Crystal Structure Transformations in Binary Halides

Home , Bismuth trifluoride, Carbon tetrabromide, Metal halides, Rubidium bromide, Silver chloride, Thallium halides

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Crystal Structure

Transformations in

Binary Halides u.s. ARTMENT OF COMMERCE National Bureau of -QC*-| 100 US73 ho . 4 1^ 72. NATIONAL BUREAU OF STANDARDS

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Crystal Structure Transformations in Binary Halides

C. N. R. Rao and M. Natