ). In between these important ternary oxides like perovskites, spinels, extremes we have a large number of oxide semicon- bronzes, and garnets. Many of these oxides show ductors. The mechanism of conduction in such semi- interesting phase transitions from one crystal struc- conducting oxides may involve the hopping of charge ture to another accompanied by changes in magnetic, carriers (as in Nb205) or the excitation from the electrical, and other properties. valence band to the conduction band (as in SrTiOs Many transition metal oxides show a wide range and Sn02). Of special interest are the transition of non-stoichiometry as in the case of oxides of metal oxides which exhibit insulator (or semicon- titanium, vanadium, iron, and niobium. Deviations from stoichiometry can be due to cation deficiency (as in (as in Fes0 4 ) or anion deficiency ZnO). Large deviations from stoichiometry cause marked changes in unit cell dimensions or in crystal structure. In many oxides, the defects are so highly ordered that we cannot really consider them as ordinary defect solids (e.g., TiO, NbO). Similarly, many oxide phases which may appear as nonstoichiometric could truly be stoichiometric compositions (with narrow ranges of homogeneity) resulting from different modes of sharing of the metal-oxygen poly-

hedra (e.g., Ti n0 2 „-i and similar Magneli phases) there are wide ranges of [ 6 , 7 ]. In oxides where homogeneity lattice constants vary with composi-

1 . tion; a typical case is that of TiO* ( 0.8 ^ x ^ 2 ). In such oxides a comparison of the pyknometric metallic oxides. density with the x-ray density will be useful; gen- Figure 1 . Resistivities of highly conducting ( )

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3 ductor)-to-metal transitions; typical of these oxides RHENIUM TRIOXIDE ROCKSALT CORUNDUM RUTILE (PEROVSKITE) are V2 0 3 , V02 , and Ti 2 0 3 (fig. 2). Conduction in MO M 2 0 3 M0 2 M03 oxides can be electronic or ionic; for example, lith-

ium-doped NiO is a pure electronic conductor while

Ca-doped Zr02 is a pure ionic conductor. d'° A number of authors have recently reviewed electron transport and related properties of transition metal oxides [3-5, 9-15]. Of the various attempts to explain these properties, the qualitative approach

of Goodenough [5, 16-18] based on principles of chemical bonding has been eminently successful. With empirically derived criteria for the overlap of cation-cation and cation-anion-cation orbitals as determined by the crystal structures, Goodenough has attempted to provide a unified understanding of the magnetic and electrical properties of variety of inorganic materials including simple transition metal oxides, perovskites, spinels, bronzes, sulfides etc. Goodenough distinguishes two classes of oxides, those Figure 3. Schematic one-electron energy level diagrams for having large interactions between d electrons on different cation electronic configurations in various crystal struc- neighboring atoms or ions because of large cation- tures (after Vest and Honig). cation interactions via a small cation-cation separa- Dotted lines show predicted Fermi levels. tion (Class I) and those having large interactions between d electrons of neighboring atoms because of large cation-anion-cation interactions via a large electrical transitions of three transition metal oxides covalent mixing of anionic orbitals into the cationic and Ti in order to illustrate the p (V203 , V0 2 , 2 02 ) d orbitals (Class II); there are also oxides belonging wealth of information available in the literature. to Class I-II where the two mechanisms coexist. There is considerable doubt concerning the mech-

Our knowledge of the band structures of transition anism responsible for the observed changes in V2 03 metal oxides is far from satisfactory. A detailed dis- and V02 . Both these transitions are related to the cussion of the theoretical models would be outside changes in crystal structure that occur simultane- the scope of this work and the reader is referred to ously. The situation is not straight-forward since the various reviews [1-5] and other articles listed the structural changes could themselves be deter- at the end of this section. The intuitive approach mined by the electronic properties of the oxides. based on molecular orbital theory provides crude Thus, it is possible that distortions occur at the but reasonable approximations. In figure 3 is shown critical temperature in such a way as to create a a summary of the over-simplified one-electron energy gap at the Fermi level. This may lower the energy diagrams for a few crystal structures with the pre- of the occupied electron states or raise the energy dicted Fermi levels for the various d electron con- of the unoccupied states. If initially the band is figurations. This figure predicts either metallic or half-filled and then split into two (accompanying semiconducting behavior for any crystal structure the crystalline distortion) there will be a transition depending on the number of d electrons in the cation. from metallic behavior of the symmetric phase

Such models will undoubtedly provide a basis for (rhombohedral in V20 3 and rutile in V02 ) to semi- further studies which may eventually turn out to be conducting properties for the less symmetric phase technologically important. We may note here that (monoclinic in V203 and V02 ). the properties of highly conducting oxides such as Several other mechanisms have been proposed to

TiO can be satisfactorily explained on the basis of account for the transition in V2 0 3 . (i) One of the such elementary band structure models. The status mechanisms invokes the onset of antiferromagnetism of our understanding of the electrical properties of (whose existence has been verified by polarized the metal oxides which exhibit electrical transitions neutron scattering). Here, the potential of the elec- is, however, in a constant state of flux; there are trons differs for the spin-up and spin-down align- many theories, none of which is entirely satisfactory ments of vanadium ions of the antiferromagnetic and the field is rapidly developing [4, 10-15]. In phase and there is a doubling of the unit cell in the figure 2 we have summarized the recent data on the magnetic superlattice. This is accompanied by a

4 splitting of the band, (ii) A combination of the two a gap of ~0.06 eV from the next higher one. The preceding mechanisms has also been invoked. The pure material is, therefore, an intrinsic semiconduc- band gap opening up under the combined action tor at liquid helium temperatures. With increase in of both these effects would be sufficiently large to temperature, the distension of the unit cell is in such cause a decrease in the carrier density and account a direction as to widen the bands and eliminate the for the increase in resistivity. This would mean that gap. The material thus gradually is converted into the mobility remains nonactivated and essentially an overlap material. All these theories of the semi- unchanged. On the other hand, if either of the two conductor-metal transitions have been discussed in mechanisms (distortion or antiferromagnetism) were the various references listed at this introduction and alone to operate, there would be a decrease in carrier they are again referred to in the respective sections density of only two orders of magnitude. In such a in the text. case a change in mobility of five orders of magnitude It was our purpose initially to review phase transi- would be necessary to account for the observations. tions of transition metal oxides in this monograph. The mobility in the low-temperature monoclinic We soon came to the conclusion that such a review phase would then be low enough to correspond to an would be incomplete unless the relevant information activated process, (iii) The possibility that band on the crystal chemistry, phase equilibria as well as overlap may occur in V 2 O 3 has been proposed, based electrical, magnetic and other important properties on transport measurements. In this event, the transi- are also included. This is because, phase transitions tion could be thought of as being driven by the in many of these oxides are associated with inter- coulomb interaction between the holes and electrons esting changes in various properties (as in fig. 2). (simultaneously present in the overlapping bands). In the phase transitions of some of the oxides like

This would result in a distortion which lifts the Ti02 and Zr0 2 electronic factors may not be very band overlap. The insulating phase, according to this important, but the transitions themselves are of mechanism, would be an 'excitonic insulator.’ academic and technological interest. The phase Accordingly, the transition can be entirely sup- equilibria and crystal chemistry of many of the pressed by application of hydrostatic pressure. Re- transition metal-oxygen systems are not fully in- sistivity studies of the metallic phase under com- vestigated. As new phases of these oxides get estab- pression down to liquid helium temperatures indicate lished, there will undoubtedly be further scope for the presence of overlapping bands, (iv) It has been the study of phase transitions and other properties. suggested that V2O3 may be an example of a ’Mott The Magneli phases of titanium and vanadium serve insulator.’ This implies that small changes in inter- to illustrate this point. We have tried to keep the atomic distances in the lattice causes sufficient altera- future possibilities in mind in presenting the avail- tions in the band overlap to change the system from able information on transition metal oxides. itinerant to localized behavior, (v) The effect of We have presented the information on the binary electron correlations has also been considered im- oxides of the 3d, 4d, and 5d transition metals by portant, since the bands in V20 3 are relatively nar- first summarizing the general features of the phase row. According to this theory, if the bandwidth is equilibria, phase transitions and physical properties within a certain narrow range and is further reduced in each metal-oxygen system followed by a detailed (by temperature changes or so) then a splitting into tabulation of the data. This tabular presentation of two sub -bands takes place. It should be possible to the data is expected to give a brief summary of the devise a single, comprehensive theory, in which the present status of knowledge on each oxide system. possibilities discussed above become special cases. The material on each oxide system forms a self- To decide among the various alternatives it is neces- contained section by itself and includes the relevant sary to obtain more detailed experimental data. The information on all the important properties. Such case of the VO2 transition is similar to that of V2O3, a review of transition metal oxides is not presently with the exception that VO2 does not exhibit mag- available in the literature and we hope that this netic order. present effort will be well-received by chemists, solid

Ti203 differs from the cases discussed earlier in state scientists and others. that, this oxide undergoes a transition over a rather The various binary oxides discussed in the paper wide temperature range. It does not exhibit either are shown in table 1. We have listed most of the magnetic order or undergo a crystallographic transi- important references to the literature (up to 1973) tion. A band broadening model has been suggested on each oxide system and we apologize for any to account for the transition of Ti203 . At low tem- omissions due to oversight or errors in judgement. peratures, the completely filled band is separated by Crystallographic data only from the recent literature

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have been quoted and well-established data on stand- istry, Ed. C. N. R. Rao (Plenum Press, New York, 1970). ard substances found in the papers have not been [7] Chemistry of Extended Defects in Nan-Metallic Solids, repeated. Studies on solid solutions of oxides, liquid Eds. L. Eyring and M. O’Keeffe (North Holland Publ. Co., Amsterdam, 1970). films oxides, oxide hydrates and thin have not been [8] Menyuk, N., in Modern Aspects of Solid State Chem- included and the review pertains only to the bulk istry, Ed. C. N. R. Rao (Plenum Press, New York, 1970). properties of oxides in solid state. Effects of impuri- [9] Bosman, A. J., and van Daal, H. J., Adv. Phys. 19, 1 ties have been discussed wherever results are rele- (1970). [10] Adler, D., Rev. Mod. Phys. 40, 714 (1968). vant. We must add that the information on the [11] (a) Mott, N. F., Phil. Mag. 20, 1 (1969); Rev. Mod. phase transitions of ternary oxides and other systems Phys. 40, 677 (1968). (b) Zinamon, Z., and Mott, N. F., Phil. Mag. 21, 881 is very vast, but does not fall within the scope of (1970). this monograph. We have not included any general [12] (a) Honig, J. M., IBM J. Res. Dev. 14, 232 (1970). (b) Honig, J. M., in Modern Aspects of Solid State Chem- discussion of the theory of phase transitions of in- istry, Ed. C. N. R. Rao (Plenum Press, New York, organic materials as this subject has been adequately 1970). [13] Rice, T. M., and McWhan, D. B., IBM J. Res. Dev. reviewed elsewhere [19-21]. 14, 251 (1970). [14] Doniach, S., Adv. Phys. 18, 819 (1969). [15] Hyland, G. J., J. Solid State Chem. 2, 318 (1970). [16] Goodenough, J. B., Bull. Soc. Chim. France 4, 1200 References (1965). [17] Goodenough, J. B., Czech. J. Phys. 17B, 304 (1967). [18] Goodenough, J. B., J. Appl. Phys. 37, 1415 (1966); 39 [1] Goodenough, J. B., Magnetism and the Chemical Bond 403 (1968). (John Wiley-Interscience, New York, 1963). [19] Rao, C. N. R., and Rao, K. J., in Progress in Solid State [2] Howe, A. T., and Fensham, P. J., Quart. Rev. (Chem. Chemistry, Yol. 4, Ed. H. Reiss (Pergamon Press, Soc. London), 21, 507 (1967). Oxford, 1967). [3] Adler, D., Solid State Phys., Eds. F. Seitz, D. Turnbull, [20] Rao, C. N. R., in Modern Aspects of Solid State Chem- and H. Ehrenreich 21, 1 (1968). istry, Ed. C. N. R. Rao (Plenum Press, New York, [4] Rao, C. N. R., and Subba Rao, G. V., phys. stat. solidi 1970). (a) 1, 597 (1970). [21] Rao, C. N. R., and Natarajan, M., Crystal Structure [5] Goodenough, J. B., in Progress in Solid State Chemistry, Transformations in Binary Halides, IN at. Stand. Ref. Vol. 5, Ed. H. Reiss (Pergamon Press, Oxford, 1971). Data Ser., Nat. Bur. Stand. (U.S.), 41, 53 pages [6] Anderson, J. S., in Modern Aspects of Solid State Chem- (July 1972).

I. Oxides of 3 d Transition Elements

1.1. Scandium Oxide by 8.3 percent) accompanied by an increase in coordination number from six- to seven-fold for two Scandium apparently forms only one oxide, SC2O3. thirds of the metal atoms. No change in the crystal

Petru and Dufek [1] reported a grey-black oxide, structure is noticed at 30 or 80 kbar pressure at which they believed to be a monoxide, ScO, obtained 1273 K (or at 89 kbar at 1573 K) and at 110 kbar by the hydrogen reduction of SC2O3 at 2170 K; this at 1273 K, poorly crystallized high pressure phase monoxide has, however, not been characterized. SC2O3 of SC2O3 is obtained [8, 9]. is cubic [2-5] and has the C-type rare-earth oxide SC2O3 is a semiconductor at all temperatures [10 structure in which the metal atoms are distributed 13] and considerable ionic contribution to the total in the ratio 1 : 3 into two crystallographically distinct conductivity is noticed at and below 1100 K [11, sites each with six-fold coordination. The structure 12] . is relatively open and can be described as a defect SC2O3 forms a complete series of solid solutions fluorite form in which one fourth of the anions are with ln20 3 [9, 14], Y2O3 [15, 16], Yb 20 3 [15], and

- absent. No magnetic ordering is known in SC2O3 Cr2 C>3 [17] and binary systems of the type M02 and heat capacity data [6, 7] do not indicate any Sc20 3 where M = Ti [18], Zr [18-21], Hf [22, 23], Th phase transformations in the range 53 to 1800 K. [24], and U [24] and CaO-Sc2C>3 [25] have been in-

Reid and Ringwood [8] found that SC2O3 transforms vestigated in detail in the literature. Generally, solid to the monoclinic B-type rare-earth oxide structure solution formation occurs up to certain composition at 130 kbar pressure and 1273 K. The unit cell and the binary systems exhibit considerable ionic volume decreases by 7.8 percent (density increases conductivity.

7 .

Scandium oxide

Oxide and description Data Remarks and inferences References of the study

SC2O3

Crystal structure and melting point. Ordinary form: This relatively open C-type [4, 8, 26]. Cubic; space group, Ia3; Z = 16; structure is adopted by many a =9.844 ±0.002 A. High-pres- rare-earth oxides and is closely sure form: Monoclinic; space related to the fluorite structure.

group, C2/m; Z = 6; a = 13.173 The high pressure form is denser ±0.01 A; 6=3.194±0.005 A; and is akin to the B-type struc- c = 7.976±0.01 A; 0 = 1OO.4O°± ture adopted by the rare-earth 0.05°. Melting point =2740 K. oxides. The transformation does not involve the intermediate corundum structure.

Electrical properties. Semiconductor behavior in the [10-13]. range 370-1870 K; reliable data on pure and single crystal material are not available.

Optical properties. Optical energy gap = 5.4 eV; in- [27, 28] frared stretching frequency = -1 635 cm .

References [14] Schneider, S. J., Roth, R. S., and Waring, J. L., J. Res. Nat. Bur. Stand. (U.S.), 65A (Phys. and Chem.), No. 4, 345-374 (July-Aug. 1961). [1] Petru, F., and Dufek, V., Z. Chem. 6, 345 (1966). [15] Hazek, B., Petru, F., Kalalova, E., and Dolezalova, J., [2] Milligan, W. O., Vernon, L. W., Levy, H. A., and Z. Chem. 6, 268 (1966). Peterson, S. W., J. Phys. Chem. 57, 535 (1953). [16] Trzebiatowskii, W., and Horyn, R., Bull. Acad. Polon. [3] Reid, A. F., and Sienko, M. J., Inorg. Chem. 6, 521 Sci., Ser. Sci. Chim. 13, 311 (1965). (1967). [17] Tresvyatskii, S. G., Pavlikov, V. N., and Lopato, L. M., [4] Geller, S., Romo, P., and Remeika, J. P., Z. Crystallogr. Izv. Akad. Nauk SSSR Neorg. Mater. 2, 269 (1966). 124, 136 (1967); 126, 461 (1968). [18] Collongues, M. R., Queyroux, F., Perez, M., Jorba, Y., [5] Norrestam, R., Arkiv for Kemi 29, 343 (1968). and Gilles, J.-C., Bull. Soc. Chim. France 4, 1141 [6] Weller, W. W., and King, E. G., US Bur. Mines Rept. (1965). Invest. No. 6245 (1963). [19] Lefevre, J., Rev. Hautes Temp. Refract. 1, 229 (1964). [7] Pankratz, L. B., and Kelley, K. K., US Bur. Mines [20] Thornber, M. R., Bevan, D. J. M., and Graham, J., Rept. Invest. No. 6198 (1963). Acta Cryst. 24B, 1183 (1968) [8] Reid, A. F., and Ringwood, A. E., J. Geophys. Res. 74, [21] Spiridonov, F. M., Popova, L. N., and Popils’kii, R. Ya., 3238 (1969). J. Solid State Chem. 2, 430 (1970). [9] Prewitt, C. T., Shannon, R. D., Rogers, D. B., and [22] Komissarova, L. N., and Spiridonov, F. M., Dokl. Akad. Sleight, A. W., Inorg. Chem. 8, 1985 (1969). Nauk SSSR 182, 834 (1968). [10] Noddack, W., and Walch, H., Z. Elektrochem. 63, 269 [23] Kalinovskaya, C. A., Spiridonov, F. M., and Komissa- (1959). rova, L. N., J. Le8s-Comm. Metals 17, 151 (1969). [11] Schmalzried, H., Z. Physik. Chem. 38, 87 (1963). [24] Trezebiatowskii, W., and Horyn, R., Bull. Acad. Polon. [12] Tripp, W. C., and Tallan, N. M., Bull. Am. Ceram. Soc. Sci., Ser. Sci. Chim. 13, 303 (1965). 47, 355 (1968). [25] Trezebiatowskii, W., and Horyn, R., Bull. Acad. Polon. [13] Zyrin, A. V., Dubok, V. A., and Tresvyatskii, S. G., Sci., Ser. Sci. Chim. 13, 315 (1965). Khim. Vysokotemp. Mater., Tr. Vses. Soveshch. 2nd [26] Noguchi, T., and Mizuno, M., Solar Energy 11, 90 (1967). Leningrad 59 (1965, Publ. 1967); Chem. Abstr. 69, [27] Companion, A. L., J. Phys. Chem. Solids 25, 357 (1964). 62562t (1968). [28] Petru, F., and Muck, A., Z. Chem. 6, 386 (1966).

8 1.2. Titanium Oxides vacancies [3-6, 12-16], a low temperature ordered

monoxide recently characterized by Watanabe et al.

The Ti-0 system has been studied exhaustively by [17], an ordered TiOi. 2 B [18-20], a sesquioxide Ti2Os various workers over the past several years. DeVries with a narrow homogeneity range, Ti 306, Magneli and Roy [1] reviewed the available literature up to phases of the general formula Ti n02n_i (4^71^10)

1954 and gave a tentative phase diagram. Since then, and Ti0 2 . The phase diagram of Porter and Roy [5] different groups of investigators [2-6] have con- is reproduced in figure 1.1. Recent studies have re- tributed much to the understanding of the various vealed the complicated nature of the Ti-0 phase phases in the Ti-0 system. The important phases diagram; according to Hirabayashi and co-workers of the systems are the following: TieO (reported by [21], below 670 K, there is an almost infinite number

Kornilov and Glazova [7, 8] and characterized by of ordered phases in the composition range 0.09 <

Yamaguchi et al. [9]), Ti 30 (reported by Kornilov O/Ti<0.40 and also a number of higher Magneli and Glazova [7, 8] and characterized by Jostsons phases (n^ll-53; range, 1.90 <0/Ti< 2.00) (22, 23]. and Malin [10]), rhombohedral Ti20 [3-6, 8, 11], a Titanium oxides show a wide variety of interesting cubic monoxide, TiO*, having a wide homogeneity electrical, magnetic and optical properties and inter- range (0.75 ^ 1.30) with disordered rock salt esting phase transitions. We shall now critically dis- structure at high temperatures containing many cuss some important aspects in this section.

Figure 1.1. Ti-TiO* phase phase diagram showing various

oxide phases ( after Porter and Roy [5]). TiO z (a: <0.70): Metallic titanium exists in two allo- orthorhombic structure and is observed in specimens tropic modifications, a (low temperature, hexagonal) which were rapidly cooled from the melt (of TiO*; and (3 (high temperature, bcc). The a—>j3 transition cubic high temperature modification) and not well occurs at 1155 K [2, 6]. Oxygen addition to Ti stabi- annealed at low temperatures as well as in thin foils lizes the a phase and oxygen can be dissolved (of TiO x) annealed at temperatures greater than interstitially up to ~30 atom percent [6]. From 870 K or heated by an intense electron beam [17, measurements of resistivity [7, 24], Seebeck co- 31]. Thus, the high temperature cubic TiO -TiO 0 . 7 1.26 efficient [7] and rate of oxidation [25], it was sug- phase transforms, on cooling, to various ordered gested that a stable phase, TieO, may exist. Recently, phases having different crystal symmetries by means

Yamaguchi et al. [9] have fully characterized this of vacancy (both cation and anion) disorder-order phase. Ti 6 0 is hexagonal with ordered superlattice mechanism. It is important to note, however, that structure. Electron/neutron diffraction and DTA it is possible to retain the cubic phase at room studies show an order-disorder transition at ~710 K; temperature by quenching from above 1270 K and the phase is stable up to ~1100 K. ' a great deal of work has been done on these quenched

Ti 30 and Ti 2 0 have a hexagonal ordered structure cubic phases. and they differ from the solid solutions of Ti and Detailed investigations on well-characterized sam- oxygen by the ordered arrangement of the oxygens ples by Banus and Reed [28] and Taylor and Doyle in the suboxide lattice and by the greater stability [29] have shown that the cubic lattice parameter of the chemical bond [3, 4, 7, 8, 10]. Andersson [26] and the density increase with increasing * in TiO* found that the so-called 5-phase of TiO x (* = 0.65) (in the range 0.86-1.24); p and dp/DT (negative) has a hexagonal structure and the isotropy in TEC are small and increase slightly, a (n-type) increases values are probably connected with the occurrence and xm is extremely small and unchanging, indi- of the close metal-metal contacts, which are present cating that TiO is a typical metallic oxide [14, 28- both parallel and perpendicular to the c axis. 30, 33]. TiO becomes superconducting at liquid

helium temperatures [15-34]; Hulm et al. [15] found

TiOx (0.70 ^ ^ 1.30): The rock salt type TiO that Tsc varies from 0. 1-1.0 K as * increases from phase exists over a wide composition range depend- 0.92 to 1.10; for other values of *, at atmospheric ing on the temperature, e.g., from TiOo .70 to TiOi .25 pressure, the materials are normal conductors down at 1673 K and TiOo.9 to TiOi.26 at temperatures to T <0.08 K. below 1260 K [3-6]. An important feature of this Annealing TiO* at 50-60 kbar pressure and at phase, as shown from density and lattice parameter 1573 K decreases the total number of vacancies con- measurements, is that its crystal structure has vary- siderably (by ~17 percent) [28, 35]; according to ing proportions of both titanium and oxygen vacan- Taylor and Doyle [29], complete removal of vacancies [3, 4, 12-14, 16, 27-30]. At the composition cies in TiO* is possible by proper selection of tem- TiOo.70, the titanium lattice is almost perfect (-~4 perature and pressure (e.g., 77 kbar and ~1850 K). percent vacancies, extrapolating to zero at ~TiOo.B) The removal of vacancies is accompanied by a rise and about 30 percent of the oxygen sites are vacant. (for a given *) in lattice parameter, density and Tsc

lattice is In TiOi. 30 , the oxygen perfect and about (from 0.7 to 2.3 K for TiOi. 00)* slight increase in p 25 percent of the Ti sites are vacant. Even in the and a and decrease in compressibility and TEC. stoichiometric oxide, TiOi.oo* about 15 percent of These changes in properties are completely reversi- both Ti and O sites are vacant. These vacancies are ble and are unambiguously due to the pressure arranged randomly at temperatures above the equi- effect because reheating the samples in the absence librium temperature, 1260 K; at lower temperatures, of applied pressure (say at ^1570 K) causes them however, ordering of the vacancies occurs and the to revert to the original vacancy concentration and cubic rock salt structure of TiOi .00 transforms to lattice parameter [28, 29]. Recently, Iwasaki et al. monoclinic phase [3, 4, 12, 17, 20, 31]. This mono- [36] have studied the effect of pressure on the lattice clinic phase was first noticed by Naylor [32] from parameters of TiOo .82 and TiOi. 2 B and essentially enthalpy studies. The oxygen-rich TiOi .26 becomes obtained the same results as those of Banus and ordered after long annealing treatments at 1070 K Reed [28] and Taylor and Doyle [29]. The change and the crystal symmetry changes from cubic to in a parameter is small for both the samples; for tetragonal [18]. There is another ordered structure, TiOo .82 the decrease is considerable for smaller pres- the so-called 'transition structure’ [17] observed over sures while TiOi .26 shows a decrease at relatively a wide composition range -TiO this has an higher pressures (10-60 kbar). TiO . 0 . 7 1 2 B;

10 5 TiO* forms solid solutions with VO* and NbO* have noted that in the range 100 to 300 K, the ma- but the solubility is low (^10 percent) [30, 37]. terial behaves as an intrinsic semiconductor with a On the other hand, Gel’d et al. [37] noticed that the band gap of 0.03-0.05 eV (depending on the sample)

oxygen -rich phases TiOi .20 and VO 1.25 have an appre- and the drop in p beginning at ^400 K and extend- ciable mutual solubility (~20-25%). BeO, MgO, ing up to 470 K. The drop in p in the 400 to 470 K and CaO also appear to form solid solutions (up to range was about hundred-fold. The resistivity of

^50%) [37], Loehman, Rao, and Honig [30] found Ti203 goes through a shallow minimum at ^800 K

that the vacancy concentration is decreased in TiO x and rises again with further increase in temperature.

after solid solution formation; both TiO and (Tio .96 Hall data [52] below 273 K indicated the charge Vo.os)0 show metallic behavior. carriers to be positively charged and with high mobil-

Since the low temperature ordered TiO z phases ities. Seebeck coefficient of pure Ti203 is positive at

have been characterized only recently, reliable data ordinary temperatures [51, 53] (Loehman [53] re- on the physical properties are lacking at present on ports smaller values compared to Yahia and Fred-

these materials. erikse [51] for pure Ti203 ) and drops to smaller values in going through the transition region (400 to

Ti203 : Ti203 has a narrow range of homogeneity 470 K).

(*=1.49-1.51 in TiO x ) [3, 4] and has the rhombo- The nature of the semiconductor-to-metal transi-

hedral crystal structure at room temperature tion in Ti203 has been discussed by various workers [12, 38, 39] and shows an unusual behavior in in the literature [54-64]. The gradual decrease in p the thermal expansion of the lattice parameters and a in the region of the transition, the broad

in the range 450 to 650 K [12]. A more recent nature of the anomalies in x, heat capacity and

study [40] on pure material shows that the abrupt DTA measurements indicate that Ti203 is a relatively change in lattice parameters sets in at a lower wide band material and that the semiconductor-to-

temperature (~400 K); the c/a ratio increases metal transition is of higher order [52, 54, 58-64]. and there is a 7 percent decrease in the volume Since the absence of antiferromagnetism is definitely of the unit cell in the range 390 to 470 K. There established in this material [47], a plausible mechan-

is no change in the crystal symmetry in this ism appears to be that of band broadening or shift-

temperature range. Magnetic susceptibility does not ing [58, 61, 62, 64-66]. The presence of c axis pairs in show any significant anomaly in the range 400 to the corundum structure to produce a cation sub-

550 K even though a two fold increase in the magni- lattice band [54] together with a strong trigonal tude has been noted [12, 41-43]. The existence of crystal field will produce a filled band with one 3d 3+ long range magnetic order in Ti20 3 has been much electron per Ti ion (figure 1.2). The rapidly in- debated on the basis of various experimental in-

vestigations [38, 44-46], but recent neutron diffrac-

tion study by Moon et al. [47] have clearly confirmed

the absence of antiferromagnetism in Ti2 03 below 400 K. A specific heat anomaly has been reported

in Ti 203 in the range 450 to 600 K [32, 48]. Rao et al.

[49] have found a broad endothermic peak (centered around 410 K) in the DTA curve (see figure 2 in introduction section). Electrical properties have been examined by many

workers in the literature [12, 33, 38, 50, 51]. Morin

[33] found that below 450 K Ti203 is a semiconductor with a small energy gap (E ~0.05 eV); above this ff temperature it behaves like a metal with a ten-fold

drop in resistivity at T t . Yahia and Frederikse [51] observed the semiconductor-to-metal transition at 450 K, the drop in resistivity being of a factor of 40.

Abrahams [38] noticed the transition, but found that

T t was ~660 K. Honig and Reed [52] have carried out a detailed investigation on highly pure and nearly stoichiometric single crystals of Ti203 ; they Figure 1.2. Schematic band strurture of T^Oa ( after [65]).

11 creasing c a ratio in the vicinity of 400 to 470 K and Keys [73] have suggested that the transition

[40] brings the anions and cations much closer and in Ti 3 0 5 may correspond to a semiconductor -to- thus provides for a progressively greater overlap metal transition. X-ray studies of Asbrink and among the corresponding wave functions (Note that Magneli [70] showed the presence of metal-metal c is a ratio at room temperature anomalously small; bonding in the low temperature form of Ti 30 5 and see figure 1.5). This, in turn, leads to narrowing above T t , these bonds get broken. Based on con- and then to a disappearance of the valence band- siderations of the localized versus collective electron conduction band separation (figure 1.2). The p-type behavior of the d electrons in Tin 02n _i Magneli nature of the charge carriers can be explained by phases, Goodenough [74] has suggested that a assuming that either the carriers in the valence semiconductor-to-metal transition may exist in band dominate the conduction or with a model in- Ti305 .

volving joint participation of electrons and of holes Bartholomew and Frankl [75] made resistivity with comparable effectiveness. measurements on flux grown Ti 3 0 5 single crystals Honig and co-workers [52, 67] have shown that a and confirmed the semiconductor-to-metal transition slight amount of impurity remarkably affects the in this material. Recently, Rao and co-workers electrical properties of this material. A variety of [49] have examined the transition in Ti 30 5 in detail values of T t as well as the magnitude of the change employing high temperature x-ray diffraction, DTA, in p (at T t) reported in the literature [33, 38, 51] electrical resistivity, ESR and Mossbauer techniques. may as well be due to the small amounts of impurities These workers established the first order nature of in the samples of Ti 2 03 employed in the investiga- the transition in Ti 30 5 with appreciable latent tions. heat and hysteresis; no magnetic ordering was

Ti2 03 forms solid solutions with V2O3 throughout noted in the low temperature phase. At T t (448 K), the composition, but the solid solutions so formed p drops by two orders of magnitude and below are not ideal and do not obey Vegard’s law [30, 68]. T t , Ti 3 05 is semiconducting with Ea of ~0.3 eV.

The interesting observation is that at rc — O.l in Slight amounts of Fe and W reduce the transition

(Tii-j Fz) 203 the values of o, c, and a are the same temperature and the magnitude of resistivity as the corresponding values for pure Ti 2 03 after it discontinuity and at about 5 percent Fe dopant undergoes the electrical transition [30, 40]; the c concentration completely suppress the transition. parameter of Ti2 03 is increased by ^3 percent by Since metal-metal bonding in Ti 30 5 has been the incorporation of ~20 percent V2 C>3 while the definitely established [70], it appears that Good- corresponding a parameter contracts by —T.5 per- enough’s model [54, 55, 61, 74] would be applicable cent. This strongly lends support to the idea [39] in explaining the transition in this material. Ac- that strong metal-metal bonding across the octa- cording to Goodenough, in the titanium and hedral edges takes place in Ti 2 C>3. Recent resistivity vanadium oxides, in the high temperature high data [69] on (Tii-zV^Os show that for 0^x^0.04, symmetry phases, the cation -cation distance is p undergoes a change in almost the same temperature greater than the critical cation-cation separation range as undoped Ti 2 0 3 ; samples with high x are (see [61] for details) and the charge carriers move metallic at room temperature and exhibit no transi- in partially filled cation sublattice d bands and tion. This observation is consistent with the crystal metallic behavior is exhibited only in the materials data of Loehman, Rao, and Honig [30]. with well-defined Fermi surface. As the temperature

is lowered through T t , however, the band breaks

Ti 3 0 5 : Ti 30 5 , the precursor of the Ti„02n _i Magneli up into pairs of atoms that are bonded by homopolar shear structures, is monoclinic at room temperature. bonds in which the conduction electrons are trapped

Asbrink and Magneli [70] reported that Ti 30 5 and a change to lower crystal symmetry (and transforms to a distorted-orthohombic (more exactly, semiconducting phase) occurs to produce a filled monoclinic) structure reversibly at ~390 K, where band separated from an empty band by a finite

as Naylor [32] from his enthalpy data, indicated a energy gap. The atomic rearrangements at T t are transition at ~450 K. Keys and Mulay [43], Mulay subtle in Ti 305 [49, 70] and it appears that these are and Danley [71], and Vasilev and Ariya [72] showed sufficient to bring about an electrical transition in that xm of Ti 3 0 5 shows a sharp discontinuity at this material. According to Goodenough’s model, ~460 K, with a 30 K hysteresis on cooling; both we expect electrons to be the charge carriers in

the is in- Ti available coefficient data above and below T t , xm temperature 3 05 and the Seebeck dependent. Keys, Mulay, and Danley [43, 71] [76] seem to confirm this prediction.

12 n

Iwasaki et al. [77] have reported the existence of a croscopy and diffraction by Hyde and co-workers

metastable form of Ti 30 5 (designated as D) which [22, 23] showed the existence of a new family of

is identical with the high temperature form of T 13O 5 ordered phases, Ti„02 n-i (15^n^36) derived by

1 [49, 70] (D' form), hut the two differ in their proper- CS planes parallel to (132). Possibility of the exis-

ties with respect to oxidation. D and D' forms can tence of another Ti n 02 „_i (n> 35) family (Oil) shear be prepared by the hydrogen reduction of anatase planes was suggested by these workers since con-

and rutile respectively at ~1520 K and give almost tinuous streaking along (Oil ) directions was observed identical x-ray diffraction patterns, but on oxidation occasionally in the electron diffraction patterns of at ~920 K, D gives rutile while D' gives a mixture reduced rutile and detailed studies are yet to be of anatase and rutile modifications of Ti02. The low made. Studies are also necessary in the composition

temperature form (M form of Iwasaki et al. [77]) range 9

can be prepared by the vacuum sintering of TiCh The rutile based Ti n02 „_i series with 4 ^ n ^ 9 have (anatase form) and Ti metal at ''-'1520 to 1570 K periodic stacking faults introducing extra planes of and yields rutile and anatase on oxidation. For the metal ions for every n planes in one crystallographic phase transition D'—»M, Iwasaki and his co-workers direction (121) and Magneli [79, 82] has pointed

report a T t slightly greater than 373 K. The D form out that the condensation of the defects into com- could easily be changed to the M form by cooling mon planes can minimize the elastic energy of the from 1620 K under vacuum. These observations sug- crystal. Goodenough [74, 85] has recognized that gest that differences in the oxygen coordination such systems exhibiting shear structures may con- around Ti may give rise to stable and metastable tain collective d orbitals and that the regularity of

modifications of ThOs and the T t as well as electrical the spacings between these shear planes is due to and related properties may also differ accordingly. the existence of a Fermi surface and to the energy From this point of view, careful reinvestigation of stabilization achieved by either creating or increas-

the transition in Ti 30 5 is worthwhile on well char- ing the energy discontinuity across a Brillouin zone

acterized samples of this material. surface at the Fermi surface. This stabilization is greater, the narrower the bands are, and hence

Ti nC> 2n-i: The existence of Magneli phases in the spontaneous crystallographic distortions could result Ti-0 system was first reported by Andersson et al. at lower temperatures. Also, since the existence of a

[3, 4] in 1957. These are a homologous series of tri- Fermi surface is a collective electron property [61], clinic phases of composition Ti„02n-i with 4 ^ n ^ 9. it can be argued that crystallographic distortions in

They are bounded by monoclinic Ti 3Os on one side the Magneli phases may also be associated with the

and by reduced rutile (TiCh-x) on the other. The metal-to-semiconductor transitions (on cooling) . As a

structures with 4 ^ n ^ 9 can be described [3, 4, 22, matter of fact, the available data indicate that elec- 23, 78-84] as being built-up of slabs of rutile-type tronic transitions do occur in these materials associ- structure which have infinite extension parallel to ated with crystal distortions; however, the crystal

the (121) plane and have a characteristic finite width distortions appear to be minor (at least in Ti 4C> 7 , corresponding to n TiOe octahedra (where n is the for which detailed high temperature x-ray data are

integer in the chemical formula, Tin02n-i) in the third available [75, 86]) and do not involve structural direction. The adjacent slabs are mutually related by changes but only minor discontinuities in the lattice crystallographic shear (CS), the shear plane and parameters at the transition temperature. shear vector being (121) and [Oil] respectively in Detailed x-ray [75, 86], xm-T [43, 71, 72], DTA

the idealized Tin02n-i structure. This CS causes the [75] and P-T [75, 87] studies show that there are

'— titanium atom positions in one slab to correspond two phase transitions in Ti 407 , at '125 and at to unoccupied or interstitial positions in the next 150 K. There is 8 to 10 K hysteresis associated with slab and decreases the crystal symmetry from tetra- the first transition and some structural changes; gonal to triclinic while decreasing the size of the the 150 K transition is associated with a change

unit cell. However, the structure of TiioOi 9 ( = 10) from semiconductor to metal behaviour as well as cannot be explained in this way and the structure magnetic susceptibility and DTA anomalies, but no is not yet known. apparent structural change. TuOg exhibits transi- Anderson and Hyde [81] suggested that rutile- tions at 130 and 136 K and resistivity measurements based shear structures other than the (121) family indicate discontinuities at these temperatures, but

probably exist, in particular, a (132) family in re- the sample remains semiconducting above T t [43, duced rutile. Detailed studies using electron mi- 71, 75]. DTA and xm-T measurements indicate a

13 '

transition in TigOn at 122 K [43, 71, 75], but the stood. There is considerable evidence for the forma- resistivity data indicate a T; of ~130 K and semi- tion of both titanium interstitials and oxygen va- conductor behavior above and below this tempera- cancies [61, 104-108]; the atmosphere in which the ture [75]. According to Bartholomew and Frankl defects are formed seems to be an important factor.

[75], Ti 80i5 exhibits semiconductor behavior in the Since the anatase-rutile transformation involves an range 78 to 295 K and no transitions exist in this overall contraction or shrinkage of the oxygen struc- material, xm -T data are not available at present. ture as indicated by a volume shrinkage of ^8 per-

The detailed physical properties of Ti 7 0i3 and TigOn cent and a cooperative movement of ions (displacive are not known at present. and reconstructive transformation [94, 109, 110]), the removal of the oxygen ions would be expected

Ti02 : Titanium dioxide, Ti02 , has a narrow homge- to accelerate the transformation; formation of tita-

- interstitials neity range ( x — 1.983-2.000 in TiO x) [3-6, 88], and nium would inhibit the transformation. exists in three polymorphic forms, namely, anatase This implies that a reducing atmosphere or doping 2+ 2+ (tetragonal), brookite (orthohombic) , and rutile with an ion of lesser valence state (Cu or Co ) ; (tetragonal) at atmospheric pressure. In each of would accelerate the transition as indeed found ex- 4+ these, Ti ion is surrounded by an irregular octa- perimentally. Vacuum or doping with anions will hedra of oxide ions, but the number of edges shared similarly give rise to Ti3+ interstitials and hence the by the octahedra of oxide ions, but the number of inhibition of the transformation process and would edges shared by the octahedra increases from two increase the temperature of the anatase-rutile transi- (out of twelve) in rutile to three in brookite and to tion. four in anatase. The rutile form of Ti02 is the most Yoganarasimhan and Rao [94] noticed that the stable form and both anatase and brookite transform particle size and crystallite size of anatase increase to rutile on heating. High pressure modifications of markedly in the region of the crystal structure trans-

Ti0 2 have been reported by Dachille and Roy [89], formation; smaller particle size and larger surface

Bendeliani et al. [90], and McQueen et al. [91]. area of anatase samples favor the transition. De- Phase transformations involving various modifica- tailed x-ray studies indicated that the unit cell ex- tions of Ti0 2 have been extensively studied in the pands prior to the transformation to rutile. literature. Recently, Vahldiek [111] examined the effect of Anatase transforms irreversibly and exothermally pressure on the anatase-rutile transformation. Elec- to rutile in the range 880 to 1200 K depending on trical resistance and quenching experiments indi-

the method of preparation of the sample and at cated that T t and AH for the transition decrease atmospheric pressure, the transformation is time with the applied pressure; at a pressure of 1 bar, j and temperature dependent and is also a function AjFT-— — 2.8 kcal/mol and his observations corrobo-

} of the impurity concentration. Rao and co-workers rated the data of earlier workers. Discontinuous l in [92-96], Sullivan and Cole [97], Iida and Ozaki [98], change p was observed at T t at various applied Kotera and Shannon and Pask pressures and the irreversibility of the anatase-rutile Suzuki and [99], j [100] have carried out systematic investigations of transition at high pressures (up to 24 kbar) was the anatase-rutile transformation. The rate of trans- confirmed. formation of spectroscopically pure Ti0 2 in the anat- Kubo et al. [112] and Tekiz and Legrand [113] ob- j ase modification was found to be immeasurably slow served the transition of anatase to rutile by grind- below 885 K and extremely rapid above 1000 K ing; anatase-type Ti0 2 obtained from TiCR method [92, 101]. The transition is a nucleation-growth pro- is changed to rutile after grinding for 96 h, while cess, and follows the first order rate law with an Ti0 2 (anatase form) obtained from the H2 S04 method activation energy of ~90 kcal/mol (fig. 1. 3a). Im- changed to an amorphous state. purities like CuO, CoO, Li 2 0, and Na 2 0 and re- Brookite, the orthorhombic modification of Ti02 ducing atmospheres accelerate the transformation occurs in nature; it has not been possible to prepare while sulphate, phosphate, and fluoride anions, P5+ this polymorph in pure form in the laboratory. and S 6+ ions and vacuum stabilize the anatase modi- Brookite-rutile transformation was first reported by fication and inhibit the transition process [94-96, 98, Schossberger [114] and examined in detail by Rao

100, 102-104]. Values of T t and activation energy and co-workers [115]. Below 990 K, the rate of are generally higher in impure samples than in the transition is extremely slow and sluggish and above pure sample. this temperature, it follows a first order rate law

The defect structure of Ti02 is not yet fully under- with little or no induction time (fig. 1. 3b). The

14 Figure 1.3. Transformation of anatase (a) and brookite (6) as a function of temperature at different times ( after Rao and co-workers

/A And /b are the fractions of anatase and brookite forms in the mixture. Kinetics of transformation of anatase and brookite to rutile are shown in (c) and (d)

respectively (/r = fraction of rutile).

15 transition is irreversible with an energy of activa- resistivity, Seebeck coefficient and Hall mobility tion ~60 kcal/mol and — (100=fc75) cal/mol. studies over a range of temperatures have not been DTA studies give 7\« 1020 K. able to provide a clear understanding of the mecha-

As regards the comparison of the anatase-rutile nism of conduction in pure and doped Ti02 [1 lb- and brookite-rutile transformations, it may be men- multiple 120; 122b— 122d] ; band conduction seems tioned that the kinetics and mechanism are exactly to be conclusively proved, but the existence of band similar (fig. 1. 3c and 1. 3d), but the energy of activa- or hopping (large or small polaron) type of conduc-

tion is larger in the anatase-rutile transition; the tion is still in dispute. entropy of activation is large and negative for the As mentioned earlier, the defect structure of Ti02 brookite-rutile transition while it is small and has a is not yet completely understood and evidence is in positive value for the anatase-rutile transition. This favor of both anion vacancies and Ti interstitials discrepancy may be understood in terms of the [61, 104-108, 123, 123a]. Hauffe [123a] and Rudolph change of symmetry (lower to higher) in the former [123b] noted that in the range 882 to 1273 K, the absence of such a change in the latter a -1/n varies case and p Po 2 where n from 5.7 to 4.4 whereas case. Greener et al. [123] obtained n = 4 above 1470 K A new high pressure modification of TiOa was and no dependence of p on p 02 below 1170 K (ideally, 4+ synthesized by Dachille and Roy [89] at pressure 15- n should be 6, 5, and 4 for anion vacancy, Ti and 100 kbar and temperatures 298 to 773 K. It forms Ti3+ interstitial mechanisms respectively). High tem- easily from anatase, but appears to be metastable perature Ea values (1.8 eV) of both workers agree with respect to rutile. The structure of this modifica- very well. Internal friction measurements [123c] on tion is not known. The new form converts slowly to vacuum-reduced rutile (for 1-3 h at 1530 K), on rutile on heating between 770 to 973 K via a short- the other hand, indicate that the predominant point- range order phase. defects are neither anion vacancies nor cation inter-

According to McQueen et al. [91], rutile at ordinary stitials and the observed data in the range 50 to 300 temperatures transforms to an orthorhombic modi- K are best explained by assuming that Ti interstitials fication at 330 kbar pressure with considerable de- are associated in pairs in TiC>2 . Resistivity studies crease in volume (^21%). Bendeliani et al. [90] on rutile under various pressures [124] indicate a have synthesized the orthorhombic modification at large decrease in p up to ^6 kbar; at higher pres- ~70 kbar pressure and 970 K (pressures 40-120 sures (up to ^100 kbar), the resistance change was kbar and 670 to 1770 K, under hydrothermal condi- small suggesting that Ti02 retains semiconductor tions). This phase reverts to rutile on heating to behavior over a large pressure range with the possibil- 720 to 870 K. The detailed physical properties of ity of exhibiting metallic conduction at very high this phase are not known at present. pressures [124, 125]. Preliminary results of Kawai Even though the anatase and brookite modifica- and Mochizuki [125a] actually indicate a highly tions have not been investigated in detail, extensive conductive state in Ti02 at pressures greater than 2 literature is available on various physical properties Mbar. of the rutile modification of TiC>2 [43, 116-120] and TiC>2 in the rutile modification forms solid solu- have been treated in detail in many review articles tions with other transition metal oxides with rutile

[57, 61, 63, 116, 121, 122]. Stoichiometric TiC>2 shows related structures; thus, solid solutions with VO2 a temperature independent susceptibility consisting [126-131], Zr02 [132, 133], Nb02 [134-136], Mo02 of a diamagnetic and a Van Vleck contribution [43, [134], Ta02 [135], W02 [137, 138], Ir02 [139] are

121, 122a]. Ti0 2 has a high room temperature re- known and the physical properties are well investi- sistivity and exhibits semiconducting behavior; in- gated. In most of the cases, undistorted rutile struc- trinsic conduction seems to set in only at very high tures are formed and the properties of ZO2 temperatures (>1500 K) with a large energy gap (Z = transition metal) are modified by the presence

(~3 eV). Doped or reduced Ti0 2 has a lower re- of Ti(>2 . sistivity and exhibits impurity conduction. Detailed

16 Titanium oxides

Oxide and description Data Remarks and inferences References of the study

TiO* (*<0.70) TieO

Crystal structure and Hexagonal; space group, P31C; a = 5.14 A, The transition appears to be an [9] phase transformation. c = 9.48 A. DTA shows a transition at order -disorder type with respect ~710 K; AH = 360 cal/mol; AS = 0.51 to the interstitial positions of the e.u. oxygens; no superlattice reflections are noted in electron diffraction patterns of a sample quenched from 970 K. TiaO

Crystal structure. Hexagonal; space group, P312; a =5.1418 The structure consists essentially of [10]. A, c = 14.308 A. a close-packed hexagonal arrangement of Ti atoms with every second layer of octahedral interstices normal to the c axis vacant; one third of the oxygen sites in the occupied layers are empty and these vacancies have an ordered arrangement in the direction of the c axis. Physical properties of

Ti 30 are not known in detail at present; it appears to be stable up to ~1670 K. TijO

Crystal structure. Rhombohedral; space group, P3ml; a = The ordered structure, described as [4, 11, 26] 2.959 ±0.003 A; c =4.845 ±0.004 A. TEC TioOi-y exhibits a homogeneity 10-«/ o C (293-773 K): ||“: 10.3 X C; || :9.2X range which increases as the tem- o 10“«/ C. perature is lowered.

TiOo.es (5-oxide) Hexagonal; space group, P6/mmm; Z — 3; This is a deficient interstitial solu- [26]. a=4.9915 A; c = 2.8794 A. TEC: (293- tion of oxygen in a metal atom -6 e ||° = 773 K): 10.0Xl0 /°C; || =9.0X lattice. io-«/ 0 c.

TiO* (0.70^ *0.25)

Crystal structure and [28] physical properties.

17 Titanium oxide—Continued

Oxide and description Data Remarks and inferences References of the study

High temperature quenched (cubic, Fm3m) phases:

Decrease in Composition Density Lattice % Vacancies vacancies Tsc* P (300 K) 3 4 X (g/cm ) parameter, (Total) (after pres- (K) (10 fi cm) (mV/°C) .(A) sure treatment), %

0.86 5.057 4.189 31.5 14 <1.3 2.8 4 0.91 5.025 4.186 29.9 12 <1.25 ~3.0 5 1.00 5.000 4.184 28.0 17 1.35 2.8 8 1.18 4.907 4.173 24.7 11 1.7 3.4 11 1.24 4.870 4.170 23.3 22 2.0 3.6 12

• Pressure treated samples (at 50—60 kbar and 1370 to 2070 K). 6 No magnetic susceptibility was observed for any TiO z samples down to the limit of the senstiivity of the magnetometer (Xji~150 X 10" emu); Hall I voltage observed was very small and no meaningful results could be obtained.

Vacancy filling in TiOi.oo and physical properties. [29].

TEC Conditions of Density Lattice para- % Vacancies Compressi- (X10 6 7a Tsc 3 13 -1 study (g/cm ) meter, a (A) (total) bility (X10 deg ) (K) cm2 /dyne) at 298 K

1773 K, 1 bar; 2 days, 4. 97 ±0.003 4.1796 28.8 5. 5 ±0.08 6.73 1.18 0.7 quenched.

1923 K, 77.4 kbar; 5. 69 ±0.009 4.2062 0.0 4. 2 ±0.008 6.30 — 2.3 sudden quench.

The gradual increase in density and lattice parameter with decrease in vacancy concentration under various pressure and temperature treatments are studied in detail. An empirical equation correlating the lattice parameter, pressure and temperature i6 deduced from experimental data.

18 a

Titanium oxide— Continued

Oxide and description Data Remarks and inferences References of the study

Compressibilities of Experimental values are 1.6(4) and 2.8(7) X These values compare well with [34]. TiOz. 10 -13 cm 2 /dyne for x=0.82 and 1.25 re- those of other transition metal spectively. monoxides.

Seebeck coefficients of Small and negative ( — 5 to —10 juY/°C) for These results are in agreement with [140, 141].

TiOz (0.8 1.2). all x in TiOz • a increases with x and tem- those of Banus and Reed [28]. perature. Qualitative description of the

band structure is given; Ef changed appreciably with the composition x.

Plasma resonance in Reflectance data show the plasma edges in The studies indicate the metallic [14, 142]. TiOz. TiOz in the range 3. 6-4.0 eV and the ac- nature of TiOz. A band of 2.5 eV

tual value depends on x to some extent. in the reflectance spectrum is as-

The color of TiOz is correlated with these signed to an interband transition. plasma edges.

Band structure of TiO. Tight binding and AP^ methods of theo- Calculations on TiOz taking into ac- [143-145]. retical calculation of the band structure count the vacancy concentration of TiO indicate that the 3 d bands are and composition do not seem to wide (~6 eV) and the electrical conduc- be satisfactory in explaining the tion can be assumed to take place in the observed data. As pointed out by

wide band. Most of the transport proper- Honig et al. (ref. [4], p. 130) cation- ties are predicted in good agreement with anion-cation overlaps also should experiment. be taken into account in comput- ing the band structure in addition to the direct cation-cation overlap in order to get a reasonable picture of the band model in oxide materials. Results of Kawano [141] indicate that the Fermi level and density of states shift with the stoichiometry in TiOz.

Low temperature ordered structures

Phase, stability range and conditions of observation:

TiOo.r-TiOo.g: thin Orthorhombic; space group, 1222; Z=6; This structure results from the or- [17, 19, 31]. foils (of TiO z ) heated a' = a V2; b —3a \/2; c' — a ( is the lat- dering of the oxygen vacancies above 870 K in the tice parameter of the original, high tem- only since there are more oxy- electron microscope or perature, cubic cell of TiOz). gen vacancies than titanium heating by an intense vacancies in the oxygen deficient electron beam. alloys; all oxygen vacancies are distributed on every third (110) plane in the original rock salt type lattice while the Ti vacancies are distributed randomly.

19 Titanium oxide—Continued

Oxide and description Data Remarks and inferences References of the study

TiOi.oo (composition Monoclinic; space group, A2/m; Z = 12; a = The transition temperature is [14, 17, 19, 5.855 6=9.340 c = 4.142 = 1190-1220 -TiO . A; A; A; around but the range TiO0 . 90 1 10 ); 0 K; 20, 28, 31]. quenched from 1773 K 107.53°. Vacancy concentration, 16.7%. process is slow (and requires long ~ and annealed for two p (300 K) = 2X10 4 V cm; p (77 K)=7X periods of annealing) since this is -5 days (1170-1220 K). 10 fi cm. Positive temperature coeffi- a disorder -order transformation. cient; Absolute value of p is much lower This structure can be described than that of the cubic phase. in terms of vacancy ordering:

i.e., in every third (110) plane of the original cubic cell, half of the % titanium and half of the oxygen atoms are missing alternately. The spacing of this (110) plane is slightly larger than that of the other {110} planes, and as a result, the symmetry of the original

cubic structure is lowered to monoclinic. Essentially the same structure has been observed in - the composition range TiOo. 90 TiOj.io which implies that a small excess or deficiency of titanium and oxygen vacancies can be tolerated in TiOi.oo structure.

TiOi .19

Samples annealed for Both types of ordered structures, TiO and Electron diffraction patterns indi- [18, 31].

long periods below TiOi. 25 , are noticed. cate that the ordered phases co- 973 K. exist as alternate thin layers of

TiOi.oo and TiOi .25 parallel to

(210 } c planes.

TiOi .25

Alloy annealed for 7 Tetragonal; space group I4/m; Z = 10; a = The type of ordered structure in [18, 31,

days at 973 K and then ao-\/lO/2 =6.594 A; c = ao = 4.171 A. (ao is TiOi .25 is different from that in 146, 147]. at 673 K. the original cube cell edge). [Vacancy TiOi.oo; the oxygen sublattice is concentration, ~22%]. fee and fully occupied while the titanium sites get ordered similar

to Ni4 Mo and Au 4 Mn type super structures.

Transition structure Both ordered TiO and TiO 1.25 structures The appearance of the transition [31]. (range for . of the latter increas- is of importance TiO0 7 -TiO 1 . 25). coexist, the amount structure Samples rapidly cooled ing with the oxygen content of the speci- understanding the process of from the melt and not men. The transition structure observed transformation from the dis-

well annealed at lower occasionally in the TiOi .25 specimen seem ordered to the ordered form. temperatures. to be due to insufficient ordering treat- When the disordered specimen of

ment and it is considered that the transi- any composition is cooled to the tion structure would ultimately transform equilibrium temperature, the

to the ordered TiOi .25 structure. vacancies concentrate firstly on every third atomic plane parallel to the (110) plane of the original

20 Titanium oxide—Continued

Oxide and description Data Remarks and inferences References of the study

rock salt structure, but are randomly distributed on these planes giving a diffraction pattern of the transition structure. On further annealing, the vacancies are distributed into the proper positions to make the

ordered structure of TiOi .26

type. Watanabe et al. [31] point out that the high temperature cubic and orthorhombic TiO*

observed by Hilti [19] are the same as the 'transition structures.’

TisOj

Crystal structure and T = 300 K: Hexagonal (corundum); space There is no change in the crystal [30, 39, 40]. high temperature group, R3C; 6; a ^5.1572 ±0.0002 A; symmetry in the range 300=500 x-ray studies. c* 13.600 ±0.001 A. K but the thermal expansion T*540 K: parameters behave in an Hexagonal; space group, R§C; Z = 6; anomalous way; the a parameter 5.122 A; c- 13.90 A. AV (on heating from decreases while c parameter in- 300 to 500 K) = -7%. creases. The unit cell volume decreases.

-4 Magnetic properties. Xm (300 K)~1.0X10 cgs units and shows Xm -T data and polarized neutron [12, 41-43, a two fold increase in the range 400—550 diffraction data do not indicate 47]. K. Xm below 400 K is almost temperature magnetic order in T^Oa below independent. 400 K.

Magnetic moment <0.03 /ub-

Thermal properties. Cp (400 K)~0.2 cal/mol, deg. and shows a These data show the second (or [32, 48, 49]. broad X-type anomaly in the range 450- higher) order nature of the

600 K. AH (calc.)~36 cal/mol [48]; 215 transition cal/mol [32], DTA shows a broad endothermic peak (centered at ~410 K).

21 8

Titanium oxides—Continued

Oxide and description Data Remarks and inferences References of the study

Electrical properties This is a careful study on well [52, 53]. of TiOi.501. characterized pure single crystals.

Temp- Rc a Various ranges of p and R c values 3 era- P (cm / Ap/ Wl are encountered because of vari- a ture (ft cm) C) P K) able sample purity; common (K) impurities like C, N affect the properties. The study by Honig

and Reed [52] emphasizes the (b) 4.2 K (1.3-1. 8) — 3-4 — need for painstaking efforts in X104 (^200 the preparation and characteri- kG) zation of oxide materials. Slight

3 77 K 0.2-0. 0.4- <10“ anisotropy in p has been noted. 1.5 Impurity conduction seems to be 273 K ~10~ 2 0.08- <10- 3 56 predominant below 20 K; in the 0.11 range 100-300 K, intrinsic be- 10~4 600 K ~3 X — — 45° havior is noted. Hall mobilities are the highest encountered hitherto in pure oxide materials;

2 (a) Magnetoresistance; Ap/po^H . (b) at higher temperatures, ph seems Not measurable, (c) At 400 K. to decrease. Magnetoresistance

Ea ( <20 K)~0.001-0.003 eV; data indicate that a multiband

Ea (100-300 K)~0 .027-0.050 eV. conduction exists in Ti 203 . The 2 T = 4.2 K: mh (calc.)~720-840 cm / band model proposed (fig. 1.2) 10 -3 Vs; n e ~10 , cm . can explain the observed data without invoking a magnetic

ordering in Ti 2 03 at low temperatures.

UHF Dielectric UHF (~10 GHz): * (T = 77 K)=30; The imaginary part of the dielec- [148]. constant studies. p (UHF) >p (DC). tric constant shows a maximum at 225 K, but this is not connected with any transition in

Ti 2 0 3 .

Raman spectroscopy Raman spectrum of Ti 2 C>3 exhibits seven The Raman results can be inter- [149]. modes as predicted for the corundum preted in terms of the electronic structure. All the modes persist above the and structural changes during the semiconductor-metal transition tempera- transition. ture indicating absence of space group -1 change. The Ai g mode at 269 cm (300 K) shows a 16% change in frequency and increase in intensity at the transition.

TisOs

Crystal structure T = 300 K: Monoclinic; space group, C2/m; There is not much change in crystal [49]. = = and x-ray studies. Z 4; a =9.80 A; 6=3.79 A; c 9.45 A; symmetry at T t but the unit cell (3=91.75°. T>460 K: Monoclinic; a = volume increases. The transition

9.90 A; 6 = 3.78 A; c = 10.02 A; 0=90.75°. is of first order. The c parameter and the unit cell volume show jump at T< (448 K) on heating.

8 Magnetic properties. Xm (300 K)~10 cgs units; xm (600 K)~ Mulay and Danley [71] proposed a [43, 49, 71]. -4 2 X 10 cgs units, xm is temperature inde- new model to explain the mag-

22 Titanium oxides—Continued

Oxide and description Data Remarks and inferences References of the study

pendent both above and below T t ; large netic behavior of Ti 3Os and the

jump at T t is noted with hysteresis; T t Magneli phases. According to as (heating) « 462 K; Tt (cooling) 432 K. this, below T t , groups of cations Xii-T data and Mossbauer studies do not are distributed periodically indicate long range magnetic ordering in throughout the lattice; within the low temperature phase. these groups, the d electrons are

Rao et al. [49] have pointed out that the localized. However, 'constrained

Mossbauer data on Fe-doped Ti 3 Os type’ antiferromagnetism sets in should be interpreted with caution since between specific neighboring d

Fe is known to stabilize the high temper- electrons through homopolar

ature phase of Ti 3 Os. Neutron diffraction bonding of cations. Neighboring

studies are urgently needed on Ti 3 Os to groups interact via thermal ex- establish the presence or absence of mag- citation of electrons. Above the netic order in this material. transition, this type of ordering

is modified by crystal structure

changes and 3d orbital overlap is large enough to bring about a nearly complete delocalization of electrons over the entire lattice

and the behavior is described by the 'free electron gas’ model.

This approach is a slightly modified form of the one proposed by

Goodenough [74].

Thermal properties. Heat content data on Ti 3 Os exhibit an The first order nature of the transi- [32, 49, 75]. anomaly at ~450 K. DTA shows a sharp tion is established. Fe doping

endothermic peak at 450 K; the peak is reduces T t and at higher concen- shown in the cooling curve at ~415 K; trations (5%) suppresses the AH = 1.6 rt0.4 kcal/mol. transition; the transition at lower concentrations does not seem to be strictly first order.

Electrical properties. p (340 K)«20 ft cm; p (500 K)«0.6 ft cm. The first order semiconductor-to- [49, 71, 75, Semiconductor behavior below 450 K; metal transition is confirmed in 76]. = Ea 0.29 eV. Large drop in p at T ( (Bar- Ti 3 Os; Rao et al. [49] stress the tholomew and Frankl [75] report a con- the importance of the purity of tinuous change in p through Tt). Metallic the material in determining the

behavior above T t ; m* (calc., high tem- resistivity discontinuity at Tt. perature form) «20 m; a (340 K)~ —60 mV/°C [76].

ESR Studies. Multiplet structure is noted in the ESR The multiplet structure appears to [49, 73]. spectra at 77 K. Well defined spectra are be due to inequivalent cation

not obtained at higher temperatures and sites in the structure of Ti 3C> 5 . in doped samples (at 77 K).

23 Titanium oxides—Continued

Magneli phases, Ti nO„_i

n: phase Crystal structure, x-T and DTA Electrical properties References studies

(T = 298 K: All phases are triclinic; space group. Pi; Z = 2) = 4; Th07 a = 5.600A; 6 = 7.134 A; c = 12. 468 A; a Ti407 is metallic at 300 K and p [43, 71, 72, 95.05°; 0 =95.20°; 7 = 108.70°; (calc. increases by a factor of 10 3 at 75, 79, 84, 3 density, 4.32 g/cm ). 149 K on cooling and semicon- 86, 87].

T ( = 125 and 149 ±2 K. Discontinuities ductor behavior in the range 130-

in the lattice parameters are noted at 149 K (Ea ~0.04 eV). At ~125 the 149 K transition; the volume change K, p shows an increase by about is <0.001 and the extra reflections three orders of magnitude. observed below 149 K suggest a doubling No Seebeck coefficient data of the unit cell. The 125 K transition available. is associated with ~8-10 K hysteresis At 300 K, m* «*13m and Hall data and some structural changes seem to give a mobility of ~1 cm 2 /V s, take place at this temperature. increasing to 4 at -~160 K; below DTA (heating curve) indicates both the 149 K, no Hall voltage could be transitions but the cooling curve indi- measured and hence ph<0.01 cates only the 149 K transition. AH val- cm 2 /V s. ues not known. Least square refinement of the X-T data do not indicate the low tempera- structure has shown that the ture transition. Probably long range mag- sharing of octahedral faces, edges

netic order is absent. and corners between rutile layers

is similar to that in corundum structures. Average Ti-0 and 0-0 distances are 2.012 and 2.826 A respectively. Average Ti-Ti distances in a rutile layer are 2.972 and 3.576 A across a shared octahedral edge and comer respectively. = 5; Ti 509 a = 5.569 A; 6=7.120 A; c = 8.865 A; <* Discontinuities in p are noted at T t ; [43, 71, 72, 97.55°; 0 = 112.34°; y = 108.50°. however, the sample appears to 75, 78, 79, T< = 130 and 136 K. be semiconducting above 136 K. 84].

DTA and x-T studies show anomalies at Ea (<130 K)~0.04 eV; these phase transitions; no AH values are m* (>136 K) ~ 11m. known. Probably long range magnetic

order is absent. 6; TifiOn a = 5.56 A; 6 = 7.14 A; c = 24.04 A; a =98.5°; p drops by a factor of 10 at 130 K [43, 71, 72, 108.5°. 0 = 120.8°; 7 = T t = 122 K. and the sample is semiconducting 75, 79, 84].

DTA and x-T studies show this transition; both below and above T t .

no AH is known. Probably long range Ea (<130 K)~0.08 eV; m* ~ magnetic order is absent, ( > 130 K) 16 m. The T t noted here is less than that obtained (122 K) from DTA and x-T data. 7; ThOn o = 5.54 A; 6 = 7.13 A; c = 15.36 A; a = 98.9°; No data are available [79, 84]. 0 = 125.5°; 7 = 108.5°. No x-T, DTA data are available. 8; TigOis a = 5.57 A; 6=7.10 A;c = 37.46 A; a=97.2°; Semiconductor behavior in the [75, 79, 84]. 0 = 128.8°; 7 = 109.6°. temperature range 78-295 K; no DTA studies do not indicate a transition in indication of transformation.

the temperature range 78-295 K. x-T Ea ~0.09 eV. data not available.

9; Ti 90i 7 o = 5.57 A; 6 = 7.10 A; c = 22.15 A; a =97.1°; No data are available [79, 84]. 0 = 131.0°; 7 = 109.8°. No x-T, DTA data are available.

24 n Titanium oxides—Continued

Oxide and description Data Remarks and inferences References of the study

Ti02 (anatase)

Crystal structure and T =298 K: Careful work up to the transition [150]. x-ray studies. Tetragonal; space group, I4/amd; Z = 4; point; the variation in the relative a =3.7845 ±0.0001 A; c = 9.5143 ±0.0004 magnitudes of the values of TEC c : A. TEC (300-985 K) ( /°C): || 7.380 X are explained in terms of the inter- 9 -u 2 c 10-6 f t : ionic +6.620 X10" + 1.771 X 10 ; ± distances. Earlier literature 3.533 X lO-6 + 5.610 X 10"9 t + 4.315 X survey presented. -12 2 10 1 , where t is the temperature in °C.

Kinetics of trans- The rate of transformation of pure anatase The irreversible transformation of [92-98, 100, formation of anatase is exponential (fig. 1.3); energy of activa- anatase to rutile is reconstructive 102-104].

—» rutile and thermal tion is ~90 kcal/mol. The transition is and involves changes in the properties. immeasurably slow below 880 K and ex- secondary coordination. The par- tremely rapid above 1000 K and is accom- ticle size, the crystallite size and panied by ~8% decrease in the unit cell the volume of the unit cell seem to volume. AH is ~ —2.8 kcal/mol (exo- increase prior to the trans- thermic). Impurities usually stabilize the formation. The impurities seem to

the anatase modification; T t as well as act by way of creating anion the energy of activation are greater in vacancies or cation interstitials these doped materials compared to pure in stabilizing the anatase phase. rutile. Smaller particle size and larger surface area seem to favor the transition.

Effect of pressure on Discontinuous changes in the resistance of Detailed work on the effect of [111].

the transition and the anatase samples noted at T t . In the pressure on the transition. Aff

resistivity studies. range 3.8-24 kbar pressure, T t decreases for the transition also decreases

with the application of pressure; (dTt / with pressure; the oxygen defi- = —0.02° -1 dP ) bar , p (300 K, pre- ciency seems to be responsible for pressed specimens, anatase) ~10 12 Q cm; the acceleration of the rate of p (300 K, pressed at 3.8-24) kbar ~107 transition at higher tempera- Q cm. tures. The results are in accord with the volume change associated with the anatase-rutile transition.

-6 Magnetic properties X=0.02X10 emu/g (essentially inde- Not a very accurate value [122a] of anatase. pendent of temperature) because of the presence of impurities; the observed variation of x with T is expalined on the basis of the presence of Ti+3 ions in the anatase samples.

Ti02 (brookite )

Crystal structure T = 298 K Data on the naturally occurring [151]. Orthorhombic; space group, Pcab? Z = 8; pure brookite sample; contains

a =9.25±0.03 A; fi = 5.46±0.02 A; c = references to the earlier data. 5.16±0.01 A.

Kinetics of brookite The transition is irreversible and time and The enthalpy of transition is small [115]. rutile transition and temperature dependent (fig. 1.3); below compared to the anatase-rutile thermal properties. 990 K, the transition is extremely slow. transition. This indicates that

Above 990 K, first order rate law is the polymorphic transformation obeyed. Energy and entropy of activation may not be of the reconstructive

25 |

Titanium oxides—Continued

Oxide and description Data Remarks and inferences References of the study

are ~60 kcal/mol and ~ —18 cal/mol type but only a modified order- (at 1070 K) respectively. DTA shows an disorder type as applied to exothermic peak at 1020 K; A —(100 diffusionless transitions in solids. ±75) cal/mol.

Ti02 (high pressure)

Crystal structure and Orthorhombic; space group, Pban; Z — 4; The high pressure phase reported [90, 91]. transformation to a =4.529 A; 6 = 5.464 A; c = 4.905 A. This by two groups of workers pre-

rutile. phase is formed from rutile at ~330 kbar pared under different experi- pressure at ordinary temperatures; it re- mental conditions appears to be verts to rutile on heating at 720-870 K. identical. As expected, the high A lower bound of 60 kbar is necessary to pressure phase is denser than stabilize the higher pressure form with rutile and can be retained with- respect to rutile at room temperature. out changes under normal conditions of temperature and pressure after quenching for in- definite periods of time. Anatase

is metastable with respect to the rutile and the high pressure forms. Reaction boundaries for these phases have been reported. Ti02 (rutile)

Crystal structure and T — 298 K; Tetragonal, space group, P42/ Careful work up to the high tem- [150, 152]. x-ray studies. mnm; Z = 2; a =4.5941 ±0.0001 A; c = peratures. Slight anisotropy in 2.9589 ±0.0001 A. TEC (303°-923 K) the TEC parameters is obvious c -9 /°C): : 8.816 XlO-«+3.653X IO t + and is explained in terms of the ( || 12 2 c 6 t : X10~ distances in the unit 6.329 X10" ; ± 7.249 +2.198 interatomic -9 -12 2 t t is the cell rutile. X10 t + 1.298X 10 , where of temperature in °C. High pressure x-ray studies indicate volume change, (V/Vo) of 0.91 at 102 kbar [152] and 0.94 at 158

kbar pressure [91].

-6 Magnetic, optical and X = (0.067 ±0.0015) XlO emu/g for high The temperature independent x is [43, HO- dielectric properties. purity single crystal rutile and tempera- due to diamagnetic and Van 121, 122b, ture independent in the range 55-372 K; Vleck contributions. The free car- 123a, 153]. negligible anisotropy. rier absorption is due to the elec- Stoichiometric rutile shows continuous trons released by the oxygen upon absorption in the IR region [122b] but reduction; the bands in the re- reduced samples exhibit free career ab- duced samples are believed to be sorption and characteristic bands (at 0.75 due to polaron absorption. The or 1.18 eV) depending on the sample re- environment of Ti sites in rutile, sistivity or degree of reduction. Optical the anisotropy and high numerical

energy gap = 3.05 eV. eo has a high value values of eo make an interesting

and exhibits anisotropy; at 300 K, eo comparison with BaTiC>3.

(||<0 =170; eodl*) =86; n(|K> 2.903; n(||*) = 2.616.

At low temperatures, eo approaches limit- c ing values; at T—>0 K, «o(| ) = 257 ; e0 (||°) = 111. Parker’s data [153] in the range 1.6-1060 K indicate that rutile is not a ferroelectric or antiferroelectric; how-

ever, the ionic polarizability of Ti is close

26 «

Titanium oxides—Continued

Oxide and description Data Remarks and inferences References of the study

to the critical value for a ferroelectric polarization catastrophe at all temperatures.

8 Electrical properties p (300 K) ~10 ficm for stoichiometric ru- Multiple band conduction is indi- [116-125a, and high pressure tile; £ ~3.1-3.4 eV; impurity conduction cated by the Rh and a data 154]. ff p, studies. is always predominant at low tempera- but many workers feel that tures; donor levels of 0.03-0.15 eV are en- polaron type conduction is pre- countered. dominant at low temperatures.

Reduced specimens have low resistivity The defect structure of Ti0 2 is -2 (~10 to 104 Slcm), are n-type and ex- not yet completely understood. hibit anisotropy in Rh, he and p. Aniso- Kawai and Mochizuki [125a]

tropy ratios, (po/pe) and ( Ra /Rc)H vary experiments at high pressures on

from 2.0 to 3.5 and 0.2 to 2.5 respectively the p behavior of Ti0 2 are signifi- and exhibit interesting temperature de- cant; x-ray data at these high pendence. pressures are highly desirable

2 p.H~ 0.1 cm /V s. at 500 K; a (100 K) = and will indicate whether the — 1.0 mV/K. m* =20 mo. a shows large resistivity behavior actually phonon drag effect at T below 20 K. corresponds to the insulator- ~lln In the range 882-1273 K, p po2 , metal (Mott) transition or not. n 4.4 to 5.7; n =4 above 1470 K according to Greener et al. [123]; both anion vacancies and cation interstitials appear to play

significant role. Ea (T>1000 K) =1.8 eV. High pressure studies indicate a large

decrease in p up to 6 kbar (dp/dP = —0.2 4 -1 XlO flbar ); in the range 6-100 kbar, -1 change in p is small {dp/dP = 0.08 fihar ). Data of Kawai and Mochizuki [125a] indicate that at pressures >2Mbar p of

Ti0 2 drops by a factor of 10® and exhibits metallic behavior at room temperature.

3 Thermal and Cp — 1 millijoule/K, mol at 4.2 K; CP ^T It is evident that the degree and [118, 123c, mechanical below 4 K with 0 d = 758 K. type of reduction brings about 155]. properties. Reduced Ti0 2 has large additional con- marked changes in these proper tribution and independent of T below 13 ties in rutile. K and is attributed to electrons which do not become degenerate above 1 K. k (200 K)=0.1 W/cm, K and exhibits anisotropy above 25 K; goes through a maximum at ~15 K indicating phonon-

phonon scattering in Ti0 2 . -1 Internal friction goes through a ( Q ) maximum at 323° in pure rutile with Q~ l -4 (| ;°) = 1 X 10 at 1842 Hz and activation energy of 14 ±0.4 kcal/mol. Reduced rutile exhibits a pronounced maximum (()-i(||«)=12xl0— 4 at 394 K and 1838 Hz with activation energy = 23.5±1.0 kcal/mol) only along [100] direction and not along [110] and [001] direction. The results are discussed in terms of simple and associated defects in pure and reduced rutile.

27 Titanium oxides—Continued

Oxide and description Data Remarks and inferences References of the study

Band structure of Calculations using tight binding method in- Since there are no magnetic order [156]. rutile dicate relatively wide valence and con- complications in Ti02 it is worth duction bands (^-4 eV). The available while to compute the detailed transport data can be interpreted assum- band structure of Ti02 using ing multiband conduction in Ti02 even at various methods including spin- low temperatures (~100 K). orbit coupling effects.

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[129] Sakata, K., and Sakata, T., Japan J. Appl. Phys. 6, 112 V0-V2 0 6 , by Andersson [18] and Andersson [19] and (1967). in the range VO2 -V2 O 5 , by MacChesney et al. [20], [130] Kristensen, I. K., J. Appl. Phys. 39, 5341 (1968). [131] Rao, C. N. R., Natarajan, M., Subba Rao, G. V., and Theobald et al. [21], Sata et al. [22, 23], and Tilley Loehman, R. E., J. Phys. Chem. Solids 32, 1147 and Hyde [24]. The available data have been sum- (1971). [132] Ksendzov, "Va. M., and Prokhvatilov, V. G., Zhur. Fiz. marized by a few workers [25, 26]. Stringer [26] Khim. 31, 321 (1957). constructed a schematic phase diagram and sug- [133] Keler, E. K., and Andreeva, A. B., Ogneupory 25, 320 (1960); Chem. Abstr. 54, 25663i (1960). gested a systematic nomenclature of the phases. A [134] Marinder, B.-O., Dorm, E., and Seleborg, A., Acta modified phase diagram of Stringer [26] and Chem. Scand. 16, 293 (1962). et al. (in [135] Riidorff, W., and Luginsland, H. H., Z. anorg. allgem. MacChesney [20] the region VO2 -V2 O 5) Chem. 334, 125 (1964). are shown in figure 1.4. [136] Sakata, K., Nishida, I., Matsushima, M., and Sakata, -0 T., J. Phys. Soc. Japan 27, 506 (1969). The V system is quite a complex one. In addi- L. L. Y., Scroger, M. G., and Phillips, B., [137] Chang, tion to the stable compounds like V2O3, VO2, and J. Less-Comm. Metals 12, 51 (1967). [138] Chang, L. L. Y., J. Am. Ceram. Soc. 51, 295 (1968). V2 0 5 having narrow homogeneity ranges, compounds [139] McDaniel, C. L., and Schneider, S. J., J. Res. Nat. Bur. with wide homogeneity range like VO, Magneli pha- Stand. (U.S.), 71A (Phys. and Chem.), No. 2, 119-123 (Mar. Apr. 1967). ses of the general formula Vn02 n-i (4^n^8) and [140] Samakhvalov, A. A., and Rustamov, A. G., Sov. Phys. = Vn 02n+i (n 3, 4, and 6 ) exist. Several new unidenti- Solid State (English Transl.) 5, 877 (1963). [141] Kawano, S., Japan J. Appl. Phys. 8, 1264 (1969). fied phases also exist in the region VO2-V2O5 [21-24]. [142] Rao, C. N. R., Wahnsiealer, W. E., and Honig, J. M., Oxygen forms a solid solution with metallic vanad- J. Solid State Chem. 2, 315 (1970). [143] Yamashita, J., J. Phys. Soc. Japan 18, 1010 (1963). ium up to about 11 mol percent (VO0.4) distorting [144] Ern, V., and Switendick, A. C., Phys. Rev. 137A, the cubic lattice of V [1, 2, 5 ]. Range VO0.6-VO0.8 1927 (1965). [145] Schoen, J. M., and Denker, S. P., Phys. Rev. 184, 864 has not been accurately studied. (1969). Vanadium monoxide has a wide homogeneity [146] Harker, D„ J. Chem. Phys. 12, 315 (1944). range all the [147] Watanabe, D., Acta Cryst. 10, 483 (1957); J. Phys. (VO 0 . 85 -VO 1 . 27 ) and compositions have Soc. Japan 15, 1251 (1960). been investigated experimentally. VO 1.5 (V2 C>3 )ap- [148] Samokhvalov, A. A., Sov. Phys.—Solid State (English Transl.) 3, 2613 (1962). pears to have a very narrow range of homogeneity Mooradian, A., and Raccah, P. M., Phys. Rev. B3, [149] [27]; different phases have been detected in alloys 4253 (1971). [150] Krishna Rao, K. V., Nagender Naidu, S. V., and having compositions VO 1.44 and VOi.es. The existence Iyengar, L., Am. Ceram. Soc. J. 53, 124 (1970). and stability of the Magneli phases, VnChn-i* with re- [151] Yoganarasimhan, S. R., and Rao, C. N. R., Anal. Chem. 33, 155 (1961). spect to the oxygen partial pressure and tempera- [152] Clendenen, R. L., and Drickamer, H. G., J. Chem. ture have been examined by various workers [9, 11, Phys. 44, 4223 (1966). = 00 [153] Parker, R. A., Phys. Rev. 124, 1719 (1961). 12 , 17, 18]. VO2 (V n02 „_i, n ) has a narrow [154] Hernandez, W. C., and Kahn, A. H., J. Res. Nat. Bur. homogeneity range [10, 28]. Besides V2 O 5 , in the Stand. (U.S.), 67A (Phys. and Chem.), No. 4, 293-299 (July-Aug. 1963). series V„02 „+ i, V3O 7 [29], V 40 9 [30] and V 60i3 [18] [155] Keesom, P. H., and Pearlman, N., Phys. Rev. 112, 800 have been prepared and characterized. (1958). [156] Kahn, A. H., Frederikse, H. P. R., and Becker, J. H., in Transition Metal Compounds, Ed. E. R. Schatz VO: VO is unique in having an extremely wide (Gordon and Breach, Science Publishers, New York, 1964). composition range essentially symmetric about the stoichiometric composition together with an extra- 1.3. Vanadium Oxides ordinarily high vacancy content (^20%). It has a cubic rock salt structure and the lattice parameter a The vanadium-oxygen system has been extensively increases with increasing oxygen content but the studied by several workers in the different homo- density decreases; however, the increase in a is not geneity ranges by employing a variety techniques. linear with x in \O x whereas the density versus x

In the range V-VO, by Seybolt and Sumsion [1], plot is linear. According to the recent study on well-

Schonberg [2], Rostoker and Yamamoto [3], Ozerov characterized and annealed VO x samples, the lower

al. to [4], and Cambini et [ 5 ]; in the range V-V2O3, by limit for the single phase cubic region appears 1.30 Klemm and Grimm [ 6 ], and Westmann and Nord- be between 0.75 and 0.79 and the upper limit,

mark [ 7 ]; in the range V2O3-V2O4, by Morozova et al. [31]. Mathewson et al. [32] first noticed that the 3 is approxi- [ 8 ], Katsura and Hasegawa [9], and recently by x-ray density of VO 1.00 (6.50 g/cm )

Kimizuka et al. [10], Okinaka et al. [11], and Ander- mately 15 percent higher than the pycknometric

12 in 3 lattice is highly son and Khan [ ]; the range V2O3-V2O5, by value (5.6 g/cm ) suggesting that the Hoschek and Klemm [13], Burdese [14], Grossman defective at the stoichiometric composition. The

30 Figure 1.4. (a) V-0 phase diagram (after [26]). The various phases and stability ranges have been discussed in detail by Stringer, (b) VO2-V2O5 phase diagram showing the VnChn+i

phases ( after [20]). variation of the defect concentration of the cationic Because of the wide homogeneity range and large and anionic lattices across the phase field has been number of vacancies, VO* shows interesting physical examined by many workers [7, 27, 31, 33-36]. At and chemical properties. With increase in * in VO x , the oxygen rich phase limit the anion lattice is p, dp dT, and xm increase linearly with abrupt in- nearly perfect with almost zero anion vacancies while creases in slopes at x = 1.00, while a changes smoothly at the vanadium rich phase limit, however, the cation from n-type to p-type at *=1.02. The metal rich lattice still contains ~13 percent vacancies. Banus VO is a semimetal with weak temperature-inde- and Reed [31] and Taylor and Doyle [36] have found pendent paramagnetism and higher * changes to a that annealing the VO x at high temperatures (~1570 semiconductor with a small energy gap and a strong- K) and high pressures (^50-60 kbar) decreases the er temperature-dependent paramagnetism. Con- total number of vacancies (by about 6-13%) re- trary to the earlier reports [37-40], recent studies on sulting in an increase in the density by ~1 percent well characterized samples of VO x indicated no and a 0.1 percent increase in the lattice parameter. metal-to-semiconductor transition for any composi- These changes are unambiguously the result of the tion on decreasing the temperature to 77 K [31, high pressure treatment, since reannealing the sam- 35, 41-43]. Oxygen rich VO shows [31] only a weak ples at 1 atm pressure at 1570 K gave samples with temperature dependent paramagnetism in the range the same lattice parameter as the material before 4.3 to 273 K with no evidence of magnetic ordering, pressure treatment. Loehman, Rao, and Honig [35] in contrast to the findings of Kawano et al. [41] have found that the vacancies in VO can be reduced who observed definite evidence of antiferromagnetic by solid solution with TiO. ordering for *=1.15 and 1.26.

31 The decrease in vacancy concentration on anneal* ing at high pressure results in slight changes in the electrical and magnetic properties. The pressure annealing of VO does not significantly change the

magnitude of p, the slope of p versus T or Xm. At all compositions, there is a 10-20 percent decrease in Xm in the range 4.2-100 K; a decrease in anion vacancies (caused by pressure anneal) affects the Xm in an opposite direction to that caused by increase in x. This indicates that xm is more sensitive to the cation vacancies than the anion vacancies. The magnitude of a increases after pressure annealing and VO1.02 changes from n-type to p-type behavior.

V2O3: Vanadium sesquioxide, V2O3, exhibits two Figure 1.5. Variation the c/a ratio with temperature transitions. Because of the dramatic changes in of of V 2O 3 [63]. physical properties accompanying these phase transi- At the transition temperature we see a drastic change in the ratio. Chromium tions and the unique behavior with respect to pres- substitution markedly affects the c/a ratio. In the Insert we have shown the variation of c/a ratio in various sesquioxides; the ratio is anomalously sure, temperature and impurity content, this oxide high in V2O3 and low in T12O3. has been studied extensively in recent years both

from theoretical and experimental points of view. V atom. Neutron diffraction [65] and magnetic sus- The metal-to-insulator transition exhibited by V2O3 ceptibility studies [66-68] indicate that the anti-

is a field of intense research activity at the present ferromagnetic ordering in V2O3 disappears at the

time (See fig. 2 in the introduction) and several transition point (154 K) where the crystallographic review articles have recently appeared [44-56] on transition also occurs. The high temperature anomaly this aspect of the problem. is associated with a discontinuity in the Xm, but the At room temperature, V2O3 has the corundum substance remains paramagnetic throughout the tem-

structure with rhombohedral symmetry [57, 58]. perature range, 150 to 1000 K [68].

There is a unique c axis perpendicular to the basal Associated with the phase transitions in V2O3, plane which contains three identical crystal axes. there occur dramatic changes in the electrical proper-

Near 150 K, a phase transition occurs to the mono- ties. At room temperature, V2O3 is a fairly good clinic structure [59-64] involving a slight tilting of metal (with the purest samples showing resistivities -3 the hexagonal c axis of the rhombohedral phase. of the order of 10 cm and a positive temperature

The transition is accompanied by discontinuous coefficient of p). When the temperature is decreased

changes in the c/a ratio (fig. 1.5) and the unit cell to 150 K (crystallographic transition point), it un-

volume (increase by about 1.4 percent) [63]. There dergoes a transition from the metallic state to an

is also a high temperature anomaly in V0O3 around insulating state with an accompanying increase in

500 K where there is no change in the crvstal param- resistivity by about seven orders of magnitude. This

eters [63]; Kosuge [17], however, finds a DTA transi- transition has been first discovered by Foex [69] tion at 430 K. Recent neutron diffraction study on and since then confirmed by many workers [37, 38,

V2O3 [65] has confirmed the presence of antiferro- 63, 66, 70-78]. The characteristic metallic resistivity magnetic ordering in the low temperature mono- in V2O3 shows an anomaly in the high temperature clinic phase (earlier measurements using various region as well. Both the c axis and basal plane re- techniques gave contradictory conclusions; see, for sistivities show a rapid increase with temperature example, reference [53]). According to Moon [65], in the region near 525 K, but keep monotically in-

the antiferromagnetic axis lies in a plane parallel to creasing up to 800 K [76]. Foex et al. [66] on the the c axis (in the hexagonal cell) and perpendicular other hand, have noted a maximum at 525 K and to one of the a axes and makes an angle of about 71° a semiconductor behavior in V2O3 above 525 K. to the c axis in this plane; the V moments are ferro- Discontinuity in a was noted at the low temperature magnctically coupled in monoclinic (010) or crystallographic transition in V2O3 [78, 79]. Above hexagonal (110) layers, with a reversal between 150 K, a was positive and small and below this tem- adjacent layers. The ordered moment is 1.2 pu per perature, it changed sign and became very small.

32 7

Rh was negative in the entire range of temperature studied (7-800 K) and did not indicate any anomaly; a did not indicate anomaly at the high temperature

transition of V2O3 [78]. X-type anomalies are noted in the heat capacities at the low and high temperature transitions in V2O3 [66, 80]; DTA studies indicate endothermic peaks

i corresponding to the transitions [17]. Anomalous behavior of V2O3 in the region of the transition point has been noted in the measurements made by a variety of techniques like NMR spectroscopy [81-

84], Mossbauer spectroscopy [74], positron annihila-

tion [85], UHF (^10 GHz) dielectric constant [86] and optical spectroscopy [76, 87]. The effect of pressure on the transition has been

studied by a few workers [38, 39, 61, 76, 77]. The

transition temperature ( \= Tn) decreases with pressure (~4 K/kbar). Application of pressures greater than 25 kbar suppresses the transition completely

[77] and V2O3 remains metallic and Pauli paramagnetic down to 2.2 K without indication of any long range magnetic order.

All the available data seem to establish that the Figure 1.6. Pressure-temperature-composition (Cr) phase dia-

gram (Vi_zCr*) 20 3 . low temperature transition in pure V2O3 is of first M, I, and AFM insulator phases have been indicated; the metal-insulator order and the high temperature one is of second phase boundary terminates at a critical point (~530 K) (after [63]).

order. The low temperature transition is sharp and

marked by a hysteresis. T is sensitive to stoichio- titanium doping. Three types of phases have been t

metry [88] and the magnitude of the discontinuity suggested to be present in V2O3: the metallic (M), in p seems to depend on sample purity and stress. the insulating (I) and antiferromagnetic insulating The properties of the metallic state of V2O3 are (AF) phases. Depending on the temperature, pres- highly anomalous and they change rapidly with sure and dopant concentration, all the three phases

pressure in the direction of making it more metallic. and the transitions between any two (M^I; M^AF;

It, therefore, appears that pure V2 Os is near a critical AF^I) can be realized experimentally. The tem- region and that the application of negative pressure perature-pressure-composition phase diagram for

would induce a transition; doping with chromium (Vi_.Cr.)i0 3 is reproduced in figure 1.6. has an empirical correspondance with the negative At room temperature and atmospheric pressure,

pressure, and indeed, it is found that a metal -to- pure V2O3 is situated in the metallic phase. As the

insulator transition occurs with Cr doping in this temperature is lowered, the undoped material passes material. through the M-AF phase surface at ~160 K. Con-

As early as 1949, Mott [89] predicted that an versely, as the temperature is raised, pure V2O8 ap- abrupt metal-insulator (collective electron-localized pears to pass through the extension of the M-I electron) transition will occur in an ionic solid when phase boundary at ^550 K. However, the M-I

the interionic separation is forced to pass through a phase boundary terminates at a critical point (fig.

critical value. Since each of the two resistance 1.6), so that the high temperature transition in pure anomalies in V2O3 is accompanied by an anomaly in V2O3 reflects a critical behavior. The phase transition

the thermal expansion, it would appear that they is no longer a well-defined, abrupt, first order transi-

may both be metal-insulator transitions of the type tion, but is a gradual change from metallic to in- envisioned by Mott. The basis for the ideas that the sulating behavior. When a small percentage of Cr

V2O3 transitions are Mott-type transitions is con- ions is substituted for vanadium in V2O3, a definite tained in the recent work of McWhan and co-workers first order phase transition between the metallic [62, 63, 84, 90]. These workers have examined the and insulating phases exists, with the transition

resistivity and the lattice distortion of V2 0 3 as a temperature rapidly decreasing as a function of function of pressure, temperature, and chromium and chromium doping. At a critical concentration (^3

33 I n

percent), the M—>1 and M—»AF transition tempera- faults into which extra planes of metal atoms are tures are equal (~160 K). For Cr dopings greater introduced. Because of the shear planes, metal-metal than 3 percent, an intermediate metallic state does bonding can occur leading to interesting electrical not exist and the only observed transition is a direct and magnetic properties. Whenever these metal- I—>AF transition (at atmospheric pressure). Similar- metal bonds are broken up, anomalies in the physical ly, at a given temperature (say, 298 K) and Cr- properties occur, but there appears to be no case concentration (say, 4 percent), I—»M transition can where structural transformation occurs; only a dis- take place with the application of pressure without continuity in the lattice parameters is noticed. the intervening AF phase. Doping with increasing Nagasawa and co-workers have grown single crystals amounts of Ti has the empirical correspondance to of these Magneli phases by the vapor transport the application of pressure (3.6 kbar/atom %) and method [94-97] and have carried out various meas- the system tends to become more and more metallic urements.

[79]; at about 7 atom percent Ti, both the transitions V3O5, even though it corresponds to V n02n_i with in V2 O 3 are suppressed and the system is metallic in n = 3, is not a Magneli phase since the structure is the entire range of temperatures as is the case in different (monoclinic). It shows a discontinuity in pure V2O3 above 25 kbar pressure [62]. the x-T plot around 133 K [95], but in contrast to Various theories have been put forward to explain the earlier studies [73] (which showed a semiconduc- the metal-insulator transitions in oxide materials tor-to-metal transition in the range 150 to 180 K), ([91-93]; for a review of various earlier theories see only a semiconductor behavior was noted in the

[53]), but each of them is able to explain the experi- range 130 to 300 K [72, 98]. Kosuge [17] does not mental findings on a specific oxide material and fail observe a DTA peak, or a x-T anomaly. X-ray to explain the behavior of other oxides. Any discus- studies of Asbrink et al. [99] up to 1270 K, also do sion of the theories would be outside the scope of not indicate a phase transition. this review and it would suffice to point out that Magnetic susceptibility and electrical transport there is still a need for unifying theoretical appro- (on single crystal materials), x-ray, DTA and Moss- aches to the understanding of metal-insulator transi- bauer studies indicate semiconductor-to-metal tran- tions. In figure 1.7 we have shown the schematic sitions (on heating; first order) in V4O7 at 250db5 K band structure of V2 O 3 (following Goodenough) in [17, 94, 100, 101]; in V 50 9 at 135±5 K [17, 96, 102]; the rhombohedral, nonmagnetic monoclinic and in V6Ou at 177 ±2 K [17, 95, 103]; in V 8015 at 70 K magnetic monoclinic structures to illustrate the na- [97, 104, 105]. Detailed studies [17, 94, 103] indicate ture of changes accompanying the low-temperature that V7O13 is paramagnetic (Pauli ?) and metallic transition. in the range 4.2 to 300 K. In their recent study published in 1973, Kachi, Kosuge, and Okinaka [105] = Vn0 2 „_i: The Magneli phases, Vn02n-i, (4^n^8), have presented the data on all the Vn 02n-i ( have a triclinic structure based upon the rutile struc- 3-9) systems and find that all but V3O5 and V7O13 ture with periodic sheared planes resembling stacking show metal -sinulstor transitions. These workers have established the phase relationships and structures of these oxides.

bgeji.j-ri.j — t i c*p It is difficult to rationalize this wealth of experi-

« I ttH — mental data without a proper understanding of the -iteTCH m "1— c»d theory underlying the semiconductor-to-metal transitions; unfortunately, present day theory has not yet developed to such sophistication, to satisfactorily explain the transitions even in simple substances like V2O3 and VO2. However, in the V„02n_i series,

two important generalizations may be noted: (i) all the phases are triclinic and detailed x-ray study on V4O7 indicate no crystal structure change at 7\, but only discontinuity in lattice parameters. Probably the same case may be true for other Magneli phases monoclinic as well, (ii) Recent studies [105] show that there is

antiferromagnetic ordering in all the Vn02 n-i phases Figure 1.7. Changes in the d-electron band strurture of V2O 3 transi- accompanying the transition at ~150 K ( After Goodenough). at low temperatures in addition to first order

34 t t

tions in many of them at higher temperatures; in the stabilization of this phase by way of Magneli

2 these respects, these phases are similar to V2O3. It defects [115, 116] . appears that the first order transitions in these The effect of pressure on the VO 2 transition has in Magneli phases are explainable terms of Good- been examined by a few workers [39, 61, 140]. T t enough’s qualitative picture ([44, 106, 107]; for a de- increases linearly with pressure at a rate of 0.082 dt tailed discussion, see [53]) of metal-metal bonding 0.005°C/kbar. The conductivity activation energy and trapping of the conduction electrons in the in the semiconducting phase decreases linearly with semiconducting phase and the breaking of these pressure typically at a rate of 1-2 mV/kbar. Assum- bonds and creation of Fermi surface at T t in the ing intrinsic behavior below T t for V02 , this indi- high temperature metallic phase. Like VO2 , Vn 02„_i cates that the carrier concentration increases with oxides also show only a single transition (V2O3 and pressure but no metallic conduction is encountered T14O7 on the other hand, show two transitions). up to pressures of ~300 kbar at 300 K. The effect of various impurities on the temperature

VO 2 : Vanadium dioxide, V02 , exhibits a crystallo- and nature of the transition in VO 2 has been in- graphic transition at 340 K. The transition is accom- vestigated extensively in the literature. Many ele- panied by interesting changes in physical properties ments like Ti, Nb, Mo, Tc, Ta, W, and Re, form

(see fig. 2 in introduction). In view of the possible complete solid solutions with VO2 [113, 116, 120, practical applications of this material in electrical 121, 133, 141-148] and at appreciable concentrations switching circuits and in other devices, the crystal stabilize the high temperature rutile from of V02 growth and physical properties of this material have at room temperature; the T t of VO2 is lowered been extensively studied and reviewed by various appreciably and the nature of the transition (at not workers in recent years [44-56, 108]. too high impurity concentrations) is changed from

V02 is monoclinic at room temperature [101, 109- that of a semiconductor-to-metal to that of a semi- 113] and transforms to a tetragonal rutile structure conductor -to-semiconductor. The same effect is pro- at 340 K [101, 114-116]. The transition is of first duced by the addition of ions like Fe, Co, and Ni,

order and is accompanied by hysteresis effects and but Al, Cr, and Ge raise the T t appreciably [133, changes in latent heat and volume [101, 114-119]. 146, 148]. Unfortunately, each system is compli-

Magnetic susceptibility [15, 114, 117, 119-122], cated in itself and no generalizations are possible on

Mossbauer [75, 123] and NMR [84, 124, 125] studies the effect of impurities on the transition in VO2 have failed to show any evidence of long range until the detailed mechanism of the semiconductor- magnetic ordering in V0 2 in the temperature range metal transition in pure VO 2 is clearly understood.

in is Since is 1.7 to 400 K; however, a jump x noted at T t long range magnetic order absent in VO2 , and x is almost temperature independent both above it might be considered that the Adler-Brooks crystal and below T t . The electrical characteristics of VO2 distortion model ([149, 53]) would be applicable to change from those of a semiconductor to those of a the transition in V0 2 , but quantitative agreement

metal at T t with a large drop in resistivity (by a between theory and experiment is lacking [56, 108,

5 factor of ~10 for pure and single crystalline ma- 134]. Since the metal -metal bonding in V02 and terial) [37, 73, 75, 113, 114, 117, 126-134]. Seebeck other rutile type oxides at low temperatures has been coefficient [117, 128, 135, 136] and Hall effect [130, established beyond doubt, Goodenough’s approach

137] also show anomalies at T t . Studies of DTA seems to be applicable (for detailed discussion, see

[17, 113, 116], heat capacity [114, 117, 138] as well [53]), but the treatment is only qualitative. Paul as infrared spectra and other optical properties [130, [108], Hyland [56], and others [150, 151] strongly 139] clearly show7 the first order nature of the transi- argue for an electron-phonon interaction mechanism tion in this material, but thermal conductivity does and phonon instability [152] as the basic driving

not show an anomaly at T t [117]. force for the transition in V02 . In figure 1.8 we have Recently, some workers have found evidence for shown the elementary band structure scheme for the existence of an intermediate triclinic phase of tetragonal and monoclinic VO 2 to illustrate the na-

VO 2 in the range 325 to 340 K by means of NMR ture of changes accompanying the transition. [125], DTA [116] and x-ray diffraction [115] studies.

The temperature range of stability of this inter- 2 Goodenough, [ 1 16a] J. Solid State Chem. 3: 490 (1971), has recently identified two distinguishable mechanisms of the monoclinic-tetragonal mediate phase may become vanishingly small for transition of VO 2 : an antiferroelectric-paraelectric transition at a tempera-

ture Tt and a change from homopolar to metallic V-V bonding at T t • Both pure V02 [101], but impurities like Al, Fe, Cr, Mo, T and T/ are at 340 K in pure VO 2 . In the presence of impurities the two for components seem to get separated with T t

35 .

hombic whereas has a monoclinic w w [30] V3O 7 crystal J { symmetry [29, 154, 155] at room temperature and decomposes at 930 K. No DTA peak was noticed by Kosuge [17] in the range 130 to 950 K. The other physical properties are not yet known. Vanadium pentoxide has a narrow range of homogeneity [26]. It is orthorhombic at room temperature and has a corrugated sheet type structure and is built up from distorted trigonal bipyramidal co- (a) (b) ordination polyhedra of oxygens around vanadium, Monoclinic V 0 Tetragonal V02 2 4 sharing edges to form zig-zag double chains along [001] and are cross-linked along [100] through shared corners, thus forming sheets in the xz plane [156-

Figure 1.8. Schematic band structure of VO 2 in the monoclinic 159]. This special type of structure predicts aniso-

and tetragonal phases ( after Goodenough ) tropy in thermal expansion of the lattice parameters and is observed experimentally [160-162]. V2O5 does

V n Oon+i 5 VeOi 3 is monoclinic at room temperature not show any phase transformations up to the melt-

[16, 18]. Kosuge [17] reported an endothermic peak ing point (~940 K) at one atmosphere [17, 163].

in DTA at 177 K where a X-T anomaly has also been Minomura and Drickamer [39] have noticed a phase

noted. Recently, Kanazawa [153] has noticed a semi- transition in V2 0 5 at pressures ~100-105 kbar, conductor-to-metal transition (on heating) in this associated with a large increase in the resistivity. material at 149 K with a drop in resistivity by a V2O5 remains semiconducting both below and above 4 factor of 10 . The transition may be first order but the transition. Electrical properties and the mech-

the structure of the low temperature phase is not anism of conduction in V2O5 have been discussed yet established. by a few workers [164-169]. Small amounts of alkali

In the series V n02n+i, in which V2O5 and V 60i3 and other metals form 'vanadium bronzes’ with

are known members, V4O9 and V3O7 have been re- V2 0 5 . These important materials have been discussed cently prepared and characterized. V4O9 is orthor- in detail in the literature [53, 170].

Vanadium oxides

Oxide and description Data Remarks and inferences References of the study

0 a, A Density % Va- P a Xm 3 Oxide (cubic, (g/cm ) cancies (300 K) (mV/ (273 K) rock salt) (Total) (Ocm) °C)

- - Crystal structure and VO0.86 4.034 5.736 37.0 6 .OXIO 4 -10.9 0.5X10 3 [31, 35]. - physical properties of VO* VO0.99 4.068 5.602 30.8 2.2 X 10~ 3 -4.2 O. 6 XIO 3 - - 3 - 3 (0.8

^Oi .23 4.133 5.329 21.2 + 22.1 0.8X10-3

36 Vanadium oxides—Continued —

Oxide and description Data Remarks and inferences References of the study

Melting point (VOZ )~2170 K Most comprehensive study on 30 well-characterized samples; contains literature survey up to 1970; High pressure annealing studies conducted; VO z (0.86 ^ x ^ 1.02) is metallic and show temperature-independent xm while samples of VO z (1.02 ^ x ^ 1.23) are semiconducting and show weak temperature- dependent xm- No metal-to- semiconductor transition nor magnetic ordering noticed in any sample in the range 4.2-300 K for all x.

Vacancy annealing in VO. High temperature (~1920 K) and A critical value of PAT (>90,000 [36]. high pressure (~56 kbar) pro- kbar. °C) where P is the ap- duced substantial decrease in the plied pressure and AT is the vacancies (vacancy filling); In temperature above ambient at common with other workers, which the pressure is applied, difficulties were encountered in seems to eliminate the vacancies the preparation of pure single completely in TiO. A similar phase VOi.oo- (but higher) value is predicted for complete vacancy filling in VO.

NMR studies of VO. V 51 NMR studies on single crystals Results indicate that the bulk of [42].

and pressure annealed VO z the temperature-dependent (x=0.86, 1.02 and 1.23) using magnetization comes from local cw and spin echo techniques in moments on a minority of sites fields of 9 -50 kOe in the range whose nuclear resonances are 1.4-300 K. Resonance was ob- unobservable. servable in all compounds at all temperatures indicating the absence of metal-insulator transition in the observed materials.

Resistivity and Hall VO z (0.82 ^ x ^ 1.0) samples studied An overlapping band model is [43]. coefficient studies. at 4.2, 77, and 300 K. p (300 K) suggested to explain the ob- -3 -4 is in the range 10 -10 flcm served properties. with small negative values of -4 dp/dT; Ru is ~5X10 cm3 /C and positive Ap/p at 4.2 K. The samples did not exhibit metal- insulator transition.

37 -

Vanadium oxides—Continued

Oxide and description Data Remarks and inferences References of the study

Seebeck coefficient For VO* samples with a: <1.0, a is The data indicate that electrons [171]. studies on VO*. negative; for *>1.0, a is positive and holes are competing to-

and for * * 1 .0, a ~ 0. The gether for the contribution to a

numerical values are small ( — 5 and the consequent complica- to +20 mV/°C). tions in the band structure related to a large concentration of metal and anion vacancies.

Crystallography and The solubility of TiO in VO is It appears that VOo.8 does not be- [35]. defect chemistry of VO small (~10%). The concentra- long to the same family of cubic and solid solutions -with TiO. tion of defects are lower than in VO* with (0.9 ^ ^ 1.2) and pure VO* at any given composi- higher disorder in cation sub- tion. Semiconductor behavior was lattice exists compared to anion noticed for VO (* ^ 1.0) and its sublattice. The low solubility of solid solutions in the range 100- TiO in VO is explained in terms 300 K. No metal-semiconductor of the different degree of disorder transition was noticed for VO or in the two crystal lattices. in the solid solutions in the temperature range investigated.

Electronic structure and Mutual solubility of VO and TiO at The observed behavior is explained [172]. solubility of other oxides 1870 K is ~18-20%; in the in terms of the electronic struc- in VO. oxygen rich phases (*>1.0), ture and valence states of the solubility is greater (20-25%). metal in these compounds. Solubility of BeO, CaO, and MgO are small in VO.

Plasma resonance in VO. Reflectance minima are found in The reflectance minima correspond [173]. the range 3. 8-4.0 eV in VO* to the plasma edges in these depending on the composition. materials.

Infrared studies of VO*. VOi.od and VOi.u give rise to The band corresponds to the metal- [174]. bands at 1075±15 and 1080±10 oxygen vibrations and results -1 cm respectively. indicate that the band position is not much sensitive to the nature of the cation or the phase composition.

Band structure of VO. Band structure calculations by the The energy bands in VO seem to be [175]. tight binding method indicate sensitive to the degree ionicity that the d bands are about 7 eV assumed in the calculation; the wide, lying below the vanadium density of states at the Fermi

4s band so that conductivity in level is large, suggesting that VO

.VO is primarily due to the d might exhibit some sort of mag-

electrons. The Fermi level falls netism. See also (ref. [9], p. 130) in the d band. for band structure calculations. v2 o 3

Crystal structures of the 77-160 Earlier interpreted [63, Range K: workers [59, 176] 64]. j phases and x-ray studies. Monoclinic; space group, 12 /a; the low temperature monoclinic Z = 4; a = 7.255±0.003, A; b = distortion in terms of a ortho 5.002 ±0.002 A; c = 5.548 ±0.002; hexagonal cell or c-centered 0 =96.75 ±0.02°; E = 99.97±0.08 monoclinic cell but the present A». choice in terms of a hexagonal -6 TEC of this phase is <2X10 cell appears to be the most

1 K- . easily conceived. Single crystal

38 Vanadium oxides—Continued

Oxide and description Data Remarks and inferences References of the study

Range 160-700 K: growth conditions established.

Rhombohedral (pseudo hexa- V 2 O 3 has an unusually high c/a [58, 62, 63]. gonal); space group, R3 C; Z = 2, ratio (at 298 K); detailed studies oh =4.9515 ±0.0003 A; ch = indicate that in going to the 14.003 ±0.001 A; c/a =2.8281; monoclinic structure, nearest

3 V =98.61 ±0.05 A . neighbor V-V distances show a

TEC (/K) at 298 K: ||“ = (20.2± discontinuous expansion while -6 c = 0.3) X 10 ; || -(8.6 ±0.3) X the average V-O distance re- io-«. mains unchanged. Contrary to The c/a ratio (pseudo c/a for the earlier interpretations [45, 76, monoclinic cell) shows a sharp 177], pair --wise contraction of the jump at ~160 K and slight basal plane cations does not, in nonlinear decrease in the region fact, occur but the oxygen octa- 450—600 K; the 4% Cr doped hedra become skewed up about samples show the exact opposite the central metal atom and the behavior. triply twinned monoclinic struc-

AF (mono.—»rhombo.) = —(1.4 ture is a logical consequence of

± 0 . 1 )%; the value is much less the distortion. for doped samples (4%). With pressure, volume decreases gradually with increasing pressure but doped samples give discontinuity depending on the concentration. Lattice parameters vary smoothly with the application of pressure;

din (c/a) /rf (In V) = -0.7. Melting point of pure V 203 ~ 2240 K.

Magnetic susceptibility. Range 4.2-1000 K: The ordering of magnetic dipoles [65, 67, 68 , ~4 magnetic ordering and Xxi = 1.0X10 (cgs units) in V 2 O 3 is of special type com- 76, 77, 79, neutron diffraction studies. (T <150 K) pared to the other transition 178-181]. xxi=2.1X10-4 +1.40/(T+600) metal sesquioxides. (180°

T t while heating and shows Curie -Weiss behavior; a hysteresis of ~12° = was noted. T t 155 K. A shallow maximum is indicated in xxi-T curve in the range 400—500 K. This seems to correspond to the high temperature

anomaly in pure V 2O 3 . The magnetic moment changes from 2.37 MB to 2.69 mb in going through this transition (spin only value for 3+ V is 2.83 mb). Xxi -T studies were also made on doped samples. Internal field value ~175±15 kOe was

obtained in V 2 0 3 at 105 K using inelastic spin -flip

39 Vanadium oxides—Continued

Oxide and description Data Remarks and inferences References of the study

scattering of neutrons. The

ordered moment (calculated) is 1.25 hb- The existence of antiferromagnetic ordering in the low temperature phase

is confirmed. The observations are in accord with a model in which the vanadium moments are ferromagnetically

coupled in monoclinic (010 ) layers with a reversal between adjacent layers. The ordered

moment is ( 1.2 ± 0 . 1 ) mb per V atom. Neutron diffraction study shows that the magnetic ordering disappears at 154 K which is the Neel temperature (Tn). Applica- tion of pressure decreases the transition temperature (dT^/dP = —3.78 K/kbar). Above 26 kbar pressure, the magnetic order disappears and the material remains paramagnetic down to 4.2 K.

Electrical properties. Semiconductor behavior below T t The low temperature behavior is [38, 63, 71, (151±3 K); P (125 K) ~ 10 6 usually interpreted in terms of 76-78, 182; ficm; Eo~0.12-0.18 eV; below intrinsic behavior. According to

40 K, evidence of impurity con- McWhan et al. [63], the proper-

duction with Ea ~0.001 eV. Be- ties of both the high temperature low a is ~ —3 and phases (metallic, M and insulat- T t, nV/°C f?n<0.3 cm 8 /C (at p~ 10 8 ftcm). ing, I, phase) are anomalous and

Metallic behavior above T <; are modified by the impending p drops by a factor 107 at 7\ while transition in the range 500--600 heating; hysteresis of ~12 K K. The order of the transition noted, a becomes positive and may be second or higher order 0 jumps to ~12 mV/ C and shows but can be made first order under slight decrease with temperature proper conditions (by doping

thereafter, p shows an anomaly with Cr). A generalized picture

(broad maximum) in the range of the transitions in V 2Q 3 has 500-600 K. No anomalous be- emerged from the work of these havior in a and Rh in this tem- investigators. perature region. Resistance decreases with the application of pressure: 1 atm: p=227±1.4 T (150 < T< 350 K). 26 kbar: p = 182 ±0.7 T (200

Data Remarks and inferences References

Endothermic peaks in DTA were The thermal properties are showing [17, 62, 76, noted at 168 and 430 K; no AH up the two transitions in V2O3 80, 183]. value reported. X-type anomaly but there is need for quantitative was noted in the heat capacity of studies on the AH and AS of V2O3 at 169 K; AH = 692 cal/mol; transition. AS for the transition = 4.1 e.u. McWhan et al. [62] estimate a value of AS = 2.6 e.u. A nuclear specific heat measurement in the temperature range below 0.5 K indicated a Schottky anomaly varying with T~ 2 for pure V2O3.

Energy gap of ~0.1 eV is obtained The fundamental absorption edge [76, 87]. from the optical measurements in the antiferromagnetic phase at 77 K in pure V2O3. The re- appears to be very soft and flectivity of pure V2O3 at 300 K shifts to lower energies on is small but has the general shape entering the high temperature characteristics of a metal; plasma insulating phase.

edge is at ~1 eV.

Knight shift (%): This study is a strong evidence for [81-84].

fc = 1.52 - 18.1/(T +684) (T>150 magnetic ordering at low tem-

K). An abrupt change in k is peratures in pure V 2O 3 . From the noted at the high temperature 'NMR-phase diagram’, Ruben-

transition, while the resonance stein [83] concludes that the signal vanishes at the low- metal-insulator (high tempera- temperature transition (150 K). ture region) phase boundary is

No anisotropy in k is noted nor not sharp and uniform below the any observed nuclear quadrupole critical point, but corresponds to splitting. an inhomogeneous transition. It

Antiferromagnetic state is sup- is postulated that the high tem-

pressed in pure V2O3 at high perature phase transition is pressure and low temperatures. produced by an instability in a V 51 NMR signal is observable normal mode vibration of the down to 4.2 K. k = ( — 1 .0 ±0.2) + crystal lattice and that the low- (0.01 ±0.005) P (20

The lattice instability is believed to be caused by the close proximity of a van Hove singu- larity to the Fermi energy in the metallic state of the crystal. The pressure dependence of the Knight shift implies that the d spin component of the suscepti-

bility (xd) is unusually strongly dependent on volume with (d In xd/d InK) =8±5.

Studies of Fe-doped V2O3 in the The calculated Ts was ~200 K. It [ 74 ]. range 4.2-300 K: Magnetic is inferred that the larger dis-

hyperfine splitting was noted be crepancy between T t and Tn t

Vanadium oxides—Continued

Oxide and description Data Remarks and inferences References of the study

low T (~140 K) which disap- (calc) might indicate that mag- peared above and netic ordering is not a major T < the substance was paramagnetic. The cause of the transition. internal field decreased with tem-

perature (400 and 315 kO e at 4.2 and 131 K respectively) and

abruptly disappeared at T t ;

isomer shift showed a jump at T t .

UHF dielectric properties. Wave guide method using a fre- The high value of e indicates the 86 [ ] quency of 9.5 GHz: e~18 (T< relative importance of nonpolar

160 K); e~36 (T>160 K). e type of bonding in V 2 O 3 and the

slowly varies with temperature; transition is accompanied by a in the region of the transition a change in the type of chemical

jump by a factor of 2 is noted. bonding, electron -energy spec- Complex conductivity also trum and consequently the

exhibits an anomaly at T t . dielectric properties.

Electric field effect on the When the applied field reaches a [184] transition. critical value (~10 4 V/cm at 77 K) the current increases rapidly; this threshold field depends weakly on the sample thickness. Switching behavior

also is noted in the samples of

pure V 2 O 3 .

Effect of stoichiometry on For low stoichiometric deviations [88]

the low temperature in VO 1 . 5 +X, both a and c param- transition. eters of hexagonal form at room temperature, decrease linearly

and c/a is a constant. With large x (x> 0.015), a decreases more rapidly than c suggesting a probable change in the distribution of defects involved in the crystals. The magnitude of the discontinuous change in xm at TV and Tn become smaller with increasing x; no transition is observed observed for x =0.034 either in terms of a change in the crystal structure or in Xm-

Band structure of V 2O 3 . The band structure of the hexa- The model appears to account for [185]

gonal V 2O 3 lattice is examined by a transition from a distorted, the tight -binding method. The insulating state to an undis- stability of the structure in rela- torted semimetallic one, for tion to the distortions and the certain values of the d-d

splitting of the d bands is interactions. investigated.

V3 O 5

Crystal structure Monoclinic; space group, C2/c or — (99) Cc; Z = 4; a =9.98 A; 6 = 5.03 A;

42 Vanadium oxides—Continued

Oxide and description Data Remarks and inferences References of the study

c = 9.84 A; 0 = 138.8°. X-ray studies do not indicate any phase transition up to ~1270 K.

Physical properties. X-T plot shows an anomaly at Earlier studies [73] might have [17, 72, 95,

~133 K, but DTA does not contained some V 5O9 impurity; 98, 105]. indicate a peak at this tempera- the cause of x-T anomaly is not ture. Probably the substance is known. paramagnetic in the range 77— 300 K. Semiconductor behavior

in the range 130-300 K; Ea =0A eV. a data (~280 MV/°C at 273 K) indicates re-type behavior; a decreases with T.

Magneli phases, V nOin-i Crystal structure and transition Magnetic, electrical, DTA References temperature and Mossbauer studies

All triclinic; ; phase phases are space group. Pi; Z—2.

4; V<07 o = 5.502 A; b =6.997 A; X-T anomaly at T t . Semicon- [17, 94, 100, c = 12.248 A; a =95.09°; 0 = ductor -to -metal transition; Ap = 101, 105, 109.28°. = 2 95.17°; 7 = 7\ 250± 10 at T t ; a shows discontinuity 186].

5 K. Discontinuity in the lattice at T t ; a is negative in the range

parameters at T t ; no change in 120-300 K; a above T t is ~10-12 the crystal structure; the volume mV/°C. Endothermic DTA peak.

- change for the semiconductor -to No magnetic order below T t ; metal transition = + 0.0011 ± Isomer shift and quadrupole 0.0006; the pressure dependence splitting show discontinuities at — 3+ of T ( is 0.2d=0.1 K/kbar. T t ; T t decreases by Fe doping but jump in x remains almost the same.

5; Vi09 0 = 5.475 A; 6=6.994 A; c = 8.718 X-T anomaly at T t . Semiconductor- [17, 96, 102, = A; a =97.53°, 0 = 112.44°; 7 to-metal transition at T <; A 105, 186, 4— 6 = - 108.99°. T t = 135 ±5 K. 10 10 ; Ea below T t 0 . 1 0.2 187]. eV. a decreases with increase in

T; anomaly at T t ; above T t , 10-20 mV/°C (re-type). Endo-

thermic DTA peak at T t . No

magnetic order below T t ; Isomer shift and quadrupole splitting show discontinuities at T<; Fe3+

doping decreases T < slightly.

6 ; = = . VgOn o 5.44 A; 6=6.99 A; c 23.66 A; X-T anomaly at T t Semiconductor- [17, 95, 103, a =98.5°; 0 = 120.9°; 7 = 108.9°. to-metal transition at T Ap~ 105, 186]. = 4 T t 170-177 K. 10 ; Ea below T<=0.12 eV. a is negative in the range 120-300 K and goes through a sharp maxi- = mum at T t . Above T t , a 10-13 mV/°C. DTA endothermical peak.

43 a ±

Vanadium oxides—Continued

Oxide and description Crystal structure and transition Magnetic, electrical, DTA References of the study temperature and Mossbauer studies

= 7; V7 O 1 , a 5.43 A; 6 = 7.00 A; c = 15.16 A; X continuously decreases with in- [17, 94, 103,

a =98.9°; /3 = 125.5°; 7 = 108.9°. increase in T. Metallic in the 105, 186]. No transition. range 4.2-300 K; p (300 K)-~ -3 10 Dcm. a is negative and small in the range 4.2-300 K, decreases slightly with T and below 120 K, nearly constant (~1 pV/°C). No endothermic peak in DTA.

= = . 8; VgOi5 a 5.43 A; 6=6.99 A; c 37.08 A; X-T anomaly at T t Semiconductor- [97, 104,

a =99.0°; /3 = 128.5°; 7 = 109.0°. to-metal transition at T t: Ap ~ 105, 186]. -3 7\ = 70 K. 10; p (200 K) ~10 ftem. a is negative and drops from —70 to — 5 /zV/°C at T t ; Above T t , a is constant.

The general characteristics of all the Magneli phases, Vn02n-i are: (i) All have triclinic crystal structure at room temperature, (ii) Antiferromagnetic ordering is present at low temperatures and semiconductor-metal transitions are present at higher temperatures, T t [105]. Ty is 70, 40, 30, 23, 43, and 7 K respectively, when n is 3, 4, 5, 6 , 7,

. = and 8 V0O3 (n— 2 ), V„02n-i (3

VO*

Crystal growth. Single crystals of V0 2 are grown Depending on the conditions large [ 10 , 28,

from V 20 6 at high temperatures single crystals of varying stoi- 133]. under various oxygen partial chiometry can be grown. pressures.

Crystal structures of the T <340 K: Crystal symmetry changes at the [61, 101 , phases and x-ray studies. Monoclinic; space group, P2i/c; transition; volume change is very 109, 111, = 3 Z 4; density, 4.65 g/cm (obs.); small. Above T t both a and c 112, 115, 4.67 g/cm 3 (cacl.). a = 5.7517 ± parameters increase but the rela- 116, 118, 30 A; 6=4.5378±25 A; c = tive increase in the c parameter 125, 189].

5.3825 ±25 A; /3 = 122.646±96°. is greater. NMR evidence shows T = 346 K: the triclinic phase formation at Tetragonal; space group, 325 K. Cr, Al, and Fe are sup- P4/mnm; Z = 2; a = posed to stabilize this phase. 4.551 ±0.001 A; c = 2.851±0.001 Also see [116a] for structural aspects

A. AV at the transition = 0.02 of V0 2 . 0.05 cm 3 /mol. TEC (350-690 K)/°C: e -9 -8 : || 29.638 XIO -2.930 X10 1+ -11 -6 2.576 X10 1*; ||«: 5.828 X10 - - * — 12 2 7.091 XIO t+6.946X 10 t . 325 < T <340 K: Triclinic, a — 5.80 A; 6 =4.52 A; c = 5.38 A.

a =91.5(5)°; /3 = 122.7(8)°; 7=90.0°.

44 t t

Vanadium oxides—Continued

Oxide and description Data Remarks and inferences References of the study

Magnetic properties. Paramagnetic in the range 1.7-400 Jump in x at Tt is noted; the exact [17, 117, - K; x below and above T are cause of this is not understood 120 122 ]. almost temperature independent; clearly. slight anisotropy in x is shown. X» 1.0X10'" (emu/g) (T<340K); X-8.5X10"6 (emu/g) (T>340 K).

Electrical properties. T <340 K: Semiconductor be- The wide variation in the proper- [39, 61, -3 — ~ havior. p~10 10 ftcm; Ea ties of VO 2 in the low and high 120, 128, 0.1-0.65 eV; a~30-1000 n\/°C temperature region are believed 130, 134- (negative); mhM). 1-1.0 em“ 2 /V s; to be due to the purity, method 137, 140]. ~10 l8-10 19 3 (T of etc. Slight aniso- n fl cm" <340 preparation K); m*~ 1.6-7 m; ap*^2. tropy exists in all the properties.

T>340 K: Metallic behavior Discontinuities at Tt in p, a, and -4 2-5 (X10 ) ficm; p linearly Rn are noticed by various 0 increases with T; a^23 /*V/ C workers. A hysteresis of '>-'2° is _2 (negative); mh~ 1-10 cm /V s; noticed in p-T data. The pres- 21 -10 23 -3 n«'*^10 cm ; m*« 0.5 m. sure effects on T indicate quali- dT/dP m + (0.082 ±0.005) °C/ tatively different behavior than kbar; ~dEa /dP~ (0.001-0.002) that encountered in V2O3. mV/kbar. p decreases with Slight increase in carrier concen- pressure at 300 K but no metallic tration is noticed with pressure behavior noted up to 300 kbar. but the effect is small and superpressures seem to be re- quired to produce metallic con-

duction in VO 2 at room temperature.

Optical properties. V0 2 exhibits infrared active phonon The data are consistent with the [130, 139].

modes for T

3 Above T t , the infrared spectra cm in the metallic state and a show sudden increase in reflec- filled oxygen 2p band ~2.5 eV tivity. Reflectivity and trans- below from the partially filled mission measurements in the bands arising primarily from

range 0.25-5.0 eV below T t indi- vanadium 3d orbitals. Transitions cate prominent absorption peaks from the filled 2p bands are re- at 0.85, 1.3, 2.8, and 3.6 eV. sponsible for the high-energy

Above T t , metallic free carrier peaks in optical absorption in absorption is observed below 2.0 both the high- and low-tempera- eV, but the same two absorption ture phases. In the metallic phase

peaks near 3 and 4 eV are present there is evidence of overlap in the high temperature phase. among the 3d bands such that at e. = 5.54 (300 K); 4.17 (355 K). least two bands are partially

According to Kawakubo [150], e occupied by the extra d electron

of VO 2 measured at 24 GHz in- per vanadium ion. In the semi- creases monotonieally from 22 to conducting phase, a band gap of 26 in the range 100=300 K and ~0.6 eV opens up within the 3d attaing a high value of 60 within bands separating two filled bands a few degrees below T< (340 K). from higher-lying empty bands. This seems to indicate the onset of an intermediate ferro- or anti -ferroelectric phase [151] possessing a distinct triclinic structure [115, 125],

45 Vanadium oxides—Continued

Oxide and description Data Remarks and inferences References of the study

Thermal properties. DTA of pure VO 2 shows an endo- First order nature of the transition [17, 80, thermic reversible peak at 340 K. is confirmed. DTA evidence is 113, 116, Some w'orkers [116] have noticed presented for the existence of a 117, 119,

two peaks at 342 and 346 K with triclinic phase of VO 2 just below 138]. slow heating rates (0.5°/min). the transition.

AH = 750 cal/mol; Cp = 13 cal/ mol, °C (300 K) and shows a X-

type anomaly at T t ; 0d = 75O K; k~40-70 mW/cm°C at 300 K and is independent of temperature in the range 300-360 K.

Mossbauer studies. No magnetic hyperfine splitting is No long range magnetic order exists [75, 123,

noticed at any temperature in in VO 2 and the absence of quad- 190].

67 Fe doped VO 2 . Isomer shift rupole splitting in the high tem-

shows an anomaly at T < while the perature phase indicates shielding quadrupole splitting goes to zero. of the nucleus by conduction electrons.

Electron and optical The boundaries between the mono- This is a direct observation of the [115, 191]. microscopic study of the clinic and tetragonal phases dur- transition and various domains

phase transition. ing the transition in VO 2 have formed according to the different been observed in an electron modes of transformation can microscope by the diffraction easily be distinguished.

contrast at the interfaces which It appears that the T < and the are revealed as fringe patterns. rate of growth of domains are The growth of transformed regions dependent on the mechanical

from sites of nucleation is ob- strain. served in micrographs.

-1 Infrared study of VO 2 . Bands at 720 and 1115 cm are These seem to correspond to [174]. noted. metal-oxygen vibrations.

Vn02n+i phases

v«o 13

Crystal structure and T = 300 K: Monoclinic; space The transition may be first order [16-18,

properties. group, C2/m; a = 11.90 A; fe = but the detailed properties are 153].

3.67 A; c = 10.12 A; /3 = 100.87°. not known at present. Low temperature structure not known. DTA endothermic peak and x-T anomaly at 177 ±2 K; semiconductor -to -metal transition at 149 K.

V4 O9

Crystal structure. T = 300 K: Orthorhombic; space The structure appears to be closely [30].

group, Pnma; Z = 4; a = 17.926 =fc related to that of V 2O 6 . The

0.004 A; fe =3.631 ±0.001 A; physical properties are not c= 9.396 ±0.002 A. known at present.

'1 v8 o7

Crystal structure and T = 300 K Monoclinic; space Detailed physical properties are not [17, 29, properties. group, C2/C; Z = 12; a =21.92 A; known at present. 154, 155].

46 Vanadium oxides—Continued

l

MagnSli phases, V n02n-i Crystal structure and transition Magnetic, electrical, DTA References temperature and Mossbauer studies

6 = 3.68 A; c = 18.34 A; 0 =95.62°. DTA does not indicate any transition in the range 130- 950 K.

v2 o5

Crystal structure and T = 300 K: Orthorhombic; space V2O5 has a corrugated sheet type [157, 162]. magnetic properties. group, Pmnm; Z = 2; density, structure and predicts anisotropy 3.357 g/cm 3 (obs.); 3.37 g/cm 3 in the TEC values; this is ob- (calc.); a = 11.519 ±0.006 A; b = served experimentally. 4.373 ±0.002 A; c = 3.564±0.002 A. x = 3.7X10-® cgs/g. TEC

(300-870 K) (X10®/°C): ||“, 2.0;

6 c || , 55.4; || , 8.0; average, 21.8; polycrystalline, 13.0.

Electrical properties. Range, 77-450 K: Semiconductor; Anisotropy in electrical transport [168, 192]. p~10 3 Ocm; a~10 pV/°C (nega- properties is understandable be- tive). p-T plot shows breaks at cause of the peculiar sheet type 250 and 390 K. a goes through a structure of V2O6. The breaks in maximum at 250 K. Anisotropy p-T plots may correspond to the

in p, Ea and a are noted, po in- change in the mechanism of creases with T in the range conduction (extrinsic to 350-390 K. intrinsic).

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48 —

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49 n

1.4. Chromium Oxides work has shown [51, 54] that Cr02 can be held for considerable period of time without decomposition Many binary oxides are known in the Cr-0 system. at temperatures 1170 to 1770 K at pressures of 60-65 Schonberg [1] described the existence of C^O, but kbar. detailed investigations have not yet been carried

Cr02 has a tetragonal rutile structure and its out. CrO does not exist in the stable state and at- lattice constants do not vary with slight changes in tempts have been made to stabilize it in the form the O/Cr ratio [48, 51, 52] and the rutile structure of solid solutions [2]. The known oxides of chromium exists in the range 300 to 673 K [55, 56]. Magnetic are Cr2 03 [3], Cr02 [4], Cr BOi 2 [5], an oxide of the measurements [4, 57-59] and Mossbauer data [60] composition Cr02>44 [6], Cr 60i5, or Cr2 05 [6-8], a indicate that the oxide is ferromagnetic with ~ nonstoichiometric oxide and T c Cr02 . 6 5 [6], Cr 3Os [8] 395 K. p-T data in the range 80-575 K indicate Cr03 [6-9]. Cr30 4 has been reported by a few workers that it exhibits a positive temperature coefficient of [10, 11], but recent work [12] has failed to indicate and is a typical metal 48, 51, 61-64]; dis- the existence of this material as a stable phase. p [4, 49,

continuities in p and a are noted at T c . Optical Cr 2 03 and Cr02 have been extensively investigated and heat capacity data [65, 66] are also consistent in the literature.

with the metallic nature of Cr02 .

Cr2 (>3: Cr 2 C>3 is rhombohedral (pseudo hexagonal) Cr0 2 forms solid solutions with V0 2 [67, 68] and and is antiferromagnetic below 308 K. Even though it Mn02 [69] with the associated changes in the physi- has the same corundum structure as a-Fe the spin 2 03, cal properties. structure differs and precludes the possibility of Cr 5Oi2 : Cr60i2 is formed under high pressure condi- superimposed ferromagnetism below Tn [13-19]. The tions at ~500 K and has orthorhombic symmetry crystal structure does not change at Tn, but dis- [6, 7]; the substance appears to be nonferromagnetic continuities in the lattice parameters are noted but detailed physical properties are not known at in particular, the body diagonal c parameter is [20]; present. contracted slightly on heating through Tn. Below Cr02 44 : This oxide is obtained by the decomposition Tn, x (perpendicular to the [111] direction) is almost of OO3 at a pressure of 600 bar and ^510 K [6]; temperature independent whilst parallel to the x the structure and other properties have not yet been direction approaches (but does not become) [111] investigated. zero at 0 K [14, 21]. Cr2 03 exhibits magnetoelectric Cr 6Oi5 (Cr2 0B) : Kubota [8] reported the formation effect below Tn and various workers have studied of Cr2 0 B by the decomposition of CrOs at high oxy- this phenomenon [22-27]. Anomalies in [28, 29], p gen pressures but the structure is not known. On ultrasonic sound velocity [30-32], Young’s modulus the other hand, Wilhelmi [6, 7] reported the forma- [33] and dielectric constant [25, 26, 34] have been tion of orthorhombic Cr60i6 at pressures > 1 kbar noted in Cr at n. 2 03 ~T and 490 K; pertinent physical properties, however, High pressure x-ray studies indicate no [35] are not known at present. crystallographic transition in Cr2 03 ; however, Tn Cr0 2 .65: This nonstoichiometric oxide reported by decreases linearly with pressure up to ~13 kbar Wilhelmi [6] is obtained by the decomposition of [36]. T also seems to depend on particle size of Cr0 3 in air at 540 to 570 K. Detailed data are lack- Cr to a small extent. Recently, Kawai and 2 03 [37] ing. Mochizuki [38] noticed a transition from insulator Cr3 Og: This oxide is formed by the decomposition to highly conductive state in Cr2 03 under static of anhydrous Cr03 [8]. It is not very stable and high pressure conditions Mbar). (>2 decomposes at 650 K. The crystal structure and C>3 forms solid solutions with other oxides of Cr2 other properties are not known. corundum structure like Al 2 03 [3, 39-41], Fe2 03 [18, CrOs: Chromium trioxide, CrC>3, is a stable material

42] and Mn2 0 3 [42] and the nature of magnetic inter- but absorbs moisture. It is orthorhombic and the actions and other physical properties have been in- structure consists of linear chains of corner-sharing vestigated in detail. Cr0 4 tetrahedra [6, 9, 70]. Electrical transport stud-

Cr0 2 : The dioxide of chromium, Cr02 , has a number ies [71] indicate it to be a semiconductor over a wide of interesting properties which include ferromagne- temperature range. No crystal structure transitions tism and metallic electrical conductivity. Many at ordinary pressures are known but p studies at methods have been reported in the literature for the high pressures [72] indicate a first order transition preparation of but the material at ~ 140-145 kbar pressure and the sample retains Cr02 [2, 6, 8, 43-52] is actually a metastable form and will, on heating to the semiconductor behavior. Other properties of Cr0 3

~720 K, transform irreversibly to Cr2 (>3 [53]. Recent have not been investigated in detail.

50 Chromium Oxides

Oxide and description Data Remarks and inferences References of the study

Cr203

Crystal structure and Hexagonal; space group, R3C; No change in the crystal structure [6, 20, 35, x-ray studies. Z = 6. T = 291 K: a=4.9575 A; takes place through the transi- 73]. c = 13.5976 A. T = 313K: a = tion region. 4.9605 A; c = 13.5945 A. Discon- tinuities are encountered in a and c parameters at ~Tn (308 K); c axis contracts on heating through Tn. Pressure also decreases c parameter.

X-T data. T = 4.2 K: x=22.4XlO- 6 emu/g X-T plot is typical of an antiferro- [14, 21, 36, (parallel to [111] direction). The magnetic material. Increase of 74, 75]. sublattice magnetization drops pressure seems to bring about the

from ~1.5 to zero at 7’n- Mag- collapse of the antiferromagnetic netic moment = 2.76 ±0.03 mb- spin structure at a given Anisotropy in x is shown. Tn = temperature. 308 ±1 K; Tn decreases linearly with pressure; dTx/dP = — 1.6±0.3 K/kbar.

Electrical properties. Semiconductor behavior in the The available data indicate that [28, 29, 71, range 300-1600 K. p~10 5 flcm the conduction is extrinsic below 76-82].

at 300 K; Eo ~0.4 eV (T<1500 1200 K dominated by impurities

K); £a ~2.5 eV (T>1520 K). or native defects with hopping of a — 0.2-0.4 mV/°C at ~500 K; holes between Cr4+ and Cr3+ p type; md~ 10 5 cm 2 /V s at sites. Intrinsic conduction sets in ~500 K. at T ^ 1500 X.

Dielectric properties. e«*8 (300 K) and the e value at The magnetoelectric effect associ- [25, 26, 34].

zero and high magnetic fields ated with Cr 203 below Tn is and at high frequencies show because of the magnetic ordering anomalies at Tn. of the Cr3+ spins.

Mechanical properties. Young’s modulus « 96 dyn/cm 2 at The anomaly in the Young’s [33, 40].

273 K and shows anomalous modulus in Cr 2 C>3 is quite differ- variation in going through Tn. ent from the one encountered in -4 Compressibility ~4. 5 X 10 CoO and NiO and is attributed -1 (kbar) at 300 K. to the domain phenomena and differences in the magnetic order.

Optical properties. Bands are noted at 14 390, 16 610, The data indicate the localized [83]. 21 690, 27 030 and 39 220 cm" 1 nature of the electrons in and are attributed to the crystal Cr2C>3. field transitions. Well defined infrared bands are encountered in region 300-750 cm -1 and are due to vibrations of the Cr-0 bond. -1 10 Dq = 16 610 cm ; 5=478 -1 cm .

C1O2

Crystal structure and Tetragonal; space group, P4/mnm; Typical rutile structure; TEC [4, 8, 51, x-ray studies. Z = 2; a=4.421 A; c = 2.916 A. shows high anisotropy and in 52, 55, 56].

51 Chromium oxides—Continued

Oxide and description Data Remarks and inferences References of the study

TEC (/°C): T = 303 K: ||‘» fact e axis is negative. || No a = -14.84X10-*; || +18.61 X crystal structure change in going 10"*. T-640 K: fl - -0.1 through T«. || X 10-« a = ; || +13.52X10-*.

Magnetic properties. Ferromagnetic; T«~392-398 K Typical ferromagnetic behavior [4, 48, 57- depending on the sample. Mag- exhibited by single and polycrys- 59, 84, 85]. netic moment =2.07 ±0.03 mb- talline samples. The properties of Specific magnetization = 138.3 Cr02 are adequately explained at 0 K. on the basis of Goodenough’s X^2X10- 4 emu/g. at 500 K. model [48, 84, 85].

Electrical properties. Metallic in the range 80-575 K; Typical metallic behavior; the [4, 48, 49, -4 p~10 Qcm (300 K); a* —10 electrical properties are ex- 51, 61-64, pV/K (300 K); exhibits discon- plained in a rational manner by 84, 85].

tinuities in p, a at Te . Semi- the Goodenough’s model [48, conductor behavior is not en- 84, 85].

countered above Te contrary to the expectations. Magnetoresist- ance effects noted; at 298 K in a field of 29 kOe (applied parallel

to [001] axis), (Ap/p) = ( — l.Bsfc 0.2)%.

87 3+ Mossbauer studies of Fe Fe atoms exist as Fe ; hyperfine A novel study of the transition in [60].

doped CrOj. splitting of the spectra dis- Cr0 2 ; Te agrees well with other appears at To (397 K). Mag measured values; internal field netic moment = 2ps per Cr4+ drops to zero at T« as expected. ion at 0 K; internal magnetic field = 530 kOe. CnOu

Crystal structure at 300 K. Orthorhombic; space group, Pbcn; [6, 7]. Z = 4; o = 12.044 A; b = 8.212 A; c = 8.177 A. The structure contains pairs of CrOg octahedra joined by sharing edges; by sharing corners, the pairs of octahedra are linked with CrCh tetrahedra to form a three dimensional frame work. Trivalent and hexavalent Cr atoms seem to be present. Cr«Oi6 (C^Os)

Crystal structure at 300 K. Orthorhombic; space group, Cmcm Detailed structural and other data [6-8] or C2cm or Cmc2; Z = 4; a = are lacking. 8.47 A; b = 12.90 A; c = 10.08 A.

Kubota [8] reports that C^Og has 10 a high value of p (~10 ftcm) at 300 K. CrO,

Crystal structure at 300 K. Orthorhombic, space group, C2cm; CrOg has a chain structure. [6, 9, 70]. £ = 4; a -4.789 ±0.005 A; 6- 8.557 ±0.005 A; c = 5.743 ±0.004 A.

52 Chromium oxides—Continued

Oxide and description Data Remarks and inferences References of the study

The structure consists of linear chains of comer-sharing CrO, tetrahedra.

5- Electrical properties. Semiconductor behavior; p~10 Detailed data are, however, lacking [71, 72]. 6 10 ficm, p studies at high pres- on Cr0 3 at present. sures indicate a transition at ~140-145 kbars with considerable decrease in the value of p but

C 1O 3 remains semiconducting up to very high pressures.

References [31] Street, R., Munday, B. C., Window, B., and Williams, I. R., J. Appl. Phys. 39, 1050 (1968). [32] Tani, H., J. Phys. C (Solid State Phys.) 3, 1597 (1970). [33] Street, R., and Lewis, B., Phil. Mag. 1, 663 (1956). [1] Schbnberg, N., Acta Chem. Scand. 8, 221 (1954). [34] Samokhvalov, A. A., Sov. Phys.—Solid State (English [2] Brauer, G., Reuther, H., Walz, H., and Zapp, K. H., Transl.) 3, 2613 (1962). Z. anorg. allgem. Chem. 369, 144 (1969). [35] Lewis, G. K., Jr., and Drickamer, H. G., J. Chem. Phys. [3] Bunting, E. N., J. Res. Nat. Bur. Stand. (U.S.), 6, 947 45, 224 (1966). (1931). [36] Worlton, T. G., Brugger, R. M., and Bennion, R. B., [4] Guillaud, C., Michael, A., Binard, J., and Fallot, M., J. Phys. Chem. Solids 29, 435 (1968). Compt. Rend. (Paris) 219, 58 (1944). [37] Gunsser, W., Hille, W., and Knappwost, A., Z. Natur- [5] Wilhelmi, K.-A., Acta Chem. Scand. 19, 165 (1965), forsch. 23a, 781 (1968). [6] Wilhelmi, K.-A., Acta Chem. Scand. 22, 2565 (1968). [38] Kawai, N., and Mochizuki, S., Phys. Letters 36A, 54 [7] Wilhelmi, K.-A., Nature (Lond.) 203, 967 (1964). (1971). [8] Kubota, B., J. Am. Ceram. Soc. 44, 239 (1961). [39] Saalfeld, H., Z. Krist. 120, 342 (1964). [9] Bystrom, A., and Wilhelmi, K.-A., Acta Chem. Scand. [40] Rossi, L. R., and Lawrence, W. G., J. Am. Ceram. Soc. 4, 1131 (1950). 53, 604 (1970). [10] Hilty, D. C., Forgeng, W. D., and Folkman, R. L., [41] Schultz, A. H., and Stubican, V. S., J. Am. Ceram. Soc. J. Metals 7, 253 (1955); Trans. AIME 203, 253 53, 613 (1970). (1955). [42] Wretblad, P. E., Z. anorg. allgem. Chem. 189, 329 [11] Layden, G. K., J. Am. Ceram. Soc. 48, 219 (1965). (1930). [12] Jonnson, R. E., and Muan, A., J. Am. Ceram. Soc. 51, [43] Michel, A., and Benard, J., Compt. Rend. (Paris) 200, 430 (1968). 1316 (1935); Bull. Soc. Chim. 10, 315 (1943). [13] Brockhouse, B. N. J., J. Chem. Phys. 21, 961 (1953). [44] Schwartz, R. S., Funkuchen, I., and Ward, R., J. Am. [14] McGuire, T. R., Scott, E. J., and Grannis, F. H., Phys. Chem. Soc. 74, 1676 (1952). Rev. 102, 1000 (1956). [45] Ariya, S. M., Shchukarev, S. A., and Glushkova, V. B., [15] Li, Y. Y., Phys. Rev. 102, 1015 (1956). Zhur. Obschchei. Khim. 23, 1241 (1953). [16] Goodenough, J. B., Phys. Rev. 117, 1442 (1960). [46] Thamer, B. J., Douglass, R. M., and Staritzky, E., [17] Osmond, W. P., Proc. Phys. Soc. (Lond.) 79, 394 (1962). J. Am. Chem. Soc. 79, 547 (1957). [18] Cox, D. E., Takei, W. J., and Shirane, G., J. Phys. Chem. [47] Kubota, B., Nishikawa, T., Yanase, A., Hirota, E., Solids 24, 405 (1963). Mihara, T., and Iida, Y., J. Am. Ceram. Soc. 46, 550 [19] Pratt, G. W., Jr., and Bailey, P. T., Phys. Rev. 131, (1963). 1923 (1963). [48] Chapin, D. S., Kafalas, J. A., and Honig, J. M., J. Phys. [20] Greenwald, S., Nature (Lond.) 177, 286 (1956). Chem. 69, 1402 (1965). [21] Foner, S., Phys. Rev. 130, 183 (1963). [49] Nakayama, N., Hirota, E., and Nishikawa, T., J. Am. [22] Asrov, D. N., Sov. Phys.—JETP (English Transl.) 13, Ceram. Soc. 49, 52 (1966). 729 (1961). [50] DeVries, K. J., Naturwiss, 54, 563 (1967). [23] Shtrikman, S., and Treves, D., Phys. Rev. 130, 986 [51] Chamberland, B. L., Mat. Res. Bull. 2, 827 (1967). (1963). [52] Shibasaki, Y., Kanamura, F., Koizumi, M., Ado, K., [24] Hornreich, R., and Shtrikman, S., Phys. Rev. 161, 506 and Kume, S., Mat. Res. Bull. 5, 1051 (1970). U967). [53] Rodbell, D., and DeVries, R., Mat. Res. Bull. 2, 491 [25] Lai, H. B., Srivastava, R., and Srivastava, K. G., Phys. (1967). Rev. 154, 505 (1967). [54] DeVries, R. C., Mat. Res. Bull. 2, 999 (1967). [26] Srivastava, K. G., Kant, P., and Srivastava, R., Ind. J. [55] Siratori, K., and Iida, S., J. Phys. Soc. Japan 17B Pure and Appl. Phys. 8, 755 (1970). (Suppl.), 208 (1962). [27] O’Dell, T. H., and White, E. A. D., Phil. Mag. 22, 649 [56] Krishna Rao, K. V., and Iyengar, L., Acta Cryst. 25A, G970). 302 (1969). [28] Roche, J., and Jaffrey, J., Compt. Rend. (Paris) 240 [57] Flippen, R. B., J. Appl. Phys. 34, 2026 (1963). 2212 (1955). [58] Rodbell, D. S., J. Phys. Soc. Japan 21, 1224 (1966). [29] Hagel, W. C., J. Appl. Phys. 36, 2586 (1965). [59] Stoffel, A. M., J. Appl. Phys. 40, 1238 (1969). [30] Shapira, Y., Phys. Letters 24A, 361 (1967); J. Appl. [60] Shinjo, T., Takada, T., and Tamagawa, N., J. Phys. Phys. 42, 1588 (1971). Soc. Japan 26 1404 (1969).

53 Kubota, D., and Hirota, E., J. Phys. Soc. Japan 345 [61] 16, MnO are arranged in ferromagnetic sheets parallel (1961). [62] Rodbell, D. S., Lommel, J. M., and DeVries, R. C., J. to {111} planes, with neighboring planes coupled Phvs. Soc. Japan 2430 (1966). 21, in an antiferromagnetic arrangement. The long range [63] Nishikawa, T., Nakayama, N., and Hirota, E., Yogyo Kyokaishi 74, 256 (1966); Chem. Abstr. 69, 81508w order parameter increases progressively with de- (1968). creasing T below Tn. Neutron diffraction measure- [64] Nygren, M., and Magneli, A., Arkiv for Kemi 28, 217 (1967). ments show considerable short range magnetic order Chrenko, R. M., and Rodbell, D. S., Phys. Letters 24A, [65] in MnO above Tn [16, 22]. Even though many work- 211 (1967). [66] Druilhe, R., and Bonnerot, J., Compt. Rend. (Paris) ers have argued for a continuous (higher order) 263B, 55 (1966). transition in MnO because no hysteresis effects are [67] Kubota, B., Nishikawa, T., and Yanase, A., J. Phys. Soc. Japan 16, 2340 (1961). noted at TN , the available x-ray diffraction [14], [68] Villeneuve, G., Bordet, A., Casalot, A., and Hagen- X-T [21], exchange striction [14, 15], TEC [25], Moss- muller, P., Mat. Res. Bull. 6, 119 (1971). [69] Stradley, J. G., Shevbn, T. S., and Everhart, J. O., bauer [26], internal friction and Young’s modulus Ohio State Univ. Res. Found. Prog. Rept. 806-1, [27], acoustic attenuation [28], elastic moduli [29] Contract AF 35(616)-5515 (1958). [70] Stephens, J. S., and Cruickshank, D. W. J., Acta Cryst. and NMR [21, 30-32] data indicate that the anti- 26B, 222 (1970). ferromagnetic-paramagnetic transition is indeed first [71] Cojocaru, L. N., Costea, T., and Negoescu, I., Z. Phys. Chem. (N.F.) 60, 152 (1968). order. High pressure studies on MnO have been [72] Minomura, S., and Drickamer, H. G., J. Appl. Phys. carried out and the x-ray data [33] indicate a transi- 34, 3043 (1963). [73] Newnham, R. E., and de Haan, Y. M., Z. Krist. 117, tion in MnO at ~100 kbar to a phase which is 235 (1962). tetragonal or of lower symmetry. Bartholin et al. [74] Corliss, L., Hastings, J. M., Nathans, R., and Shirane, G., J. Appl. Phys. 36, 1099 (1965). [34] found that the Tn increases with P up to ~3 [75] Samuelson, E. J., Hutchings, M. T., and Shirane, G., kbar. Physica 48, 13 (1970). [76] Chapman, P. R., Griffith, R. H., and Marsh, T. D. F., Electrical properties of pure [9, 35-41] and doped Proc. Roy. Soc. (Lond.) 224A, 419 (1954). (with Li or Cr) [9, 42-44] MnO have been examined [77] Fischer, W. A., and Lorenz, G., Z. Phys. Chem. (N.F.) 18, 308 (1958). in detail. MnO is unique among the transition metal [78] Hagel, W. C., and Seybolt, A. U., J. Electrochem. Soc. monoxides in that there is a maximum in the p versus 108, 1146 (1961). Appl. plot indicating a transition from to type [79] Crawford, J. A., and Vest, R. W., J. Phys. 35, po 2 p n 2413 (1964). semiconducting behavior at ~1370 K; changes in [80] Vaisnys, J. R.’, J. Appl. Phys. 38, 2153 (1967). [81] Cojocaru, L. N., Z. Phys. Chem. (N.F.) 64, 255 (1969). sign of ph and a have also been noted [5, 35, 38-40] Subba Rao, G. V., Wanklyn, B. M., and Rao, C. N. R., [82] to corroborate the resistivity data. The electron J. Phys. Chem. Solids 32, 345 (1971). [83] Subba Rao, G. V., Rao, C. N. R., and Ferraro, J. R., mobility in MnO is higher than the hole mobility Appl. Spec. 24, 436 (1970) and references therein. at high temperatures (p£/p^«10 at ~1450 K) [45, [84] Rao, C. N. R., and Subba Rao, G. V., Phys. Stat. Solidi (a) 1, 597 (1900). 46] and the defect structure seems to involve doubly Honig, M., in Modern Aspects of Solid State Chemistry, [85] J. ionized cation vacancies at high temperatures and Ed. C. N. R. Rao, (Plenum Press, New York, 1970). at low oxygen partial pressures [5, 9, 35-37, 41, 42, 45, 46]. The charge carriers are localized in pure 1.5. Manganese Oxides and doped MnO and at low enough temperatures (<700 K) hopping type of conduction occurs by Detailed phase relation studies of the Mn-0 system holes in the 3d-band with a small E a whereas the [1-5] have shown that and MnO, Mn304 , M^Oa, dominant mechanism of conduction is by holes in MnOa are the stable solid oxides. All the oxides show the oxygen 2p-band [9, 47, 48]. The EMF data on interesting crystallographic and magnetic transitions MnO at high temperatures [10] indicate second or and have been widely investigated in the literature. higher order transitions within the homogeneity region. MnO: MnO shows slight deviations from stoichio- Optical properties of MnO have been studied in metry (<2%) at high temperatures (~1400 K) the literature [49-51]; the absorption edge appears [5-10]. It is cubic with the rock salt structure at to be ~3.8 eV. Far infrared antiferromagnetic room temperature and undergoes a trigonal distor- resonance studies [52-54] show that the resonance -1 tion as well as a volume contraction on cooling to frequency at 27.5 cm is dependent on temperature ~118 K and changes from a paramagnetic to an and impurity content. antiferromagnetic state [6, 11-15]. The structure in MnO forms solid solutions with other transition the antiferromagnetic phase has been examined by metal monoxides having the rock salt structure [55, a few workers [11, 15, 16-24] and detailed x m T 56] and the physical properties show variations with measurements made. The magnetic moments in the composition.

54 M 113O4 : M 113O 4 has a tetragonal structure at room involves a shift in the symmetry center of the struc- temperature and can be considered as a distorted ture (characteristic of a diffusionless, but probably, spinel because of the Jahn-Teller distortion at the first-order transition). 3+ Mn sites [57]; the ionic formula is probably very Mn2 C>3 behaves as a semiconductor in the range 2+ + close to Mn [Mnj! O 4 corresponding to a 'normal’ 400 to 1000 K [85, 86]. It decomposes at high tem- ]

spinel. Preparation of Mn304 in both polycrystalline peratures. and single crystal forms has been reported in the Trivalent cations can be doped successfully into

literature [6, 58-65]. The degree of tetragonal dis- Mn203 and the structural and magnetic ordering tortion in Mn304 decreases with increase in T and aspects have been examined by Geller and co-work -

a rapid structural transition to cubic phase takes ers [80-84] and by Hase et al. [87].

place at ~1435 K [66, 67]. The high temperature Mn02 : Mn02 apparently exists only in the rutile

cubic form can not be preserved by quenching and modification and found in nature as pyrolusite [6]. the thermodynamic parameters indicate the first An orthorhombic modification has been reported

! order nature of the transition [10, 66, 67]. in the naturally occurring samples, but detailed data

II Mns04 is ferrimagnetic below 43 K which is the are lacking; laboratory preparation of the orthor-

curie temperature [65, 68, 69]. At 33 K a rearrange- hombic form has, however, not been successful [6].

ment of the moments occurs such that the chemical Mn02 is always associated with slight nonstoichiom-

and magnetic unit cells become identical. Measure- etry [6, 88, 90] and decomposes above 800 K to

ments of Cp [70] in the range 20-49 K show that Mn203 .

there are no anomalies except for a peak at Tc . The usual wet chemical methods of preparation

The material is a semiconductor [71-73] and has a of Mn02 result in nonstiochiometry and water of

higher resistivity than the analogous material Fe 3C> 4 . hydration and are less characterized [6, 88, 89].

Mn304 forms complete series of solid solutions Atomic or molecular oxygen is evolved on heating

with Fe304 and C03O4 [67, 74]. these 'disperse’ forms of Mn02 and serve as good

Mn 2 03 : The common form of manganese sesquioxide oxidizing agents in organic chemical reactions; most

is the a-modification which is closely related to the probably the 'active’ oxygen is present in the lattice bixbyite, as defects or surface. (Mn, Fe) 2 03 , structure [6]. 7-Mn203 having adsorbed on the The activity a structure nearly identical to that of Mn304 has 'seems to depend on the particle size and the method

been reported [6, 75], but detailed data are lacking. of preparation etc.

Moore et al. [6] have noted that 7-Mn2 03 transforms No crystal structure transitions are known in

to the a -modification either by heating in vacuum Mn02 . It is antiferromagnetic below 92 K [90-94] for 48 h at ~770 K or by keeping at room tempera- and has a screw-type spin structure such that the ture for one year. spins, aligned parallel in the c plane, screw along the

Among the transition metal sesquioxides, a-Mn203 c axis; the magnetic unit cell is as large as 7 times is the only oxide which does not possess a corundum the chemical unit cell.

structure and instead, it assumes a deformed cubic Mn02 has a low resistivity (~0.H2 cm at 300 K) C-type rare-earth oxide structure [76-78] possibly and shows anomalous temperature dependence in

as a result of the large Jahn-Teller distortion associ- the range 4 to 300 K [90]. Anomalies in p and Cp 3+ ated with the Mn ion [79, 80]. Geller and co- have been noted at Tn [90, 94, 95]. Bhide and

workers [79-84], employing x-ray diffraction, x-T Damle [96] reported Mn02 to be ferroelectric with a

and Mossbauer studies have shown that a-Mn2 03 Tc of 325 K and Samokhvalov [97] finds a (UHF) undergoes a crystallographic transition (cubic to dielectric anomaly at around this temperature; how- orthorhombic) at 302 K, a paramagnetic-antiferro- ever, the anomaly does not correspond to that of a magnetic transition at 80 K and another magnetic typical ferroelectric material. Ferroelectricity in

transition (of the Neel type) at 25 K. Crystal struc- Mn02 is doubtful since no other relevant data are

ture of a-Mn203 does not change below 300 K and available in the literature.

x-ray data at 6.5 K show no additional lines or sym- Solid solutions, (Mn _ a: Cr a.)02 have been reported 1 , metry change implying that the transition at 25 K by Siratori and Iida [98].

55 Manganese oxides

Oxide and description Data Remarks and inferences References of the study

MnO

Crystal structure and T = 296 K: Cubic; space group, The cubic lattice distorts to [6, 11-15,

x-ray studies. Fm3m; Z = 4; a =4.4457 ±0.0002 rhombohedral structure at T t on 25, 33]. A. T ~i K: Rhombohedral; o = cooling; slight volume changes 4.4316 ±0.0003 A; a=90.624± also occur. TEC also touches 0.008°. T t (= Tn) = 118±2 K. maximum at T t . The structure of 9 TEC (/°C): 64X10- at ~TN . the high pressure form is not High pressure x-ray studies at known. 300 K indicate a phase transition at ~100 kbar to a tetragonal or lower symmetry phase.

Magnetic properties. x (122 K)= 83Xl0-« emu/g; In the antiferromagnetic state the [11, 15, 16- -6 X (4.2 K)=79X10 emu/g. magnetic moments are arranged 24, 34]. Typical antiferromagnetic be- in ferromagnetic sheets parallel havior with Tn = 118±3 K. to (111) planes and the direction Tn increases with pressure; of magnetization in neighboring -3 (dTn/dP) =0.3X10 deg/bar. In planes is antiparallel. Neutron the presence of a magnetic field, diffraction shows the persistence below Tn, the direction of anti- of short range magnetic order ferromagnetism is modified which above Tn. The magnetic transi-

leads to an increase in x- tion appears to be first order.

Electrical properties. Pure MnO: The data on pure MnO can be in- [5, 9, 35- 16 p (300 K)~10 ftcm in stoichio- terpreted as due to hopping of 37, 42, 45, metric MnO. Semiconductor be- charge carriers with a band gap 46-48, 99].

havior up to ~1500 K; Ea ~l.Q of ~2.2 eV or with a band con- eV; a (500 K)~400 pV/K; duction for electrons with a Changes from p- to n-type be- smaller band gap. The change

havior as a function of po a (at from p- to n-type behavior is due ~10“12 atm), at ~1370 K; a to the greater mobility of elec- changes sign at this temperature; trons compared to holes at high (Md/md) * 10. temperatures and low oxygen partial pressures and due to the Li doped MnO: cation vacancy defects. In doped -3 -1 p~10 -10 ficm; £a ~0.7 eV; materials, conduction is always Mh =0.007 cm 2 /V s (400-700 K); extrinsic at ordinary temperatures Md = (calc.)~0.03 cm 2 /V s; m* = and mainly occurs by hopping; 7 mo at 700 K. p decreases with intrinsic conduction seems to set pressure and no transitions are in at very high temperatures. indicated up to 200 khar.

-1 -1 Optical properties. Dq = 1010 cm ; B = 601 cm ; Optical data indicate the localized [49-54]. Absorption edge is at ~3.8 eV. nature of the charge carriers in Far infrared antiferromagnetic MnO. -1 resonance is at ~27.5 cm at 2 K; the frequency decreases with rise in T and varies with the impurity content.

Mossbauer studies of 67 Fe Tn = 116.9 K. Magnetic hyperfine The magnetic transition appears to [26]. doped MnO. splitting noted below Tn. Internal be first order but no tempera-

magnetic field is maximum (170 tures hysteresis is noted in the kG) at 90 K and decreases to 97 transition. kG at 4.2 K.

56 Manganese oxides=C©ntinued

Oxide and description Data Remarks and inferences References of the study

NMR studies of MnO. The 66Mn NMR frequency shifts Guenther et al. [32] speculate that [21, 30-32]. are found to be temperature de- weak ferromagnetism may exist pendent and proportional to x* in MnO at very low magnetic field dependence also is temperatures. noted. Hyperfine coupling constant = — (81.5±2.5)X10-4 -1 cm .

7 -1 Mechanical properties. Compressibility = 6.48 X10“ bar . The discontinuous changes in the [14, 15, 27, Elastic moduli (measured at 30 elastic moduli, Young’s modulus 29]. MHz): Cn = (1.768 ±0.006) X10 12 and internal friction are taken as 2 u dyn/cm ; Cu = (6.8±0.1) X 10 strong evidence for a first order dyne/cm 2 (at T>Tn). At Tn, magnetic transition in MnO. Cu drops by ~19% and greater decrease is shown by Cu. Young’s modulus (at Tn)=2X10u dyn/cm and internal friction « 20 10~3 X ; both the properties show anomalous behavior near Tk. M113O4

Crystal structure and T = 300 K: Tetragonal; space MmO* can be considered as a dis- [6, 58-67]. x-ray studies. group, I4i/amd; Z = 4; 0 = 5.762 torted spinel because of the A; c = 9.463 A. T, = 1435 K. Jahn -Teller distortion at the 3+ T>T ( : Cubic; a = 8.42 A. Mn sites; the ionic formula can 2 + 8 + be written as Mn /[Mn 2 04 . ] The transition is first order and the high temperature phase cannot be quenched to room temperature.

Magnetic properties. Ferrimagnetic below 43 K (To); a The observed ferrimagnetism is [60, 65, 68, rearrangement of the spin mo- consistent with the spinel struc- 69]. ments occurs at 33 K such that ture. 1/x-T behavior is of the the magnetic and chemical unit Neel type above Tc and con- cells become identical, xm (290 sistent with the Yafet-Kittel K)~12XlO-3 emu; Magnetic model. moment = 5.27 mb below To.

Electrical and thermal Semiconductor behavior; p (300 K) Detailed electrical data are lack- [10, 66, 67, 7 properties. ~10 ficm; a«0.2 mV/°C in the ing; Mna0 4 has a higher value of 70, 71-73]. range 500-900 K (p-type); AH = p than FesCh. The enthalpy and = — (5.9±0.7) kcal/mol; AS Cp data indicate that the transi-

(4.2 ±0.5) et at T<; Cp shows an tion is first order.

anomaly at T t .

-1 Infrared spectra. Bands at 474 and 550 cm are The data are correlated with the [100]. noted. spectra of other oxide spinels. MniOs

Crystal structure and T = 298 K; Orthorhombic; space Earlier workers concluded that [76-84], x-ray studies. group, Pbca; Z = 4; a =9.414 A; only cubic structure exists in 6=9,424 A; c = 9.405 A. T< = MnjOs; this oxide does not have 302 K. T = 314 K: Cubic; space a corundum structure and in- group, Ia3; Z = 16; a =9.414 A. stead has a distorted C-type rare- earth oxide structure. The transi-

57 1

Manganese oxides—Continued

Oxide and description Data Remarks and inferences References of the study

tion appears to be first order but detailed data on other properties are lacking.

-3 Magnetic properties. Xm (100 K)~7X10 emu. Para- Geller et al. [82] proposed a mag- [79-84, 101, magnetic—antiferromagnetic netic structure in the antiferro- 102]. transition, Tni, at 80 K and magnetic phase. Chevalier et al. another magnetic transition at [102] find that the low tempera-

25 K (Ts 2 ). No crystal structure ture (25 K) transition is at 50 K transition below 300 K up to 4 K; and is of second order. Both the low temperature phases are antiferromagnetic. Tni and

Tn 2 are greatly affected by impurities.

-1 Optical and electrical Band at ~20,000 cm is ascribed Detailed electrical data on single [85, 86, properties. to the spin allowed transition. crystal material are lacking. 103]. Infrared bands are in the range 320-670 cm -1 and are ascribed to the stretching and bending vibrations of the Mn-O bond. Semi- conductor behavior in the range 400-1100 K: p (200 K)~7X10 3

Qcm; p (500 K)~50 £2cm; Ea = 0.6 eV.

M11 O2

Crystal structure and T = 298 K: Tetragonal; space No crystal structure transitions are [6, 33, 88- x-ray studies. group P42/mnm; Z = 4; a =4.3980 known and it is very difficult to 90, 104, ±0.0002 A; c = 2.8738 ±0.0002 A. prepare exactly stoichiometric 105]. 7 TEC (300-670 K) (X10 /°C); MnC> 2 . c = 69 a =67. With pressure, c 1 ; || axis expands at low pressures, then passes through a maximum (at ~40 kbar) and ultimately contracts; a axis, on the other hand, decreases smoothly with

pressure. The compressibility is low at low pressures and then increases at pressures beyond the maximum in c axis.

Magnetic properties. Antiferromagnetic with Tn= 92±2 The characteristic screw structure [90-94]. -6 K. x (100 K) «36X 10 emu/g; is exclusive to MnC> 2 . -8 X (300 K) —32X10 emu/g. Curie-Weiss Law is satisfied above 150 K. A screw type spin structure exists below Tn and the magnetic unit cell is 7 times the chemical unit cell.

Electrical and other Resistivity is low; p (4.2 K) ~2.6X The anomalous p-T behavior is not [90, 94, 96]. 2 properties. 10" ficm; p (300 K)»l.lXl0~k clearly understood; Mn02 may ficm. Anomalous T dependence be classified as a degenerate in the range 4-300 K. Anomalies semiconductor [57]. The magnetic

58 Manganese oxides—Continued

Oxide and description Data Remarks and inferences of the study

in p and Cp noted at Ts- Ferro- transition may be classified as electricity with a Tc~325 K second order. The details of the reported. ferroelectric behavior are not

understood and ferroelectricity is doubtful.

References [32] Guenther, B. D., Christensen, C. R., and Daniel, A. C., Phys. Letters 30A, 391 (1969). [33] Clendenen, R. L., and Drickamer, H. G., J. Chem. [1] Hahn, W. C., Jr., and Muan, A., Am, J. Sci. 258, 66 Phys. 44, 4223 (1966). (1960). [34] Bartholin, H., Bloch, D., and Georges, R., Compt. [2] Otto, E. M., J. Electrochem. Soc. Ill, 88 (1964). Rend. (Paris) 264B, 360 (1967). [3] Schmahl, N. G., and Stemmier. B.. J. Electrochem. Soc. [35] Hed, A. Z., and Tannhauser. D. S., J. Chem. Phvs. 47, 112, 365 (1965). 2090 (1967). [4] Muan, A., Nucl. Sci. Abstr. 20, 50 (1966). [36] Duquesnoy, A., and Marion, F. C., Compt. Rend. [5] Hed, A. Z., and Tannhauser, D. S., J. Electrochem. Soc. (Paris) 256, 2862 1963). 114, 314 (1967). [37] Eror, N. G., Ph.D. Thesis, Northwestern L'niv., Illinois, [6] Moore, T. E., Ellis, M., and Selwood, P. W., J. Am. U.S.A. (1965). Chem. Soc. 72, 856 (1950). [38] Gvishi, M., Tallan, N. M., and Tannhauser, D. S., [7] Davies, M. W., and Richardson, F. D., Trans. Faraday Solid State Commun. 6, 135 (1968). Soc. 55, 604 (1959). [39] DeWit, H. J., and Crevecoeur, C., Phvs. Letters 25A, [8] Turnock, A. C., J. Am. Ceram. Soc. 49, 382 (1966). 393 (1967). [9] O’Keeffe, M., and Valigi, M., J. Phvs. Chem. Solids [40] Bocquet, J. P., Kawahara, M., and Lacombe, P., Compt. 31, 947 (1970). Rend. (Paris) 265C, 1318 (1967). [10] Fender, B. E. F., and Riley, F. D., in The Chemistry [41] Le Brusq, H., Oehhg, J.-J., and Marion, F., Compt. of Extended Defects in non-Metallic Solids, Eds. L. Rend. Paris) 266C, 965 (1968). Eyring and M. O'Keeffe (North Holland Publ. Co., [42] Nagels, P., Denayer, M., DeWit, H. J., and Crevecoeur, Amsterdam, 1970), p 54. C., Sohd State Commun. 6, 695 (1968). [11] Roth, W. L., Phvs. Rev. 110, 1333 (1958); 111, 772 [43] Ah, M., Fridman, M., Denayer, M., and Nagels, P., (1958); J. Appl. Phvs. 31, 2000 (1961). Phys. Stat. Sohdi 28, 193 (1968).

[12] Rodbell, D. S., Osika, L. M. t and Lawrence, P. E., [44] Crevecoeur, C., and DeWit, H. J., J. Phys. Chem. J. Appl. Phvs. 36, 666 (1965). Solids 31, 783 (1970). [13] Bloch. D., Charbit. P., and Georges, R., Compt. Rend. [45] Wagner, J. B., Jr., in Mass Transport in Oxides, Nat. (Paris) 266B, 430 (1968). Bur. Stand. (U.S.), Spec. Publ. 296, (1968), p. 65. [14] Morosin, B., Phvs. Rev. B 1, 236 (1970). [46] Tannh auser, D. S., Tallan, N. M., and Gvishi, M., in [15] Bartel, L. C., Phvs. Rev. B 1, 1254 (1970): Bull. Am. Mass Transport in Oxides, N.B.S. Special Public. Phvs. Soc. 15, 271 (1970); J. Appl. Phys. 41, 5132 296, 1968. (1970). [47] Adler, D., Sohd State Phvs. 21, 1 (1968). [16] Shull, C. G., Strauser, W. A., and Wollan. E. O.. Phvs. [48] Adler, D., and Feinleih, f., Phys. Rev. B2, 3112 (1970). Rev. 83, 333 (1951). [49] Iskenderov, R. N., Drabkin, I. A., Emel’yanova, L. T., [17] Kaplan, J. I., J. Chem. Phys. 22, 1709 (1954). and Ksendzov, Ya. M., Sov. Phys. —Sohd State 1 [18 Keffer, F., and O’Sullivan, W., Phvs. Rev. 108, 637 (Enghsh Transl.) 10, 2031 (1969). (1?57). [501 Huffman, D. R., J. Appl. Phys. 40, 1334 (1969). t [19] Uchida, E., Kondoh, H., Nakamuzi, Y., and Nagamiya, [51] Huffman, D. R., Wild, R. L., and Shinmei, M., J. T., J. Phvs. Soc. Japan 15, 466 (1960). Chem. Phys. 50, 4092 (1969). [20] Slack, G. A., J. Appl. Phys. 31, 1571 (1960). [52] Sievers, A. J., and Tinkham, M., Phys. Rev. 129, 1566 [21 1 Lines, M. E., and Jones, E. D., Phvs. Rev. 139A, 1313 (1963). s (1965 . [53] Richards, P. L., J. Appl. Phys. 34, 1237 (1963). [22] Blech, I. A., and Averbach, B. L., Phvs. Rev. 142, 287 [54] Hughes, A. E., Phys. Rev. B3, 877 (1971).

(1966 . [55] Bizette, H., and Mainard, R., Bull. Soc. Sci. Bretagne [23] Bloch, D., Feron, J. L., Georges, R., and Jacobs, I. S., 42, 209 (1967). J. Appl. Phys. 38, 1474 (1967). [56] Brezny, B., Rvall, W. R., and Muan, A., Mat. Res. Bloch, I. [24] D., Georges, R., and Jacobs, S., J. de Phvsique Bull. 5, 481 (1970). 31, 589 (1970). [57] Rao, C. N. R., and Subba Rao, G. V., Phvs. Stat. [25] Foex, M., Compt. Rend. (Paris) 227, 193 (1948). Sohdi (a) 1, 597 (1970). [26] Siegwarth, J. D., Phys. Rev. 155, 285 (1967). [58] Scott, E. J., J. Chem. Phys. 23, 2459 (1955). [27] Belov, K. P., Katayev, G. I., and Levitin, R. Z., [59] Kleinert, P., Z. Chem. 3, 353 (1963). J. Appl. Phys. Suppl. 31, 153 (1960). H., Phvs. Stat. Sohdi 563 [28] Evans, R. G., and Daniel, M. R., Phvs. Letters 26A, [60] Perthel, R., and Jahn, 5, 276 (1968). (1964). [29] Cracknell, M. F., and Evans, R. G., SoUd State Com- [61] Rozdestvenskaya, M. V., Mokievskii, V. A., and mun. 8, 359 (1970). Stogova, V. A., Sov. Phvs.—Crvstallogr. (Enghsh [30] Jones, E. D., Phys. Rev. 151, 315 (1966). Transl.) 11, 765 (1967). [31] Tompa, K., Toth, F., and Gruner, G., Phvs. Stat. [62] Smith, B. A., and Austin, I. G., J. Cryst. Growth 1, 79 Solidi 22, Kll (1967). (1967).

59 [63] Buhl, R., J. Phys. Chem. Solids 30, 805 (1969). 1.6. Iron Oxides [64] Caslavska, V., and Roy, R., J. Appl. Phys. 41, 825 (1970). R., and Perrin, M., Appl. [65] Boucher, B., Buhl, J. Phys. The Fe-0 system has been a subject of extensive 42, 1615 (1971). [66] Southard, J. C., and Moore, G. E., J. Am. Chem. Soc. investigation over the past several decades [1-4]. The 1769 (1942). 64, available data up to 1957 has been summarized in [67] van Hook, H. J., and Keith, M. L., Am. Mineral. 43, 69 (1958). the literature [2]. The most important iron oxides I. Phys. Chem. Solids (1959). [68] Jacobs, S., J. 11, 1 are the oxygen deficient ferrous oxide (wiistite), [69] Dwight, K., and Menyuk, N., Phys. Rev. 119, 1470 (1960). Fei-zO, magnetite, Fe 30 4 and hematite, Fe2 0 3 . The [70] Lecomte, M., quoted in [65]. dioxide, Fe02 has been recently reported [5], but [71] Verwey, E. J. W., and deBoer, J. H., Rec. Trav. Chim.

existance of stable still II 55, 531 (1936). the Fe02 appears doubtful Dani, R. H., Physica 821 (1961). [72] Bhide, V. G., and 27, [6]. [73] Oehlig, J. J., Le Brusq, H., Duquesnoy, A., and Marion,

'I

F. , Compt. Rend. (Paris) 265C, 421 (1967). [74] Wickham, D. J., and Croft, W. J., J. Phys. Chem. Fei_zO: Wiistite at ordinary pressures is always I Solids 7, 351 (1958). nonstoichio-metric and the homogeneity range ex- [75] Gattow, V. G., and Glemser, O., Z. anorg. allgem. Chem. 309, 121 (1961). tends from Fe to at ~1620 0 . 84O Feo.gsO K and the [76] Banks, E., and Kostiner, E., J. Appl. Phys. 37, 1423 (1966). material is unstable below 840 K with respect to Hase, W., Kleinstuck, K., and Schulze, G. E. R., Z. [77] decomposition into Fe 30 4 and Fe metal; the metasta- Krist. 124, 428 (1967).

ble phase can, however, be retained by quenching i [78] Norrestam, R., Acta Chem. Scand. 21, 2871 (1967). Geller, S., Cape, A., Grant, R. W., and Espinosa, [79] J. [1, 2, 7-10]. The thermodynamics and the defect G. P., Phys. Letters 24A, 369 (1967). structure at high temperatures has been examined [80] Grant, R. W., Cape, J. A., Geller, S., and Espinosa,

P., Bull. Am. Phys. Soc. 13, 462 (1968). ' G. [4, 11-20]. The deviation from stoichiometry in [81] Geller, S., Grant, R. W., Cape, J. A., and Espinosa, wiistite is due to cation vacancies, with each cation G. P., J. Appl. Phys. 38, 1457 (1967). [82] Grant, R. W., Geller, S., Cape, J. A., and Espinosa, vacancy requiring the presence of two Fe 3+ ions to G. P., Phys. Rev. 175, 686 (1968).

maintain charge balance; thus, be re- i [83] Geller, S., and Espinosa, G. P., Phys. Rev. Bl, 3763 Fei_ xO may (1970). garded as a solid solution of Fe2+ and cation vacan- [84] Geller, S., Acta Cryst. 27, 821 (1971). cies in FeO. The average structure at room tempera- [85] Drotschman, C., Batterien 18, 686 (1964); Chem. Abstr. 62, 8472g (1965). ture is cubic [7-10] and the material is paramagnetic. [86] Subba Rao, G. V., Wanklyn, B. M., and Rao, C. N. R., Magnetic ordering into antiferromagnetic state oc- J. Phys. Chem. Solids 32, 345 (1971). ; [87] Hase, W., Bruckner, W., Tobsisch, J., Ullrich, H.-J., curs at 198 K accompanied by a change from cubic and Wegerer, G., Z. Krist. 129, 360 (1969). to rhombohedral structure. Below Tn, the magnetic [88] Wadsley, A. D., and Walkley, A., Rev. Pure & Appl. Chem. 1, 203 (1951), and references therein. moments are arrayed in ferromagnetic sheets parallel Malinin, G. V., and Tolmachev, Yu. M., Russ. J. Phys. [89] to (111) planes and the direction of magnetization Chem. (English Transl.) 43, 1129 (1969).

in neighboring planes is antiparallel; the magnetic : [90] Rogers, D. B., Shannon, R. D., Sleight, A. W., and

• Gillson, L., Inorg. Chem. 841 (1969). J. 8, axis in Fei_ xO is perpendicular to the ferromagnetic [91] Bizette, H., J. Phys. Rad. 12, 161 (1951). sheets [9, 10, 21]. [92] Bacon, D., Usp. Fiz. Nauk. 8, 335 (1963). [93] Yoshimori, A., J. Phys. Soc. Japan 14, 807 (1959). Detailed studies employing x-ray and neutron [94] Ohama, N., and Hamaguchi, Y., J. Phys. Soc. Japan diffraction techniques of the quenched phases of 30, 1311 (1971). Fei_ xO indicate defect cluster formation [10, 18, 22- [95] Kelley, K. K., and Moore, G. E., J. Am. Chem. Soc. 65, 782 (1943). 24]; it appears that the oxygen sublattice is continu- Bhide, V. G., and Damle, R. V., Physica 26, 33 (1960). [96] ous and the defect cluster is a region in which there [97] Samokhvalov, A. A., Sov. Phys.—Solid State (English Transl.) 3, 2613 (1962). are vacancies in cation sites and interstitial ions in [98] Siratori, K., and Iida, S., J. Phys. Soc. Japan 15, 210 some tetrahedral sites. The results are in agreement (1960). | with a model which assumes that the average defect ' [99] Minomura, S., and Drickamer, H. G., J. Appl. Phys. 34, 3043 (1963). cluster contains about 4 vacancies and 2 interstitials Hafner, S., Z. Krist. 331 (1961). [100] 115, in powder samples and 13 vacancies and 4 inter- [101] Banks, E., Kostiner, E., and Wertheim, G. K., J. Chem. Phys. 45, 1189 Q966). stitials in single crystalline samples. Order-disorder R., Bertaut, F., Solid [102] Chevalier, R. Roult, G., and E transitions have been noted in Fei_ xO at high tem- State Commun. 5, 7 (1967). peratures [19]. [103] Subba Rao, G. V., Rao, C. N. R., and Ferraro, J. R., Appl. Spec. 24, 436 (1970). lattice varia- The cubic parameter of Fei-zO shows ; Bradt, R. C., and Wiley, S., Electrochem. Soc. [104] J. J. tion with the stoichiometry and generally, a increases 109, 651 (1962). [105] Kirchner, H. P., J. Am. Ceram. Soc. 52, 379 (1969). with increasing Fe content [8]. Magnetic cluster

60 formation [22] also affects the spin structure and transition is commonly referred to as the Verwey

directions in Fei_ xO below Tn, but Tn is almost transition and detailed investigations have been car-

unaffected by variation in x [21]. This behavior is ried out to understand the nature of the transition.

indicative of the existence of defect clusters of cation Magnetic properties of Fe 30 4 are not affected vacancies by virtue of which the remaining part of below the Verwey transition, but x shows aniso-

the cation sublattice is left intact thereby not weak- tropy [41, 43-46]. Fe 30 4 has a low resistivity at room antiferromagnetic interaction; random temperature the of ions ening the and ordering the below T t vacancies, on the other hand, would be expected to (119 K) appears as a sudden increase of p by a factor

1 weaken the antiferromagnetic interaction and lower of >90 and also causes an anisotropy in p [41, 43]. the Tn. Verwey et al. [39, 40] have explained this observation

Stoichiometric FeOi.oo lies outside the phase field as due to the fast electron exchange in Fe 3 0 4 which is

under ordinary pressure conditions [24], but under inhibited below T t because of the ordering process. high pressure conditions and in the presence of ex- Many workers [47, 48] have proposed that this

cess metallic iron, the cation deficient structure of phenomenon is similar to a metal-semiconductor

wiistite might not be formed; Katsura et al. [25] transition, but the actual electrical behavior above

have indeed successfully synthesized FeOi.oo from T t is more complex and does not correspond to the FeOo.95 and Fe at a pressure of 36 kbar and at typical metallic behavior. Semiconductor behavior

1040 K. Hentschel [26] has recently concluded on is noted in the range 80 to 119 K and 119 to 250 K; the basis of x-ray and Mossbauer data that FeOi.oo p is essentially constant in the range 250 to 1700 K

is formed as a metastable intermediate in the de- showing slight maxima and minima at 350 and ~850 composition of wiistite at relatively low tempera- K respectively [43, 49]; the low value of p and con- tures (~500 K). stancy over a wide temperature range (250 to 1700

Resistivity measurements [4, 13, 14, 27-29] on K) seem to indicate that Fe 30 4 can be classified as a

Fei_;rO indicate that p decreases with temperature degenerate semiconductor above room temperature as well as the oxygen content. Low oxygen content [35, 50-53]. Discontinuities in a, Rh and mh have samples are type as evidenced by the a data; a also been noted at [53-56]. p T t p - to n-type transition is noted at higher oxygen Mossbauer studies of the ordering transition in system. in the content or with higher po 2 in the The defect Fe 3 0 4 have been reported by many workers structure is interpreted as due to doubly ionized literature [57-67], Careful studies on untwinned

cation vacancies. crystals of Fe 3 0 4 have revealed that the Mossbauer

is Anomalous behavior of TEC [30], C p [31] and spectrum composed of five components below T t 3+ UHF dielectric constant [32] have been noted at corresponding to a tetrahedral Fe site, two octa- 3+ 2+ Tn in Fei-xO. High pressure x-ray studies [33] do hedral Fe and two octahedral Fe sites. One not indicate a transition up to several hundred would expect, on the basis of the Verwey mecha- kilobars. nism, a three component spectrum of equal intensity Solid solution formation of Fei_xO with MnO and (due to octahedral Fe2+ and Fe 3+ and tetrahedral 3+ MgO has been reported in the literature [8, 34]. Fe sites). In order to explain the observed spectrum, it is proposed [64, 65] that the Verwey-order-

Fe304 : This oxide is encountered in nature as the ing is not entirely correct and the unit cell must at magnetic oxide of iron or magnetite; it can also be least be doubled to enable the different sites to con- prepared in the laboratory and has a narrow range tribute to the hyperfine component. This is cor- of homogeneity [1-3]. Fe 30 4 has the cubic inverse roborated by the recent studies employing neutron spinel structure [35] at room temperature where diffraction [68] and electron microscopy [69]. 3+ Fe ions occupy all the A sites and one-half the B Several properties of Fe 3 0 4 exhibit discontinuities 2+ sites, while the Fe ions occupy the other half of at T t ; among them are TEC [70], saturation mag- the B sites. The material is ferrimagnetic with Tn~ netization [41], magnetostriction [70], Young’s mod-

858 K [36-38]. ulus and internal friction [71], elastic constants and

On cooling to 119 K, Fe 3 0 4 undergoes a crystallo- sound wave velocity [72], magnetocaloric effect [73] graphic transition to an orthorhombic modification and NMR relaxation [74, 75]. However, T t deter- [39-44]. This has been explained by Verwey et al. mined by different techniques differ slightly, possibly 2+ [39, 40] as due to the ordering of the octahedral Fe due to slight nonstoichiometry or sample purity 3+ and Fe ions into perpendicular rows with the [66]. Discontinuities in the ultrasonic sound velocities associated reduction in crystal symmetry. This have been noted near Tn in Fe 30 4 [76]. High pres-

61 m m

sure studies indicate that T t decreases where- that a [47, 77] of pure antiferromagnet; above TM , the crys- as Tn increases in a linear fashion. tal exhibits weak ferromagnetism with canted spins

Fe3C>4 forms solid solutions with Mn304 and C03O4 lying in the (111) plane. On application of a suffici- giving spinel type structures [34, 78]. ently large magnetic field in the 2-direction below

Tm, a field induced first order spin-flip transition is

Fe20s: Iron sesquioxide, Fe2C>3 (hematite), exists in observed. The Morin transition in Fe2 03 has been several modifications: a-Fe2 03, the most stable form examined by a variety of experimental techniques having a corundum structure [1, 2, 4], 0-form (sup- like neutron diffraction [100, 107, 108], static mag- posedly metastable) possessing a cubic bixbyite netization [106, 109-113], AFMR [110, 114, 115], structure [79, 80] and the 7-modification having a Mossbauer spectroscopy [102, 116-120], NMR [121], tetragonal structure which is metastable at all con- ultrasonic attenuation [122], Cotton-Mouton effect ditions of temperature and pressure [2, 79, 81-84]. [123] and inelastic neutron scattering [124-127]. The The structure of 6-Fe203 is not well established and effect of pulsed magnetic field has also been studied even its composition is open to doubt (a better de- [128]. scription might be 5-FeOOH) [85-88]; the c-form Several theoretical approaches have been attempt- having a monoclinic structure has been recently ed to gain an insight into the mechanism of the characterized [89] and a high pressure form of Fe203 Morin transition [113, 128, 129-132]. The effect of having a hypothetical perovskite structure has been pressure on the transition indicates that T in- deduced from shock wave experiments [90]. creases in a linear fashion; (dTu/dP) = +3.6°/kbar [133-135]. T m also depends on the particle size, the

7-Fe 2 0 3 : 7-Fe2 03 has a tetragonal structure and method of preparation and the purity of the sample consists of three spinel blocks stacked on top of [136, 137]. The effect of doping Fe203 (with impuri- each other; the unit cell has the formula Fe 6 40 96 ties) on Tm has been studied by a few workers; 3+ 3+ 3+ 3+ 4+ 4+ instead of Fe 720 96 to be expected with a spinel com- Al , Ga , Cr , Sc , Ti , and Sn ions seem to pound. The eight vacancies are distributed among the lower Tm (broadening the range of existence of weak 3+ octahedral spinel sites in an ordered manner [79, 83, ferromagnetism) [112, 138, 139] while Rh raises 91]. The material is ferrimagnetic [91, 92] and trans- the Tm considerably [140-142]. forms to <*-Fe2 0 3 above ~670 K [2, 79, 81, 92-94]; Electrical properties of pure [143-146] and doped the transition occurs gradually and it is easy to [147-150] Fe203 have been examined by various control the conversion. Goto [95] investigated the workers in the literature. The material is a semi- phase transformation of y-Fe203 up to 33 kbar conductor in the entire temperature range investi- pressure and up to 570 K. The rate constant prim- gated (200 to 1600 K). Highly pure Fe 2 C>3 shows arily depends on the thermo-dynamic parameter some what complex behavior with different activa- PA V where A V (~7%) is the volume difference tion energies in the region 300 to 700K, 700 to 950K, between a - and 7-modifications. and 950 to 1600 K whereas impure Fe 2 03 shows linear variation of lnp with 1/T in the range 400 to e-Fe2 03 : The monoclinic e-Fe 203 has a dark brown 1500 K with Ea^T.O eV. Anomalies in p and a at color and is ferromagnetic with Tc ~485 to 500 K T t and Tn have not been noted, but n-type Fe 2 C>3

[89, 96]. It decomposes to a-Fe 2C>3 above 770 K. samples show a sign reversal in Rn at ~Tn while Ph goes through a minimum around this tempera- a-Fe2C>3: This is the most stable sesquioxide of iron ture. Because of the ease with which the valence and has a very narrow range of homogeneity [1-4, state of Fe can be changed by slight impurities, it is

97]. It has a rhombohedral structure [98] and is highly improbable that the conduction is intrinsic antiferromagnetic with a high Neel point (Tn~960 in Fe 20 3 . K) [99-104]; discontinuities in the lattice parameters High pressure x-ray studies do not indicate any are noted at Tn [105]. Of special interest in Fe203 is phase transition [151], but recent resistivity studies the magnetic transition which takes place at ^263 K at ultrahigh pressures seem to indicate a transition (Tm) and is usually referred to as the Morin transi- from the insulating phase to a highly conducting tion [106] and has been extensively studied in recent state (at ^2 Mbar pressure) [152]. years. Below T and in the absence of magnetic Solid solutions of Fe2C>3 with Mn2 03 [153-157], 3+ field, the Fe spins are aligned along the (111) or z Cr2 0 3 [158], Ga 2 0 3 [159, 160], and ln2 0 3 [160-162] axis of the crystal and the magnetic structure is have been examined in detail in the literature.

62 Iron oxides

Oxide and description Data Remarks and inferences References of the study

Fei_*0

Crystal structure and T = 300 K: Cubic; space group, At ordinary pressures, FeO is [1, 2, 7, 8, x-ray diffraction studies. Fm3m; Z = 4. a varies from always nonstoichiometric due to 10, 18, 19, 4.2886 to 4. 3012 A with in- the cation vacancies. Studies on 22-25, 33].

creasing Fe content in Fei_x O quenched high temperature

(Feo.8*0 to Feo.950 ). For FeOi.oo phases (Fei_xO is unstable below synthesized at high pressure, a = 850 K) indicate defect cluster

4.323 ±0.001 A. T t = 198 K. formation. Below T t , the

Below T t, the structure is rhombohedral distortion varies

rhombohedral. The rhombohedral with the stoichiometry but T t

distortion is sensitive to the Fe itself remains unaffected.

content; at 90 K, a «60° for high Hentschel [26] found evidence for

Fe content Fei_xO and « 59.04° the formation of intermediate for low Fe content sample. High stoichiometric FeOj.oo in the de- pressure x-ray studies do not composition of wiistite at rela- indicate any phase transition up tively low temperatures. to several hundred kilobars pressures.

Magnetic properties. Antiferromagnetic below 198 K The spins in FeO are perpendicular [9, 10, 21, (= Tn) and the magnetic mo- to the (111) plane unlike in NiO 22]. ments are arrayed in ferromag- and MnO which also show anti- netic sheets parallel to (111) ferromagnetism and have rock planes and the direction of salt structure above Tn. magnetization in neighboring

planes is antiparallel; the mag-

netic axis is perpendicular to the ferromagnetic sheet, xm obeys a Curie-Weiss law; xm =3.56/ (T+136). Magnetic cluster for-

mation is noted anologous to the cationic defect clusters which affect the spin structure and spin directions below Tn. Tn is not much affected by variation of x in Fei-zO.

Electrical properties. Semiconductor behavior (100-1100 Unusually low resistivity exhibited [4, 13, 14,

K); p (300 K) «0.05 ficm; p de- by Fei_x O compared to other 3d 27-29]. creases with increase in the oxy- transition metal monoxides and

gen content in Fei_xO. Three controlled by stoichiometry;

distinct regions in lnp-T behavior: p-T behavior is extrinsic.

Ea = 0.008 eV (<100 K); Ea =

0.3 eV (125-175 K); Eo =0.07 eV (200-1100 K). The samples are p-type but high oxygen content samples are n-type. The de-

fect structure is of the doubly ionized cation vacancy type.

TEC and UHF dielectric TEC and UHF (~10 GHz) dielec- [30, 32]. constant studies. trie constant data indicate anomalies at Tn. TEC (186 K) = 41X10~ 8 /°C; e« = 12.7.

63 t t t t t

Iron oxides=Centinued

Oxide and description Data Remarks and inferences References of the study

FeiO«

Crystal structure and T = 295 K: cubic; space group, The substance is an inverse spinel. [40-43, 47, x-ray data. Fd3m; Z = 8; a =8.394 ±0.0005 A. The Verwey transition in FegC^ 70]. T« = 119 K. T = 78 K: Ortho- changes the lattice symmetry rhombic; space group, Imma; from cubic to orthorhombic and a=5.912 A; 6 = 5.945 A; c = causes an ordering of the Fe 2+ 8.388 A. TEC (128-295 K): 7.7X and Fe3+ ions on the B sites _6 10 /°C. TEC is negative near which are randomly distributed -6 T ( ( — 20 X 10 /°C) with a peak above the transition temperature.

at T t . T decreases with pressure; The octahedral ferric ions lie

(dT t /dP)= — (0.46 ±0.02) along the orthorhombic a axis K/kbar. and the ferrous ions along the 6 axis in alternate rows that are mutually perpendicular.

Magnetic properties. Ferrimagnetic; Tn«860 K. x does The magnetic behavior is unaffected [36-38, 41,

not show anomaly at T t (119 K) at the ordering transition. The 43-46, 77]. but anisotropy behavior noted magnetic moment corresponds to with or without a magnetic field. the spin-only value (since FegO*

(Aff /Mo) =0.026 at TN . Mag- is an inverse spinel), the contri- 3+ netic moment = 4 mb* Tn in- bution from the Fe ions must

creases with pressure; ( dTn/dP ) vanish and the entire net mo- = (2.05 ±0.10) K/kbar. ment must be due to the Fe 2+ ions.

Electrical properties. Below T (119 K) semiconductor The electrical properties of FegC^ [39-41, 43,

behavior; Ea increases from 0.03 are a little bit complex; some 49, 53-56]. to 0.15 eV just below T<. At Tt workers [47, 48] assume that it (on heating), p drops by a factor exhibits a semiconductor-metal -2 of 10 2 to the order of 10 flcm. transition at T (119 K). How-

Above T t , p continues to de- ever, the p-T behavior above T crease (with Ea ~0.06 eV) and in indicates that of a typical de- the range 250-1700 K, p is es- generate semiconductor. The sentially constant ('~4X10-8 maxima and minima at 350 and 12cm) with a slight maximum at 860 K in the p-T plot may be ~350 K and a minimum at spurious and requires confirma- ~Tn. tion by a detailed investigation

a (500 K) « —60 mV/°C. Rh , on well-characterized, stoichio- however, is +ve. At 300 K, metric FeaCh. The abrupt drop

-4 3 Rh ~5 X 10 cm /C; mh~0.1 in p at T t is explained as due to 2 cm /V s. a shows discontinuities the rapid electron exchange be- at T and Tn. Rh and mh de- tween the octahedral Fe 2+ and 3+ crease with T below T t and Fe ions; this is inhibited below

remain constant or show an in- T ( because of the ordering

creasing trend above T t . process.

Mttssbauer studies. Careful results on untwinned crystals Mossbauer studies have provided [57-67]. of FeaO« indicate that the spectrum the most direct evidence of the can be resolved into five indi- electron exchange and ordering

vidual components corresponding process in FegO* at T t< 2+ to a tetrahedral Fe , two octahedral Fe,+ and two octahedral Fe 2+ sites. The lines below

T t are broadened. The results are interpreted in terms of a modi-

64 Iron oxides—Continued

Oxide and description Data Remarks and inferences References of the study

fied Verwey model consisting of twice as large as the unit cell originally proposed. The relaxation time of the electron hopping

is estimated to he 1.1 ±0.2 ns.

Mechanical properties. Young’s modulus drops by ''-'30% These observations indicate that [71, 72, 76]. at 108 K; internal friction ex- the 119 K transition is of first hibits a peak at ~95 K. At 300 order. K, cl = 6.75 X 10 5 cm/s; is — 3.69 X10 5 cm/s. Both cl and cs along [100] axis show sharp increases of 7.5 and 0.2% respectively at 119 K. cl also exhibits an anomaly at 130 K where there is an easy axis change in FejCX from the [100] to the [111] axis. cl and cs also go through a minimum at ~Tn.

Magnetocaloric effect. The temperature change of a sam- [73]. ple upon applying magnetic field

has been examined through T t in

FejO*. A T is negative below T t

and reaches a maximum at T <; AT passes through zero and becomes -fve. The sign reversal shifts to lower temperatures as the applied field gets stronger.

-1 Infrared spectra. A band at 595 cm is noted and [163]. correlated with the bands noted in other oxide spinels. -FesOj

Crystal structure and Tetragonal; space group P4j; Z = The structure consists of three [79, 83, 91, properties. 32; a =8.33 A; c = 24.99 A. spinel blocks stacked on top of 92—95]. 7 -FejOa is ferrimagnetic and each other; the unit cell has transforms to a -modification cation vacancies arranged in an above 670 K. AF~7% and high ordered manner. pressure studies indicate no abrupt transition at any given P and T. Detailed physical properties not known. FesOj

Crystal structure and Monoclinic; Z = 20; a = 12.97 A; Detailed properties not known. [89, 96]. properties. 6 = 10.21 A; c = 8.44 A; 0 =95.33°. Ferromagnetic; Tc~485-500 K. Transforms to a -form above 770 K. •FejOj

Crystal structure and T = 300 K: Rhombohedral Typical corundum structure as- [98, 105, x-ray studies. (pseudo-hexagonal); space sumed by many transition metal 151, 160]. group, R3c; Z = 6; a = 5.0351 ± sesquioxides and a-AljOj.

65 m

Iron oxides—Continued

Oxide and description Data Remarks and inferences References of the study

0.0003 A; c = 13.750 ±0.001 A. Discontinuities in lattice parameters noted at Tn (»960 K). High pressure studies do not indicate any phase transitions up to several hundred kilobars.

Magnetic properties. Antiferromagnetic; Tn~960 K. The superimposed weak ferro- [99-104, -6 X~1.7-2.3X10 emu/g. Mag- magnetism in the range 260-960 106-113, netic moment = 2.45 mb- Above K is characteristic of only 133-137].

263 ±2 K (Tm), Fe203 exhibits a-Fe 203 and the mechanism weak superimposed ferromag- which causes the Morin transition

netism due to slight canting of is the change in the spin- the spins in (111) plane. The direction from parallel to per- spin -flip transition has been pendicular to the c-axis. The observed in the presence of a observed pressure dependence of

magnetic field. Careful neutron T ji can be explained if we as- diffraction studies show that the sume a change in sign of the

transition is spread over a 25-30° total magnetic anisotropy energy. range, centered at 261 K. Tm depends on the particle size and stoichiometry of the sample. T increases with pressure; (dTu/dP) =3.6 ±0.3 K/kbar.

Electrical properties Semiconductor behavior; p (300 K) The conductivity behavior ap- [143-150]. (200-1600 K). ~10 3-10 6 ficm. Pure Fe2C>3 shows pears to be extrinsic or con- a three-region behavior with trolled by native defects in the

various Ea values in the range entire temperature range investi- 300-1600 K; usually p type be- gated. The reason for the sign havior but some samples show reversal of Rh is not exactly change of sign of a on heating. known. Impure or doped samples exhibit

linear lnp-T plots with Ea ~1.0 eV. Both p- and n-type samples can be obtained by doping. For n-type Fe2C>3, at 1000 K, p~l-10 12cm; a~ — (400-700) pV/K; -2 Ph~T0 cm 2 /V s. Rh changes sign at Tn.

Mossbauer studies Characteristic six -line spectra noted Results indicate that Fe 20 3 in the [102, 116- (100-1050 K). below Tn. Anomalies in isomer form of fine particles exhibit 120]. shift and quadrupole splitting superparamagnetic behavior. noted at ~T m. The electric field gradient shows an abrupt increase at ~Tn. The temperature dependence of the internal field

is in agreement with the pre- dictions of the molecular field theory.

Optical properties. Absorption hands noted at 1.4, 1.9, The 0.7 eV peak observed by [164-166]. 2.4 and 3.2 eV. Morin [164] ob- Morin seems to be associated served a band at 0.7 eV which with donor levels. The higher

66 Iron oxides—Continued

Oxide and description Data Remarks and inferences References of the study

increases strikingly as Ti is energy hands are ascribed to the added. Infrared hands noted in crystal field transitions [166]. the range 285-560 cm -1 and are associated with the metal oxygen vibrations.

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68 Y., and Horne, R. A., Chem. [148] LegsofF, H., Kersey, J. antiferromagnetic behavior below Tn [16-19]. CoO Phys. 31, 1141 (1959). is all [149] Nakau, T., J. Phys. Soe. Japan 15, 727 (I960). semiconducting at temperatures and discon- 150] van Daal, H. J., and Bosnian, A. J., Phys. Rev. 158, tinuities in p, a, and Rn are exhibited at the N6el 736 (1967). [151] Lewis, G. K., Jr., and Drickamer, H. G., J. Ghem. temperature by pure and Li-doped CoO [20-22], Phys. 45, 224 (1966). Similarly, anomalies in Cp [23-26], k [25, 27], elastic [152] Kawai, N., and Mochizuki, S., Phys. Letters 36A, 54 (1971). wave velocity [27, 28-30], elastic constants, internal Geller, S., Grant, R. W., Cape, A., and Espinosa, [153] J. friction and Young’s modulus [27-29, 31] and an G. P., J. Appl. Phys. 38, 1457 (1967). [154] Chevalier, R. R., Roult, G., and Bertaut, E. F., Solid [25] have been noted at ^Tn in CoO. The continuous State Commun. 5, 7 (1967). nature of the transition as indicated by k, Cp and [155] Grant, R. W., Geller, S., Cape, J. A., and Espinosa, G. P., Phys. Rev. 175, 686 (1968). neutron scattering experiments [9, 23-27] seem to [156] Hase, W., Bruckner, W., Tobsisch, J., Ullrich, H.-J., infer that the antiferromagnetic—^paramagnetic tran- and Wegerer, G., Z. Krist. 129, 360 (1969). [157] Geller, S., J. Appl. Phys. 42, 1499 (1971). sition in CoO is of the order greater than unity. CoO Takei, and Shirane, G., Phys. [158] Cox, D. E., W. J., J. was one of the first compounds to be studied by Chem. Solids 24, 405 (1963). [159] Marezio, M., and Remeika, J. P., J. Chem. Phys. 46, Mossbauer spectroscopy and many workers have 1862 (1967). examined the spectrum as a function of temperature [160] Prewitt, C. T., Shannon, R. D., Rogers, D. B., and Sleight, A. W., Inorg. Chem. 8, 1985 (1969). [32-39]. Below Tn, a six-line pattern indicating mag- Schneider, S Roth, R. S., and Waring, L., [161] J., J. J. netic hyperfine splitting is seen; this disappears in Res. Nat. Bur. Stand. (U.S.), 65A (Phys. and Chem.), No. 4, 345-374 (July-Aug. 1961). the paramagnetic state. The Neel temperatures ob- [162] Geller, S., Williams, H. J., and Sherwood, R. C., J. tained from Mossbauer studies agree excellently with Chem. Phys. 35, 1908 (1961). 162] Hafner, S„ Z. Krist. 115, 331 (1961). those obtained by other techniques. 164] Morin, F. J., Phys. Rev. 93, 1195 (1954). Effect of pressure on the magnetization and mag- 165] Bailey, P. C., J. Appl. Phys. 31, 39S (1960). 166] Subba Rao, G. V., Kao, C. N. R., and Ferraro, J. R., netic ordering temperature of CoO has been in- Appl. Spec. 24, 436 (1970). vestigated by a few workers [40, 41]; the results indicate that Tn increases with increasing pressure 1.7. Cobalt Oxides and decreasing volume. X-ray diffraction [8] and resistivity [42] studies do not indicate any high pres-

In the Co-0 system, CoO and C 03O 4 are the stable sure transitions in CoO up to pressures of ~250 oxides which have been investigated in detail. Al- kbar. though many workers have reported the preparation According to Ok and Mullen [37, 43], there exist and properties of a sesquioxide, Co2 Og, characteriza- two forms of cobalt monoxide, CoO (I) and CoO (II). tion with respect to stoichiometry and method of Both have cubic rock salt structure at room tem- preparation seems to be still in doubt and many a perature but CoO (II) has a lesser density and the time, hydrated forms of the oxides are obtained. lattice has ^25 percent vacancies. CoO (II) can Cobalt monoxide, CoO, has a narrow range of only be prepared at room temperatures and is very homogeneity [1-4]. It has a cubic rock salt structure reactive with atmospheric oxygen at ordinary tem- at room temperature and is paramagnetic [5-8]. Be- peratures. The two forms show different Mossbauer low 289 K, it becomes antiferromagnetic followed patterns and their Neel temperatures differ. CoO (II) by a crystal distortion to tetragonal symmetry [5-7, transforms to CoO (I) above '—'573 K with a trans-

9] and the tetragonality increases with decrease in formation rate that increases with temperature; bn temperature. Detailed x-ray diffraction studies [10] the other hand, the transition is not complete on at low temperatures indicated that a rhombohedral heating CoO (II) in an argon atmosphere up to deformation is superimposed on the tetragonal dis- ~1270 K for several hours and generally CoO (I, tortion and at 123 K, the true symmetry is mono- II) is obtained which can be treated as a mixture of clinic. The magnetic structure in the low tempera- forms I and II in varying amounts. Ok and Mullen ture phase of CoO has been examined by various point out that pressures of ~10 kbar are not workers [11-13]. Although a number of non-collinear sufficient to transform II—*1 at 300 K in an inert spin-axis structures are possible in CoO [12, 14], atmosphere. These authors indicate that the usual available experimental evidence points to a collinear chemical methods of preparation of CoO yield al- structure [10, 13, 15] with the moments tipped out of ways CoO (I, II). the {111} planes 7.85° toward the c axis. It should be mentioned that the Mossbauer spec- The transition in CoO has been studied by a tra of the oxides reported by Ok and Mullen [37] variety of techniques in recent years. x m T data on differ nontrivially with the work of others; complete powder and single crystalline CoO indicate typical characterization of CoO (I) and CoO (II) is less

69 unequivocally established [43]. Further detailed work manent moment as a consequence of the splitting of is urgently needed. the 3d levels by the octahedral cubic field. At ~40 K,

Optical properties of CoO have been examined by C 03O 4 undergoes a transition to form an ordered many workers over a wide energy range (0.02-26.0 antiferromagnetic state [58]; the magnetic structure eV) [44-53]. Studies in the UV region have pro- is due to antiferromagnetic ordering of spins in the 2+ vided an understanding of the density of states in A sites of the spinel structure [60] and each Co CoO [44, 45, 49, 50, 52]; some of the bands observed ion in an A site is surrounded by four nearest neigh- in the far infrared region have been ascribed to the bors with oppositely directed spins, xm -T curve fol- antiferromagnetic resonance [19, 46, 47, 53] but the lows a Curie-Weiss law with a deviation at high resonance frequencies are insensitive to temperature temperatures in the range 90-500 K and the ab- [53]. normally low value of xm in C03O4 is ascribed to the 8+ Small amounts of Li and Ti can be incorporated fact that the material is a 2-3 spinel in which Co in CoO and a great deal of work has been carried give no contribution to the magnetic moment [61, out on these doped materials [3, 20-22, 50, 54, 55]. 62].

Generally, doping with Li decreases the resistivity C 03O 4 is a semiconductor [42, 63, 64] at ordinary compared to the pure stoichiometric material. CoO and at high pressures [42]; however, detailed studies forms solid solutions with other metallic oxides like on single crystal materials have not been made.

MnO, FeO, NiO and MgO [24, 51, 56, 57]. The Mossbauer studies on C 03O 4 have been reported by magnetic, electrical, optical and thermal properties a few workers [38, 39, 65]. Above Tn ( = 33±1 K), show variations depending on the composition of the two quadrupole split lines corresponding to A and B solid solution. site Fe 3+ are noted while below Tn the A site Fe 8+

C 03O 4 is a normal spinel and has the ionic structure spectrum is magnetically split due to the antiferro- 2 3+ 2+ +[Co it is cubic [58, and paramagnetic magnetic Co sublattice. NMR studies [66, 67] of Co ] 2 04; 59] but not ferrimagnetic, although it crystallizes in the C 03O 4 fully confirm the spinel structure and different magnetite structure. The Co 3+ ions have zero per- oxidation states of cobalt ion at the A and B sites.

Cobalt oxides

Oxide and description Data Remarks and inferences References of the study

CoO

Crystal structure and T = 290 K: Cubic, space group, The low temperature structure (at [5-7, 10]. x-ray study. Fm3m; Z = 4; a =4.260 A. T = and below 123 K) is shown to be 250 K: Tetragonal; a =4.263 A; strictly monoclinic and not c = 4.247 A. T = 123 K: Mono- tetragonal. clinic; space group, C2/m; Z = 2; a = 5.18±0.03 A; 6 =3.01 ±0.05

A; c = 3.01±0.07 A; /3 = 125.55± 0.01°. X-ray studies conducted in the range 210-330 K indicate the change in crystal symmetry at 284 ±1 K which is recognized as the Tn.

-3 Magnetic susceptibility in the Xm~ 10 cgs units; slight aniso- The calculated values of xm on the [16-19, 41]. range 90-500 K and the tropy noted. Tn =287.25 K and basis of Kanamori’s theory [19] effect of pressure. Tn increases with pressure; are in agreement with the ex- (dTu/dP) =0.6d=0.03 K/kbar. perimental values.

70 Cobalt oxides—Continued

Oxide and description Data Remarks and inferences References of the study

7 n Electrical properties. Pure oxide: p~10 -10 flcm in the Typical semiconductor behavior in [1, 3, 20- range 200-300 K; p(1300 K)~ the temperature range investi- 22, 42, 68]. 0.5 ficm; pd (1300 K)~0.4 cm 2 /V gated. Band and polaron theories

s; Ea changes at Tn and slightly have been applied to explain the depends on the stoichiometry. mechanism of conduction in pure Li doped CoO (0.05-0.15%): and doped CoO and the weight p~0.5 ficm; a~ + 500 pV/K; of experimental evidence seems p D = 0.25 cm 2 /V s (1200 K); to point out the validity of band pd~6 cm 2 /V s (300 K); Rn i9 model in this oxide. + ve; p and Rn show discontinuities at Tn. ph is constant in the range 200-1500 K (~0.1 cm 2 /V s); p type behavior, p decreases with pressure but no transition occurs up to 250 kbars.

Optical reflectance spectra Optical reflectance spectra give The low energy bands are inter- [44, 49, 50, (range 1-26 eV). bands at 5.5, 7.5, 12.6 and 17.5 preted as the exciton transitions [52]. eV. The observed bands at 0.97 whereas the higher energy hands and 2.28 eV are attributed to may be due to the transitions in 4 T —*T, and 4 T -»«T (P) the conduction band. The data lc ff lff lff respectively [49]. show the localization of the d electrons in CoO and the 2p hand of oxygen seems to lie ~8.5 eV below the vacuum level.

Infrared spectra Prominent bands are at 146, 215, The data are in agreement with the [19, 45-49,

1 (100-300 cm" ) 222, 233, 243, 250, 260, and 297 theoretically predicted values 53]. -1 cm . Most of the bands are pre- using time-dependent molecular sumed to be the antiferromag- field theory. netic resonance modes but temperature variation in the range 2-300 K shows the band to be insensitive.

Mossbauer studies of 57 Fe 1 atmospheric pressure: The hyper- Quantitative features of the spectra [32-40, 43]. doped CoO at ordinary and fine splitting disappears at Tn seem to differ for various investi- high pressures. ( 291 K); Isomer shift, screening gations but the general agreement parameter and magnetic hyper- with the values of Tn is fine field show discontinuities at excellent. Tn. High pressures: Tn increases = with pressure; (d \nTn/d \nV ) — 3. Isomer shift and screening parameter show small changes on the application of pressure. Ok and Mullen [43] report different values of Mossbauer parameters for CoO (I) and CoO(II).

Thermal properties. Dilatometric study shows that the The discontinuous nature of the [24-27, 69].

coefficient of expansion passes transition indicates that it is through a sharp maximum at probably of a higher order 292 K for CoO; TEC at 292 K = transformation. -8 3.0Xl0 /K. Cp and k show

anomalies at Tn; Cp (290 K) =32 cal/mol, K.

71 Cobalt oxides—Continued

Oxide and description Remarks and inferences References of the study

Mechanical properties. Sound velocities in various crys- [28, 29, 31]. tallographic directions change dis- continuously above Tn by 10- 20%. Elastic moduli (X10 11 2 dyn/cm ) (300 K): cn=26.17±

0.03; C 12 = 14.5zt0.2; C44 = 8.32ifc 0.02. During phase transition, Cn

and C 12 exhibit jumps. In the region of the transition. Young’s modulus increases rapidly from 6.3 to 17X10 11 dyn/cm, with increase in temperature and the moduli are approximately step functions of temperature; on the other hand, the internal friction in CoO falls from 91 to 2 -4 (X10 ) at Tn for a constant -7 strain amplitude of 10 . In- creasing the strain amplitude has little effect on the Young’s modulus and internal friction. C03O4

Crystal structure and T = 300 K: Cubic; a = 8.0835 A; The crystal structure does not [58, 59, 61, magnetic properties. space group: Fd3m (T>Tn); change in going through Tn but 62]. F43m (T < Tn). Tn =40 K. xm the space group changes. The low 2 is (100 K) =1.4X10- ; xm (300 K) value of xm believed to be due 2 2+ = 0.71X10~ . The Co moment to the absence of permanent 3+ below Tn is 3.02 mb- moment of Co and the magnetic structure is due to the antiferromagnetic ordering of the spins of Co 2+ ions on the A sites of the spinel structure.

Electrical properties. Semiconductor; p~10 3-104 f2cm; [42]. p decreases with P and a minimum is noted at 215 kbars in the p-P plot. Detailed data not available.

Mossbauer studies. Tn is found to be 33 ±1 K; above Tn obtained from Mossbauer work [38, 39, 65]. Tn, two quadrupole split lines are is less than that reported by observed corresponding to A and Xm-T measurements. The be- B spinel sites whereas below Tn, havior of ultrafine particles of 3+ the A site Fe spectrum is mag- C03O4 towards Mossbauer spectra netically split due to the anti- is interesting and needs further ferromagnetic Co 2+ sublattice. detailed study.

72 t *

Cobalt oxides—Continued

Oxide and description Data Remarks and inferences References of the study

Above Tn, the spectra of the ultrafine particles (diameter =

100 A) of C03O 4 is found to be identical to that of the bulk material. Below Tn, the fine particles display superparamagnetic behavior and the B site spectral broadening is enhanced. The anisotropy constant estimated

from the superparamagnetic -

antiferromagnetic transition is 4 3 = 4X10 erg/cm . Effective 0 d 185 K.

NMR studies of 59 Co in the The Co signal is identified with the The T-independent shift (1.5%) is [66, 67]. paramagnetic state. Co3+ ions at the B sites and the interpreted as the second order line shift is separated into T- chemical shift due to the mixing dependent and T-independent of the low-lying excited state;

terms; the line shapes are studied the T-dependent shift is at- and the thermal relaxation time tributed to the hyperfine coupling -5 is estimated to be ~1.5X10 s. between the Co nuclear spin and the electron spins on the Co 2+

ions at the A sites in C03O 4 . The super -exchange mechanisms are

examined and it is concluded that 2 % fractional spin density in 3+ the Co e g orbital mainly contributes to the super -exchange.

Diffuse reflectance Absorption bands at 0.81 and 1.85 It is concluded that the forbidden [49].

spectral studies. eV are noted. These are attrib- band widths in CoO and C03O 4 2+ 4 — uted to Co at A sites ( A 2 are almost identical. 4 3+ T2 ) and Co at B sites QTig—*

4 Ai g ) transitions respectively; corresponding Dq values are 0.08 and 0.19 eV.

Infrared spectral studies. The prominent bands are at 570 The spectra have been discussed in [48, 70, 71]. -1 and 665 cm ; other bands are at relation to other oxide spinels and -1 350, 460 and 635 cm . and covalency effects.

C02O3

° Crystal structure and t phase Low -spin phase (with 2ff e s The high -spin phase appears to be [72]. transition configuration) is obtained at the stable phase at atmospheric high pressures. This phase has pressure. corundum structure (a =4.782 A and c = 12.96 A). The low-spin phase transforms to the high-

2 spin corundum phase ( 2se s ) around 670 K (a =4.8882 A, c = 13.34 A).

73 References [38] Murin, A. N., Lur’e, B. G., and Seregin, P. P., Sov. Phys.—Solid State (English Transl.) 10, 1000 (1968); 10, 2006 (1968).

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74 n ;

1.8. Nickel Oxides [50] indicate that the samples of NiO remain semiconducting and that the d electrons are localized.

Nickel monoxide, NiO, is the only stable oxide in Even though it is definitely known that the d

the Ni-0 system. Earlier literature [1, 2] contains electrons in NiO are localized, the detailed mecha- reports of the existence of some 25 different oxides nism of electrical conduction is not yet understood;

of nickel including Ni304 , Ni2 03 , and Ni02 and the various properties of this material have been

Kuznetsov [3] has recently suggested that all the reviewed in several articles [51-56]. The available higher oxides and oxide solid solutions are merely data on transport properties indicate that: (a) be- partial hydroxide decomposition products. Detailed tween 200 to 1000 K, the predominant conduction

studies made recently [4-6] have shown that the mechanism is the motion of holes in a band with an

monoxide is the only material known in the Ni-0 m* = 6m; (b) the carrier concentration in lightly system and slight deviations from stoichiometry are doped samples is thermally activated with an accep-

allowed [4-7]. The possibility of synthesizing Ni 30 4 tor ionization energy of 0.3-0.4 eV; (c) Li acceptors

at high pressure has been pointed out [8]. are always self-compensated by donors; (d) hd 2 NiO is one of the extensively studied transition («0.5 cm /V s at 300 K) decreases with rise in T metal oxides. In the paramagnetic state it has the and ph is proportional to hd in the range 300 to 600 fee rock salt structure [9-17] and as the temperature K and (e) intrinsic conduction is observable at is lowered through T (~523 K), there is a trigonal T> 1000 K in the more highly compensated samples. distortion in the unit cell [9-14, 16, 17] and the It appears that small polaron hopping by 3d-holes

material becomes antiferromagnetic [11-13, 18]; in (with low Ea) contributes little to the total con- addition, there is an isotropic volume contraction ductivity at ordinary temperatures (100-1000 K)

associated with the magnetic behavior. In the anti- whereas the major contribution is from the holes in ferromagnetic state, the spins are ferromagnetically the oxygen 2p-band of NiO. We may note here that alligned in sheets parallel to (111) planes and anti- until 1963 it was generally taken for granted that parallel to those in adjacent sheets [11-13, 18]. This charge -transfer in doped NiO involved activated magnetic structure is characteristic of all the transi- mobilities. Since then the weight of the evidence

tion metal monoxides and is explained as due to the seems to have shifted in the other direction; for antiferromagnetic superexchange through the p orbi- example, the studies of Bosman and Crevecoeur [42] tals of the intervening oxygen anions [19]. Below Tn, are in favor of the band regime. Since most of the the unit cell contracts along the [111] axis perpen- studies have been on doped or nonstoichiometric dicular to the ferromagnetic sheets and the crystal NiO, we have to view the various claims with some

becomes rhombohedral; the distortion from cubic is, caution. Needless to mention, there is continuing however, small and the rhombohedral angle increases need for careful experimental studies on pure NiO.

from 90° at Tn to 90.06° at 300 K [17]. x-T data Optical absorption measurements have been car- definitely indicate the antiferromagnetic behavior ried out on pure and doped NiO from far infrared

of NiO [20]. Magnetic anisotropy and antiferromag- through x-ray frequencies [44, 57-68] and the results

netic domains in NiO have been studied by a few indicate that the d electrons are localized. There is

workers [13, 19, 21-24]. a peak at 0.24 eV which begins to broaden and de- Pure, stoichiometric NiO is green in color and the crease in energy above 400 K and disappears above reported black and grey colors are due to slight devia- 600 K [58, 63]; it appears to be connected with anti- tions from stoichiometry or the presence of impuri- ferromagnetism since the band vanishes just above ties [25-27]. Stoichiometric NiO is a good electrical Tn and moves towards lower energies with decrease 13 insulator (p^l0 flcm) but the p decreases drastic- in sublattice magnetization by either increase of ally by slight changes in the stoichiometry or by temperature [58] or dilution with MnO or CoO [59].

lithium doping at small concentrations. Electrical The main feature in the optical spectrum of NiO is

properties of pure and doped NiO have been in- the absorption edge at ~3.8 eV [57, 58, 64, 67]. vestigated by various workers for the past few years Infrared studies show restrahlen bands at ^0.05 eV

[4, 5, 28-48]. Discontinuities in p, E0 , a and jRh are the antiferromagnetic resonance peak is at 0.04 eV encountered at Tn', Rh changes sign from positive below Tn [46, 69-71]. The band structure that em- to negative values at around Tn, but the exact cause erges from the optical absorption, electroreflectance

of this behavior is not understood. Measurements in [72] and photoemission [73] studies is consistent the range 10 to 1300 K and at various dopant con- with the theoretically calculated band structure of centrations (up to 40% [49]) and at high pressures NiO [74-76].

75 Moiibauer studied of 67 Fe doped NiO have been NiO forms solid solutions with other oxides of reported in the literature [77, 78] and accurate value rock salt structure [59, 66, 84, 85] and the properties

k, of Tn obtained. Digcontinuitiei in C p , am. Young’s are slightly modified depending on the nature of the modulus and internal friction and TEC have been oxide. noted in pure and Li doped NiO [79-83],

Nickel oxides

Oxide and description Remarks and inferences References of the study-

NiO

Crystal strucrure, T—>0 K: Rhombohedral; a =4.1705 The rhombohedral distortion is [9-17]. x-ray studies. A; a =90.075°. T = 298 K: small and the transition at Tn o Rhombohedral; a =4.1759 A; appears to be second order. a =90.058°. T = 525 K: Cubic; space group, Fm3m; Z = 4; a = 4.177 A. Slight volume change at Tn (523 K). High pressure studies up to several hundred kilobars do not reveal any phase transition. TEC (/K): 7.93X10“®; 0 d = 9OO K.

X-T studies and observation x(Tn) =11.7X10“® emu/g. The The antiferromagnetic-paramag- [11-13, of the magnetic domains by antiferromagnetic T (twin) and netic transition appears to be of 19-24]. optical and neutron S (spin -rotation) walls studied. order greater than unity and diffraction. Crystals with only T or S walls slight magnetic order has been can be produced; these can be noticed above Tn by neutron displaced by the application of diffraction. The anomalous x in small mechanical stress or mag- (111) at higher magnetic fields is netic fields. Magnetic field de- caused by spin rotation. pendence of x also is noticed.

13 Electrical properties. Pure NiO: p (300 K)~10 ftcm; At very low temperatures, im- [4, 5, 28- “ pap 1/4 is 0s (po 2 . 10-M atm.; purity conduction noticed 50]. 1170-1473 K); p«por1/6 (po«, whereas in the range 100-1000 K 10-2-10+3 atm; 1320 K); p« 3d holes contribute to conduction lli -1 Po»~ (pog^lO atm; 1070- (only a small part of the total

electrical properties. Ea ~0.9 eV gested to be due to the forma-

(200-500 K); above Tn, Ea drops tion or small phase separation of

to 0.6 eV and increases to 1.0 eV n type Ni 3 0, or small regions of

at 1000 K; Range 10-100 K, Ea ~ metallic Ni in the samples. 0.004 eV. mh~0.5 cm a /V s, at 200 K and hole like behavior; no striking temperature variation in the range 200-400 K but mh gradually decreases with rise in T. Above 400 K, rapid decrease is shown and at ~600 K, mh re- verses sign, a ^0.5 raV/K at ~200 K and above 300 K, has

76 Nickel oxides—Continued

Oxide and description Data Remarks and inferences References of the study

the same slope as log p as a function of T. Doped NiO: p~l

12cm at 300 K and Ea is much

smaller than in pure samples; Ea varies with dopant concentration and slightly with T. ph and a behave in a qualitatively similar manner as the undoped samples. Application of pressure decreases p but the sample remains semiconducting up to the highest pressures measured (~500 kbar). Neutron irradiation seems to increase p and decrease a of pure NiO.

Optical properties The main features are (i) anti* Most of the observations are con- [44, 57-72]. (0.03-26 eV). ferromagnetic resonance peak at sistent with other body of data '**'0.04 eV observed in NiO below and the band picture that Tx (and which disappears above emerged is in reasonable agree- Tx), (ii) restrahlen peaks in the ment with the theoretical pre-

vicinity of 0.05 eV, (iii) a peak at dictions. The peak at 0.24 eV 0.24 eV which begins to broaden appears to be connected with and decrease in energy above antiferromagnetism in NiO. 400 K and disappears at ~600 K, (iv) absorption edge at ~0.38 eV and (iv) several high energy peaks in the range 5-18 eV connected with interband transitions. eo»12; e«»5.

Mossbauer studies of 67 Fe An electric held gradient which The order of the transition in NiO [77,-78]. doped NiO (4.2-550°K). appears at the 67 Fe nucleus below appears to be greater than unity. Tx grows as T decreases. The

internal magnetic held is maximum (216 kG) at ^300 K and decreases to ~55 kG at 4.2 K. Tx is found to be 524 K and the + direction of spins of Ni* is found to be close to <112>.

Thermal and mechanical Cp shows a X-type anomaly at Tx; Striking anomalies are noted inspite [79-83]. properties. maximum value of Cp = 65 cal/ of the negligible volume change mol. aid and k also show anoma- and only smooth variation of the lies at Tx. Large increase in lattice parameter at Tx in NiO. Young’s modulus noted at ~Tx (Maximum value = 8.5 X10 -11

2 dyn/cm ). Elastic wave velocity also increases at ~Tx in NiO.

77 —

References [41] Snowden, D. P., and Saltsburg, H., Phys. Rev. Letters 14, 497 (1965). [42] Bosman, A. J., and Crevecouer, C., Phys. Rev. [1] Bogatskii, D. P., Zhur. Obshchei Khim. 21, 3 (1951); 144, 763 Chem. Abstr. 46, 7861d (1952). (1966). [43] Cojocaru, L. N., Phys. Stat. Solidi 22, 361 (1967). [2] Bogatskii, D. P., and Mineeva, I. A., Fiz. Tverd. Tela Akad. Nauk SSSR, Sbornic Statie 2, 361 (1959); Zhur. [44] Ksendzov, Ya. M., Avdeenko, B. K., and Makarov, Obshchei Khim. 29, 1382 (1959). V. V., Sov. Phys.—Solid State (English Transl.) 9, 828 (1967). [3] Kuznetsov, A. N., Russian J. Inorg. Chem. (English Daal, Transl.) 13, 1050 (1968). [45] Van H. J., and Bosman, A. J., Phys. Rev. 158, 736 (1967). [4] Mitoff, S. P., J. Chem. Phys. 35, 882 (1961). [46] Austin, I. G., Springthorpe, A. J., Smith, B. A., and [5] Cox, J. T., and Quinn, C. M., J. Mat. Sci. 4, 33 (1969). Turner, C. E., Proc. Phys. Soc. (Lond.) 90, 157 (1967). [6] Drakeford, R. W., and Quinn, C. M., J. Mat. Sci. 6, 175 Uno, R., Phys. Soc. (1971). [47] J. Japan 22, 1502 (1967). [48] Kabashima, S., and Kawakuho, T., J. Phys. Soc. Japan [7] Tretyakov, Y. D., and Rapp, R. A., Trans. Met. Soc. 493 (1968). AIME 245, 1235 (1969). 24, [49] Toussaint, C. J., and Vos, G., J. Appl. Cryst. 1, 187 [10][8] Reed, T. B., in The Chemistry of Extended Defects in non-Metallic Solids, Eds. L. Eyrinff and M. O’Keeffe (1.968). Minomura, S., and Drickamer, H. (North Holland Publ. Co., Amsterdam, 1970), pp 21- [50] G., J. Appl. Phys. 34, 3043 (1963). 35. [51] Adler, D., Solid State Phys. 21, 1 (1968). [9] Rooksby, H. P., Acta Cryst. 1, 226 (1948). [52] Goodenough, J. B., J. Appl. Phys. 39, 403 (1968). Shimomura, Y., Tsubokawa, I., and Kojima, M., J. [53] Austin, I. G., and Mott, N. F., Adv. Phys. 18, 41 (1969). Phys. Soc. Japan 9, 521 (1954). Rao, C. N. R., and Subba Rao, G. V., Phys. Stat. Solidi [ll]Roth, W. L., Phys. Rev. 110, 1333 (1958); 111, 772 [54] (a) 597 (1970). (1958). 1, [55] Bosman, A. 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L., in The Chemistry of Extended Defects in Gielisse, P. Plendl, N., Mansur, L. Marshall, non-Metallic Solids, Eds. L. Eyring and M. O’Keeffe [62] J., J. C., R., Mitra, S. S., Mykolajewycz, R., and Smakula, A., (North Holland Publ. Co., Amsterdam, 1970), pp 455- Appl. Phys. 487. J. 36, 2446 (1965). [63] Austin, 1. G., Clay, B. D., and Turner, C. E., J. Phys. C. [20] Singer, J. R., Phys. Rev. 104, 929 (1956). (Ser. 2) 1, 1418 (1968). [21] Kondoh, K., Uchida, E., Nakazumi, Y., and Nagamiya, [64] Rossi, C. E., and Paul, W., J. Phys. Chem. Solids 30, T., J. Phys. Soc. Japan 13, 579 (1958). 2295 (1969). [22] Roth, W. L., J. Appl. Phys. 31, 2000 (1960). [65] Cherkashin, A. E., and Vilesov, F. I., Sov. Phys.—Solid [23] Kondoh, H., and Takeda, T., J. Phys. Soc. Japan 19, State (English Transl.) 1068 (1969). 2041 (1964). 11, [66] Jacono, M. L., Sgamellotti, A., and Cimino, A., Z. Phys. [24] Yamada, T., J. Phys. Soc. Japan 21, 650 (1966); 21, 664 Chem. (N.F.) 70, 179 (1970). (1966). [67] Powell, R. J., and Spicer, W. E., Phys. Rev. B2, 2182 [25] Francois, J., Compt. Rend. (Paris) 230, 1282 (1950); (1970). 230, 2183 (1950). [68] Brown, F. C., Gahwiller, C., and Kunz, A. B., Solid [26] Teichner, S. J., and Morrison, J. A., Trans. Faraday Soc. State Commun. 9, 487 (1971). 51, 961 (1955). [69] Kondoh, H., J. Phys. Soc. Japan 15, 1970 (1960). [27] Tourky, A. R., Hanafi, Z., and Salem, T. M., Z. Phys. Sievers, A. and Tinkham, M., Phys. Rev. 1566 Chem. (Leipzig) 243, 145 (1970). [70] J., 129, (1963). [28] Morin, F. J., Phys. Rev. 93, 1199 (1954). [71] Richards, P. L., J. Appl. Phys. 34, 1237 (1963). [29] Snowden, D. P., Saltsburg, H., and Perene, J. H., Jr., [72] McNatt, J. L., Phys. Rev. Letters 23, 915 (1969). J. Phys. Chem. Solids 25, 1099 (1964). [73] Cherkashin, A. E., Vilesov, F. I., Keier, N. P., and [30] Nachman, M., Cojocaru, L. N., and Ribco, L. V., Phys. Bulgakov, N. N., Sov. Phys. Solid State (English Stat. Solidi 8, 773 (1965). — Transl.) 11, 506 (1969). [31] Pizzini, S., and Morlotti, R., J. Electrochem. Soc. 114, Yamashita, Phys. Soc. Japan 1010 (1963). 1179 (1967). [74] J., J. 18, [75] Switendick, A. C., MIT Quart. Progr. Rept. *No. 49, [32] Aiken, J. G., and Jordan, A. G., J. Phys. Chem. Solids 1963 (unpublished, quoted in 29, 2153 (1968). [51]). [76] Wilson, T. M., J. Appl. Phys. 40, 1588 (1969); Intern. [33] Eror, N. G., and Wagner, J. B., Jr., Phys. Stat. Solidi Quant. Chem. IIIS, 757 (1970). 35, 641 (1969). J. [77] Bhide, V. G., and Shenoy, G. K., Phys. Rev. 143, 309 [34] van Houten, S., J. Phys. Chem. Solids 17, 7 (1960). Ksendzov, Ya. M., Ansel’m, L. N., Yasil’eva, L. L., and (1966). [35] Rev. Latysheva, V. M., Sov. Phys. —Solid State (English [78] Siegwarth, J. D., Phys. 155, 285 (1967). [79] Zhuge, V. P., Novruzov, O. N., and Shelykh, A. I., Transl.) 5, 1116 (1963). Sov. Phys. Solid State (English Transl.) 11, 1044 [36] Zhuge, V. P., and Shelykh, A. I., Sov. Phys. —Solid — (1969). State (English Transl.) 5, 1278 (1963). 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78 1.9. Copper Oxides as hydrogen-like series of narrow- lines at the absorption edge and towards the long-wavelength re- Phase equilibrium studies [1-3] on the Cu-0 sys- gion (yellow series). Similar series of lines (called the tem indicate that Cu2 0 and CuO are the only stable green, blue-green and blue series) also appear near oxide phases in the solid state; both have narrow- the other interband transitions [23-25] and are due ranges of homogeneity. Frondel [4] has reported the to the optical excitation of the excitons in Cu2 0. existence of a tetragonal nonstoichiometric oxide of Detailed investigation of these excitonic spectra con- 2+ + the formula (Cu i_2 xCu 2z)Oi_*, x= 0.116, but de- tributed to the understanding of the photoconduc- tails are not know-n. tivity and band structure of Cu2 0 [23, 26].

Cu 2 0: Cuprous oxide, Cu 2 0, has a cubic structure

[5] and no phase transformations are known up to CuO: Cupric oxide, CuO, has a monoclinic structure the melting point [3, 5]. Because of the extensive [27, 28] and has considerable covalencv associated use of Cu 2 0 as a barrier layer photoelectric cell, wdth the Cu-0 bond [28]; no phase transformations many workers have successfully grown single crystals are known at ordinary pressures. CuO is a semi- of the oxide [6, 7]. CU2 O has considerable covalency conductor [29, 30] and resistivity studies at high associated with the Cu-0 bond and exhibits plastic pressure [29] seem to indicate a phase transition; behavior above 870 K [5, 8]. The material is a semi- however, detailed data are lacking. conductor and the bulk and surface resistivities and CuO forms solid solutions with PdO [31, 32] and

f photoconductivity have been examined by several PtO’ [33] having a tetragonal structure and with workers [9-18]. Optical absorption studies [19-22] MgO, NiO and CoO having cubic rock salt structure indicate several reflection bands with an absorption [32]; the lattice constant-composition curve extrapo- edge, corresponding to the optical energy gap, of lated to pure CuO suggest the existence of a

~2.02 eV. Cu2 0 exhibits the interesting phenomena tetragonal or cubic metastable (crypto-) modification of exciton spectra at low temperatures which appear of CuO [31-33].

Copper oxides

Oxide and description Data Remarks and inferences References of the study

Cu2 0

Crystal structure and Cubic; space group, Pn3m; Z — 2; Cu 2 0 has an unusual structure de- ]5, 8]. x-ray studies. a =4.268 ±0.001 A. TEC (120- scribed as two interpenetrating 470 K), /°C): 2X10-0. Melting and identical frame works of point ~1510 K. copper and oxygen atoms which are not cross connected by any primary Cu-0 bonds. Consider-

able convalency is associated with the Cu-0 bond.

Electrical properties. Semiconductor (300-670 K); p (400 The conductivity behavior in Cu 2 0 [9, 15]. 3 K)~2X10 flcm; Ea = 0.26 eV; is complicated by surface and a (400 K)~+l mV/°C; mh photoconductivity effects. 2 (300 K)«0.08 cm /V s. Cu 20 may be nearly an ionic conductor and extrinsic hole conduction can occur by activated hopping.

Elastic properties by pulse Room temperature values of elastic [34].

12 2 echo technique constants (XlO dyn/cm ): Cn =

(4.2-300 K). 1.165 ±0.005; ci 2 = 1.003 ±0.04;

c 4 4 = 0.121 ±0.003. Limiting low- temperature values (~4.2 K) 12 2 (XlO dyn/cm ): Cn = 1.211±

0.005; Ci 2 = 1 .054 ±0.04; C44 =

79 ±

Copper oxides—Continued

Oxide and description Data Remarks and inferences References of the study

0.109 ±0.003. 0d = 181 K; com- possibility (300 K) = 0.946 X 10“6 cm 2 /kg.

Optical properties and Reflectivity bands at 2.67, 3.85, The data are discussed in terms of [14, 19-

band structure. 4.64 and 5.00 eV are noted. the band structure of CU 2O. 26]. Energy gap is estimated to be

2.02 eV. €o = 7.60; €» = 6.46. Ex- citon spectra are noted at the band edge at low temperatures. 3 CuO

Crystal structure and Monoclinic; space group, C2/c; The structure can be described in [3, 27, 28]. x-ray studies. Z = 4; a = 4.6837 ±0.005 A; b = the following manner: the build- 3.4226 ±0.0005 A; c = 5.1288 ing elements are the oxygen co- 0.0006 A; /3= 99.54 ±0.01°. TEC ordination parallelograms, which (293-973 K, /°C) «6.5XlO-«. form chains by sharing edges. [010] Each copper atom is coordinated Such chains traverse the structure to four coplanar oxygen atoms in the [110] and [110] directions, situated at the corners of an al- the two types of chains alternat- most rectangular parallelogram; ing in the [001] direction. Each

the oxygen atom is coordinated to type of chain is stacked in the four copper atoms situated at the direction with a separation corners of a distorted tetra- between the chains of ~2.7 A; hedron. each individual chain in a group Estimated ionic character of stacked chain of [110] type is 40%; hence the bonding in CuO linked to each chain in the two appears to be predominantly adjacent groups of [110] type by covalent. corner sharing.

Electrical properties. p (296 K)~2-4X10 Slcm at 20 The high pressure data seem to [29, 30]. kbar pressure. High pressure indicate a solid -solid phase studies indicate no transition up transition in CuO but detailed to 450 kbar at 300 K or up to studies are yet to be made. 419 K at 17 kbars; however, at 391 K, p shows discontinuous drop at 187 kbar or 462 kbar at 360 K. a data indicate p type behavior. Detailed data are lacking.

S., Parker, H. S., Cryst. Growth References [7] Brower, W. Jr. and J. 8, 227 (1971), and references therein. [8] Vagnard, G., and Washburn, J., J. Am. Ceram. Soc. 51, 88 (1968). [1] Roberts, H. S., and Smyth, F. H., J. Am. Chem. Soc. [9] Toth, R. S., Kilkson, R., and Trivich, D., Phys. Rev. 43, 1061 (1921). 122, 482 (1961). [2] Gadalla, A. M. M., and White, J., Trans. Brit. Ceram. [10] O’Keeffe, M., and Moore, W. J., J. Chem. Phys. 35, Soc. 63, 39 (1964). 1324 (1961); 36, 3009 (1962). [3] Pranatis, A. L., J. Am. Ceram., Soc. 51, 182 (1968). [11] Campbell, R. H., Kass, W. J., and O’Keeffe, M., in [4] Frondel, C., Am. Mineral. 26, 657 (1941). Mass Transport in Oxides, Nat. Bur. Stand. (U.S.), [5] Suzuki, T., J. Phys. Soc. Japan 15, 2018 (1960). Spec. Publ. 296 (1968), p. 173. [6] Toth, R. S., Kilkson, R., and Trivich, D., J. Appl. [12] Young, A. P., and Schwartz, C. M., J. Phys. Chem. Phys. 31, 1117 (1960). Solids 30, 249 (1969).

80 Zielinger, P., Tapiero, M., Roubaud, Mme. C., and [13] J. 1. 10. Zinc Oxides Zouaghi, M., Solid State Commun. 8, 1299 (1970). [14] Kuzel, R., and Weichman, F. L., Canad. J. Phys. 48, 1585 (1970); 48, 2643 (1970). ZnO is the stable oxide of zinc and it has the [15] Kuzel, R., Cann, C. D., Sheinin, S. S., and Weichman, F. L„ Canad. J. Phys. 48, 2657 (1970). wurtzite (hexagonal) structure at room temperature. Fortin, E., Zouaghi, M., and Zielinger, P., Phys. [16] J. It is piezoelectric. No phase transformations are Letters 24A, 180 (1967). [17] Zouaghi, M., Caret, A., and Eymann, J. 0., Solid State known in ZnO in the range 4.2 to 890 K. At high Commun. 311 (1969). 7, pressures, it transforms to the rock salt structure at [18] Zouaghi, M., Tapiero, M., Zielinger, J. P., and Burgraf, R„ Solid State Commun. 8, 1823 (1970). room temperature. [19] Balkanski, M., Petroff, Y., and Trivich, D., Solid State Yannerberg [1] has reported a cubic form of zinc Commun. 5, 85 (1967). [20] Tandon, S. P., and Gupta, J. P., Phys. stat. solid! 37, 43 peroxide, Zn0 2 (a = 4.871 ±0.006 A), but apparently (1970). no other studies on this material are available in [21] Dahl, J. P., and Switendick, A. C., J. Phys. Chem. Solids 27, 931 (1966). the literature. [22] Smirnov, V. P., Sov. Phys.—Solid State (English Transl.) Because of the technical importance, single crys- 7, 2312 (1966); 8, 2020 (1967). [23] Gross, E. F., Sov. Phys. Uspekhi (English Transl.) 5, tals of ZnO have been grown by a variety of tech- 195 (1962). niques including hydrothermal, flux growth, travel- [24] Deiss, J. L., Daunois, A., and Nikitine, S., Solid State Commun. 7, 1417 (1969); 8, 521 (1970). ing solvent, vapor phase reaction and oxidation Forman, A., Brower, W. S., Jr., and Parker, H. S., [25] R. methods [2-7]. Lattice parameters and thermal ex- Phys. Letters 36A, 395 (1971). [26] Elliot, R. J., Phys. Rev. 108, 1384 (1957); 124, 340 pansion coefficients have been determined by various (1961). workers in the range 4.2 to 890 K [7-13]. ZnO is a [27] Tunell, G., Posnjak, E., and Ksanda, C. J., Z. Krist. semiconductor the electrical transport properties 0 90, 120 (1935). and Norrby, Acta Cryst, 26B, (1970). [28] Asbrink, S., and L.-J., 8 and optical characteristics have been examined in [29] Minomura, S„ and Dnckamer, H. G., J. Appl. Phys. 34, 3043 (1963). detail in the literature [6, 14-18], Zuev, K. P., and Dojgintsev, V, D., Izv. Akad. Nauk [30] High pressure x-ray diffraction [19, 20] and optical SSSR, Neorg. Mater. 4, 1498 (1968). [31] Schmahl, N. G., and Minzl, E., Z. Physik. Chem. (N.F.) studies [21] indicated that ZnO transforms from 47, 142 (1965). hexagonal to cubic structure at pressures '--'100-125 [32] Schmahl, N. G., and Eikerling, G. F-, Z. Physik. Chem. (N.F.) 62, 268 (1968). kbar at 300 K (Bates et al. give a value 670 K). [33] Muller, O., and Roy, R., J, Less-Comm. Metals 19, Studies of Bates and co-workers [20] indicate that 209 (1969). [34] Hallberg, J., and Hanson, R. C., Phys. stat. solidi 42, NH 4 C1 acts as a catalyst in the formation of the 305 (1970). high pressure phase and the transformation is sluggish. The cubic phase does not revert back to the wurtzite structure even after prolonged standing (several weeks) at room temperature; however, at

^390 K, it reverts within three weeks time.

Zinc oxides

Oxide and description Data Remarks and inferences References of the study

ZnO

Crystal structure and Range, 4.2-890 K; Hexagonal; Reeber [9] has summarized all the [9, 11, 12]. x-ray studies. space group, P6amc; Z = 2; 6 d = earlier crystal data on ZnO. 370 K. T = 297 K: a = 3.2497 dh Slight anisotropy in TEC values

0.0009 A; c = 5.206 ±0.0009 A. is evident. TEC (300-890 K; °/C)i a - || = 6.05X 1O“ 8 +2.20X 10 ® J+2.29X 18 =e 10= *»; 1® = 3.53 10 1 X H=2.38 X =# =ls 1 § 10 t+9.24X10 t (t in C).

Ibach [12] and Reeber [9] find that in the range 4.2=300 K, a goes through a minimum at 93 K.

81 Zinc oxides—Continued

Oxide and description Data Remarks and inferences References of the study

Pressure transition High pressure x-ray study shows T t The 4:4 coordination of the [19-21]. to be 673 or 300 K at pressures wurtzite phase of ZnO changes to ~105-110 kbars. Cubic phase; 6:6 coordination in the rock salt space group, Fm3m; Z = 4; a = phase; the latter phase is denser. 4.28 A. AH = 785 cal/mol; AS = -2.5 e.u.; dP/dT = 42.5 X10~ 3 bar/°C; AV = —2.55 cm 3 /mol. High pressure optical absorption indicates transition at 125 kbars

at 300 K. Absorption edge is at

3.14 eV ( = E 0 , at 1 bar);

d(AE g )/dP = 0.6-1.9X10-" eV/ bar (wurtzite); = 1.9X1O -0 eV/bar (rock salt).

Electrical properties. Pure undoped crystals have p~l- Eg is ~3 eV but donor levels of [14-18]. 100 ficm but doping increases the ~0.3 eV below the conduction

p by a factor '"-•'10. The p of band always create charge car- sintered and unsintered speci- riers thermally and contribute to mens behave differently with to the conductivity. respect to temperature. Inter- stitial metal atoms appear to play a dominant part.

Hutson [16] obtained the following In the range 150-900 K, T de- transport parameters at 300 K: pendence of mh has been ex- a«1.7 mV/K; mh = 180 cm 2 /V s; plained by the optical mode 16 -3 n c =2 X 10 cm ; m* =0.5 m; scattering; however, below 150 K,

trip* = 0.27 m; a?* = 0.85; «o = 8.5; ionized impurity scattering and e» = 3.73; 0 d =92O K. phonon -drag effects play a significant role.

References [11] Khan, A. A., Acta Cryst. 24A, 403 (1968). [10] [12] Ibach, H., Phys. Stat. solidi 33, 257 (1969). [1] Vannerberg, N. G., Arkiv Kemi 14, 119 (1959). [13] Kirchner, H. P., J. Am. Ceram. Soc. 52, 379 (1969). [2] Weaver, E. A., J. Cryst. Growth 1, 320 (1967). [14] Hahn, E. E., J. Appl. Phys. 22, 855 (1951). Kleber, Tech. [3] W., and Mlodoch, R., Krist. 1, 245 (1966); [15] Hutson, A. R., Phys. Rev. 108, 222 (1957). Chem. Abstr. 69, 46561m (1968). [16] Hutson, A. R., in Semiconductors, Ed. N. B. Hannay Hirose, M., and Kubo, I., Japan Appl. Phys 402 [4] J. 8, (Reinhold Publ. Corp., New York, 1959), p. 541. (i969). [17] Seitz, M. A., and Whitmore, D. H., J. Phys. Chem. [5] Cleland, J. W., in Mass Transport in Oxides, Nat. Bur. Solids 29, 1033 (1968). Stand. (U.S.), Spec. Publ. 296, (1968), p. 195. [18] Hideo, H., and Kasae, K., Nippon Kagaku Zasshi 90, [6] Nielsen, K. F., J. Cryst. Growth 3-4, 141 (1968). 112 (1969); Chem. Abstr. 70, n821z (1969). [7] Heller, R. B., McGannon, J., and Weber, A. H., J. Appl. Phys. 21, 1283 (1950). [19] Class, W., Iannucci, A., and Nesor, H., Norelco Reporter (1966). [8] Abrahams, S. C., and Beinstein, J. L., Acta Cryst. 25B, 13, 87 (1966); 13, 94 1233 (1969). [20] Bates, C. H., White, W. B., and Roy, R., Science 137, [9] Reeber, R. R., J. Appl. Phys. 41, 5063 (1970). 993 (1962). Sirdeshmukh, D. Ef., and Deshpande, V. T., Curr. Sci. [21] Edwards, A. L., Slykhouse, T. E., and Drickamer, H. G., (India) 36, 630 (1967). J. Phys. Chem. Solids 11, 140 (1959).

82 j II. Oxides of 4d Transition Elements

II. 1. Yttrium Oxides indexed on a hexagonal basis but a monoclinic symmetry could not be ruled out.

The fully oxidized form, Y2O3, is the only solid The C-type structure of Y2O3 is relatively loose oxide of yttrium [1-3]; substoichiometric yttrium and is of lower density compared to the B- (monoclinic) oxide, Y0 i. 5_ x , has, however, been reported by a and A- (hexagonal) type structures (usually few workers [4-6]; with x = 0.009, the oxide possesses adopted by rare earth sesquioxides). One would, intense green color and is paramagnetic in nature, therefore, expect that application of pressure should but the crystal structure is the same as that of favor the denser structures. Accordingly, Hoekstra stoichiometric Y2O3. Miller and Daane [5] report a and Gingerich [11], Hoekstra [12] and Prewitt et al. higher lattice parameter and a lower density for [13] noticed that C—*B conversion occurs in Y2O3 at YO1.491 compared to YOi.soo* A monoxide, YO, was ^25 kbar and at ~T270 K. The high pressure phase reported [7] as an impurity in Y metal on the basis can be quenched and the reversibility of the phase of x-ray pattern, but it is likely to have been a sur- transformation has been confirmed by heating the face film; the existence of YO as a bulk phase has B-form in air at ^1270 K or annealing in air at not been reported. ambient pressure and ^1170 K for several hours

Y2O3: Y2 0 3 has a cubic C-type rare-earth oxide [11, 12]; from the observed pressure -temperature structure at room temperature and 1 atmospheric data, the enthalpy and entropy changes accompany- pressure and can be described as a modified fluorite ing the C—>B transformation in Y2O3 have been phase having one anion in four missing to balance estimated [1, 14]. the tri valent cationic charge with a slight readjust- Electrical properties of Y2O3 have been examined ment of the positions of the remaining ions. The by a few workers [15-22], Y2O3 exhibits p-type semi- resulting unoccupied anion sites may be considered conduction with considerable ionic contribution to as forming nonintersecting strings along the four the total conductivity. The ionic conductivity has

(111) directions of the cubic cell [1-3]. been ascribed to the anionic migration [17, 18, 20- A hexagonal (H) form of Y2O3 at high temperatures 24] but the contribution is smaller the larger the

(~2650 K) has been reported by Foex and Traverse temperature and is less than 1 percent above 1500 K;

[8]. This H-form of Y2O3 is closely related to the impurities in slight amounts seem to play a dominant

A-type rare-earth oxide structure. The C—>H trans- role in ionic conduction [18, 24]. formation has been found to be reversible [8] and the Y2O3 forms solid solutions with oxides of the type two modifications are usually observed to coexist MO (M = Sr [25]), M 2 03 (M = B [26], Al [27], Sc over a remarkably extended range of temperatures. [28, 29], La [30] and Nd [30]) and M02 (M = Ti

Noguchi et al. [9] report a crystal structure transi- [31, 32], Zr [9, 31, 33-36], Hf [37-42], Ce [23], tion (probably C—>H) at ~2550 K. Mehrotra et al. Th [43, 44] and Re [45, 46]) and their properties [10] have reported a high temperature transformation have been examined extensively. Some of the TI1O2- in Y2O3 which is reversible and occurs over the range Y2O3 solid solutions exhibit 100 percent ionic con- 670 to 1170 K. The high temperature phase has been ductivity and are used as solid electrolytes [47, 48].

83 Yttrium oxides

Oxide and description Data Remarks and inferences References of the study

Y2O3

Crystal structure (including T = 300 K, 1 atm pressure: Cubic The cubic structure of Y2O3 is the [1-3, 8, high temperature high (C-type); space group, Ia3; Z = usual structure type adopted by 10-12]. pressure x-ray studies. 16; a = 10.604 A. High tempera- the heavy rare earth oxides. It is ture phases. T = 1170 K: Hexa- remarkable that the structure gonal (A-type?); a =9.80 A; change on heating is to that of a c = 10.17 A. T = 2603 K: Hexa- lower symmetry and is due to gonal (H-type); a =3.805 A; c = the changes in coordination and 6.085 A. Cubic-monoclinic (B- increase in density. The pressure type) transition at ~25 kbar transition does not go through and at ~1270 K. High pressure the corundum phase and the de-

phase is monoclinic; space tailed mechanism has been dis- group, C2/m; Z = 6; a = 13.91 A± cussed by Prewitt et al. [13]. 0.01 A; b = 3.483 ±0.003 A; c =

8.593±0.008 A; /3 = 100.15± 0.05 A. AH (estimated) ~2 kcal/ mol; AS (estimated) ~1.5 e.u. Melting point = 2959 ±20 K [49].

Electrical properties Semiconductor; p (970 K)~10 7 Y2O3 at low temperatures can be [15-24].

(300-1500 K). J2cm; Ea ~ 1 eV; a (900 K)~2 considered to be almost an in- mV/°C; p-type to n-type change sulator having a very high re- at low oxygen partial pressures. resistivity. Impurities exert a lot Considerable ionic conductivity of influence in the ionic conduc- below 1000 K; above 1500 K, tivity of the material; purest pure electronic conduction. sample has a very low ionic contribution. The p- to n-type change in Y2O3 may be due to loss of oxygen and creation of anion vacancies at high temperatures.

Dielectric and infrared eo=9.77; e«, = 3.58. Several bands [50-53]. -1 studies. in the region 300-560 cm are noted; weak bands in the region -1 800-1070 cm also exist. The bands are attributed to metal- oxygen vibrations.

Heat capacity studies Cp (1300 K) =31.17±19 cal/deg The cause of the Cp anomaly at [54, 55]. (300-1800 K). mol. An anomaly has been noted 1330 K is not known. at 1330 K with a AH = 310 cal/ mol; AH (estimated, by extra- polation) = 249 ±66 cal/mol; AS =0.19±0.05 e.u.

and Maslen, E. N., Acta Cryst. 19, 307 References [3] Paton, M. G., (1965). [1] Brauer, G., Progr. in the Science and Technology of [4] Haefling, J. A., Schmidt, F. A., and Carlson, O. N., Rare Earths Ed. L. Eyring (Pergamon, Oxford), 1, J. Less-Comm. Metals 7, 433 (1964). 152 (1964); 2, 312 (1966); 3, 434 (1968). [5] Miller, A. E., and Daane, A. H., J. Inorg. Nucl. Chem. [2] Fert, A., Bull. Soc. Franc. Mineral Crist. 85, 267 (1962). 27, 1955 (1965).

84 .

6] Miller, A. E., Thesis, Iowa State Univ. of Sci. and Tech. [43] Wimmer, J. M., Bidwell, L. R., and Tallan, N. M., 1964; Nucl. Sci. Abstr. 20, 880 (1966). Nucl. Sci. Abstr. 19, 42896 (1965). [7] Huber, E. J., Jr., Head, E. L., and Holley, C. E., Jr., [44] Subbarao, E. C., Sutter, P. H., and Hrizo, J., J. Am. J. Phys. Chem. 61, 497 (1957). Ceram. Soc. 48, 443 (1965). [8] Foex, M., and Traverse, J.-P., Compt. Rend. (Paris) [45] Roy, R., McCarthy, G. J., Muller, O., and White, W. B., 261, 2490 (1965). Nucl. Sci. Abstr. 20, 36784 (1966).

[9] Noguchi, T., Mizuno, M., and Yamada, T., Bull. Chem. [46] Roy, R., Kachi, S., McCarthy, G. J., Muller, O., and Soc. Japan 43, 2614 (1970). White, W. B., Nucl. Sci. Abstr. 20, 13260 (1966). [10] Mehrotra, P. N., Chandrashekar, G. V., Rao, C. N. R., [47] Wimmer, J. M., Bidwell, L. R., and Tallan, N. M., and Subbarao, E. C., Trans. Faraday Soc. 62, 3586 J. Am. Ceram. Soc. 50, 198 (1967). (1966). [48] Patterson, J. W., Bogren, E. C., and Rapp, R. A., [11] Hoekstra, H. R., and Gingerich, K. A., Science 146, J. Electrochem. Soc. 114, 752 (1967). 1163 (1964). [49] Noguchi, T., and Kozuka, T., Solar Energy 10, 203 [12] Hoekstra, H. R., Inorg. Chem. 5, 754 (1966). (1966). [13] Prewitt, C. T., Shannon, R. D., Rogers, D. B., and [50] McDevitt, N. T., and Davidson, A. D., J. Opt. Soc. Sleight, A. W., Inorg. Chem. 8, 1985 (1969). Amer. 56, 636 (1966). [14] Westrum, E. F., Jr., Progr. in the Science and Tech- [51] Goldsmith, J. A., and Ross, S. D., Spectrochim. Acta nology of Rare Earths, Ed. L. Eyring (Pergamon, 23A, 1909 (1967). Oxford) 3, 459 (1968). [52] Subba Rao, G. V., Rao, C. N. R., and Ferraro, J. R., [15] Noddack, W., and Walch, H., Z. Elektrochem. 63, 269 Appl. Spec. 24, 436 (1970). (1959); Z. Physik. Chem. (Leipzig) 211, 194 (1959). [53] Nigara, I., Ishigame, M., and Sakurai, T., J. Phys. Soc. [16] Neuimin, A. D., and Pal’guev, S. F., Chem. Abstr. 59, Japan 30, 453 (1971). 941 7g (1963). [54] Pankratz, L. B., King, E. G., and Kelley, K. K., U.S. [17] Tare, V. B., and Schmalzried, H., Z. Physik. Chem. Bur. Mines, Rept. Invest. No. 6033, 1962. (NF) 43, 30 (1964). [55] Holley, C. E., Jr., Huber, E. J., Jr. and Baker, F. B., [18] Tallan, N. M., and Vest, R. W., J. Am. Ceram. Soc. Progr. in the Science and Technology of Rare Earths 49, 401 (1966). Ed. L. Evring (Pergamon, Oxford), 3, 343 (1968). [19] Zyrin, A. V., Dubok, V. A., and Tresvyatskii, S. G., Chem. Abstr. 69, 62562t (1968). II. 2. Zirconium Oxides [20] Chandrashekar, G. V., Mehrotra, P. N., Subba Rao, G. V., Subbarao, E. C., and Rao, C. N. R., Trans. The solubility of oxygen in a -zirconium (hexagon- Faraday Soc. 63, 1295 (1967). al, a = 3.231 A, c= 5.148 A) is appreciable and with [21] Subba Rao, G. V., Ph.D. Thesis, Indian Institute of Technology, Kanpur, India, 1969. increasing oxygen content, the c-axis of the metal [22] Subba Rao, G. V., Ramdas, S., Mehrotra, P. N., and elongates, the change being marked around ZrOo Rao, C. N. R., J. Solid State Chem. 2, 377 (1970). .3 [23] Subba Rao, G. V., Ramdas, S., Tomar, M. S., and [1]; the a axis passes through a maximum around Rao, C. N. R., Ind. J. Chem. 9, 242 (1971) and refer- - = enccs therein. ZrOo.25 The Zr30 phase (a = 5.629 A, c 5.197 A) [24] Etsell, T. H., and Flengas, S. N., Chem. Rev. 70, 339 is completely ordered, the compositions richer than (1970). Zr being analogous to Zr [1]. Electrical and [25] Lopato, L. M., Yaremenko, Z. A., and Tresvyatskii, 30 30 S. G., Dopovidi Akad. Nauk Ukr RSR, 1493 (1965). mechanical properties of the solid solutions of oxygen [26] Levin, E. M., Phys. Chem. Glasses 7, 90 (1966). in Zr have been measured and Zr phase with a [27] Bondar, I. A., and Toropov, N. A., Izv. Akad. Nauk 60 SSSR, Ser. Khim. 212 (1966). decomposition temperature of 1200 K has been iden- [28] Hajek, B., Petru, F., Kalalova, E., and Dolezalova, J., tified in addition to the more stable Zr both Z. Chem. 6, 268 (1966). 30 [2]; [29] Trezebiatowski, W., and Horyn, R., Bull, Acad. Polon. Zr60 and Zr30 are reported to be semiconductors. Sci., Ser. Sci. Chim. 13, 311 (1965). Both the Zr Zr phases have been examined [30] Andreeva, A. B., and Keler, E. K., Zh. Prikl. Khim. 39, 60 and 30 489 (1966). by electron microprobe and electron microscopic [31] Collongues, M. R., Queyroux, F., Perez, M., Jorba, Y., techniques and the ordered nature of the latter phase and Gilles, J.-C., Bull. Soc. Chim. France 4, 1141 (1965). confirmed [3]. The structures of the solid solutions in [32] Ault, J. D., and Welch, A. J. E., Acta Cryst. 20, 410 (1966). the composition range 11-28.5 atomic percent oxy- [33] Rouanet, A., Compt. Rend. (Paris) 267C, 1581 (1968). gen have been examined by x-ray, neutron and elec- [34] Takahashi, T., and Suzuki, Y., Deuxiemes Jour. Int. des tron diffraction techniques [1, 4, 5]. The compositions piles a Combustible, Bruxelles 378 (1967). de- [35] Strickler, D. W., and Carlson, W. G., J. Am. Ceram. in the range 15-25 atomic percent oxygen can be Soc. 48, 286 (1965). scribed in terms of a one -dimensional long-period [36] Duwez, P., Odell, F., and Brown, F. N., J. Electrochem. a periodicity increases Soc. 98, 356 (1951). superstructure with which [37] Volchenkova, Z. S., and Pal’guev, S. F., Trans. Inst. with oxygen content [4]. The superstructure can be Elektrokhim. Akad. Nauk SSSR, Ural. Flilial 133 5, expressed in terms of the stacking sequence of octa- (1964).

[38] Robert, G., Deportes, C., and Besson, J., Deuxiemes hedral interstitial sites in the lattice of a-Zr with Jour. Int. des piles a Combustible, Bruxelles 368 ABCABC ... in ZrO* (0.25

85 revealed superstructure patterns due to Zr20 [6]. haviour of evaporated films of Zr02 has been in-

Zr 2 0 crystallizes in the hexagonal system with a = vestigated by electron microscopy and diffraction

16.16 A and c= 5.148 A. Six oxygens form a ring and [29]. As mentioned earlier, the mechanism of the a total of four such rings form the supercell. monoclinic-tetragonal transition of Zr02 has been The most stable oxide of zirconium is the import- examined by several workers [10-14, 21-24]. The ant ceramic dioxide, Zr0 2 which normally crystal- kinetics of this transition with particular reference to lizes in the monoclinic structure (P2i/c) possessing its martensitic nature have been studied by Suk-j

4 Zr0 2 units in the unit cell [7-9]. The monoclinic harevskii et al. [30]. The mechanism of the transition structure reversibly transforms to a tetragonal struc- has also been studied with single crystals [31]. ture around 1420 K and to a fluorite type cubic Preparation of finely-divided samples of the meta- form at very high temperatures around 2500 K stable phases of Zr02 has been described by several

[10-14]. Zr0 2 melts at 2995±20 K [15]. workers [10, 11, 21, 32-34]. Zr02 prepared by de-

Oxygen equilibria over nonstoichiometric Zr0 2 composition of alkoxides is cubic at room tempera- (with oxygen deficiency) have been examined by ture and transforms to the metastable tetragonal

Aronson and others [16] in the 1100 to 1400 K range phase around 570 K which then transforms to the -12 20 at 10 -10~ atm pressures. The compositions of monoclinic phase in the range 580 to 670 K [32]. -6 the oxide in the 1670 to 2170 K range at 1-10 atm Occurrence of the metastable tetragonal phase in have been investigated exhaustively by Carniglia et high surface area. (small crystallite size) samples of = al. [17] who find the ZrOo-^-Zr boundary to be x Zr0 2 has been examined by Garvie [33] and the -6 0.014 at 2070 K and 3.5 XlO atm of oxygen. A kinetics of transformation of the metastable phase black form of Zr02 has been prepared by heating to the monoclinic phase have been investigated by hydrated Zr0 2 in argon or vacuum; this has a tetra- Dow Whitney [34] and Surkarevskii et al. [30]. The gonal structure [18, 19]. Amorphous hydrated Zr02 low temperature metastable cubic and tetragonal prepared by precipitation methods has been shown phases possess lattice parameters identical to those to possess short range order [19]. of the corresponding high temperature phases [11,

The phase transitions of Zr0 2 have been investi- 21]. Thin evaporated films of Zr02 also exhibit the gated extensively by several workers. Although there cubic structure at room temperature as shown by were some inconsistencies and uncertainties in the electron diffraction [29]; the cubic form transforms earlier literature [20] recent studies have clearly to the tetragonal and monoclinic forms on heating. established the transition temperatures [21] and Electrical conductivity measurements of Vest and there appears to be no doubt regarding the mono- co-workers [35, 36] have clearly shown that Zr02 is a clinic-tetragonal and tetragonal-cubic transitions semiconductor, the sign of the majority carriers de- mentioned earlier [10-14]. The monoclinic-tetragonal pending on the oxygen partial pressure and tempera- transition shows appreciable hysteresis similar to ture; some evidence of ionic conduction in tetragonal martensitic phase transitions [22] and is predomin- Zr02 has been presented. Electrical conductivity of antly athermal [22, 23]; the formation of the cubic the tetragonal phase has been measured by other form is reversible [22]. Anomalous intensity changes workers [37, 38] as well. Electrical conductivity in the x-ray pattern are noticed prior to the mono- measurements have been employed to study the clinic-tetragonal transition (in the 1200 to 1370 K monoclinic-tetragonal transformation under pressure range) and hybrid crystal formation has been no- [26].

ticed [24]; orientation relationship between the mono- Phase equilibria, phase transitions, defect chem- ! clinic and tetragonal phases has been established. istry, electrical properties and other aspects of solid

The tetragonal-cubic transition around 2550 K (ob- solutions of Zr0 2 with a variety of oxides have been served by x-ray studies in vacuum), is undoubtedly reported in the literature [39]. Solid solutions like accompanied by partial reduction of Zr02 and the Zr02 -CaO [39, 40] are well-known solid electrolytes.

- phase boundary between the cubic and tetragonal Typical of the solid solutions investigated are Zr02

- forms is affected by the stoichiometry or oxygen MgO [41, 42], Zr02 -SrO [43], Zr02 -MnO [44], Zr02 I 45- deficiency [17, 25]. The lattice dimensions of mono- R 2 03 where R = Sc, Y, or a rare earth [10, 39, clinic and tetragonal Zr0 2 decrease with increase in 54], Zr02 -Ce0 2 [55], Zr02 -Ge02 [56], Zr02 -Ti02 [57- oxygen deficiency, but the phase does not appear to 59] and Zr02 -Hf02 [60-62]. The monoclinic tetragonal be significantly affected. transition of Zr0 2 is markedly affected in such solid

Pressure induced monoclinic-tetragonal transition solutions; as can be expected the cubic phase is I of Zr02 has been reported [26-28]. Polymorphic be- stabilized in many of the solid solutions.

86 Zirconium oxides

Oxide and description Data Remarks and inferences References of the study rO

Crystal structures of the The stable room temperature mono- The monoclinic structure can be [7-9]. monoclinic, tetragonal and clinic phase has the P2i/C space considered to be a distorted

cubic phases of Zr02 by group with 4 Z1O 2 in unit cell: fluorite structure. Detailed x-ray diffraction. a = 5.1454 ±0.0005 A; b = atomic positions in the mono- 5.2075 ±0.0005 A; c = 5.3107± clinic phase as well as structural

0.0005 A and /3 =99°14'±0°05\ rearrangements in polymorphic The Zr-0 distances vary from transitions to tetragonal and 2.05 to 2.28 A (for the near cubic phases have been dis-

neighbors) and there is a strong cussed. The structures of ZrC>2

tendency to twin in the (100) and HfC>2 have been compared. direction and polymorphism can be understood in terms of the (fluorite type) layers parallel to (100). The tetragonal phase (D4®-P42/ The lattice dimension of the cubic [11-13]. nmc) has dimensions: a = 3.64 A phase will be a function of stoi- and c = 5.27 A at 1520 K with 2 chiometry which in turn will molecules in the cell. The depend on the temperature and fluorite type cubic phase has oxygen partial pressure. o*5.26 A at the transition temperature.

Phase transitions by The phase transition temperatures The transitions of HfC>2 and ZrC>2 [11-14, x-ray diffraction. reported by different workers are similar. 21-25]. vary to some extent. The ranges are: monoclinic—tetragonal, 1320-1490 K; tetragonal—* cubic, ~2 550-2650 K.

There is considerable thermal The transition is martensitic in [14, 22-24]. hysteresis in the monoclinic- character. tetragonal transition just like in

the transition of HfC> 2 . For example, the tetragonal—monoclinic transition (on cooling) has been reported in the range 1320-1170 K. Coexistence of monoclinic and The thermal hysteresis in the [24]. tetragonal phases as well as hy- monoclinic-tetragonal transition brid crystal formation have been arises from the differences be- observed in the range 1370-1490 tween the forward and reverse

K. The orientation relationship transitions (i.e., heating versus

between the two phases is found cooling) The pretransformation

in terms of the parallelism of the region is between 1200 and (100) and (010) planes of the 1370 K. tetragonal phase; the b axis of the

monoclinic phase is similar to the c axis of the tetragonal phase.

-8 Stoichiometry and its The tetragonal-cubic transition At ~2670 K and 10 torr of oxy- [25]. relation to the tetragonal- temperature found by x-ray gen, the composition will be

cubic transition. diffraction (in vacuum) varies ZrOi. 96o*o. oo 2 » At ~2670 K and -3 widely due to the changes in the 2 X 10 torr of oxygen, the

stoichiometry of Z 1O 2 . For a composition will be ZrOi.gge.

87 -

Zirconium oxides—Continued

Oxide and description Data Remarks and inferences References of the study

composition of ZrOi, 97 , the tetragonal-cubic transformation temperature corresponds to cross-

ing of the (tetragonal Z 1O 2 + cubic ZrOj)-(cubic ZrOj) phase boundary.

The stoichiometry of ZrOi as a ZrC>2 is nearly stoichiometric at [16, 17, 25]

function of temperature and 1270 K and 1 atm poa . The oxygen partial pressure has been Zr02_*-Zr phase boundary is at

# investigated and the cubic- x =0.018 at pot »3.5X10“ atm tetragonal transition temperature and T« 2070 K.

is significantly lowered in samples containing a-Zr and Zr02.

ZrC>2 is oxygen-deficient over the Lattice thermal expansion is not entire temperature range of 1370- affected by the oxygen deficiency 2170 K at /?oj<1 atm. In the although the dimensions of the 1670-2170 K range, oxygen de- monoclinic and tetragonal phases

ficiency is given by log x » decrease; the monoclinic -tetra- - 0.890[ (0.400 Xl0-«)/T]- gonal transition is not signifi- (logP)/6] where x is in ZrC> -*, cantly affected. No change in [ 2 T is K and p is in atmospheres Young’s modulus and strength of of oxygen. ZrC>2 is noticed due to oxygen defciency in sintered samples.

Metallographic observations. The monoclinic-tetragonal phase Based on these observations the [23]. transition has been examined on transformation has been shown a vacuum hot -stage microscope to be diffusionless and (up to 1570 K). In the 1320- athermal as in martensite 1420 K range, the platelet sub- transformations.

structure within ZrC>2 grains forms rapidly. Photomicrographs and motion pictures were taken through the transformation.

High pressure studies. Monoclinic Z 1O 2 reversibly trans- dT/dP of the transition has been [26-28]. forms to the tetragonal form at reported to be — 0.01°/bar in

298 K on application of pressures [27] and -0.003°/bar in [26]. greater than 37 kbar. The tetragonal phase cannot be retained at ambient conditions.

Kinetics of monoclinic M artensite (monoclinic^tetragonal) Transformation velocity decreases [30]. tetragonal transformation. transformations in ZrC>2 show no with decreasing temperature in principle difference between isothermal runs; the energy of

athermal and isothermal kinetics. activation is 150 kcal/mol. At a certain supercooling, the transformation velocity increases sharply and the transformation becomes athermal. In the tetragonal-monoclinic transition, a change in the kinetics of dif-

fusionless transition is observed. This phenomenon has also been studied with the metastahle tetragonal phase prepared by decomposition of the oxychloride.

88 Zirconium oxides—Continued

Oxide and description Data Remarks and inferences References of the study

Thermal expansion. Thermal expansion of the lattice There is a critical Zr-0 distance [14, 63]. parameters up to 1370 K have above which the monoclinic been reported. The monoclinic- structure is no longer stable. tetragonal transition occurs around 1370° and the reverse transition occurs at much lower temperatures (980-1250 K).

Thermal expansion data for the The data are probably applicable [64]. tetragonal phase are: a =3.5882 + up to 2270 K. At 1420 K, a = -5 4.50 X10 1 within O.OOI3 A; 3.639 A and c = 5.275 A and at -5 c = 5.188 2 +7.57X10 t within 1970 K, a=3.678 A and c =

0.0022 A where t is in degrees 5.339 A. centigrade between 1150 and 1700 °C.

Thermal expansion data for mono- The monoclinic -tetragonal transi- [65]. clinic ZrC>2 have been measured tion is accompanied by a con- in detail along the three axes and traction of 3.42%. found to be highly anisotropic.

Heat of transition. The monoclinic -tetragonal transi- [66]. tion is associated with an enthalpy change of 1420 cal/mol.

Superplasticity Enhanced plasticity during the [67]. monoclinic -tetragonal transition has been studied.

Infrared spectra. Cubic phase gives one band at 490 [52, 68]. -1 cm . Monoclinic phase shows several bands as expected.

Neutron induced The monoclinic -cubic transforma- Impurities stabilize the high [69]. transformation. tion by neutrons depends on the symmetry phase. method of preparation.

Metastable cubic and Finely divided samples of Z1O2 Cubic phase lattice parameter from [10, 11, 21, tetragonal phases and their prepared by the decomposition of electron diffraction is ~ 5.07 A. 29, 32, 33, phase transitions. alkoxides, oxychloride and other Thin evaporated films of Z1O2 52, 70]. salts give rise to the metastable (as shown by electron diffrac- cubic and tetragonal phases. The tion) also exist in cubic form and metastable cubic form prepared the transformation to tetragonal

from alkoxides transforms to the and monoclinic form is affected metastable tetragonal form by annealing, oxygen pressure (~570 K), and then to the mono- etc. The cubic lattice parameters clinic form (580-670 K). The are around 5 A, the actual value temperatures of transformation varying with annealing. seem to vary with the method of sample preparation and some workers have reported cubic- monoclinic transitions. The lattice parameters of these low temperature cubic and tetragonal forms agree with those of the high temperature phases. The metastable tetragonal phase pre-

89 Zirconium oxides—Continued

Oxide and description Data Remarks and inferences References of the study

pared by precipitation of alkaline aqueous solution or low -temperature calcination of zirconium nitrate has been found to be stable at room temperature due to the small crystallite size (or large surface area).

The kinetics and mechanism of No induction period was seen in the [34]. transformation of the metastable kinetics. tetragonal phase to the monoclinic phase have been studied and the kinetic data have been interpreted in terms of Avrami’s equation and order -disorder theory.

The cubic-tetragonal transformation [29].

in thin films is monotropic while the tetragonal-monoclinic transformation exhibits the nucleation and growth mechanism.

Electrical properties. Electrical properties have been re- There is considerable disagreement [35, 36, 39].| ported by several workers. Mono- regarding the ionic transport clinic Zr02 is an amphoteric numbers in monoclinic and tetra-

semiconductor at 1270 K and at gonal Z 1O 2 and the available data high oxygen pressures (1-10-6 have been nicely reviewed by

atm), the predominant defects are Etsell and Flengers [39]. There is

ionized Zr vacancies which give no doubt that ZrC>2 is a mixed rise to low mobility holes. The conductor. The conductivity defect structure of tetragonal minima have been reported at

-9 16 ZrC>2 has been examined by IO and 10~ atm at 1270 K measuring the temperature and for the monoclinic phase. For the oxygen pressure dependence of tetragonal phase the minima are -4 -6 -7 conductivity and the electronic reported at 10 , 10 and 10 transference number. At high atm at 1570 K. The minima are

temperatures and low po2 , ZrC>2 at higher oxygen pressures at is an n-type semiconductor (due higher temperatures. to ionized oxygen vacancies).

Tetragonal ZrC>2 is a mixed electronic and ionic conductor. The conductivity change accompanying the monoclinic-tetragonal

transition is isothermal and the rate of change depends on thermal and sample history.

90 References [36] Vest, R. W., and Tallan, N. M., J. Am. Ceram. Soc. 48, 472 (1965). [37] Anthony, A. M., Guillot, A., and Nicolau, P., Compt. [1] Holmberg, B., and Dogerhamm, T., Acta Chem. Scand. Rend., Ser. A 262B, 896 (1966). 15, 919 (1961). [38] McClaine, L. A., and Coppel, C. P., J. Electrochem. Soc. [2] Kornilov, I. I., Glazova, V. V., and Kenina, E. M., 113, 80 (1966). [4] Dokl. Akad Nauk SSSR 169, 343 (1966). [39] Etsell, T. H., and Flengers, S. N., Chem. Revs. 70, 339 [3] Ericson, T., Osteberg, G., and Lehtinen, B., J. Nucl. (1970). [5] Materials 25, 322 (1968). [40] Subbarao, E. C., in Non-Stoichiometric Compounds, Ed. f [6] Fehlmann, M., Jostsons, A., and Napier, J. G., Z. Krist. L. Mandelcorn (Academic Press, New York, 1964). [7] 129, 318 (1969). [41] Grain, C. F., J. Am. Ceram. Soc. 50, 288 (1967). Yamaguchi, S., J. Phys. Soc. Japan 24, 855 (1968). [42] Viechnicki, D., and Stubican, U.S., J. Am. Chem. Soc. [8] [9] Steeb, S., and Rickert, A., J. Less Common Metals 17, 48, 292 (1965). 429 (1969). [43] Noguchi, T., Okubo, T., and Yonemochi, O., J. Am. McCullough, J. D., and Trueblood, K. N., Acta Cryst. Ceram. Soc. 52, 178 (1969). [10] 12, 507 (1959). [44] Bayer, G., J. Am. Ceram. Soc. 53, 294 (1970). Rogers, D., Acta Cryst. (1959). Thornber, M. R., Bevan, D. M., and Graham, [11] Adam, J., and M. 12, 951 [45] J. J., Smith, D. K., and Newkirk, H. W., Acta Cryst. 18, 983 Acta Cryst. 24B, 1183 (1968). [12] (1965). [46] Spiridonov, F. M., Popova, L. N., and Popilskii, R. Y., Kuznetsov, A. K., Keler, E. K., and Fan, F. K., Zh. J. Solid State Chem. 2, 430 (1970). [13] Isupova, E. N., Glushkova, V. B., and Keler, E. V., [14] Prikl. Khim. 38, 233 (1965). [47] Boganov, A. G., Rudenko, V. S., and Makarov, L. P., Izv. Akad. Nauk SSSR Neorg Mater. 4, 399 (1968). [15] Dokl. Akad Nauk SSSR, 160, 1065 (1965). [48] Collongues, R., Queyroux, F., Jorba, M. P. Y., and Smith, D. K., and Cline, C. F., J. Am. Ceram. Soc. 45, Gilles, J. C., Bull. Soc. Chim. France 1141 (1965). [16] 249 (1962). [49] Rouanet, A., Compt. Rend. 267C, 1581 (1968). [17] Teufer, G., Acta Cryst. 15, 1187 (1962). [50j Takahashi, T., and Suzuki, Y., Deuxiemes Jour. Int. Ruh, R., and Corfield, P. W., J. Am. Ceram. Soc. 53, des Piles a Combustible, Bruxelles, 378 (1967). [18] 126 (1970). [51] Mazdiyasni, K. S., Lynch, C. T., and Smith, J. S., T., Solar. 203 Ceram. Soc. [19] Noguchi, T., and Kozuka, Energy 10, J. Am. 50, 532 (1967). (1966). [52] Glushkova, V. B., and Koehler, E. K., Mat. Res. Bull. [20] Aronson, S., J. Electrochem. Soc. 108, 312 (1961); Kof- 2, 503 (1967); see also, Sci. Ceram. 4, 233 (1967), [21] stad, P., and Ruzicka, D. J., ibid. 110, 181 (1963). CA, 70, 60405r. S. I. A., B., Keler, [22] Carniglia, S. C., Brown, D., and Schroeder, T. F., [53] Davtyan, Glushkova, V. and E. K., [23] J. Am. Ceram. Soc. 54, 13 (1971). Izv. Akad Nauk SSSR Neorg Mater. 2, 890 (1966). Livage, J., and Mazieres, C., Compt. Rend. 261, 4433 [54] Strickler, D. W., and Carlson, W. G., J. Am. Ceram. [24] (1965). Soc. 48, 286 (1965). Casselton, stat. solidi (a) [25] Livage, J., Doi, K., and Mazieres, C., J. Am. Ceram. [55] R. E. W., Phys. 1, 787 (1970). Soc. 51, 349 (1968). [56] Lefevre, J., and Collongues, R., Bull. Soc. Chim. France [26] Weber, B. C., J. Am. Ceram. Soc. 45, 614 (1962). 1959 (1966). [27] Ruh, R., and Rockett, T. J., J. Am. Ceram. Soc. 53, 360 [57] Ksendzov, Y. M., and Prokhvatilov, V. G., Zhur Fiz. (1970). Khim. 31, 321 (1957). [28] Wolten, G. M., J. Am. Ceram. Soc. 46, 418 (1963). [58] Keler, E. K., and Andreeva, A. B., Ognewpory 25, 320 [29] Fehrenbacher, L. L., and Jacobson, L. J., J. Am. Ceram. (1960); CA, 51, 15237d; CA, 48, 6798h. Soc. 48, 157 (1965). [59] Shakhtin, D. M., Levintovich, E. V., Pivovar, T. L., [30] Patil, R. N., and Subbarao, E. C., Acta Cryst. 26A, and Eliseeva, G. G., Izv. Akad. Nauk SSSR Neorg. [31] 535 (1970). Mater. 4, 1603 (1968). Nicholson, P. S., J. Am. Ceram. Soc. 54, 52 (1971); see [60] Gavrish, A. M., Susharevskii, B. Y., Krivoruchko, P. P., [32] also Ruh, R., and Garrett, H. J., ibid. 50, 257 (1967). and Zoz, E. I., Izv. Akad. Nauk SSSR Neorg. Mater. [33] Dow-Whitney, E., J. Electrochem. Soc. 112, 91 (1965). 5, 547 (1969). [34] Yahldiek, F. W., and Lynch, C. T., Proc. Intern. Conf. [61] Ruh, R., Garret, H. J., Domagala, R. F., and Tallan, [35] on Sintering and Related Phenomena, Ed. G. C. N. M., J. Am. Ceram. Soc. 51, 23 (1968). Kuczinski, (Gordon and Breach, New York, 1966). [62] Stansfield, O. M., J. Am. Ceram. Soc. 48, 436 (1965). Kulcinski, G. L., J. Am. Ceram. Soc. 51, 582 (1968). [63] Grain, C. F., and Campbell, W. J., U.S. Bur. Mines El-Shanshoury, I. A., Rudenko, Y. A., and Ibrahim, Rept. Invest. 5982 (1962). I. A., J. Am. Ceram. Soc. 53, 264 (1970). [64] Lang, S. M., Sci. Tech. Aerospace Rept. 2, 2065 (1964). Sukharevskii, B. Ya., Alapin, B. G., and Gavrish, A. M., [65] Filatov, S. K., and Frank -Kamenetskii, V. A., Soviet Dokl. Akad. Nauk SSSR 156, 677 (1967). Phys. -Cryst. 14, 696 (1970). Grain, C. F., and Garvia, R. C., U.S. Bur. Mines Rept. [66] Coughlin, J. P., and King, E. G., J. Am. Chem. Soc. 72, Invest No. 6619, 19 (1965), CA, 63, 5027A. 2262 (1950). Mazdiyasni, K. S., Lynch, C. T., and Smith, J. S., [67] Hart, J. L., and Chakladar, A. C. D., Mat. Res. Bull. J. Am. Ceram. Soc. 48, 372 (1965); ibid. 49, 286 (1966). 2, 521 (1967). Garvie, R. C., J. Phys. Chem. 69, 1238 (1965). [68] White, W. B„ Mat. Res. Bull. 2, 381 (1967). Dow-Whitney, E., Trans. Faraday Soc. 61, 1991 (1965). [69] Adam, J., and Cox, B., J. Nucl. Energy 17, 435 (1963). Vest, R. W., and Tallan, N. M., and Tripp, W. C., [70] Mazdiyasni, K. S., and Brown, L. M., J. Am. Ceram. J. Am. Ceram. Soc. 47, 635 (1964). Soc. 53, 43 (1970). II. 3. Niobium Oxides argued [32, 33] that this oxide should exhibit a

semiconductor-metal transition anologous to V0 2 . Phase relations in the Nb-0 system have been The available data are all on polycrystalline samples examined and reviewed over the past vears by many and may not be reliable. Rao et al. [28] have studied workers [1-12]. In addition to the well-established the transitions in Nb0 2 and its solid solutions with monoxide, dioxide and pentoxide, several suboxides V0 2 , Nbi_ x F x0 2 (0

NbO: Niobium monoxide is easily made by the Homologous series, Nb3n+i08n_2 (5^n^8): Stud- oxidation of the metal and has a narrow range of ies by Norin and Magneli indicate that in the 14- [38] homogeneity (0.982^x^1.008 in NbO x) [1, 11, composition range between Nb0 2 and Nb 2 0 5 there 17]. It has a cubic rock salt structure with ~25 exist several nonstoichiometric oxides shown by percent vacancies in the lattice which are ordered Gatehouse and Wadsley [39] (and in many later [1]. No phase transformations are known in this papers) to belong to the general homologous series material; high temperature, high-pressure studies of the formula Nb3n+i0 8ra _ 2 (rc = 9 would be Nb 2Os if [18, 19] indicate no change in the vacancy concentra- it formed, but H-Nb 2 0 5 actually has a slightly differ- tion up to 77 kbar and at 1920 K. It appears that

ent structure). In addition, oxides Nbi 2 0 2 g (Nb0 2 . 4 n) the nature of bonding in the ordered NbO structure and Nb 470ii 6 (Nb0 2 464 ) have also been reported is significantly different from that in the disordered [40, 41]. All these oxides have structures similar to vacancy structures of TiO and VO. Detailed re- but different from, the basic high temperature sistivity studies by many workers [14-16, 20-22]

(monoclinic) structure of H-Nb 2 0 5 [42]. Physical i indicate the metallic behavior of NbO in the range properties of these systems have not been investi- 2 to 900 K. The material becomes superconducting gated in detail. at ~1.5 K [20]. Solid solution formation and the solubility of other monoxides in NbO have been

Nb2 0 5 : Niobium pentoxide, Nb 20 5 , has a narrow examined by Gel’d et al. [23]. homogeneity range [1, 43] and the stable phase is

Nb02 : Niobium dioxide has a distorted rutile struc- the high-temperature (H) form. The polymorphic ture with a narrow homogeneity range at room behavior of this oxide has been examined by various temperature [11, 12, 15, 24-29] and exhibits a first workers over the past many years; however, a de- order transition at ~1070 K transforming to a tailed picture is far from clear at the present stage

(perfect) rutile phase [26-29]. Discontinuities in x and several metastable phases of Nb 205 are being and p have been noted at T t , but the material is reported from time to time and several reports are paramagnetic with no magnetic ordering in the range contradictory in nature.

300 to 1200 K [30]. Reports of the electrical proper- There are at present eight crystalline modifica- ties are conflicting: Janninck and Whitmore [31] tions of Nb 205 known and many of them can be and Roberson and Rapp [21] reported an apparent grown by vapor trar sport methods. Schafer et al. || semiconductor-to-metal transition at T <, but Sakata [44] have reviewed various preparation methods of

[27] finds semiconductor behavior throughout the the Nb 2 Os polymorphs while Wadsley and Andersson range 300 to 1270 K with a ten-fold jump in a at T t . [42] elucidated the structural relationships involved Metal-metal bonding in the low temperature phase and pointed out that many new modifications are of Nb0 2 has been established [26, 27] and it may be feasible and may be discovered in future.

92 The low-temperature form, TT-Nb 2 0 5 , first re- at 1070 K without first going into the B-form. The ported by Frevel and Rinn [45]; is hexagonal and detailed mechanism is not known but probably im- can be best prepared by the oxidation of Nb0 2 in purities play a significant role. air at 590 K for 24 h [44]. Holtzberg et al. [46] and B-Nb 2 05 is the densest of all the polymorphs of

Shafer and Roy [47] obtained this (5) phase by care- niobium pentoxide [55-57] and obtained as char- fully heating the niobic acid hydroxide gel or amor- acteristic plate-like crystals having monoclinic sym- phous Nb 2 05 at 770 to 870 K or at low temperatures metry by vapor transport methods [44]. Polycrystal- for prolonged periods of time. This metastable form line material can be obtained easily by the oxidation [49] has also been realized by vapor transport reactions of Nb0 2 in air (24 h heat treatment at 590 and at with Nb 2 0 5 [48] and by rapid-quenching of the melt 1120 K) [44]. This polymorph is depicted as the

. The TT form transforms continuously into high-pressure phase by Wadsley and Andersson [42]

T-Nb 2 05 on heating at 870 to 1073 K [50]; annealing and thermal studies by Schafer et al. [44] indicate at 1670 K results in the high temperature (H) it to be thermodynamically stable at low tempera- polymorph [49]. tures.

The orthorhombic T-Nb 2 0 5 has been obtained by DTA studies indicate a B—>H endothermic con-

Brauer [1], Holtzberg et al. [46] (y-form) and Shafer version at 1230 K but isothermal experiments in [50] and Roy [47] (Ill-form), by heating the hydroxide NbOCh and Cl 2 atmospheres reveal that the transi- gels or amorphous Nb 2 0 5 at 870 to 1073 K. It is tion is reversible and occurs at lower temperatures also produced by the oxidation of Nb or Nb alloys (T/=1020 K; Tr — 920 K) with AH~2 kcal/mol. below 1270 K [51] or Nb0 2 at 870 to 1070 K in air The tetragonal M-Nb 2Os has been first reported

. Under hydrothermal conditions (170 atm, 600- by Brauer [1] and can be obtained by heating either 650 K), amorphous niobic acid also changes to T- the amorphous or T form at 1170 to 1220 K for

Nb 20 5 [47]. Single crystals of the T-form can be some hours or for shorter periods at higher tempera- obtained either by vapor transport or quenching the tures [46, 58]. Single crystals have been grown by supercooled melt [52]. vapor transport [59] which indicate the true sym-

The T-form changes to the B-form on heating by metry to be tetragonal rather than monoclinic [57]. an exothermic irreversible process and DTA scan This phase transforms to H-Nb 20 5 continuously on

indicates a T t of 1090 K; heating to higher tempera- heating at 1170 K. tures, however, leads to the formation of M and H Monoclinic N-Nb 2Os can be prepared by vapor forms of Nb 205 . However, Goldschmidt [53] con- transport and also by the thermal decomposition cluded from his x-ray data that T-M3 conversion of Nb0 2 F [48, 60-62]. It has a needle-like habit - is time -dependent and that the T form transforms and F ion appears to stabilize the structure. Trans- slowly to H modification even at room temperature. formation to H-Nb 2 05 takes place slowly on heating

The detailed crystal structure of the T-Nb 20 5 is to 1170 to 1270 K [44]. not known at present but it appears that TT and T P-Nb 205 has been obtained by Schafer et al. [48] modifications are closely related since TT—>T process and Lavers et al. [56] by vapor transport in the range is continuous and TT may be a less ordered form of 1020-1120 K. Water vapor and chloride ions seem - T-Nb 20 5 and it is likely that substitution of F or to have stabilizing effect on this phase [44]. This - 2- (OH) for O is necessary for their stability [42]. tetragonal modification has an idealized V 2 0 5 struc-

On the other hand, these two phases may well be ture and transforms to H-Nb 20 5 on heating at 1120 members of a family with complex compositions K; the process is endothermic and irreversible [44]. and large unit cells and not true polymorphs of Gruehn [63] has reported the formation of R-Nb 2Os

Nb 20 5 [54]. Schafer and co-workers noticed the by vapor transport. It has a monoclinic symmetry

'memory’ effect in T-Nb 2 Os [42]. Thus, the sample and is supposed to have a simple structure compared prepared by heating Nb0 2 in air at 770 to 870 K is to all other polymorphs. Thermochemical behavior converted to the B-form sharply at 1090 K whereas is not known at present. the T-Nb 20 5 obtained by reaction of Nb0 2 with Cl 2 The stable H-Nb 2C>5 is easy to obtain and all (9-10 h, 540 to 570 K and further heating at 770 K other forms of the niobium pentoxide transform to for 15 min in air) exhibited a slow T—»B transition this polymorph on heating in air to ~1370 K. The and not to completion even after heating for 165 h same result is obtained on heating the amorphous at 1120 K. On the other hand, T-Nb 20 5 obtained by forms of Nb 205 or the oxidation of Nb, Nb alloys wet-precipitation-and-ignition methods transformed or Nb0 2 or slow cooling of the molten Nb 2C>5 [64] to a mixture of M and H forms after heating for 16 h or under hydrothermal conditions (170 atm, 1350

93 T

K) [47]. The detailed crystal structure of this mono- Schafer and co-workers [44] reported the formation clinic modification has been worked out by Gate- of several modifications of Nb 205 under nonequilib- house and adsley ^ [39, 42]. rium conditions from the NbO x phases (Ox i tc ,

Several metastable phases of Nb 2 0 5 have been Oxvi); the powder x-ray data are almost identical reported in the literature whose identity is not very to the initial NbO^ phases from which they are ob- well established. Reisman and Holtzberg [64] obtained but differ from one another and from all " a served high-temperature e form which crystallizes other polymorphs of Nb 20 5 . Detailed structural in- without supercooling from the molten NboOs at 1708 formation is lacking at present and it is not known K. The e form transforms on cooling irreversibly whether they can be classified as the true polymorphs

(without evolution of much heat) to the H-form at of Nb 2 0 5 . Ox i to Ox vi change to H-Nb 20 5 above 1370 K. 1670 to 1470 K; e-Nb 20 5 melts at 1708 K whereas

Electrical properties of Nb 2 Os have been examined the equilibrium melting point of H-Nb 2 05 is 1764 K. h >' Greener and co-workers [66, 67], The stoichio X-ray and DTA studies by Shafer and Roy [47] metric Nb2° 5 is a semiconductor and the defect indicated the existence of a metastable H' form of

. structure is interpreted to be due to the oxyger tvt, ^ , , tt tvti /-x i r . JSboOs obtained on heating the H-JNboOs above 1560 . . . , , . vacancies; highly reduced material appears to bti,, T _ T _ T / . i , . . ® —>11 is reversible transi- •, , . J K; H an endothermic and ^ t-.. , , degenerate. Dielectric properties have been reported tion. Recently, Jonejan and Wilkins [65] reported a by a few workers in the i iterature [68> 69] . ; transition in taking high-temperature phase Nb 20 5 Systems -PbO Nb 2 0 5 [70], -B 2 O s [71], -Ln 2 0 : place at 1756 K; the true melting point of Nb 20 5 (Ln=Rare earth) [72], -Ge0 2 [73], -Ti0 2 [65], is hot- obtained 1780 K as evidenced by DTA and — P 2 0 5 [74], — V 20 5 [75], — Ta 2 0 5 [76, 77], — WO, stage microscopic data. [78] have been examined in the literature.

Niobium oxides

Oxide and description Data Remarks and inferences References of the study

Suboxides

Crystal structures. NbOx (Nb 6 0): forms at 570-620 K; These suboxides form only by oxi- [11, 12]. tetragonal; a = 3.387 A; c = 3.27 dation of metal or by ageing of A. NbOy (NluO): forms at 620- Nb that is supersaturated with 670 K; orthorhombic or tetra- oxygen in definite ranges of gonal; details not known. NbOx temperature and oxygen pres-

(Nb 20): forms at 670-970 K; sures. They are metallic in tetragonal; a =6.645 A; c = nature but the detailed structure 4.805 A. and properties are not known. NbO

Crystal structure and Narrow homogeneity range; cubic; No crystal structure transforma- [15-19, 79, physical properties. space group, Fm3m; Z = 4; a = tions are known in this material. 80]. 4.2101 A. Contains 25% cation The low value of k and the mag- and anion vacancies. TEC (°/K): nitude of 70 indicate that NbO is 4.8 Xl0-« at ~297 K; k (cm 2 / metallic in nature. Banus and 13 dyn): 4.2 ±0.06 X10~ ; Cv (erg/ Reed [18] suggest that NbO can i 1 7 mol, deg at 297 K): 41.06X10 ; be considered as a simple cubic 1 \ 7G = 1.26. Melting point = 2218 oxide (with Z = 3 and Nb and O K. Pressure and temperature do in four-fold coordination and not not produce any change in the octahedral type) with no vacan- concentration of vacancies indi- cies in the structure which would eating that these are either explain the results of pressure greatly ordered or form an in- experiments. herent part of the structure and are not true lattice defects in NbO.

94 t

Niobium oxides—Continued

Oxide and description Data Remarks and inferences References of the study

Electrical and optical Metallic behavior in the range Detailed band picture and vacancy [14-16, 20- properties. 2-900 K with low resistivity; ordering effects not known. 22, 81]. p (77 K)~10-« ficm; p (300 K)~ Available superconductivity data -8 10 ficm. Seebeck coefficient and are qualitative and need rein- Hall data not available. Exhibits vestigation. superconductivity below 1.5 K. Plasma edge from reflectance data = 4.3 eV; IR frequency = -1 1080 ±10 cm .

Nb02

Crystal structure, x-ray Rutile structure; space group, The first order nature of the [15, 24-29]. and DTA studies. I4i/a; Z = 32; a = 13.690 ±0.001 transition is established. Super- A; c = 5.9871 ±0.0003 A. Super- structure lines of the a sublattice structure and metal-metal bond- of rutile structure and metal-

ing below T t . T t »1070 K; metal bonding disappears above

thermal hysteresis ~10°; AH = T t .

600 cal/mol. The T t obtained by x-ray data is higher (~50°) than the DTA value. Melting point = 2190 K.

Magnetic and electrical Paramagnetic in the range 100- Paramagnetic semiconducting [21, 27, 28, -8 properties. 1300 K; xm s»20X10 emu/mol; characteristics are confirmed below 30, 31, 37].

XM exhibits jump at T t . T and above TtNb0 2 appears Semiconductor behavior below 7\; to be metallic. There is also

5 P'-’-'lO ficm (300 K; polycrystal- discrepancy in the absolute line material); p~14 12cm (300 K, magnitude of p for polycrystalline single crystal). Ea «0.26 eV (both and single crystal samples of

samples). Drop in p by a factor of Nb0 2 .

10 at T f. Temperature independent p observed by Janninck and Whitmore [31] above Tt while Sakata [27] noted semi-

conductor behavior with Ea ~ 0.15 eV in the range 1100- 1270 K.

-1 Infrared studies. Bands at 715 and 1080 cm are — [81]. noted and are correlated with the metal-oxygen vibrations.

Homologous series, Nbsn+lOgn-l

Crystal structure. All phases are monoclinic; n =5; These homologous series have [11]. = 21.2 6 = Nb > 3 o A; 3.82 structures to the 0 2 7 ; A; somewhat c = 12.0 A; 0 = 132.2°. n=6; high -temperature structure of

Nb0 2 ,«; o=20.73 A; 6=3.835 A; Nb 2Os even though n =9 actually c- 15.67 A; 0 - 112.93°. n= 7; corresponds to this oxide.

Nb0 8 ,4«; 0=21.19 A; 6-3.822 A; c- 15.75 A; 0-124.51°. n -8; NhOj.u; o -21.20 A; 6-3.824 A; c -29.97 A; 0-95.07°.

95 .

Niobium oxides—Continued

Oxide and description Remarks and inferences References of the study

NbOj^nCNbuOig)

Crystal structure. Appears to exist in orthorhombic [40]. and monoclinic forms. Ortho- rhombic form: space group. Amma; Z = 4; a =28.99 A; 6 = 3.825 A; c = 20.72 A. Physical properties not known.

NbO* .464 (NR47O116 )

Crystal structure. Monoclinic: a = 57.74 A; 6 = 3.823 [41]. A; c =21.18 A; 0 = 105.32°. The oxide has been prepared using

NbCla/NbOCl 3 at 1520 K and the

structure is probably a combina-

tion of Nb 220a4 and Nb 2aOe 2 of the homologous series. Physical properties are not known.

Nl^Os

Crystal structures and Eight modifications known. The polymorphic behavior of [39, 42,

transformations TT-Nb 2Oa: best prepared by the Nb 2 C>6 is complicated. Wadsley 44-49, 54-

oxidation of Nb0 2 in air at 590 K and Andersson [42] have been 57, 59-63]. for 24 h; pseudohexagonal; Z = able to explain the stability and 0.5; a =3.607 A; c = 3.925 A: structure of the stable poly- Transforms continuously to morphs using the principle of

T-Nb 2Oa on heating at 870-1073 crystallographic shear and even K [50]; annealing at 1670 K re- predicted the existence of

sults in H-Nb 206 [49]. T-Nb 206: hitherto unknown forms: How- best prepared by heating the ever, various metastable and amorphous oxide at 870-1073 K nonequilibrium phases need ex-

[46, 47] or oxidation of Nb0 2 at planation. Hydrothermal be- 670 K for 4 to 5 h. Two forms havior, various impurity effects

are reported. Orthorhombic: a = and 'memory’ of Nb 2Os warrant 6.19 A; 6 = 3.65 A; c = 3.94 A; careful and comprehensive Z = 12. Monoclinic: a = 7.13 A; investigation. 6 = 15.72 A; c = 10.75 A; 0 = 120.7°; Z = 12. Changes to

B-Nb 206; T t = 1090 K; exothermic and irreversible; heating to higher temperatures produces

H-Nb 206. TT and T modifications appear to be structurally related.

B-Nb 2Os: Best prepared by the

oxidation of Nb0 2 in air (24 h at 590 and at 1120 K) or by vapor transport. Monoclinic; space group, C2/c; Z = 4; a = 12.73 A; 6=4.88 A; c = 5.56 A; 0 = 105.1°; 3 density = 5.29 g/cm . Densest of all the modifications; depicted as the high-pressure form;

thermal studies indicate it to be

96 t

Niobium oxide8=Gontinued

tb© thermodynamically stable form at low temperatures, DTA studies show endothermic B—»H transition at 1230 K; however, isothermal experiments in

NbOCb and Cl a atmospheres . indicate the transition to be reversible and lower T< with ^//~2 keal/mel.

M-NbiOi: Pure phase is difficult to obtain and partial transformation to H-form always occurs; prepared by heating oxide gels or amorphous form to 1170= 1220 K or by vapor transport of NbgOg with NbOGls or NbQBr® (7\ = 1120=1170 K and 7V>7\ = 100 K). Tetragonal; space group, 14/mmm; a =20,01 A; c = 3.84 A.

Transforms to H-Nb aOs on heating at 1170 K; no thermal effects noticeable, N-NbaQs: Best prepared by heating NbQaF at 1270 K in - vacuum for some hours; F ion appears to stabilize the structure. Monoclinic; space group, C2/m; a = 28.51 A; 6 = 3.83 A; c = 17.48

A; /3 = 120.8°. Structures of M and N forms appear to be basically related [42]. N-NbaOe transforms to H-NbaCb slowly on heating at 1170-1270 K. P-NbgOs: Prepared by vapor

transport (Cl a ; Ti = 1020 K; 7V-7W00 K). Cl= ions and water vapor appear to have stabilizing effect. Tetragonal: space group, 14i/22; Z= 4; a = 3.896 A; 6=23.43 A. P=>H & 1120 K; endothermic and irreversible. B-NbiOiS Prepared by vapor transport but usually associated with other polymorphs, MonoeUnie; space group, G2/m; a = 12.79 A; 6 = 3,826 A; e = 3.983 A;| = § 90,75 . Thermal behavior not known. H-NbgOii Easy to obtain; any other form heated in air at 1370 K produces this form; also under hydrothermal conditions. Mono- clinic; space group, P2; Z-14; a-21.16 A; 6-3.822 A; c-19.35

97 Niobium oxides—Continued

Oxide and description Data Remarks and inferences References of the study

A; /3 = 119.83°. Most stable form; structure based on the crystallographic shear has been worked out by Gatehouse and Wadsley

[39, 42]. The melting points of Nb 206 reported are inconclusive and the values quoted are 1758, 1764, and 1780 K [46, 65, 78].

Electrical properties. Semiconductor;

^1000 K; E0 “1.65 eV. Reduced NbjOj tends to behave as a degenerate semiconductor. Defect

structure is interpreted to be due to anion vacancies.

IR studies. Bands are noted at 790 and 1010 [81]. cm -1 for H-NbjOj and are interpreted in terms of metal-oxygen vibrations.

20 Meissner, W., Franz, H., and Westerhoff, H., Ann References [ ] Phys. (Leipzig) 17, 593 (1933).

[ 21 ] Roberson, J. A., and Rapp, R. A., J. Phys. Chem. Solids [1] Brauer, G., Naturwiss. 28, 30 (1940); Z. Elektrochem. 30, 1119 (1969).

46, 397 (1940); Z. anorg. allgem. Chem. 248, 1 (1941). [ 22 ] Chandrashekar, G. V., Moyo, J., and Honig, J. M., 2] Elliot, R. P., Trans. ASM. 52, 990 (1960). J. Solid State Chem. 2, 528 (1970). 3] Brauer, G., Mueller, H., and Kuehner, G., J. Less- [23] Gel’d, P. V., Shveikin, G. P., Alyamovskii, S. I., and Comm. Metals 4, 533 (1962). Tskhai, V. A., Russ. J. Inorg. Chem. 12, 1053 (1967). " 4 Norman, N., J. Less-Comm. Metals 4, 52 (1962). [24] Marinder, B.-O., Arkiv Kemi 19, 435 (1962). 5 Blackburn, P. E., J. Electrochem. Soc. 109, 1142 (1962). [25] Rogers, D. B., Shannon, R. D., Sleight, A. W., and 6 Kofstad, P., and Espevik, S., J. Electrochem. Soc. 112, Gillson, J. L., Inorg. Chem. 8, 841 (1969). 153 (1965). [26] Sakata, T., Sakata, K., and Nishida, I., Phys. Stat. [7] Taylor, A., and Doyle, N. J., J. Less-Comm. Metals 13, Solidi 20, K155 (1967). 313 (1967). [27] Sakata, K., J. Phys. Soc. Japan 26, 582 (1969); 26, 867 [8] Fromm, E., and Jehn, H., J. Less-Comm. Metals 15, 242 (1969); 26, 1067 (1969). (1968) [28] Rao, C. N. R., Rama Rao, G., and Subba Rao, G. V., [9] Schafer, H., Bergner, D., and Gruehen, R., Z. anorg. J. Solid State Chem. 6, 340 (1973). allgem. Chem. 365, 31 (1969). [29] Rao, C. N. R., Natarajan, M., Subba Rao, G. V., and 10 Terao, N., Japan J. Appl. Phys. 2, 156 (1963). Loehman, R. E., J. Phys. Chem. Solids 32, 1147 11 Niebuhr, J., J. Less Comm. Metals 11, 191 (1966). (1971). 12 Chang, L. L. Y., and Phillips, B., J. Am. Ceram. Soc. [30] Sakata, K., Nishida, I., Matsushima, M., and Sakgta, T., 52, 527 (1969). J. Phys. Soc. Japan 27, 506 (1969). [13] Nakayama, T., Osaka, T., and Kitada, A., J. Less- [31] Janninck, R. F., and Whitmore, D. H., J. Phys. Chem. Comm. Metals 19, 291 (1969). Solids 27, 1183 (1966). [14] Pollard, E. R., Ph.D. Thesis, MIT, USA (1968). [32] Adler, D., Solid State Phys. 21, 1 (1968). [15] Subbarao, G. V., Ph.D. Thesis, Indian Institute of Tech- [33] Rao, C. N. R., and Subba Rao, G. V., Phys. Stat. nology, Kanpur, India, 1969. Solidi (a), 1, 597 (1970). [16] Rao, C. N. R., Wahnsiedler, W. E., and Honig, J. M., [34] Marinder, B.-O., Dorm, E., and Seleborg, M., Acta J. Solid State Chem. 2, 315 (1970). Chem. Scand. 16, 293 (1962). [17] Taylor, A., and Doyle, N. J., J. Appl. Cryst. 4, 103 [35] Riidorff, W., and Luginsland, H. H., Z. anorg. allgem. (1971). Chem. 334, 125 (1964). [18] Banus, M. D., and Reed, T. B., in The Chemistry of [36] Kristensen, I. K., J. Appl. Phys. 40, 4992 (1969). : Extended Defects in Non-metallic Solids, Eds. L. [37 Rao, C. N. R., unpublished results (1971). Eyring and M. O’Keeffe (North Holland Publ. Co., [38] Norin, R., and Magneli, A., Naturwiss. 47, 354 (1960). Amsterdam, 1970), p. 488. [39] Gatehouse, B. M., and WadsleV, A. D., Acta Cryst. 17, [19] Taylor, A., and Doyle, N. J., in The Chemistry of 1545 (1964). Extended Defects in Non-metallic Solids, Eds. L. [40] Norin, R., Acta Chem. Scand. 17, 1391 (1963). Eyring and M. O’Keeffe (North Holland Publ. Co., [41] Gruehn, R., and Norin, R., Z. anorg. allgem. Che Amsterdam, 1970), p. 523. 367, 209 (1967).

98 Wadsley, A. D., and Andersson, S., in Perspectives in [42] II. 4. Molybdenum Oxides Structural Chemistry, Vol. Ill, Eds. J. D. Dunitz and A. Ibers (John Wiley & Sons, New York, 1970), J. Phase studies on the Mo-0 system have been pp. 1-58. [43] Blumenthal, R. N., Moser, J. B., and Whitmore, D. H., carried out by various workers [1-6]. In addition to J. Am. Ceram. Soc. 48, 617 (1965). the well-known dioxide and trioxide, many mixed [44] Schafer, H., Gruehn, R., and Schulte, F., Angew. Chem. Intemat. Edit. 5, 40 (1966). valence phases (between Mo02 and M0O3) have been [45] Frevel, L. K., and Rinn, H. W., Anal. Chem. 27, 1329 reported [3-7]; of these, (1955). Mo 4On and Mo9026 seem

i Holtzberg, F., Reisman, A., Berry, M., and Berkenblit, [46] to be stable [1, 5, 6] whereas M 017O 47 , M05O 14 and M., J. Am. Chem. Soc. 79, 2039 (1957). Mo802 3 seem to be metastable and can be stabilized [47] Shafer, M. W., and Roy, R., Z. Krist. 110, 241 (1958). [48] Schafer, H., Schulte, F., and Gruehn, R., Angew. Chem. by incorporation of other elements like Ti, V, and Internat. Edit. 3, 511 (1964). W [8]. These mixed valence oxides can not be con- [49] Sarjeant, P. T., and Roy, R., J. Am. Ceram. Soc. 50, 500 (1967). sidered to be truly nonstoichiometric, since they Brendel, C., quoted in ref. [44]. [50] have narrow ranges of H., Benesovsky, F., Rudy, E., and Wittmann, homogeneity; some of them ! [51] Nowotny, A., Mh. Chem. 91, 975 (1960). have tunnel and layer structures. No oxide of molyb- [52] Mertin, W., and Jagusch, W., quoted in ref. [44]. [ denum in the range Mo-Mo02 is known and the [53] Goldschmidt, H. J., J. Inst. Metals 87, 235 (1958/59). L., and Andersson, S., Acta Chem. Scand. [54] Jahnberg, reported existence of Mo30 [2] is in doubt [1, 5, 6, 9]. 21, 615 (1967). [55] Laves, F., Moser, R., and Petter, W., Naturwiss. 51, M0O2 : Molybdenum dioxide, Mo02 , has a mono- 356 (1964). [56] Laves, F., Petter, W., and Wulf, H., Naturwiss. 51, 633 clinic structure [1, 7, 10-12] and exhibits metallic (1964). behavior in the range 4 to 300 K [12, 13]. No phase [57] Terao, N., Japan J. Appl. Phys. 4, 8 (1965). [58] Schulte, F., Gruehn, K., and Gorbing, M., quoted in transitions are known in this material. Other proper- ref. [44]. ties are not known. [59] Mertin, W., Andersson, S., and Gruehn, R., J. Solid State Chem. 1, 419 (1970). Mo02 forms solid solutions with Ti02 [14], Nb02 [60] Andersson, S., and Astrom, A., Acta Chem. Scand. 19, [14] and V02 [15-17] which have rutile structures. 2136 (1965). Lundberg, M., and Andersson, S., Acta Chem. Scand. [61] Mixed Valence Phases: has a monoclinic 21, 615 (1967). M04O 11 [62] Andersson, S., Z. anorg. allgem. Chem. 351, 106 (1967). structure (stable below 890 K) and assumes an Gruehn, R., Less-Comm. Metals 119 (1966). [63] J. 11, orthorhombic symmetry when synthesized in the [64] Reisman, A., and Holtzberg, F., J. Am. Chem. Soc. 81, 3182 (1959). range 890 to 950 K; both the forms are closely re- [65] Jonejan, A., and Wilkins, A. L., J. Less-Comm. Metals lated [3, 7, 18] in structure and T t appears to be 1 19, 185 (1969). ^900 K. 04O 11 appears to be metallic with no [66] Greener, E., Whitmore, D., and Fine, M., J. Chem. M [3] ' Phys. 34, 1017 (1961). significant change in p at T t . M 017O 47 has an or- i [67] Greener, E. H., and Hirthe, W. M., J. Electrochem. Soc. thorhombic symmetry [3, 18] whereas M06O 14 is 109, 600 (1962).

[68] Robinson, M. L. A., and Roetschi, H., J. Phys. Chem. tetragonal [8]; Mo802 3 is monoclinic [3, 7, 18-20], Solids 29, 1503 (1968). 1 M 09O26 is dimorphic and can assume a triclinic or [69] Emmenegger, F. P., and Robinson, M. L. A., J. Phys. Chem. Solids monoclinic structure depending on the method of , 29, 1673 (1968). [70] Roth, R. S., J. Res. Nat. Bur. Stand. (U.S.), 62, 27 preparation [5, 9, 18, 19]. Phillips and Chang [5]

I (1959). report a phase transformation in Mo902 e at 1040 K; [71] Levin, E. M., J. Res. Nat. Bur. Stand. (U.S.), 70A i (Phys. and Chem.), No. 1, 11-16 (Jan.-Feb. 1966). the low-temperature form is monoclinic, but the Godina, N. A., Savchenko, E. P., and Keler, E. K., [72] structure of the high-temperature form is not ex- Russ. J. Inorg. Chem. 14, 1162 (1969).

' actly exhibits low resistivity and [73] Levin, E. M., J. Res. Nat. Bur. Stand. (U.S.) 70A (Phys. known. M 017O 47 and Chem.), No. 1, 5-10 (Jan.-Feb. 1966). appears to be metallic [3]. M 04O 11 and Mo902 6 melt 1 [74] Levin, E. M., and Roth, R. S., J. Solid State Chem. 2, at high temperature and decompose to 250 (1970). (>1000 K) [75] Waring, J. L., and Roth, R. S., J. Res. Nat. Bur. Stand. lower oxides [5]. Detailed physical properties are (U.S.), 69A (Phys. and Chem.), No. 119-129 (Mar.- 2, not known for these mixed valence phases. Apr. 1965). [76] Holtzberg, F., and Reisman, A., J. Phys. Chem. 65, : trioxide, 0 has an or- 1192 M0O3 Molybdenum M O3 , , (1961). [77] Mohanty, G. P., Fiegel, L. P., and Healy, J. H., J. Phys. thorhombic symmetry and consists of a layered i Chem. 68, 208 (1964). (or chain) structure [1, 5, 7, 21]. Detailed studies [5] [78] Roth, R. S., and Waring, J. L., J. Res. Nat. Bur. Stand. (U.S.), 70A (Phys. and Chem.), No. 4, 281-303 (July- indicate no phase transitions in this material up to Aug. 1966). the melting point (^1055 K). MoOs is a semicon- [79] Taylor, A., and Doyle, W. J., J. Appl. Cryst. 4, 109 ductor 22 ]. . (1971). [

[80] Gel’d, P. V., and Kusenko, F., Izv. Akad. Nauk SSSR, Molybdenum bronzes are formed from M 0O3 and Otdel. Tekhn. Nauk Met. i Toplivo 2, 79 (1960). 1 other metallic oxides; these interesting materials [81] Alyamovskii, S. I., Shveikin, G. P., and Gel’d, P. V., Russ. J. Inorg. Chem. 12, 915 (1967). have been discussed in detail in the literature [23]

99 Molybdenum oxides

Oxide and description Remarks and inferences References of the study

Mo0 2

Crystal structure and Monoclinic; space group, P2i/c; The structure is closely related to [1, 7, 10- electrical properties. Z = 4; a = 5.6109 ±0.0008 A; 6 = that of rutile; the metallic 13]. 4.8562 ±0.0006 A; c = 5.6285 ± properties have been interpreted 0.0007 A; 0 = 120.95°. No phase in terms of a modified Good- transitions are known. Metallic enough’s model. behavior; p (4.2 K)»5.4X10~7 (2cm; p (300 K) *8.8X10" 8 12cm.

Mixed Valence Phases

M04O11

Crystal structure and Monoclinic (<900 K); a =24.54 A; Both the modifications are closely [3, 7, 18]. electrical properties. 6 = 5.439 A; c = 6.701 A; 0 = related structurally and are

94.28°. Orthorhombic (>900 K); built up of M0O 4 tetrahedra and

a =24.49 A; 6 = 5.459 A; c = distorted MoOe octahedra; the

6.752 A. T t » 900 K. Monoclinic difference lies in the relative and orthorhombic forms can be orientation of the substructures obtained at ordinary tempera- in nieghboring slabs, being tures depending on the method of parallel in the monoclinic and preparation. Both forms show alternating in the orthorhombic metallic behavior; p (300 K)~0.2 form. Detailed data on the 12cm; p does not show much physical properties are lacking.

change at T <.

M017O47

Crystal structure and Orthorhombic; space group, Pba2; This has a tunnel structure which [3, 18]. electrical properties. Z-2; 0-21.61 A; 6-19.63 A; may be considered as built up

c- 3.951 A. Metallic behavior; from MoO« octahedra and M0O7 p (300 K)~5X10“* (2cm. Detailed pentagonal bipyramids. Metal- data are lacking. metal bonding is noted and probably accounts for the low measured resistivity. MosOu

Crystal structure. Tetragonal; a = 11.50 A; c = 3.937 The structure is related to that of [ 8 ].

A. Metastable form and de- M017O 47 . Detailed data are composes on prolonged heat lacking. treatment; Ti, V, and W seem to stabilize the phase.

Mo»02j

Crystal structure. Monoclinic; a = 16.88 A; 6 > = 4.052 The structure is that of distorted [3, 7, 18-

A; c = 13.39 A; 0 = 106.19°. ReOs-type in which recurrent 20 ]. Properties not known. dislocations of atoms occur along the shear planes. MogOw (M018O62)

Crystal structure and Dimorphic: Triclinic phase; a — The triclinic phase has a basic [5, 9, 18,

x-ray studies. 8.145 A; 6 = 11.89 A; c = 21.23 A; M0O 3 structure. According to 19].

a = 102.67°; 0=67.82°; 7 = Kihlborg [24], MoigOs 2 is the

100 Molybdenum oxides—Continued

Oxide and description Data Remarks and inferences References of the study

109.97°. = Monoclinic phase; a first member (m = 1 ) of a

16.74 A; b =4.019 A; c = 14.53 A; homologous series, Mon03n -m+i*

0 =95.45°. Both forms can be The existence of oxides M 012O 33

prepared at room temperature. and M 026O76 is suggested. 5 Monoclinic phase shows a transition at 1040 K; the structure of

the high -temperature phase is not known in detail. Physical properties are not known.

M0O3

Crystal structure and Orthorhombic; space group, Pbnm; It has a chain structure. The break [1, 5, 7, 21, properties. Z = 4; a =3.966 A; 6 = 13.88 A; in lnp — 1/T plot at 650 K may 22]. c = 3.703 A. No phase transforma- correspond to a change in the tions exist up to the melting mechanism of conduction. point (1055 K). Semiconductor; 10 p (450 K)~10 Stem; p (900 K) ~10 Stem; lnp — 1/T plot shows a

break at ~650 K; Eo (<650 K)

«0.56 eV; Ea (>650 K)«1.83 eV.

References [12] Rogers, D. B., Shannon, R. D., Sleight, A. W., and Gillson, J. L., Inorg. Chem. 8, 841 (1969). [13] Perloff, D. S., and Wold, A., Crystal Growth, Proc. Int. Conf. on Cryst. Growth, Ed. H. S. Peiser (Pergamon, [1] Hagg, G., and Magneli, A., Arkiv Kemi, Mineral Geol. London, 1967). 19A, 1 (1945). [14] Marinder, B.-O., Dorm, E., and Seleborg, M., Acta [2] Schonberg, N., Acta Chem. Scand. 8, 221 (1954); 8, 617 Chem. Scand. 16, 293 (1962). (1954). [15] Marinder, B.-O., and Magneli, A., Acta Chem. Scand. [3] Kihlborg, L., Acta Chem. Scand. 13, 954 (1959); Adv. 11, 1635 (1957); 12, 1345 (1958). Chem. Ser. 39, 3745 (1963). [16] Israelson, M., and Kihlborg, L., Mat. Res. Bull. 5, 19 [4] Rode, E. Ya., and Lysanova, G. V., Dokl. Akad. Nauk (1970). SSSR 145, 351 (1962). [17] Rao, C. N. R., Natarajan, M., Subba Rao, G. V., and [5] Phillips, B., and Chang, L. L. Y., Trans. AIME 233, Loehman, R. E., J. Phys. Chem. Solids 32, 1147 1433 (1965). (1971). [6] Chang, L. L. Y., and Phillips, B., J. Am. Ceram. Soc. [18] Kihlborg, L., Arkiv Kemi 21, 471 (1963). 52, 527 (1969). [19] Magneli, A., Acta Chem. Scand. 2, 501 (1948). [7] Magneli, A., Andersson, G., Holmberg, B., and Kihlborg, [20] Kihlborg, L., Arkiv Kemi 21, 461 (1963). L., Anal. Chem. 24, 1998 (1952). [21] Kihlborg, L., Arkiv Kemi 21, 357 (1963). [8] Kihlborg, L., Acta Chem. Scand. 23, 1834 (1969). [22] Deb, S. K., and Chopoorian, J. A., J. Appl. Phys. 37, [9] Kihlborg, L., Acta Chem. Scand. 16, 2458 (1962). 4818 (1966). [10] Magneli, A., Arkiv Kemi, Mineral Geol. 24A, 11 (1946). [23] Rao, C. N. R., and Subba Rao, G. V., Phys. Stat. Solidi [11] Brandt, B. G., and Skapski, A. C., Acta Chem. Scand. (a) 1, 597 (1970). 21, 661 (1967). [24] Kihlborg, L., Arkiv Kemi 21, 443 (1963).

101 II. 5. Technitium Oxides ent monoclinic cell was suggested by Zachariasen

[6]; it has been pointed out [3] that Tc02 may exhibit

polymorphism like Re02 .

Studies by various workers [1-3] have shown that Tc 20 7 is orthorhombic and the structure consists in the system Tc-O, Tc02 is the only stable oxide at of an arrangement of isolated Tc207 molecules with high temperatures. A covalent heptoxide, Tc207 , is tetrahedral coordination of Tc atoms; the structure

also known. Tc0 2 has a monoclinic, Mo02 -type struc- of Tc207 is more closely related to Cr0 3 , Ru0 4 and ture [1, 3] and according to Marinder and Magneli 0s0 4 than to Re207 . Physical properties are not

[5], metal-metal bonds exist in this material. A differ- known at present.

Technitium oxides

Oxide and description Data Remarks and inferences References of the study

Tc0 2

Crystal structure. Monoclinic; space group, P2 i/C; Rutile based structure with metal- [1, 3, 5]. Z = 4; a = 5.55±0.01 A; 6 = metal bonds. 4.85 ± 0.01 A; c = 5.62 ± 0.01 A;

/3 = 121.9±0.1°.

TC 2O 7

Crystal structure. Orthorhombic; space group,- Pbca; The structure consists of isolated [4].

Z = 4; a = 13.756 A; 6 = 7.439 A; TC 2O 7 molecules. c = 5.617 A.

D., Sleight, W., and References [3] Rogers, D. B., Shannon, R. A. Gillson, J. L., Inorg. Chem. 8 , 841 (1969).

[4] Krebs, B., Angew. Chem. Int. Ed. (Engl.) 8 , 381 (1969); Z. anorg. allgem. Chem. 380, 146 (1971). [1] Muller, O., White, W. B., and Roy, R., J. Inorg. Nucl. [5] Marinder, B.-O., and Magneli, A., Acta Chem. Scand. Chem. 26, 2075 (1964). 11, 1635 (1957). [2] Rulfs, C. L., Pacer, R. A., and Hirsch, R. F., Inorg. 6 Zachariasen, W. H., ACA Program and Abstr. of Winter J. [ ] Nucl. Chem. 29, 681 (1967). Meeting, F-4 (1951). K

II. 6. Ruthenium Oxides sistivity is very low and exhibits metallic behavior

[3, 5-7, 11-14].

Phase relations in the Ru-0 system have been Fletcher and co-workers [13] have shown that the reported in the literature [1,2]. In the temperature oxidation state of Ru in Ru02 is more than 4. The range 1070 to 1770 K, Ru02 is the only stable oxide de Hass-van Alphen effect [15], magnetoresistance

(in air); Ru03 and Ru0 4 exist as vapor species. [16] and Azbel-Kaner cyclotron resonance [17] stud-

Attempts to prepare the stable nonstoichiometric ies on Ru02 suggest that the simple band picture oxide, Ru02 ± x , have not been successful [2]. Ru02 proposed by Rogers and co-workers [7] is inadequate dissociates in air at ^1680 K to the metal. to explain the observed properties. Ryden et al. [12] Several workers have studied the crystal growth have found that electron-phonon and electron -elec- of Ru02 by chemical transport [3-5] and other tron interband scattering mechanism accounts methods [6, 7]. Ru0 2 has a tetragonal rutile structure quantitatively for the observed resistivity behavior

[2, 3, 5-9] at room temperature and no phase transi- of Ru02 in the range 10 to 1000 K. tions are known in the temperature range 4.2-1270 K McDaniel and Schneider [2] report that Ru02 and

[5, 8, 10-12]. It has a low and almost temperature Ir02 can form complete solid solutions of rutile independent magnetic susceptibility [6, 13]; the re- structure in the entire composition range.

Ruthenium oxides

Oxide and description Data Remarks and inferences References of the study

Ru0 2

Crystal structure and TEC. T = 298 K: Tetragonal; space The c lattice parameter decreases [5, 10].

group, P4 2 /mnm; Z = 2; a = with rise in T; slight anisotropy 4.4910 ±0.0003 A; c = 3.1064± is evident. 0.0002 A. TEC (XlO -6/°C:

c = ||- || —1.4; = +7.0.

-4 Magnetic and electrical Xm=2.03X10 cgs units at 298 K. The metallic behavior of Ru0 2 is [3, 6, 7, 11, properties. p (4.2 K)~l .0X10-8 ftcm; p (100 evident; the Lorentz number is 12, 18]. -6 K)~1X10 ftcm; p (298 K)~[9] close to the theoretical value for 4.0X10-5 ftcm. At 77 K, Rh = typical metals. —0.79, mh=61 cm 2 /V s; at 300 K, Rn = —1.10, Mh=3.1 2 cm /V s. 0d (calc.) =900±5 K.

l/x = d/T+aT" (for T <1.5 Tm ) 2 where /3 = 18.1 cm K /W, a = 8.42X10'4 cm K^/W, n =1.66 and Tn is the temperature at which K is a maximum. Lorentz _8 2 ratio =2.6 X10 W ft/ .

References Boman, C. E., Acta Chem. Scand. 24, 116 (1970). [10] Hazony, Y., and Perkins, H. K., J. Appl. Phys. 41, 5130 [1] Bell, W. E., and Tagami, M., J. Phys. Chem. 67, 2432 (1970). (1963). [11] Osburn, C. M., and Vest, R. W., Bull. Am. Ceram. Soc. [2] McDaniel, C. L., and Schneider, S. J., J. Res. Nat. Bur. 47, 354 (1968). Stand. (U.S.), 73A (Phys. and Chem.), No. 2, 213-219 [12] Ryden, W. D., Lawson, A. W., and Sartain, C. C., (Mar.-Apr. 1969). Phys. Rev. B 1, 1494 (1970). [3] Schafer, H., Schneidereit, G., and Gerhardt, W., Z. anorg. [13] Fletcher, J. M., Gardner, W. E., Greenfield, B. F., allgem. Chem. 319, 327 (1963). Holdoway, M. J., and Rand, M. H., J. Chem. Soc. A [4] Schafer, H., Chemical Transport Reactions (Academic 653 (1968). Press, New York, 1964), p. 48. [14] Ryden, W. D., Lawson, A. W., and Sartain, C. C., [5] Butler, S. R., and Gillson, L., Mat. Res. Bull. 81 J. 6, Phys. Letters 26A, 209 (1968). U97D. [15] Marcus, S. M., and Butler, S. R., Phys. Letters 26A, 518 [6] Cotton, F. A., and Mague, J. T., Inorg. Chem. 5, 317 (1968). (1.966). Marcus, S. M., Phys. Letters 28A, 191 (1968). [7] Rogers, D. B., Shannon, R. D., Sleight, A. W., and [16] N., Phys. Letters Gillson, J. L., Inorg. Chem. 8, 841 (1969). [17] Slivka, R. T., and Langenberg, D. [8] Krishna Rao, K. V., and Iyenger, L., Acta Cryst. 25A, 28A, 169 (1968). 302 (1969). [18] Millstein, J., J. Phys. Chem. Solids 31, 886 (1970).

103 II. 7. Rhodium Oxides temperature low-pressure form can be prepared

from Rh metal at 1270 K [1], Shannon and Prewitt

In the Rh-0 system, the only well-established [6] have prepared a high-temperature high-pressure oxides are Rh2 Os and RI1O2 , even though there are form of Rh2Og (at 65 kbar pressure and 1470 K) numerous claims and counter claims regarding the which has an orthorhombic symmetry but closely existence of oxides such as Rh20 and RhO [1-5]. related to the corundum structure. The high-temper-

Rh2Os exhibits polymorphism; the low-temperature ature form of Wold et al., and the high-pressure low-pressure form belongs to the corundum struc- form of Shannon and Prewitt appear to be closely ture and the high-temperature low-pressure form has related [6]. The latter material appears to be semi- the orthorhombic perovskite structure [1, 5], Ac- conducting at room temperature [6]. cording to Wold and co-workers [1], the transition Rh02 has the rutile structure [2-4]; it has a low '"-'1020 temperature is K and is sluggish; T t also de- resistivity and exhibits metallic behavior [3, 4], Other pends on the starting materials used. The high- Physical properties are not known.

Rhodium oxides

Oxide and description Data Remarks and inferences References of the study

RI12O3

Crystal structures and Low-temperature low-pressure The low-temperature—^high-temper- [1, la, 6 , 7]. electrical properties. form: Hexagonal (Corundum ature transformation is sluggish structure); space group, R3C; and depends on the starting ma-

Z = 6 ; a =5.108 A; c = 13.810 A. terials employed; the high tem-

T t is ~1020 K. High tempera- perature form can be prepared ture form: Orthorhombic; space directly from Rh metal at 1270 K. group, Pbca; a = 5.1477 A; 6 = The orthorhombic structures are

5.4425 A; c = 14.6977 A. High- related to the corundum structure [ 6 ]. temperature high-pressure form: and may be described as contain- Orthorhombic; space group, ing layers of the corundum struc- Pbna; Z = 4; 0 = 5.1686 ±0.0003 ture cut parallel to (lOll) and A; b = 5.3814±0.0004 A; c = stacked together. 7.2486 ±0.0004 A. p (300 K)~

130 Ocm; Ea “0.16 eV.

RhOj

Crystal structure and Rutile structure; space group Rutile structure characteristic of [2-4]. electrical properties. P4 2 /mnm; Z = 2 ; a=4.4862± many transition metal dioxides. 0.0005 A; c = 3.0884 ±0.0005 A. Typical metallic behavior. -6 p (4.2 K)~2X10 ftcm; p (300 -4 K)~lXlO ficm. RI1 O 3 decomposes to RhOs at ~1120 K followed by decomposition to the metal and oxygen at ~1320 K.

D., Solid References [3] Shannon, R. State Commun. 6, 139 (1968). [4] Rogers, D. B., Shannon, R. D., Sleight, A. W., and [1] Wold, A., Arnott, R. J., and Croft, W. J., Inorg. Chem. Gillaon, J. L., Inorg. Chem. 8, 841 (1969). 2, 972 (1963). [5] Prewitt, C. T., Shannon, R. D., Rogera, D. B., and [la] Bieaterboa, J. W. M., and Horuatia, J., J. Le»>. Common Sleight, A. W., Inorg. Chem. 8, 1985 (1969). Metalt, 30, 121 (1973). [6] Shannon, R. D., and Prewitt, C. T., J. Solid State Chem. [2] Muller, O., and Roy, R., J. Leaa-Common Metala 16, 129 2, 134 (1970). [7] Coey, J. M. D., Aota Cryat. 26B, 1876 (1970).

104 1 1. 8. Palladium Oxide single crystals indicate that the material is a p-type semiconductor. No phase transformations are known in this material; it decomposes in air at ~1070 K

Palladium monoxide is the only stable oxide known [5]. in the Pd-0 system. Attempts to prepare PdsCX PdO forms solid solutions with CuO [6, 7], PbO have not been successful [1]. PdO has a tetragonal [8] and rare-earth sesquioxides [5] and the phase structure [2-4], Studies by Rogers et al. [4] on PdO relations have been described in detail.

Palladium oxide

Oxide and description Data Remarks and inferences References of the study

PdO

Crystal structure and Tetragonal; space group, P42/ The material is a p-type extrinsic [2-5]. properties. mmc; Z = 2; a = 3.0434 ±0.0002 semiconductor. A; c = 5.3363 ±0.0004 A. Semi- conductor (p-type); p (300 K)~

10-1000 ficm; Ea =0.04-0.10 eV. Decomposes to the metal and oxygen in air at ~1070 K; the

process of decomposition is reversible. No phase transitions are known.

References [5] McDaniel, C. L., and Schneider, S. J., J. Res. Nat. Bur. Stand. (U.S.), 72A (Phys. and Chem.), No. 1, 27-37 [1] Muller, 0., and Roy, R., J. Less-Comm. Metals 16, 129 (Jan.-Feb. 1968). (1968). [6] Schmahl, N. G., and Minzyl, E., Z. Phys. Chem. (N.F.) [2] Moore, W. J., Jr. and Pauling, L., J. Am. Chem. Soc. 63, 47, 142 (1965). 1392 (1941). Schmahl, N. G., and Eikerling, G. F., Z. Phys. Chem. [3] Wasor, J., Levy, H. A., and Peterson, S. W., Acta Cryst. [7] 6, 661 (1953). (N.F.) 62, 268 (1968). [4] Rogers, D. B., Shannon, R. D., and Gillson, J. L., J. [8] Shaplygin, I. S., Bromberg, A. V., and Sokol, V. A., solid State Chem. 3, 314 (1971). Russ. J. Inorg. Chem. (English Transl.) 15, 1195 (1970).

105 II. 9. Silver Oxides modification of Ag20 has been described by Kabal-

kina et al. [16]; this appears to have a layered Cdl2 structure possesing metal-metal bonds. These au- Even though the literature contains many refer- thors report that it may be a degenerate semiconduc- ences [1-8] where silver oxides have been studied, it tor or a semiconductor with a low energy gap. appears that only Ag2 0 and AgO (and probably

Ag 2 0 3 ) are the only well-characterized oxides of AgO: Silver monoxide, AgO, has a monoclinic struc- silver. Silver sesquioxide, Ag 2 0 3 , is prepared at low ture [3, 17, 18]; the oxygen content seems to vary temperature in aqueous solutions [8]; it appears to depending on the method of preparation [3], but have cubic structure [9] and decomposes to AgO at no other crystalline modifications of AgO are known. higher temperatures. The material is diamagnetic and has a very low room temperature resistivity (^TOficm), but p shows nega-

Ag 2 0: The stability range of Ag2 0 is limited [8, tive temperature coefficient in the range 230 to 290 K

10-13] and decomposes to the metal above ~500 K. [1, 18]; these properties are explained as due to the

Ag2 0 has a cubic structure [11] and exhibits semi- existance of monovalent and trivalent Ag ions in conducting behavior [14, 15], A new high pressure AgO and confirmed by x-ray structure analysis [17].

Silver oxides

Oxide and description Data Remarks and inferences References of the study

AgjO

Crystal structure and Cubic; space group, Pn3m; Z — 2; Ag 20 has the same structure as [11, 16].

x-ray studies. a *4.720 ±0.004 A. TEC~2X Cu 20 with considerable coval- 10“«/°C (not very accurate ency; the high-pressure modifica- value). Decomposes above <~500 tion has a layer -structure prob- K at atmospheric pressure. High- ably with metal-metal bonding. pressure form (prepared at 115- 125 kbar and 1670±200 K): Hexagonal; a =3.072 ±0.003 A; c = 4.941 ±0.004 A. The high

pressure phase is denser by ~30%.

Thermal properties Heat capacity data have been re- Anomalies have been noted in the [19-21]. (10-470 K). ported in the 10-500 K range. heat capacity curves around 30 DTA studies have been carried and 420 K. The low temperature out in the 300-470 K range. anomaly is difficult to understand in terms of particle size of surface area effects alone. The high temperature anomaly is probably associated with annealing of crystal defects or crystalization.

Electrical properties Semiconductor; p-type behavior a data lacking; the conduction [14, 15]. measured in oxygen -1/4 — with p «po* . p (500 K)~10* mechanism is interpreted as due atmospheres (300 and 450 bar) 10< Qcm; p (620 K)~10 l -10 a to positive holes excited from ficm; E„ *0.64 eV. traps (cation vacancies).

106 Silver oxides—Continued

Oxide and description Data Remarks and inferences References of the study

AgO

Crystal structure, magnetic Monoclinic; space group, P2i/c; AgO and CuO are isomorphous; the [1, 3, 17, and electrical properties. Z = 4; a=5.85±0.02 A; 6 = 3.47 ± unusual physical properties of 18]. 0.08 A; c = 5.49 ±0.05 A; 0 = AgO compared to CuO are ex- 107.5°. Structure analysis indi* plained as due to the coexistence cates that Ag+ and Ag8+ ions of mono- and trivalent Ag. p-T exist; two different Ag-0 dis- behavior indicates either near tances are encountered. Diamag- degeneracy or a semiconductor netic in the range 90-370 K; with a low energy gap. Detailed Xm = — 19.1X10-6 emu; Unusually studies on AgO are urgently low room temperature resistivity needed. (p~10 flem); p decreases with rise in T in the range 230-290 K.

References II. 10. Cadmium Oxides

f l] Neiding, A. B., and Kazarnovskii, I. A., Dokl. Akad. Nauk SSSR 78, 713 (1951). Jones, P., and Thirsk, H. R., Trans. Faraday Soc. [2] 50, Cadmium monoxide, CdO, is the well known oxide 732 (1954). [3] Graff, W. S., and Stadelmair, H. H., J. Electrochem. Soc. in the Cd-0 system [1-4]. Hoffman et al. [5] re- 105, 446 (1958). ported the preparation and structure of the peroxide [4] Ylach, J., and Stehlik, B., Collec. Czech. Chem. Commun.

25, 676 (1960). Cd02 , but detailed properties are not known. [5] Amlie, R. F., and Riietschi, P., J. Electrochem. Soc. 108, 813 (1961). [6] Karpov, A. A., Borisova, T. I., and Veselovskii, V. I., CdO: CdO has the cubic rock salt structure and is Zh. Fiz. Khim. 36, 1426 (1962). diamagnetic [6]. No phase transitions are known in [7] Allen, J. A., Proc. Australian Conf. Electrochem., Sydney, Hobart, Australia, 72, (1963) Publ. 1965. this oxide. The substance is usually nonstoichiome- [8] Nagy, G. D., Moroz, W. and Casey, E. Proc. J., J., Am. trie (Cd excess) with pronounced cation interstitials Power Sources Conf. 19, 80 (1965). [9] Stehlik, B., Weidenthaler, P., and Vlach, J., Chem. Listy or anion vacancies [2, 7-9], resulting in a free electron 52, 2230 (1958). 19 21 -3 concentration of ^10 -10 cm . Such high elec- [10] Garner, W. E., and Reeves, L. W., Trans. Faraday Soc. 50, 254 (1954). tron concentration induces degenerate behavior; at [11] Suzuki, T., J. Phys. Soc. Japan 2018 (1960). 15, high temperatures, CdO is either nondegenerate or [12] Weidenthaler, P., Collec. Czech. Chem. Commun . 28, 137 (1963). degenerate n-type semiconductor depending on the [13] Otto, E. M., J. Electrochem. Soc. 113, 643 (1966). history of the sample, i.e., sintering time and tem- [14] Cahan, B. D., Ockerman, J. B., Amlie, R. F., and Riietschi, P., J. Electrochem. Soc. 107, 725 (1960). perature, carrier concentration and oxygen pressure [15] Talukdar, M. I., and Baker, E. H., Solid State Commun. [1]. Donor impurities which contribute conduction 7, 309 (1969). [16] Kabalkina, S. S., Popova, S. V., Serebryanaya, N. R., electrons give rise to an impurity level a few tenths and Vereshchagin, L. F., Dokl. Akaa. Nauk SSSR of an electron volt below the conduction band [10]. 152, 853 (1963). [17] Scatturin, V., Bellon, P. L., and Zannetti, R., J. Inorg. Detailed resistivity, Seebeck coefficient and Hall Nucl. Chem. 8, 462 (1958). coefficient measurements [1, 2, 9-14] as well as opti- [18] Scatturin, V., Bellon, P. L., and Salkind, A. J., J. Electro- chem. Soc. 108, 819 (1961). cal [15, 16], NMR [17, 18] and Faraday rotation [19] Pitzer, K. S., and Smith, W. V., J. Am. Chem. Soc. 59, [19] studies indicate that depending on the carrier 2633 (1937). [20] Kobayashi, K., Sci. Rep. Tohoku Univ. 35, 173 (1951). concentration, the impurity band may overlap with [21] Pitzer, K. S., JR. Gerkin, E., Gregor, L. V., and Rao, the conduction band in CdO giving rise to quasi- C. N. R., Pure and Applied Chem. (J.IUPAC) 2, 211 (1961). metallic or metallic properties.

107 Cadmium oxides

Oxide and description Data Remarks and inferences References of the study

CdO

Crystal structure and Cubic; space group,2 Fm3m; Z = 4; CdO exhibits pronounced non- [3, 6, 7]. magnetic properties. a=4.6949 A. Slight variation of stoichiometry which is attributed a with the stoichiometry. Dia- to either Cd interstitials or oxy- -8 magnetic; x = —48.74 X 10 gen vacancies. emu/g.

Electrical properties. Degenerate semiconductor be- Nonstoichiometry gives rise to [1, 2, 9- -2 -3 havior; p (300 K)~10 -10 large number of charge carriers; 14]. flcm; n~10 18 -10 21 cm-3 depend- n can be controlled by sample ing on the sample history but treatment. The impurity level independent of temperature; belonging to the ionized donors a (300 K)~20 mV/°C; Mh (81 K) merges into the conduction band ^700-1100 cm /V s; m* = 0.15— giving rise to quasi -metallic 0.45 m. behavior in some samples. De- tailed conduction mechanism treated.

Optical properties. Band gap = 2.3 eV; a possible in- Free carrier absorption is noted in [15, 16, 19]. direct gap noted at ~1.2 eV; m* the optical spectra due to quasi- (optical) = 1.4 m. Plasma edge free electrons. The results are is ~0.2-0.5 eV depending on n. interpreted in terms of the band

eo = 18.1; Coo = 5.6. structure of CdO [20].

113 Cd NMR studies Results indicate that the nuclei [17, 18]. (1.4-300 K). interact strongly with the degenerate conduction electrons and the electrons are in the host lattice conduction band, r « il3 i; 113 n~ T~ k « n .

cao 2

Crystal structure Cubic; space group, Pa3; Z = 4; [5]. a = 5.313 ±0.003 A. Decomposes violently at ~450-470 K to CdO and oxygen. Other properties not known.

[11] Bastin, J. A., and Wright, R. W., Proc. Phys. Soc. [10] References (Lond.) 68A, 312 (1955). [12] Konak, C., Hoschl, P., Dillinger, J., and Prosser, V., Crystal Growth, Suppl. J. Phys. Chem. Solids 341 [1] Hogarth, C. A., Proc. Phys. Soc. (Lond.) 64B, 691 (1951). (1967). [2] Wright, R. W., and Bastin, J. A., Proc. Phys. Soc. [13] Wada, M., Japan J. Appl. Phys. 9, 327 (1970). (Lond.) 71, 109 (1958). [14] Koffeyberg, F. P., J. Solid State Chem. 2, 176 (1970); [3] van Houten, S., Nature (Lond.) 195, 484 (1962). Canad. J. Phys. 49, 435 (1971). [4] Hoschl, P., Konak, C., and Prosser, V., Mat. Res. Bull. [15] Finkenrath, H., and von Ortenberg, M., Z. angew. Phys. 4, 87 (1969). 23, 323 (1967). [5] Hoffman, C. W. W., Ropp, R. C., and Mooney, R. W., [16] Altwein, M., Finkenrath, H., Konak, C., Stuke, J., and J. Am. Chem. Soc. 81, 3830 (1959). Zimmerer, G., Phys. Stat. Solidi 29, 203 (1968). [6] Mookherji, T., J. Electrochem. Soc. 117, 1201 (1970). [17] Look, D. C., Phys. Rev. 184, 705 (1969). [7] Cimino, A., and Merezio, M., J. Phys. Chem. Solids 17, [18] Benedict, R. P., and Look, D. C., Phys. Rev. B 2, 4949 57 (1960). (1970). [8] Haul, R., and Just, D., J. Appl. Phys. 33, 487 (1962). [19] Zvara, M., Kocka, J., Konak, C., and Hoschl, P., Phys. [9 Koffyberg, F. P., Phys. Letters 30A, 37 (1969). Stat. Solidi 42, K5 (1970). Lamb, E. F., and Tompkins, F. C., Trans. Faraday Soc. [20] Maschke, K., and Rossler, U., Phys. Stat. Solidi 28, 577 58, 1424 (1962). (1968).

108 III. Oxides of 5d Transition Elements

III.l. Lanthanum Oxides The C-form of La203 can be prepared by decom- posing the precipitated hydroxide under vacuum The common oxide of lanthanum is the sesqui- and slow heating to 670 K [9]. DTA and resistivity oxide, La203 [1]. Warf and Korst [2] have reported studies indicate the C—»A transition at ~870 K the monoxide, LaO, (probably a surface film), but which is irreversible and apparently sluggish; mois- the material has not been prepared in bulk form.

ture seems to affect the T t considerably. According Rare-earth sesquioxides (of which La203 is the to Foex et al. [5, 6, 10], the A«=±H transition (marked first member) generally exist in the cubic (C), hex- by small thermal effects and lack of hysteresis) is of agonal (A) or the monoclinic [1, 3] modifications; the displacive type [11] whereas the H«=*X transition the C-form is a defect structure of fluorite type (marked by strong thermal effects) belongs to the where the metal ion has the MO0V2 coordination, reconstructive type. During the course of the latter where V stands for a vacancy along the body diag- transition, the two corresponding modifications are onal. The A- and B -forms are essentially close -packed usually observed to coexist over a remarkably ex- structures with MO7 coordination. All the oxides tended range of temperatures; the energy barrier have the C-type structure at low temperature. The between the corresponding modifications is relatively sesquioxides of heavier rare earths (Dy to Lu) as high. No high pressure transitions are known in well as Sc20 3 and Y203 retain the C structure even La203 . Daire and Wilier have recently reported the up to very high temperatures (^2300 K); however, existence of the B -modification of La203 [12, 13]; sesquioxides from La to Nd revert to the A-type thus, in this oxide it is possible to realize all the structure depending on the temperature and method different modifications that are generally exhibited of preparation of the sample etc. High-temperature, by the rare-earth oxides. high-pressure transitions are exhibited by almost all The magnetic behavior of La20 3 has been examined the rare-earth sesquioxides and these have been dis- in the range 290 to 1070 K by Smol’kov and cussed in detail in the literature [1, 3]. Dobrovol’skaya [14] who found that the substance is

La2 03 : The usual form of La2 0 3 that is encountered diamagnetic. Electrical properties have been exis the hexagonal (A) modification [1, 4]. Detailed amined in detail by various workers in the literature studies by Foex and co-workers [5, 6] indicate that [9, 15-21]. La203 is a p-type semiconductor and the A-form transforms on heating to another hex- shows considerable ionic conductivity at relatively agonal form (H) at 2310 K which is followed by an low temperatures (<800 K). Impurities seem to additional transformation at 2380 K to an unidenti- play a significant role in determining the contribution fied structure (X) before melting (at 2573 K); A—>H to ionic conductivity [22]; pure electronic conduction and H—>X transformations are reversible. The A

The presence of the X -modification is indicated by [29], Ga [29], Pr [31], and Y [29]); M02 (M = Ti the appearance of one or two characteristic fines in [29, 32], Zr [29, 32-35], Hf [36], Ce [18, 29], and Sn the x-ray diffraction patterns. The X-form appears [29]); M20 6 (M = Pa [37]) and M03 (M = W [38], to be denser than the H-form, but the detailed and U [39]). The crystal structures and electrical structure has not yet been established; preliminary properties of these solid solutions have been ex- data seem to indicate [7, 8] a cubic structure differ- amined in detail in the literature with particular ent from the C modification. reference to ionic conduction.

109 Lanthanum oxides

Oxide and description Remarks and inferences References of the study

La20 3

Crystal structure and C-type : Cubic; space group, Ia3; Depending on the method of [1, 4-9, 12, high temperature x-ray Z = 16; o = 11.39±0.03 A. C-form preparation of the sample, differ- 13].

studies. is metastable; C—*A transition at ent types of polymorphs of La 20 3 ~870 K; energy of activation can be realized. C—+A transition ~20 Kcal/mol. is irreversible; A—*H and H—*X transitions are reversible but A -type: Hexagonal; space group, differ in the mechanism of the P3ml; Z = 1; at T = 293 K, a = process. 3.937 A; c = 6.13 A; at T = 2213 K, o=4.028 A; c = 6.385 A. A-+H transition at 2310 K.

H-type: Hexagonal; at 2393 K, a = 4.063 A; c = 6.43 A. if-*X transition at (or above) 2380 K.

X-type : Most probably cubic; detailed structure not known. Melting point ~2570 K.

B-type: Monoclinic; space group, C2/m; Z = 6; a = 14.60 A; b = 3.717 A; c = 9.275 A; 0=99.77°.

Magnetic and electrical Diamagnetic (range 290-1070 K); The magnetic behavior is under- [9, 14-21]. -6 properties. X= — 0.23 X10 emu/g. standable because there are no 4f 6- Semiconductor; p (700 K)~10 electrons to contribute to para-

10® Ocm; Ea (C-form) ~1.1 eV; Ea magnetism. Electrical data are (A-form)~1.2 eV. C—*A transi- lacking especially at high tem- tion is shown as a change in the peratures where crystal structure slope of lnp — 1/T plot; moisture transitions occur. The con-

seems to affect T t considerably, ductivity behavior of highly pure _1/4 oc is p-type behavior; p po 2 ; and single crystalline material seems to show p- to n -transition urgently needed to elucidate the

at low po 2 and high temperature. detailed mechanism of conduction Considerable ionic conductivity and the contribution of the ionic at low enough temperatures and conductivity. probably controlled mostly by impurities.

Infrared studies. Bands are noted in the range 280- [40]. -1 430 cm and are ascribed to the metal-oxygen vibrations.

Heat capacity studies C„ (300 K) =25.92 ±0.08 cal/mol, [41]. (5-1800 K). K. There are no anomalies in the measured temperature range.

Energy References [2] Warf, J., and Korst, W., U.S. At. Comm. NP- 6078, 1956. [3] Eyring, L., and Holmberg, B., in Non-stoichiometric [1] Brauer, G., Progr. in the Science and Technology of Rare Compounds (Adv. in Chem. Series) 39, 46 (1963). Earths Ed. L. Eyring (Pergamon, Oxford) 1, 152 [4] Brauer, G., and Siegert, A., Z. anorg. allgem. Chem. (1964); 2, 312 (1966); 3, 434 (1968). 371, 263 (1969).

110 Foex, M., Z. anorg. allgem. Chem. 337, 313 (1965). [5] III. 2. Hafnium Oxides [6] Foex, M., and Traverse, J.-P., Bull. Soc. Franc. Mineral Crist. 89, 184 (1966); Compt. Rend. (Paris) 262C, 743 (1966). Phase studies on the Hf-0 system [1-3] have shown [7] Foex, M., and Pierre, J., Rev. Inst. Hautes Temp. that the dioxide, is the only stable Refract. 3, 429 (1966). Hf02 , oxide. Hf02 [8] Foex, M., Sci. Ceram. 4, 217 (1967), Publ. 1968; Chem. exhibits polymorphism and has been investigated in Abstr. 70, 60406s (1969). detail by many workers. [9] Mehrotra, P. N., Chandrashekar, G. V., Rao, C. N. R., Subbarao, E. C., Trans. Faraday Soc. 62, 3586 (1966). The stable room temperature modification of Hf02 [10] Foex, M., and Traverse, J.-P., Compt. Rend. (Paris) is monoclinic 262C, 636 (1966). [4, 5] which transforms to the tetragonal in in Solids [11] Buerger, M. J., Phase Transformations Ed. structure reversibly at ~1890 K [6, 7] with appreci- R. Smoluchowski (John Wiley, New York, 1951). able thermal hysteresis [8]. reversible tetragonal- [12] Daire, M., and Wilier, B., Compt. Rend. (Paris) 266C, A

548 (1968). cubic inversion in Hf02 has been reported at ~2970 [13] Wilier, B., and Daire, M., Bull. Soc. Franc. Mineral K, which is close to the melting point Crist. 92, 33 (1969). [7]. Even [14] Smol’kov, N. A., and Dobrovol’skaya, N. V., Izv. Akad. though the detailed mechanism of the monoclinic- Nauk SSSR, Neorg. Mater. 1, 1564 (1965). tetragonal transition in Hf0 is not known, compari- [15] Foex, M., Compt. Rend. (Paris) 220, 359 (1945). 2

[16] Rudolph, J., Z. Naturforsch. 14a, 727 (1959). son with the analogous transition in Zr02 suggests W., and Walch, H., Z. Elektrochem. 269 [17] Noddack, 63, the first order, diffusionless and athermal nature of (1959); Z. Phys., Chem. (Leipzig) 211, 194 (1959). the transformation in this oxide [18] Neuimin, A. D., and Pal’guev, S. F., Tr. Inst. Elektro- [5, 6, 8, 9]. khim., Akad. Nauk SSSR Ural’sk. Filial 133 (1963). A high pressure orthorhombic modification of Hf02 [19] Pal’guev, S. F., and Yol’chenkova, Z. S., in Electro- described Bocquillon et al. this is chemistry of Molten and Solid Electrolytes Ed. A. N. has been by [10];

Baraloshkin (Consultants Bureau, New York, 1968), obtained by heating the monoclinic Hf02 at ^1870 Yol. 6. K under a minimum pressure of ~15 kbar; this [20] Etsell, T. H., and Flengas, S. N., J. Electrochem. Soc. 116, 771 (1969). metastable phase reverts to the monoclinic form on [21] Subba Rao, G. V., Ramdas, S., Mehrotra, P. N., and prolonged heating at ~570 K. Rao, C. N. R., J. Solid State Chem. 2, 377 (1970). tetragonal or cubic modification of Hf0 can- [22] Etsell, T. H., and Flengas, S. N., Chem. Rev. 70, 339 The 2 (1970). not be quenched nor can it be stabilized at room [23] Kuo, C.-K., and Yen, T.-S., Chem. Abstr. 63, 10743e temperature and efforts to obtain these forms (1965). [7, 10]

[24] Tresvyatskii, S. G., and Lopato, L. M., Vpor. Teorii i by precipitating Hf0 2 in finely divided form have Prim. Redkozem. Metal. Akad. Nauk SSSR 155 (1964). been unsuccessful [9]. Submicron Hf02 prepared by [25] Barry, T. L., Nucl. Sci. Abstr. 20, 36784 (1966). is [26] Cassedanne, J., Anais Acad. Brasil. Cienc. 36, 413 (1964). the hydrolytic decomposition of alkoxides [9] amor- [27] McDaniel, C. L., and Schneider, S. J., J. Res. Nat. Bur. phous and transforms directly to the monoclinic Stand. (U.S.), 72A (Phys. and Chem.), No. 1, 27-37 (Jan.-Feb. 1968). form at ~600 K. However, the cubic form (and not [28] Levin, E. M., Phys. Chem. Glasses 7, 90 (1966). the tetragonal form) seems to be stabilized in thin [29] Andreeva, A. B., and Keler, E. K., Zh. Prikl. Khim. 39, films of Hf02 and recently El-Shanshoury et al. [11] 489 (1966). [30] Bonder, I. A., and Toropov, N. A., Izv. Akad. Nauk have discussed their polymorphic behavior. SSSR, Ser. Khim. 212 (1966). Electrical properties of Hf02 have been examined [31] Brauer, G., and Pfeiffer, B., Paper presented in Fifth Rare Earth Conference’, Chem. Session C, 1965. in detail by Tallan, Tripp, and Vest [12]. The ma- [32] Collongues, M. R., Queyroux, F., Perez, M., Zorba, Y., terial is a p-type semiconductor and a is essentially and Gilles, J.-C., Bull. Soc. Chim. France 4, 1141 -18 electronic in the range 1300-1800 K (po from 10 (1965). 2 [33] Lin, T.-H., and Yu, H.-C., Kuei Suan Yen Hsueh Pao to 1 atm). At low oxygen partial pressures, a small 3, 159 (1964). contribution due to the ionic conductivity has been [34] Fehrenbacher, L. L., Jacobson, L. A., and Lynch, C. T., Proc. Conf. Rare Earth Res., 4th, Phoenix, Arizona, noticed [12-14]. Measurements on polycrystalline 1964, p. 687 (Publ. 1965). Hf02 by Tallan et al. [12] did not show a discon- [35] Strickler, D. W., and Carlson, W. G., J. Am. Ceram. Soc. in the ln

SSSR, Neorg. Mater. 2, 1254 (1966). is also observed in the solid solutions of Hf02 with [40] Subba Rao, G. V., Rao, C. N. R., and Ferraro, J. R., = rare-earth sesquioxides [18]. Systems Hf02 -MO (M Appl. Spec. 24, 436 (1970) and references therein. = Sc [23, [41] Holley, C. E., Jr., Huber, E. J., Jr. and Baker, F. B.» Mg [19], Ca [19-22]), and Hf02-M 20 3 (M Progr. in the Science and Technology of Rare Earths, 24], Y [20, 25-27], La [25, 28], Nd [25, 29], Sm [30], Ed. L. Eyring, (Pergamon, Oxford) 3, 343 (1968) and references therein. and Gd [31]) have been investigated in the literature.

Ill Hafnium oxides

Oxide and description Data Remarks and inferences References of the study

HfOg

Crystal structure and fe 298 K: Monoellnie; space In many respects, HfOg resembles [4-8, 10,

x-ray studies. group, P2i/c; Z = 4; a=»5.1156± Zr0 2 even though there are subtle 32], 0.0005 A; b = 5.1722 ±0.0005 A; differences in behavior. The se- c = 5.2948 ±0.0005 A; 0= 99.02 ± quence (and the mechanism )of 0.08 A. TEC (470-670 K): [100], the transformations from the low 5.9X10-®; [001], 11.9 X10-®. symmetry to the high symmetry TEC slightly increases with tem- in the crystal structure are the

perature up to 1470 K. Mono- same in Hf0 2 and Zr0 2 except

clinic—tetragonal transition at that the T < for Hf0 2 are higher 1890 K; thermal hysteresis, 30 K. and the thermal hysteresis values The transition appears to be are lower. The high temperature endothermic associated with a modifications of HfOg cannot be volume contraction, but detailed quenched nor can they be data are not available. Boganov stabilized at room temperature by

et al. [7] report the T j to be solid solution formation. ~2170-2270 K for this transition. At 2770 K, a = 5.21 A, c = 5.35 A;

this tetragonal phase is stable up to ~2970 K and then transforms to cubic fee modification with a = 5.300 A. The tetragonal-

cubic inversion is reversible; however detailed data are lacking. High-pressure form: obtained by heating at 1870 K under 60 kbar pressure and quenching; orthorhombic; a =5.008 ±0.006 A; b = 5.062 ±0.006 A; c = 5.223 ±0.006 A. This metastable phase reverts to the monoclinic form on prolonged heating at 570 K.

Amorphous Hf0 2 obtained by the thermal decomposition of the metal alkoxide goes to the monoclinic variety on heating at ~600 K; the intermediate cubic and tetragonal forms are not realiz- able, neither they can be quenched to room temperature from the respective temperature ranges of existence. Melting

point of HfOj is 3070 K.

Electrical properties. p-type semiconductor in the range The electrical behavior is as ex- [12].

1270-1770 K;

1 1 B" cm" ; E. is 0.7 eV ( <1570 K) also behaves as an extrinsic p- 1/# and 0.2 eV ( >1570 K); a « poj type semiconductor. Apparently, (poi>10“® atm); md = 0.3-1.6X no discontinuity in the ln

ionic conductivity seems to be in Hf0 2 .

present. The defect structure is interpreted in terms of holes and fully ionized cation vacancies.

112 References and Chang and Phillips [10]. In addition to the suboxides, (Ta@0), TaO* TaO y (Ta 40) and TaO* (Ta 20), [1] Rud^, E., and Steeher, P., J. Lesa-Comm. Metals 5, 78 the monoxide TaO (whose existence is not proved

beyond doubt), dioxide, TaG2 and the pentoxide, [2] Bomagala, R. F., and Ruh, R., Trans. Am. Soe. Met. 58, 164 (1965). Ta 20 6 are known; however, Ta2 0 6 is the only oxide [3] Karnilov, I. L, Glazova, V. V., and Ruda, I. G., Izv. which is well-characterized Akad. Nauk SSSR, Neorg. Mater. 4, 2106 (1968). and extensively investi- [4] Adam, J., and Rogers, M. D., Acta Cryst. 12, 951 (1959). gated in recent years. [5] Ruh, R., and Corfield, P. W. R., J. Am. Ceram. Soc. 53, 126 (1970).

[6] Wolten, G. M., J. Am. Ceram. Soc. 46, 418 (1963). Suboxides: The suboxides form only by the oxida- [7] Boganov, A. G., Rudenko, V. S., and Makarov, L. P., Dokl. Akad. Nauk SSSR 160, 1065 (1965). tion of tantalum metal or tantalum compounds. [8] Ruh, R., Garret, H. J., Domagala, R. F., and Tallan, Ta 6 (tetragonal) forms at ~570 K; Ta (orthor- N. M., J. Am. Ceram. Soc. 51, 23 (1968). 0 40 Mazdiyasni, K. S., and Brown, L. M., J. Am. Ceram. [9] hombic) forms below 770 K and Ta20 (tetragonal) Soc. 53, 43 (1970). forms between 620 to 1470 K. complex suboxide [10] Bocquillon, G., Susse, C., and Vodar, B., Rev. Int. A Hautes, Temp. Refract. 5, 247 (1968). of unknown structure is formed above 1770 K. Elec- [11] El-Shanshoury, I. A., Rudenko, V. A., and Ibrahim, I. A., tron diffraction studies of monocrystalline samples J. Am. Ceram. Soc. 53, 264 (1970). [12] Tallan, N. M., Tripp, W. C., and Vest, R. W., J. Am. above 1770 K. Electron diffraction studies of mono- Ceram. Soc. 50, 279 (1967). crystalline samples reveal superlattices 9]. These [13] Robert, G., Deportee, C., and Besson, J., J. Chim. Phys. [8, 64, 1275 (1967). suboxides appear to be metallic, but detailed data [14] Etsell, T. H., and Flengas, S. N., Chern. Rev. 70, 339 are lacking. (1970). [15] Mazdiyasni, K. S., Lynch, C. T., and Smith, J. S., J. Am. Ceram. Soc. 48, 372 (1965). [16] Stansfield, O. M., J. Am. Ceram. Soc. 48, 436 (1965). Monoxide, TaO: Lagergren and Magn61i [1] and [17] Gavrish, A. M., Sukharevskii, B. Ya., Krivornchko, P. P., Schonberg [3] reported the existence of tantalum and Zoz, E. I., Ivz. Akad. Nauk SSSR, Neorg. Mater. 5, 547 (1969). monoxide with a narrow homogeneity range; it has Koehler, E. K., and Glushkova, V. B., Sci. Ceramics [18] 4, a cubic rock salt structure with a variable lattice 233 (1967, publ. 1968). [19] Strekalovskii, V. N., Volchenkova, Z. S., and Pal’guev, parameter. Physical properties are not known. The S. F., Izv. Akad. Nauk SSSR, Neorg. Mater. 1230 2, existence of this monoxide has not been proved be- (1966). [20] Volchenkova, Z. S., and Pal’guev, S. F., Trans. Inst. yond doubt and systematic investigations are called Elektrokhim Akad. Nauk SSSR, Ural. Filial 133 5, for. (1964). [21] Johansen, H. A., and Cleary, J. G., J. Electrochem. Soc. Ill, 100 (1964). : Rutile type is to be a stable [22] Delamarre, C., Silicates Ind. 32, 345 (1967). Ta02 Ta02 known [23] Komissarova, L. N., and Spiridinov, F. M., Dokl. Akad. oxide [1, 3, 11], but detailed data on the physical Nauk SSSR 182, 834 (1968). properties are lacking. to solid [24] Kalinovskaya, C. A., Spiridinov, F. M., and Komissarova, Ta02 appears form L. N., J. Less-Comm. Metals 151 (1969). 17, solutions with Ti02 [3, 12], although there are [25] Komissarova, L. N., Wang, C. S., and Spitsyn, V. I., changes in the cation valencies. Izv. Akad. Nauk SSSR, Ser. Khim. 1, 3 (1965). [26] Besson, J., Deportee, C., and Robert, G., Compt. Rend. (Paris) 262,327 (1966).

[27] Duclot, M., Vicat, J., and Deportee, C., J. Solid State Ta 20 6 : Tantalum pentoxide, Ta 20 6 , is a stable oxide Chem. 2, 236 (1970). with a very narrow homogeneity range [13] and has [28] Komissarova, L. N., Wang, C. S., Spitsyn, V. I., and Simanov, Yu. P., Russ. J. Inorg. Chem. 9, 385 (1964). been well-investigated by various workers. The exact [29] Komissarova, L. N., Spitsyn, V. I., and Wang, C. S., relationships of the various polymorphs of Ta 2Oa are Dokl. Akad. Nauk SSSR 150, 816 (1963). [30] Isupova, E. N., Glushkova, V. B., and Keler, E. V., not clearly understood because of the complexity of Izv. Akad. SSSR, Nauk Neorg. Mater. 4, 399 (1968). the structures of the phases, sluggish nature of the [31] Spiridinov, F. M., Stepanov, J. A., Komissarova, L. N., and Spitsyn, V. I., J. Less-Comm. Metals 14, 435 transitions and the existence of various metastable (1968). phases that can be stabilized by impurities. [32] Filatov, S. K., and Frank-Kamenetskii, V. A., Sov. Phys. -Crystallogr. (English Transl.) 14, 696 (1970). It is fairly well established that Ta2C>6 exists in

two polymorphic forms, /3 and a, with a reversible

phase transition at ~1630 K [1, 14]; the transition

III. 3. Tantalum Oxides between them is sluggish and the low temperature /3 form can persist as a metastable phase in the stabil- The Ta-0 system has been investigated by many ity field of the a form and can be melted at ~2060 workers [1-7] and the stability and phase relation- K. The high temperature (a) form melts at ^2160 K ships of the various oxides have been reviewed and [14, 15]. discussed by Steeb and Renner [8], Niebuhr [9], The proper method of indexing the powder pattern

113 of the /3 form has puzzled crystallographers for a studies have, however, indicated [15, 29] that the long time; the more intense diffraction lines can be a form actually undergoes several unquenchable indexed on the basis of a simple orthorhombic sub- phase transitions upon cooling from high tempera- cell with a = 6.20, 6 = 3.66, and c = 3.89 A. However, tures. The true high-temperature form is postulated numerous weak lines which also appear in the pat- to be tetragonal in its field of stability and several tern have not been interpreted unambiguously main- metastable phases occur in the stability field of the ly because the position of these super-structure lines low-temperature polymorph. Ta20 B when quenched is strongly dependent upon both the nature of the from above 1630 K (T\) is triclinic and transforms heat treatment and the amount of impurities [16, to a monoclinic form on heating to ~590 K and this 17]. Consequently, different workers have suggested in turn reverts to the tetragonal form at ~1220 K.

different lattice parameters and symmetries to ex- Sarjeant and Roy [30] reported a metastable high-

plain the x-ray pattern of the /3 form of Ta 2 C>6 [7, temperature hexagonal form of Ta20& obtainable by 18-25]. Systematic examination of the polymorphic rapid quenching techniques; this 5 form reverts to

behavior of Ta 2 06 by Roth and co-workers [15, 16, the /3 form on annealing at ~1470 K. Impurities — 26-28] has contributed significantly to the under- like titanium lower the T t of the /3 >a transition standing of the structure of the low-temperature and Sc 8+ in low concentrations appears to stabilize

phase. These workers found that many metallic the high-temperature tetragonal phase [16, 28]; the oxides, especially WOa, can stabilize the /3-form and crystal structure consists of a-UOa-type blocks in

that this pure phase exists in two slightly different which Ta atom is surrounded by a pentagonal bi- modifications with 6-axis multiplicities of 11 and 14 pyramid of oxygen atoms. These blocks are infinite at low temperatures and at ~1580 K respectively. in two directions and are separated from similar At intermediate temperatures, an infinite number of blocks along a third direction by shear planes and in at least partially ordered sequences of these two the vicinity of these shear planes, the Ta atoms are modifications exist in equilibrium. The addition of surrounded by distorted octahedral coordination

WO 3 (or other impurities) causes the stabilization polyhedra.

of an infinite number of phases similar in structure Semiconducting behavior of Ta 20 B has been ex-

to the low-temperature form of Ta 20 6 . amined by Kofstad [13] who finds that the material

The structure of the high-temperature (a) form is p- or n-type depending on the po2 . Differences in

of Ta 20 6 has been a subject of intensive study and the dielectric behavior of the polymorphs of Ta 206

and tetragonal, hexagonal, orthorhombic, mono- have been reported by Pavlovic [31]. Systems K 20-

clinic, and triclinic symmetries have been suggested Ta20 6 [32] and Nb206-Ta 20 B [33, 34] have been in-

by various workers [1, 14, 20, 22, 29]. Detailed vestigated in the literature.

Tantalum oxides

Oxide and description Data Remarks and inferences References of the study

Suboxidea

Crystal structure. Ta#0: Tetragonal; a *=3.36 A; c = These oxides are formed from Ta by [It 2, 3, 5,

3.25 A. Ta 40: Orthorhombic; o = the oxidation at various tempera- 9, 10]. 3.61 A; b “3.27 A; c = 3.20 A. tures; some reveal superstructure TajO: Tetragonal; a = 6.63 A; in electron diffraction patterns. c = 4.75 A. Detailed data on the physical properties are lacking.

TaO

Crystal structure. Cubic; space group, Fm3m; 2 = 4; [1. 31. 0 = 4.422-4.429 A. Formed in the range 870-1770 K; physical properties not known in detail.

114 Tantalum oxides—Continued

Oxide and description Remarks and inferences References of the study

TaOj

Crystal structure. Tetragonal; space group, P42/mnm; [1. 31- Z = 2; 0 = 4.709 A; c = 3.065 A. Physical properties are not known in detail.

TagOg

Crystal structures and Low-temperature 03) form: Sub- Systematic examination of the de- [15, 16, x-ray studies. cell: orthorhombic; Z = 1; a — tailed phase relationships by 26-28]. 6.20 A; 6 = 3.66 A; c = 3.89 A. Roth and coworkers has revealed True-cell: orthorhombic; Z = ll; a proper understanding of the o=6.198 A; 6=40.29 A; c = 3.888 structures of TagOg modifications A; 6-axis multiplicity = 11. and cleared the ambiguities of 0-form at a given temperature the earlier workers. consists of an infinite number of partially ordered sequences of two simple modifications with multiplicities 11 and 14 and these sequences are affected both by heat treatments and by impurities thus explaining considerable ambiguity encountered by various workers in the interpretation of the powder patterns. T<~1630 K; sluggish but completely reversible transition. High -temperature (a) form: tetragonal; space group; I4j/amd; Z = 6; a = 3.81 A; c* 36.09 A. This phase is unquenchable in pure TagOg; however, Sc,+ appears to stabilize this form. The structure consists of a-UOj type blocks in two directions and separated in the third direction by shear planes. Sc8+ doping seems to introduce random shear planes (Wadsley defects) which stabilize the structure. TagOg quenched from temperatures above T, is triclinic: a =3.801 A; 6 = 3.785 A;

c = 35.74 A; <*=90.91®; /3 =90.19®; 6 yss*90 . This triclinic phase transforms to monoclinic phase

on heating to ~590 K; a meao - V2 Ot,i„; 6meao “ y/% bttlei Csnooo®* cuio. Monoclinic phase reverts to the tetragonal phase on heating to '>-'1220 K. Rapid quenching of TagOg from high temperatures appears to produce a hexagonal modification; a =3.874 A; c =

115 Tantalum oxides—Continued

Oxide and description Data Remarks and inferences References of the study

3.62 A. This 6 form reverts to the

/3 form on annealing at ~1470 K. Melting pointcii2160±20 K.

Electrical properties. Semiconductor in the range 1000- The defect structure is interpreted [13].

5 0" 1 1670 K. cr (1000 K)~10" in terms of oxygen interstitials

1 cm" ; E0 = 1.79 eV; a (1300 K; (p-type) and oxygen vacancies P02 ~l atm)~l mV/K. p-type (re-type). No change in the slope

behaviour at poj^l atm and re- of bnr-l/T plot noticed at T <; the type at lower po*. p to re transi- reason is not clearly understood. tion takes place at lower po* with decreasing T.

Infrared spectra. Bands are noted at 720 and 860 [34], -1 • cm for a-Ta 206 and interpreted in terms of the metal-oxygen vibrations.

References [19] Simanov, Yu. P., Lapitskii, A. V., and Artamanova, E. P., Vetn. Mosk. Univ. 9, Ser. Fiz. Mat. i Estestven

[ 1 ] Lagergren, S., and Magneli, A., Acta Chem. Scand. 6, Nauk (6), 109 (1954).

\\\ (1952). [20 ] Zaslavskii, A. I., Zvinshuk, R. A., and Tutov, A. G., [2 Wasilewski, R. J., J. Am. Chem. Soc. 75, 1001 (1953). Dokl. Akad. Nauk SSSR 104, 409 (1955). 3 Schonberg, N., Acta Chem. Scand. 21 Lapitskii, A. V., Simanov, Yu. P., Semenen, K. N., and 8, 240 (1954). [ ] Pawel, R. E., Cathcart, J. V., and Campbell, J. J., Acta Yarembash, E. I., Vetn. Mosk. Univ. 9, Ser. Fiz. Mat. Met. 10, 149 (1962). i Estestven Nauk, (2), 85 (1959). Norman, N., Less-Comm. Metals 22 Harvey, and Wilman, H., Acta Cryst. 1278 (1961). J. 4, 52 (1962). [ ] J., 14, B Kofstad, P„ J. Inst. Metals 90, 253 (1962); 91, 209 23 Lehovec, K., J. Less-Comm. Metals 7, 397 (1964). (1963). 24 Turnock, A. C., J. Am. Ceram. Soc. 48, 258 (1965); Terao, N., Japan J. Appl. Phys. 6, 21 (1967). 49, 382 (1966). [si Steeb, S., ana Renner, J., J. Less-Comm. Metals 10, 246 [25] Wolten, G. M., and Chase, A. B., Z. Krist. 129, 365 (1966). (1969). Niebuhr, J., J. Less-Comm. Metals 10, 312 (1966). [26] Roth, R. S., and Stephenson, N. C., in The Chemistry (ioj Chang, L. L. Y., and Phillips, B., J. Am. Ceram. Soc. of Extended Defects in non-Metallic Solids, Eds. L. 52, 527 (1969). Eyring and M. O’Keeffe, (North Holland Publ. Co., [11] Rogers, D. B., Shannon, R. D., Sleight, A. W., and Amsterdam, 1970). Gillson, J. L., Inorg. Chem. 8, 841 (1969). [27] Roth, R. S., Waring, J. L., and Parker, H. S., J. Solid

[ 12 ] Riidorff, W., and Luginsland, H. H., Z. anorg. allgem. State Chem. 2, 445 (1970). Chem. 334, 125 (1964). [28] Stephenson, N. C., and Roth, R. S., J. Solid State Chem. 13 Kofstad, P., J. Electrochem. Soc. 109, 776 (1962). 3, 145 (1971). 14 Reisman, A., Holtzberg, F., Berkenblit, M., and Berry, [29] Laves, F., and Petter, W., Helv. Phys. Acta 37, 617 M., J. Am. Chem. Soc. 78, 4514 (1956). (1964). [15] Waring, J. L., and Roth, R. S., J. Res. Nat. Bur. Stand. [30] Sarjeant, P. T., and Roy, R., J. Am. Ceram. Soc. 50, 500 (U.S.), 72A (Phys. and Chem.) No. 2, 175-186 (Mar.- (1967). Apr. 1968). [31 Pavlovic, A. S., J. Chem. Phys. 40, 951 (1964). [16] Rotn, R. S., Waring, J. L., and Brower, W. S., Jr., [32 Holtzberg, F., and Reisman, A., J. Phys. Chem. 65, 1192 J. Res. Nat. Bur. Stand. (U.S.), 74A (Phys. and (1961). Chem.), No. 4, 485-493 (July-Aug. 1970). [33] Mohanty, G. P., Fiegel, L. J., and Healy, J. H., J. Phys. [17] Jahnberg, J., and Andersson, S., Acta Chem. Scand. 21, Chem. 68, 208 (1964). 615 (1967). [34] Alyamovskii, S. I., Shveikin, G. P., and Gel’d, P. V., [18] Frevel, L., and Rim, H., Anal. Chem. 27, 1329 (1955). Russ. J. Inorg. Chem. 12, 915 (1967).

elinie phas© by grinding. Th© latter phai© ii itabl© Tungsten dioxide and trioxide are the well-known in th© rang© 233 to 290 K. On cooling to 233 K, th© oxides in the W»0 system [1=3]. Two mixed valence trielinie phai© of pur© WO§ changes to a monoclinic phases, (WOg,?g) and WggOig (WOg.go) also Wi§04§ phas© with observable temperature hysteresis, TEC, exist [1, 4=7] in the range WOg-WO§ and substoiehio- AH and AS changes, discontinuities in p, g, H#, and viz., and metrie phases of WO§, WOg,g§ (WioOiu) Hu [18, 23=23, 27=29] and changes in the domain characterized in the WOg.ii (WioOiw) have been structure [29]. This monoelinie form seems to be literature 8 ]. Even though earlier workers [9, 10] [ stable in the range 77 to 233 K and has been found have claimed the existence of an oxide O, recent W 3 to be ferroelectric [28, 30, 31]. Roth and Waring [26] studies [7, 11] indicate that the oxide is not a stable obtained this phase at room temperature by quench- phase. ing a soid solution of 2 percent Nb205 in WO3 from 1500 to 1660 K; however, this solid solution does WO2: The range of homogeneity of W02 appears to not exhibit ferroelectricity whereas specimens of sin- be small (1.94-2.025) [12] and the material is stable tered W08 containing 2-4 percent Ta 2Os have been up to 1800 K after which it decomposes. It has a reported to be ferroelectric at room temperature monoclinic structure [2, 5, 13, 14] and exhibits metal- [32], Nonstoichiometry in W0 3 , by way of removal lic behavior [13]; no phase transitions are known in of oxygen from the lattice, apparently decreases the

W02 . . T t of the two low-temperature transitions and brings W02 forms solid solutions with Ti02 [15] and about changes in the electrical properties [18]. V02 giving rise to rutile based structures [14, 16] The room-temperature monoclinic modification of WO3 transforms to an orthorhombic structure re- Wi 8049: This oxide is monoclinic [4, 5] and is stable versibly at '"‘-'580 K [24, 33-37]; DTA peak [31] above 860 K; below this temperature, it decomposes and resistivity discontinuity have been noted at this to W02 and W20O 58 . Wi 8C>49 is the most refractory temperature [24]. Roth and Waring [26] obtained the oxide (stable up to 1970 K) of the W-0 system [7]. orthorhombic form at room temperature on heating Physical properties of this oxide are not known. the stabilized monoclinic W0 3 (containing 2%

-— Nb2Oe) to '1170 K and rapid cooling. The orthor- W20O68 { This monoclinic oxide seems to have a nar- hombic—^tetragonal transition in WO3 is well estab- row range of homogeneity [4, 5, 17, 18] and decom- lished and investigated by many workers [26, 33, 34, poses below 760 K to W02 and WOa [7]. Measure- 38-41]; x-ray diffraction, and resistivity ments of resistivity and Seebeck coefficient indicate 36, DTA

measurements indicate the T t to be 1010 K (accom- metallic behavior [18]. panied by thermal hysteresis). The tetragonal phase

is related to the ReOa structure and doping with Substoichiometric phases : W B oOi48 and W40O119 lowers the considerably according to are monoclinic and according to Gebert and Acker- Nb T t [26]; Roth and Waring [26], the (Nb) stabilized mono- man [8], a high-temperature polymorph of W 6 oOi4 8 exists. Detailed data are lacking. clinic phase of WOa transforms to the tetragonal modification (without going through the triclinic,

W08 : Tungsten trioxide is a stable phase and is monoclinic and orthorhombic structures as in the usually associated with slight nonstoichiometry [1, case of pure WO3) at ^1010 K and this monoclinic— 7, 18]; it is polymorphic and not less than five tetragonal transition has been found to be reversible distinct crystallographic modifications are known (fig. below 1025 K. III.l). The phase transitions among the various DTA [35, 38], heat capacity and resistivity [39] forms of WO3 have been examined using a variety of studies indicate transitions at ~1170 and ~1500 K techniques. in pure WOa, but no structural changes have been

At room temperature, WOa is monoclinic [1, 5, 7, observed (fig. III.l) [26, 35, 38]; the high temperature 12, 19-22] and this phase is stable in the range 290 phases remain tetragonal. A transformation from to 583 K [23]. On cooling to 290 K, the room tem- tetragonal to cubic structure of the ideal ReOa type perature polymorph transforms to a triclinic form; (which seems quite logical) has not been observed this reversible change is associated with some thermal in pure WOa below the melting point (1700 K). hysteresis (3-13° depending on the sample), AH Attempts to prepare the cubic phase by doping have and AS and discontinuities in p, a, 1?#, and p# [18, not been successful. 23-25]. Roth and Waring [26] noted that the mono- WO3 is diamagnetic [18, 42] in the range 77 to 292

117 250 K 293-303 K 600 K

Monoctinlc Triclinic Monoclinic Orthorhombic

(77-233 K) 230 K (233-290 K) 290 K (290-583 K) 560 K (580-1010 K) (Ferroelectric) AH = 39cal/mole AH=20 cal/moie AH=330cal/mole

1170 K

Tetragonal ~1500 K Tetragonal Tetragonal

AH=120 cal/moie ~t170 K (1010-1700 K) AH = 2 80 cal/moie (Antiferroelectric ?)

Figure III.l. Phase relations in WD 8 .

K; feeable paramagnetism has been noted at -~1.3 K deficient WO 3 has been investigated in detail by in the oxygen deficient WO3 [18]. It is semiconduc- Sienko and co-workers [18, 24, 25], ting at low temperatures, but exhibits semimetallic W03 in combination with other metal oxides form behavior at higher temperatures. As mentioned ear- very interesting mixed oxides called Tungsten bron- lier, electrical characteristics show anomalous be- zes.’ The physical properties of these bronzes have havior at the transition points. The mechanism of been examined in great detail and discussed in the electrical conduction in stoichiometric and oxygen literature [43-46].

Tungsten oxides

Oxide and description Data Remarks and inferences References of the study

wo 2

Crystal structure and Monoclinic; space group, P2i/c; The structure is that of a dis- [2, 5, 13, electrical properties. Z = 4; a = 5.5607 ±0.0005 A; b = torted rutile; metal-metal bond- 14]. 4.9006 ±0.0005 A; c = 5.6631 ± ing seems to exist and the elec-

0.0005 A; j8 = 120.44°. Metallic trical properties are consistent -4 behavior; p (4.2 K)=2.0X10 with this assumption. -3 12cm; p (300 K) = 2.9 XlO 12cm.

w 18o 49

Crystal structure. Monoclinic; a = 18.28 A; 6=3.775 [4, 5, 7]. A; c = 13.98 A; 0 = 115.14°. De- composes below 860 K but stable up to 1970 K. Detailed data are lacking.

Crystal structure and Monoclinic; a = 16.74 A; 6=4.019 The structure appears to be re- [4, 5, 7, 17, electrical properties. A; c = 14.53 A; 0 =95.45°. lated to WO 2 and metal-metal 18]. Decomposes below 760 K but bonding seems to be present, p stable up to 1820 K. Metallic anomaly at ~270 K is not ex- behavior; p (80 K)~5X10~4 plainable at present. 12cm; p-T plot exhibits a maximum at 270 K. a (90 K)~ — 15 pV/°C; a (240-320 K) 30

3 pV/°C. n«4XlO*Vcm .

118 Tungsten oxides—Continued

Oxide and description Data Remarks and inferences References of the study

W 50O 148 (W02.9#)

structure Monoclinic; space group, P2/c; a = The structure to Crystal appears be re- [ 8 ].

11.90 ±0.02 A; 6 = 3.826±0.012 lated to that of WO 3 . A; c = 59.64 ±0.06 A; 0=98.4°. A high -temperature polymorph with a unit cell having the formula

W 26O74 is found to be stable at ~1520 K. Detailed data are lacking.

W40O 119 (WOi.m)

Crystal structure Monoclinic; space group, P2i/m; [8]. Z = 4; a = 7.354 ±0.005 A; 6 = 7.569 ±0.005 A; c = 3.854 ±0.005 A; /3=90.6°. Detailed properties not known.

wo 8

= Crystal structure and Range 77-233 K: Monoclinic; a WO 3 is the only oxide which shows [1, 5, 7, 12, x-ray studies. 5.27 A; 6 = 5.16 A; c = 7.67 A; 0 = a wide variety of crystallographic 19-22, 26, 91.72°. This phase can be stabil- transitions in the easily attain- 27-29, 33- ized at room temperature by able ranges of temperatures; 41]. quenching a solid solution of 2 % many of the transitions are of

Nb 2 C>6 in W0 8 from 1500-1660 K. first order. Surprisingly, the tetra- Monoclinic to triclinic transition gonal—»cubic transition has not at 251 K (heating); hysteresis been noticed either in the pure or ~15°. The stabilized solid solu- doped or nonstoichiometric sam-

tion transforms to a tetragonal ples; neither it has been possible form at ~1010 K; transition re- to stabilize the cubic phase at

versible if the sample is heated any temperature. The effect of below 1015 K and if heated to nonstoichiometry is pronounced ~1170 K, tetragonal phase in the case of low-temperature

transforms irreversibly to an transitions. Data is lacking for orthorhombic modification. the high -temperature transitions Range 233-290 K: Triclinic; a = 7.30 and also the effect of pressure on A; 6 = 7.52 A; c = 7.69 A; a = 88.83°; the transitions. 0=90.92°; 7=90.93°. Triclinic phase not encountered in the

solid solutions of WO 3 with

Nb 20 s at any temperature. Tri- clinic to monoclinic transition at ~293-303 K (heating); hysteresis, ~3-13°. Reverse transition (mono.—»tric.) noted on grinding the specimen at room temperature. Removal of oxygen from the lattice of pure WO* reduces the

T t of the two low -temperature transitions. Range 290-583 K: Monoclinic; = space group, P2i/n; Z = 8 ; a 7.306 ± 0.001 A; 6 = 7.540 ± 0.001 A; c = 7.692 ± 0.001 A;

119 Tungsten oxides—Continued

Oxide and description Data Remarks and inferences References of the study

/3 =90.881 ±0.005°. Solid solu-

tion with Nb 206 retains the monoclinic structure but slightly changes the lattice parameters. Monoclinic to orthorhombic transition at ~600 K (heating); thermal hysteresis ~40-50°. Range 580-1010 K: Orthorhombic; the lattice parameters remain the same as in the monoclinic phase (except for the effect of

thermal expansion) and /3 ap-

90° « . proaches at T t According

to Rosen et al. [35], the b param-

eter shows an anomaly at T t in

WO 3 . The orthorhombic form can be stabilized at room tempera-

ture by heating the 2 % Nb 206 solid solution to 1170 K and rapid cooling. Orthorhombic to tetragonal transition at ~1010 K; hysteresis ~10 °. The solid solu- to tion (of 2 % Nb 206 ) transforms tetragonal form at a lower temperature. Range 1010-1700 K: Tetragonal; space group, P4/nmm; = Z 2 ; a = 5.25 A; c = 3.91 A. A transition takes place at ~1170 K in WOs but x-ray studies do not indicate any change in the crystal symmetry. Roth

and Waring [26] point out that it is possible that the tetragonal unit cell has a doubled c axis below 1170 K and only above this temperature does the powder pattern yields the correct

unit cell. Melting point of WO 3 is 1700 K and it melts under its own equilibrium oxygen partial pressure [12, 41, 47].

Magnetic properties. Diamagnetic in the range 77-300 K; Both stoichiometric and nonstoi- [18, 42]. X m (=300 K) = -21.0±1.7X chiometric samples exhibit simi- _a 10 . Below 77 K, oxygen defi- lar magnetic behavior. cient WO* shows decrease in xm eventually switching to a feeble “ paramagnetism (xm = 10 X 10 # cgs u.) at 1.28 A.

120 Tungsten oxides—Continued

Oxide and description Data Remarks and inferences References of the study

Electrical properties. Semiconductor behavior; p (200 K) The electrical properties show com- [18, 23-25, ~10 3 12cm; p (400 K)~5 !2cm; plex behavior; the actual values 31, 48]. a (300 K) 0.5 mV/°C; pH of p, a and ph vary from sample (400 K)~10 cm 2 /V s. Discon- to sample and depend on the

tinuities in p, a and ph are en- method of preparation, stoichiom- countered near the transition etry etc. However, the qualitative temperatures of the various features are reproducible and p-T phases encountered with the behavior provides a good tool for associated hysteresis. In the indicating the phase transitions. range 400-900 K, p is very small Electrical data on the doped WO3

( 0. 1-0.5 Ocm) and shows (where various high- and low- slight increase with rise in tem- temperature phases have been perature (quasi-metallic be- stabilized) are lacking.

havior). Sawada [31] has reported a resistivity anomaly at ~1500 K

in WO 3 ; this may correspond to a change in the mechanism of conduction or to a new modification. Details are not known. The mechanism of electrical conduction in the range 100-500 K is interpreted in terms of various polaron theories.

Optical and dielectric Absorption edge is at 2.7 eV. Re- Accurate dielectric constant data [18, 28, 30, properties. moval of oxygen from the lattice and the study of ferroelectric and 38, 49, 50]. causes an increase in absorption antiferroelectric properties could at all wave lengths. Absorption not be carried out because of the edge shows an anomalous shift at high conductivity effects in pure

the ~250 K transition. In the W0 3 . range 270-970 K, absorption -4 edge shows a red shift (~10 eV/K); near 1010 K transition, a sudden red shift is noted for a polarized light. The static dielectric constant at room tempera-

ture is ~10 3-10 4 (at 1 MHz) and 2 decreases to ~10 [50] at -~250 K. According to Matthias and Wood [28, 30], the low-temperature monoclinic phase (in the

range 77-233 K) is ferroelectric. On the basis of structural studies,

Kehl et al. [38] concluded that the high -temperature tetragonal may be antiferroelectric. Levine

et al. [32] reported that WO 3 containing 2-4% TajOs exhibits ferroelectricity at room temperature whereas Roth and Waring

[26] do not find this behavior in the 2% NI^Ob doped sample.

121 Tungsten oxides—Continued

Oxide and description Data Remarks and inferences References of the study

DTA studies. All the crystallographic transitions The nature of the high -temperature [18, 31]. are associated with DTA anoma- transitions is not known in de- lies; in addition, transitions are tail; the transitions seem to be also indicated at ~1170 and first order. ~1500 K (fig. III.l). Hysteresis is noted in all cases. AH (cal/ mol): Monocl. (low -temperature) —triclinic, 39; tricl.—»monocl., 20; monocl. —»orthorh., 330; orthorh. —tetra., 450; tetra.—»tetra., 280; 1500 K transition, 120. Removal of oxygen from the lattice de-

creases T t as well as the AH of the two low-temperature transitions. Data are not available for other transitions.

References [26] Roth, R. S., and Waring, J. L., J. Res. Nat. Bur. Stand. (U.S.), 70A (Phys. and Chem.), No. 4, 281-303 (July- Aug. 1966). [1] Hagg, G., and Magneli, A., Arkiv Kemi Mineral. Geol. [27] Foex, M., Compt. Rend. (Paris) 220, 917 (1945); 228, 19A, No. 2 (1944). 1335 (1949). [2] Magneli, A., Arkiv Kemi Mineral. Geol. 24A, No. 2 [28] Matthias, B. T., and Wood, E. A., Phys. Rev. 84, 1255 (1946). (1951). [3] H., Braekken, Z. Krist. 78, 484 (1931). [29] Hirakawa, K., J. Phys. Soc. Japan 7, 331 (1952). [4] Magneli, A., Arkiv Kemi 1, 223 (1949); 1, 513 (1950). [30] Matthias, B. T., Phys. Rev. 76, 430 (1949). [5] Magneli, A., Andersson, G., Blomberg, B., and Kihlborg, [31] Sawada, S., J. Phys. Soc. Japan 11, 1237 (1956). L., Anal. Chem. 24, 1998 (1952). [32] Levine, S., Corwin, R., and Blood, H. L., Bull. Am. Phys. [6] Glemser, O., and Sauer, H., Z. anorg. allgem. Chem. Soc. Ser. II, 1, 255 (1956). 252, 144 (1943). [33] Wyart, J., and Foex, M., Compt. Rend. (Paris) 232, [7] Phillips, B., and Chang, L. L. Y., Trans. AIME 230, 2459 (1951). 1203 (1964). [34] Ueda, R., and Ichinokawa, T., Phys. Rev. 82, 563 (1951). [8] Gebart, E., and Ackermann, R. J., Inorg. Chem. 5, 136 [35] Rosen, C., Banks, E., and Post, B., Acta Cryst. 9, 475 (1966). (1956). [9] Neuberger, M. C., Z. Krist. 85, 232 (1933). [36] Perri, J. A., Banks, E., and Post, B., J. Appl. Phys. 28, [10] Hagg, G., and Schonberg, N., Acta Cryst. 7, 351 (1954). 1272 (1957). [11] St. Pierre, G. R., Ebinara, W. T., Pool, and M. J., [37] Gado, P., Magy Fiz. Folyoirat 10, 347 (1962). Speiser, R., Trans. AIME 224, 259 (1962). [38] Kehl, W. L., Hay, R. G., and Wahl, D., J. Appl. Phys. [12] Chang, L. L. Y., and Phillips, B., Am. Ceram. Soc. J. 23, 212 (1952). 52, 527 (1969). [39] Sawada, S., Phys. Rev. 91, 1010 (1953). [13] Rogers, D. B., Shannon, R. D., Sleight, A. W., and [40] Fiegel, L. Mohanty, G. P., and Healy, H., Chem. Gillson, J. L., Inorg. Chem. 8, 841 (1969). J., J. and Engg. Data 365 (1964). [14] Israelsson, M., and Kihlborg, L., Mat. Res. Bull. 5, 19 9, (1970). [41] Chang, L. L. Y., Schroger, M. G., and Phillips, B., [15] Chang, L. L. Y., Scroger, M. G., and Phillips, B., J. J. Am. Ceram. Soc. 49, 385 (1966). Less-Comm. Metals 12, 51 (1967). [42] Sienko, M. J., and Banerjee, B., J. Am. Chem. Soc. 83,

[16] Nygren, M. t and Israelsson, M., Mat. Res. Bull. 4, 881 4149 (1961). (1969). [43] Sienko, M. J., in Non-stoichiometric Compounds, Adv. [17] Magn61i, A., Nature (Lond.) 165, 356 (1950). Chem. Ser. 39, 224 (1963). Berak, [18] J. M., and Sienko, M. J., J. Solid State Chem. 2, [44] Wadsley, A. D., in Non-stoichiometric Compounds, Ed. 109 (1970). L. Mandelcorn, (Academic Press, New York, 1964). [19 Andersson, G., Acta Chem. Scand. 7, 154 (1953). [45] Dickens, P. G., and Whittingham, M. S., Quart. Rev. 20 Tanisaki, S., Phys. Soc. Japan 573 (1960). J. 15, (Lond.) 22, 30 (1968). 21 Loopstra, B. 0., and Boldrini, P., Acta Cryst. 21, 158 [46] Rao, C. N. R., and Subba Rao, G. V., Phys. stat. Solidi (1966). (a) 1, 597 (1970). [22] Loopstra, B. O., and Rietveld, H. M., Acta Cryst. 25B, Levin, E. 1420 (1969). [47] M., J. Am. Ceram. Soc. 48, 491 (1965). Sawada, S., [23] Tanisaki, S., J. Phys. Soc. Japan 15, 566 (1960). [48] and Danielson, G. C., Phys. Rev. 113, 803 [24] Crowder, B. L., and Sienko, M. J., J. Chem. Phys. 38, (1959). 1576 (1963). [49] Iwai, T., J. Phys. Soc. Japan 15, 1596 (1960). [25] Crowder, B. L., and Sienko, M. J., Inorg. Chem. 4, 73 [50] Hirakawa, K., and Ueda, I., Mem. Fac. Sci. Kyusyu (1965). Univ. Bl, 112 (1954).

122 III. 5. Rhenium Oxides The simple cubic structure of ReOa is important from the view point of understanding the structures

In the Re-0 system, ReQ 2 , ReOs, and Re 2t)7 are of perovskites (see fig. III.2) [13]. Goodenough [IS- the oxides that are well characterized and examined IS] has explained the metallic behavior of ReOa by in detail. A tetragonal Re 2 Q 6 has been reported by constructing a one-electron energy band diagram.

Trabalat et al. [1]. The di- and sesquioxides are Recent band structure calculations on ReOa have stable ionic compounds whereas Re2C>7 is mostly proved that the essential features of the Good- covalent and has a low melting point (~490 K). enough’s model are correct [16, 17].

Re02 is polymorphous and when synthesized below Re2C>7 is orthorhombic at room temperature [18- 570 K, the structural modification (a) is that of 20]. It is yellow in color and is supposed to have a monoclinic Mo0 2 type [2, 3]; above ~570 K, this polymeric structure in the solid state; in the liquid oxide transforms irreversibly to an orthorhombic and vapor states, monomeric species have been iden- form (/3) with a structure characterized by zig-zag tified [19, 21]. Infrared and Raman spectra of this chains of Re atoms propagating along the c axis material have been studied in detail [21, 22] but of the unit cell. It is also possible to directly synthe- other properties are not known. size the high temperature form [2, 3] and the material is stable in the range 570 to 1320 K. Magnetic susceptibility studies [4] indicate that both the forms of Re02 are Pauli paramagnetic. Resistivity data indicate that /3-Re02 is metallic [3] and most probably the low temperature modification also is metallic. Monoclinic Re02 forms rutile type solid solutions with V02 and Mo02 [5].

Re2 05 is prepared by precipitation from aqueous solutions [1] and has a tetragonal structure. It appears to be stable up to 470 K, but decomposes to

Re2C>7 and Re02 in vacuum at ^ 520 K. Detailed properties are not known at present.

Re(>3 is cubic and no phase transformations are known in the range 1 to 300 K [2, 6-8]; the material disproportionates to Re02 and Re 20? in the neighbor- hood of 670 K. Re0 3 has a red metallic luster and magnetic susceptibility [6, 8], electrical [6, 8], optical [9], NMR [10], magneto-thermal oscillation [II], Figure XII.2. Cubic ReOa structure. Each metal atom is at the center of an octahedran of oxygen atoms. This and de Hass-van Alphen effect [12] studies confirm structure is closely related to the perovskite structure which is obtained by the typical metallic behavior of ReOs. insertion of a large cation in the center of the cube shown.

Rhenium oxides

Oxide and description Data Remarks and inferences References of the study

ReOj

Crystal structure and Low -temperature for (T <570 K): The low temperature form is [2, 23]. x-ray studies. Monoclinic: space group, P2i/c; closely related to rutile structure. Z = 4; a =5.562 A; 6=4.838 A;

c — 5.561 A; /3 = 120.87°. High temperature form (T>570 K): orthorhombic; space group, Pbcn; Z = 4; a =4.810 A; 6 = 5.643 A; c = 4.601 A.

123 Rhenium oxides—Continued

Oxide and description Data Remarks and inferences References of the study

Electrical properties. /S-ReOj: Metallic behavior is indicated; data [3]. P (4.2 K)«1.2X10“ 6 ficm; for a-Re02 are not available. -4 p (300 K) » 1.0 X 10 ficm.

RejOj

Crystal structure. Tetragonal; Z — 4; a = 5.80 A; c = The substance has been obtained [1]. 12.87 A. from aqueous solutions; exact purity and characterization doubtful; other properties not known. ReOs

Crystal structure. Cubic, space group, Pm3m; Z = 1; This simple cubic structure is [8]. a = 3.7474 ±0.0003 A. closely related to the perovskite

structure since the latter is obtained by insertion of a large cation in the center of the cube of the ReOa structure (fig. 1).

-6 Magnetic and electrical Xm (300 K) =74.5X10 emu. Weak paramagnetism is indicated [6, 8]. 7 properties. p (100 K)«6X10~ item; contrary to the earlier data [6]. p (300 K)«lXl0" 6 item.

Optical and NMR studies. Absolute reflection measurements The data confirm the metallic be- [9, 10]. on ReOa give plasma edge at 2.1 havior of ReOs in which the con- eV; interband transitions domi- duction bands are predominantly nate the optical spectrum above d like. the plasma edge, m * (calc.)~

0.86 m 0 . k - -(0.25 ±0.02)%.

Band structure of ReO». LCAO, APW, and Slater-Kostiner Honig et al. [17] point out that the [16, 17]. methods give a band structure band structure scheme will also be in essential agreement with the useful in the characterization of band diagram proposed by Good- the band structure for perovskites enough [13-15] and in reasonable which have a closely related agreement with the optical and structure. Fermi surface data. The conduction bands are predominantly d like.

RejO? T = 300 K; Orthorhombic; space The structure consists of strongly [20]. group P2i2i2i; Z = 8; a = 12.508 distorted ReOe octahedra and A; b = 15.196 A; c = 5.448 A. fairly regular ReO« tetrahedra which are connected through corners to form polymeric double layers in the ac plane. The double layers have only van der Waals contacts to neighboring

ones. RejO? is one of the few known examples where metal atoms of the same oxidation state occur with the coordination numbers 4 and 6 in the same structure. In its structure and bond

124 Rhenium oxides—Continued

Oxide and description Data Remarks and inferences References of the study

properties, Re207 represents an intermediate between the polymeric oxides MoOg and WOg and the more covalently bonded OsO*, which forms a molecular structure.

IR of Re 207 . Several bands in the region 400- [21]. 1010 cm-1 are noted and assigned to various stretching and bending vibrations.

References III. 6. Osmium Oxides

The dioxide, 0s02 and the tetroxide, Os04 , have

[1] Tribalat, S., Dalafosse, D., and Piolet, C., Compt. Rend. been studied exhaustively. Os02 forms lustrous yel- (Paris) 261, 1008 (1965). lowish brown crystals which decompose at T>770 [2] Magneli, A., Acta Cryst. 9, 1038 (1956); Acta Chem. is solid soluble in nonpolar Scand. 11, 28 (1957). K. Os0 4 a covalent and

[3] Rogers, D. B., Shannon, R. D., Sleight, A. W., and solvents; the crystals are colorless with a low melting Gillson, J. L., Inorg. Chem. 841 8, (1969). point (~310 K) and high vapor pressure. [4] Gibart, P., Bull. Soc. Chim. France 444 (1967). rutile [5] Marinder, B.-O., and Magneli, A., Acta Chem. Scand. Os02 has a tetragonal structure [1-4] and 11, 1635 (1957). exhibits slight temperature dependent magnetic sus- [6] Ferretti, A., Rogers, D. B., and Goodenough, J. B., J. ceptibility It has a very low room-temperature Phys. Chem. Solids 26, 2007 (1965). [3]. [7| Gukova, Yu. Ya., and Emolaev, M. I., Russian J. Inorg. resistivity, positive temperature coefficient of re- Chem. (English Transl.) 13, 777 (1968). sistivity and low negative Seebeck coefficient [1, 3] [8] Quinn, R. K., and Neiswander, P. G., Mat. Res. Bull. 5, 329 (1970). indicative of metallic behavior. No phase transitions

[9] Narath, A., and Barham, D. C., Phys. Rev. 176, 479 are known in Os02 in the temperature range 77 to (1968). 500 K. [10] Feinleib, J., Scouler, W. J., and Ferretti, A., Phys. Rev. 165, 765 (1968). Thiele and Woditsch [2] report two forms (black [11] Graebnev, E., and Greiner, J. E. S., Phys. Rev. 185, 992 and brown) of Os0 both have tetragonal sym- (1969). 2 ; [12] Marcus, S. M., Phys. Letters 27A, 584 (1968). metry, but the lattice parameters and x values differ [13] Rao, C. N. R., and Subba Rao, G. V., Phys. Stat. Solidi significantly. The lattice parameters of the brown (a) 1, 597 (1970). form are identical with the data reported by other [14] Goodenough, J. B., J. Appl. Phys. 37, 1415 (1966). [15] Goodenough, J. B., Czech. J. Phys. 17B, 304 (1967). workers on single crystal Os02 [3, 4]. It is possible [16] Mattheis, L. F., Phys. Rev. 181, 987 (1969). that the black form of Os02 reported by Thiele [17] Honig, J. M., Dimmock, J. O., and Kleiner, W. H., Woditsche is slightly nonstoichiometric or im- J. Chem. Phys. 50, 5232 (1969). and [18] ^ ilhelmi, K.-A., Acta Chem. Scand. 8, 693 (1954). pure. [19] Krebs, B., and Muller, A., Z. Naturforsch. 23b, 415 OsO* at room temperature is a solid and has a (1968). compound [20] Krebs, B., Muller, A., and Beyer, H. H., Inorg. Chem. monoclinic symmetry [5]. It is a covalent 8, 436 (1969). (mp, 310 K; bp, 374 K) and is soluble in polar [21] Ulbricht, K., and Kriegsmann, H., Z. Chem. 244 7, tetrahedral in vapor state. (1967). solvents. The molecule is

[22] Beattie, I. R., and Ozin, G. A., J. Chem. Soc. (Lond.) Infrared and Raman studies of 0 s04 is nonaqueous A2615 (1969). solvents have been carried out by various workers [23] Magnlli, A., Andersson, G., Blomberg, B., and Kihlborg, L., Anal. Chem. 24, 1998 (1952). [6, 7]; other physical properties are not known.

125 ± -

Osmium oxides

Oxide and description Data Remarks and inferences References of the study

Os0 2

Crystal structure. T = 300 K: Tetragonal; space Recent single crystal study; the [4).

group, P4 2 /mnm; Z = 2; a = values are in agreement with the 4.5000 ±0.0003 A; c = 3.1830 ± data of earlier workers. 0.0009 A.

-6 Magnetic properties. Xii (X10 emu) is 311, 220, and The values of Thiele and Woditsch [2].

204 at 90, 195, and 291 K re- [2] have been corrected for the

spectively. Greedon et al. [1] diamagnetic contribution. These report a temperature independent authors argue for the presence of susceptibility (77-500 K) ofxM« a magnetic ground state for -6 4+ 120 X10 emu. Os in Os0 2 .

Electrical properties. p (4.2 K) =3.2 X10"7 ficm; p (300 The data indicate the typical metal- [1. 3]. -5 K) =6.0X10 (km. a (~300 K) lic nature of Os0 2 ; estimated ~1 pV/K (negative); no Hall carrier concentrations are ~10 22- 23 3 voltage could be measured. 10 /cm . p, (calc.) ~1.5-15

cm 2 /V s; m* «8 mo. The model

proposed by Goodenough [8] and

Rogers et al. [3] seem to explain the observed behavior.

O8O 4

Crystal structure T = 300 K: Monoclinic; space The structure can be described as [5]. group, C2/c; Z = 4; a =9.379 cubic closest packing of oxygen 0.005 A; b =4.515 ±0.002 A; c = atoms, with osmium in tetra-

8.632 ±0.003 A; /3 = 116±0.05°. hedral holes. The intermolecular 0-0 distances exceed 2.98 A and are consistent with weak inter molecular forces indicated by the high vapor pressure and low melting point.

-1 Infrared studies on solid 0 s04 . Bands at 965 and 959.5 cm are IR of Os04 in CCh solution and [ 6 , 71. noted. Raman studies are also consistent with the behavior in the solid state.

References [4] Boman, C.-E., Acta Chem. Scand. 24, 123 (1970). [5] Ueki, T., Zalkin, A., and Templeton, D. H., Acta Cryst. 19, 157 (1965). [1] Greedon, J. E., Willson, D. B., and Haas, T. E., Inorg. Z. (1967). Chem. 7, 2461 (1968). [6] Krebs, B., and MuUer, A., Chem. 7, 243 [2] Thiele, G., and Woditsch, P., J. Less-Comm. Metals 17, [7] McDowell, R. S., and Goldblatt, M., Inorg. Chem. 10, 459 (1969). -625 (1971) and references cited therein. [3] Rogers, D. B., Shannon, R. D., Sleight, A. W., and [8] Goodenough, J. B., Bull. Soc. Chim. France 4, 1200 Gillson, J. L., Inorg. Chem. 8, 841 (1969). (1965).

126 III. 7. Iridium Oxides firm the metallic nature of the material; as pointed

out by Butler and Giilson [8], the simple one-band

The phase study of the Ir-0 system by Cordfunke model of Rogers and co-workers [7] may not explain and Meyer [1] and McDaniel and Schneider [2, 3] the observed behavior. Ryden et al. [6] have found indicate that Ir0 2 is the only condensed oxide phase that electron-phonon and electron-electron inter- stable in an air environment. Ir02 has a tetragonal band scattering mechanism accounts for the ob- rutile structure and no phase transformations are served temperature dependence of resistivity. known in the range 4.2 to 1000 K [4-8]. Ir0 2 forms complete solid solutions with Ru02

Magnetic properties of Ir02 have not yet been and to a limited extent with Ti02 and Sn02 [2, 3]; investigated. Electrical resistivity studies [5-8] con- these have the rutile structure.

6

Iridium oxides

Oxide and description Data Remarks and inferences of the study

Ir0 2

Crystal structure and T = 298 K: Tetragonal; space TEC is positive in Ir0 2 anisotropy ;

electrical properties. group, P4 2 /mnm; Z = 2; a = is evident. The compound is 4.4980 ±0.0003 A; c = 3.1543± metallic and most probably Pauli 0.0002 A. TEC (X10“ /°C): paramagnetic. e : || 1.7; ||“: 3.8. p (4.2 K)~1.5X 8 5 10~ Ocm; p (298 K)~3.0X10- Ocm. f?H (77 K) ~ — 3.12; juh 2 ( 77 K) =130 cm /V s; RH (300 K) ~ -2.60; mh (300 K)=7.5 cm*/V s.

References III. 8. Platinum Oxides

[1] Cordfunke, E. H. P., and Meyer, G., Rec. Trav. Chim. 81, 495 (1962); 81, 670 (1962). system is [2] McDaniel, C. L., and Schneider, S. J., J. Res. Nat. Bur. Although the literature on the Pt-0 Stand. (U.S.), 71A (Phys. and Chem.), No. 2, 119-123 extensive due to the catalytic and electrochemical (Mar.-Apr. 1967). of them [3] McDaniel, C. L., and Schneider, S. J., J. Res. Nat. Bur. applications of the platinum oxides, many Stand. (U.S.), 73A (Phys. and Chem.), No. 2, 213-219 are poorly characterized. Recent careful study by (Mar.-Apr. 1969). the existence of [4] Hazony, Y\, and Perkins, H. K., J. Appl. Phys. 41, 5130 Muller and Roy [1] has revealed (1970). in a-Pt02 , /3-Pt02 , and Pt 30 4 as the stable oxides [5] Ryden, W. D., Lawson, A. W., and Sartain, C. C., Phys. Letters 26A, 209 (1968). the Pt-0 system. Contrary to earlier reports [2-6], [6] Ryden, W. D., Lawson, A. W., and Sartain, C. C., Phys. the existence of PtO and Pt303 are doubtful. Rev. Bl, 1494 (1970). chain structure [7] Rogers, D. B., Shannon, R. D., Sleight, A. W., and Pt30 4 is cubic [1]. a-Pt02 has a Giilson, J. L., Inorg. Chem. 8, 841 (1969). and is hexagonal; /3-Pt02 is orthorhombic and has a [8] Butler, S. R., and Giilson, J. L., Mat. Res. Bull. 6, 81 (1971). distorted rutile structure [1, 7, 8], a-Pt02 transforms

127 to 0-form either by heating to 970 K at 3 kbar physical properties of a-Pt02 and Pta04 are not pressure in the presence of KCIO3 or by heating to known. Muller and Roy [10] have found that solid

1370 K at 65 kbar pressure [7]. DTA, TGA, and solutions of the form Cui_ xPt xO possessing the high temperature x-ray studies show that 0-PtO2 tetragonal structure can easily be formed. The ex- decomposes to the platinum metal and oxygen at trapolated lattice parameters for the hypothetical

~860 to 920 K [7, 8]. TtO’ do not quite agree with the values reported by

jS-PtCL appears to be a semiconductor [7, 9]. The Moore and Pauling [2].

Platinum oxides

Oxide and description Data 6 Remarks and inferences References of the study

Pt 3 0«

Crystal structure. Cubic; space group, Pm3m; Z = 2; — [1].

a = 5.585 A . a-PtCh

Crystal structure. Hexagonal; space group, C6/mmm; Samples are poorly crystallized; [1]. Z = 1; a =3.10 A; c = 4.29-4.41 A. Pt-O-O-Pt-O-O chain structure in the c direction.

0-PtO 2

Crystal structure and Orthorhombic; space group, Pnnm; Distortion from the rutile struc- [1. 7, 9].

electrical resistivity. Z = 2; a =4.487 ±0.0005 A; 6 = ture is small; semiconductor be- 4.536±0.0005 A; c = 3.137± havior (in the range 4.2-300 K). 0.0005 A. p (300 K) *10 ficm; £0=0.2 eV. Decomposes in air at ~860-920 K.

References III. 9. Gold Oxides

Although Shishakov [1] earlier reported the forma-

tion of hexagonal AU 3O 2 on the surface of heated [1] Muller, O., and Roy, R., J. Less-Common Metals 16, 129 (1968). gold metal at 770 K, recent studies by Muller and

[2] Moore, W. J., Jr. and Pauling, L., J. Am. Chem. Soc. [10] Roy [2] indicate AU2 O 3 to be the only crystalline 63, 1392 (1941). oxide phase in the Au-0 system. Attempted prepara- [3] Suzuki, T., Z. Naturforsch. 12a, 497 (1957). [4] Every, R. L„ J. Electrochem. Soc. 112, 524 (1965). tion of Au2 0 has not been successful [3]. [5] Fryburg, G. C., J. Chem. Phys. 42, 4051 (1965). Au20 3 is cubic (a = 4.832 A; space group, P43m) [6] Shishakov, N. A., Andreeva, V. V., and Andruschenko, [2, 4], The physical properties are not known in N. K., Stroenie i Mekhanism Obrazovaniya Okisnykh Plenok na Metallakh, Akad. Nauk. SSSR, Moscow, detail. The infrared band's of AU2O 3 have been re- 1959, Chap. 6. -1 ported in the range 515-660 cm . [7] Shannon, R. D., Solid State Commun. 6, 139 (1968); 7, 257 (1969). References [8] Schneider, S. J., and McDaniel, C. L., J. Am. Ceram. Soc. 52, 519 (1969). [1] Shishakov, N. A., Kristallografiya 2, 686 (1957). [2] Muller, O., and Roy, R., Am. Ceram. Soc. Bull. 46, 881 [9] Rogers, D. B., Shannon, R. D., Sleight, A. W., and (1967). Gillson, J. L., Inorg. Chem. 8, 841 (1969). [3] Suzuki, T., J. Phys. Soc. Japan 15, 2018 (1960). Muller, O., and Roy, R., J. Less-Common Metals 19, [4] Schwarzmann, E., and Gramann, G., Z. Naturforsch. 209 (1969). 25b, 1308 (1970).

128 III. 10. Mercury Oxides with zig-zag -Hg-O-Hg- chains (Hg-O = 2.03 A); there is only weak bonding between the chains. HgO is the well known oxide in the Hg-0 system Laruelle [5] has described a rhombohedral form of [1-3]. Vannerberg [4] reported the formation of two HgO obtained by precipitation from aqueous solu- different forms (a- and /3-) of the mercury peroxide, tions; it transforms irreversibly to the orthorhombic Hg02 ; a-Hg02 has a rhombohedral structure whereas form at ~470 K and at this temperature, the trans- /3-form is orthorhombic. formation is a slow process. HgO is diamagnetic

HgO: HgO exists in the red and yellow forms but [7] and the diamagnetic susceptibility is almost the these are not polymorphs; they differ in the particle same for both the yellow and red forms. HgO de- size and give identical x-ray patterns [2], The stable composes at high temperatures to the metal; other form is orthorhombic [1-3, 5, 6] and has a structure properties are not known.

Mercury oxides

Oxide and description Data Remarks and inferences References of the study

HgO

Crystal structure and Orthorhombic; space group, Pnma; HgO has a zig-zag structure associ- [1-3,5]. properties. Z = 4; a =6.6129 ±0.0009 A; 6 = ated with considerable covalency 5.5208 ±0.0007 A; c = 3.5219 ± in the Hg-0 bond. Detailed 0.0005 A. A rhombohedral form properties are not known. can be precipitated from solution and it transforms irreversibly to the orthorhombic modification at ~470 K; detailed structure not known. The red and yellow colors of HgO seem to be due to the particle size effect. HgO is diamagnetic; x (red form) = — 0.221 X10-6 emu/g; x (yellow -6 form) = — 0.2 16 X10 emu/g. X increases in the range 298-573 K. x-T plot shows peaks at 323 (red form) and 387 K (yellow form); the reason is not clear.

-1 [8, 9]. Infrared and Raman Low frequency band is at 67 cm ; spectral studies. Raman bands at 331 and 550 -1 cm . The data are interpreted in terms of the chain structure of HgO.

HgO 2

Crystal structure. a-form: rhombohedral; a =4.74 A; [4]. a ~90°. /3-from: orthorhombic; space group, Pbca; Z = 4; a = 6.080 A; 6 = 6.010 A; c =4.800 A. Detailed data on the properties are lacking.

P., Compt. Rend. (Paris) 241, 802 (1955). References [5] Laruelle, [6] Rooymans, C. J. M., Philips Res. Rep. Suppl. 1, 1 (1968). Chem. [7] Mikhail, H., Hanafy, Z., and Salem, T. M., J. (1961). [1] Zachariasen, W., Z. Physik. Chem. 128, 421 (1927). Phys. 35, 1185 D., and Armand, H., J. Chim. Phys. Physico- [2] Roth, W. L., Acta Cryst. 9, 277 (1956). [8] Edmond, Biol. 1030 (1968). [3] Aurivillius, K., Acta Chem. Scand. 10, 852 (1956); 18, chim. 65, Hall, R., Austr. Chem. 22, 1305 (1964). [9] Cooney, R. P. J., and J. J. [4] Vennerberg, N. G., Arkiv for Kemi 13, 515 (1959). 331 (1969).

129 Recent single crystal studies on V 3Oi 3 confirm a negative IV. Some Recent Studies — [7] volume change ( 0.5%), a positive enthalpy change (-f-400 cal/mol) and a semiconductor-metal transition at 150 K. Titanium Oxides There is no change in the crystal system. Marezio et al. [8] have studied some structural aspects of

Rao and co-workers [1] have recently found that pressure Vi_* Cr* O 2 in detail. has no effect on the TuOj transition; this is consistent with the band-crossing mechanism wherein the aT and eT bands References cross as the c/a ratio increases.

The very recent study of Marezio et al. [2] published in [1] Honig, J. M., Chandrasekhar, G. V., and Sinha, A. P. B.,

1973 on the three phases of Ti 4C>7 has clearly established the Phys. Rev. Letts. 32, 13 (1974). structural aspects of the transitions. The room temperature [2] Caruthers, E., Kleinman, L., and Zhang, H. I., Phys. )hase is metallic with an average valence of 3.5 for Ti. In the Rev. B7, 3753 (1973). (ow-temperature insulating nonmagnetic state, there is a [3] Goodenough, J. B., Phys. Rev. B5, 2764 (1972). 3+ 4+ separation into strings of Ti and Ti with the 3+ sites [4] Honig, J. M., Van Zandt, L. L., Board, R. D., and Weaver, forming metal-metal bonds. In the intermediate phase, there H. E., Phys. Rev. B6, 1323 (1972). is no evidence for charge separation or long range order of [5] Kachi, S., Kosuge, K., and Okinaka, H., J. Solid State bonds. Chem. 6, 258 (1973). [1] The atomic vacancies in TiO have been discussed by Good- [6] McWhan, D. B., Remeika, J. P., Maita, J. P., Kinaka, O., Kosuge, K., and Kachi, S., [2]enough [3]. H. Phys. Rev. B7, 326 ESR studies on Ti 306 have been reported by Schlenker et (1973). [3] al. [5]. [7] Saeki, J., Kimizuka, N., Ishii, M., Kawada, I., Nakamo, [4] M., Ichinose, A., and Nakahira, M., J. Cryst. Growth [5] References 18, 101 (1973). [8] Marezio, M., McWhan, D. B., Remeika, J. P., and Viswanathan, B., Devi, S. U., and Rao, C. N. R., Pramana Dernier, D. P., Phys. Rev. B5, 2541 (1972). Tewari, S., Solid State Commun. 1, 48 (1973). [9] 11, 1139 (1972). Marezio, M., et al., J. Solid State Chem. 6, 213 (1973); Phys. Rev. Letters 28, 1390 (1973). Nickel Oxides Goodenough, J. B., Phys. Rev. 135, 2764 (1972). Recent measurements on highly pure NiO crystals gave Honig, M., Wahnsiedler, W. E., and Dimmock, O., [1] J. J. 2 * = the hole mobility, /u

[1] Spear, W. E., and Tannhanser, D. S., Phys. Rev. B7, Vanadium Oxides 831 (1973).

Recent studies of co-workers on Honig and [1] Vo. 99 Cro .01 Niobium Oxides O 1.6 in the 200 to 900 K range indicate that the electrical anomaly is extrinsic and is caused by the coexistence of phases Recently, measurements of electrical properties of NbO in over a wide temperature range. If this be the case, much of high magnetic fields have been reported by Honig et al. [1] the earlier arguments based on the I-M transition will be who have interpreted the results in terms of a nearly free invalid. electron model in which holes and electrons contribute jointly Caruthers et al. [2] have recently calculated the band struc- to conduction process; magnetoresistance anomalies have also ture of VO 2 . been examined by these workers. The influence of atomic vacancies on the band structure of VO has been discussed by Goodenough [3]. Reference For a recent ESCA study on V 3Os, see [4],

For recent data on V 2O 6 and V n 02n-i, see [5]. [1] Honig, J. M., Wahnsiedler, W. E., and Ekland, P. C., J. Heat capacity studies on V„ Chn-i are given in [6]. Solid State Chem. 6, 230 (1973).

130 U.S. GOVERNMENT PRINTING OFFICE : 1974 0-522-205 4

NBS-1 14A (REV. 7*73)

U.S. DEPT. OF COMM. 1. PUBLICATION OR REPORT NO. 2. Gov’t Accession 3. Recipient's Accession No. BIBLIOGRAPHIC DATA NBS NSRDS-49 N °- SHEET

4. TITLE AND SUBTITLE 5. Publication Date TRANSITION METAL OXIDES May 1974

Crystal Chemistry, Phase Transitions and Related Aspects 6. Performing Organization Code

7. AUTHOR(S) 8. Performing Organ. Report No. C. N. R. Rao and G. V. Subba Rao 9. PERFORMING ORGANIZATION NAME AND ADDRESS 10. Project/Task/Work Unit No. NATIONAL BUREAU OF STANDARDS DEPARTMENT OF COMMERCE 11. Contract /Grant No. WASHINGTON, D.C. 20234

12. Sponsoring Organization Name and Complete Address (Street City, State, ZIP) 13. Type of Sc , Report Period Covered

16. Same as Number 9. Final

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15. SUPPLEMENTARY NOTES Library of Congress Catalog Card Number: 73-600267

ABSTRACT (A 200-word or leas /actual summary of moat significant Information. If document includea a significant bibliography or literature survey, mention it here.)

A survey is made of the data describing the thermodynamics

of phase equilibria, crystal chemistry and phase transformations

of binary oxides of 3d, 4d, and 5d transition metals. Changes in

17. electrical, magnetic, and other properties which accompany phase

transitions are discussed-Nearly complete coverage of the literature

18. is provided up to 1973.

KEY WORDS (six to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper name; separated by semicolons)

Crystal structure transformations ; critical data, transition metal oxides; electronic properties; phase equilibria; phase transitions; magnetic properties.

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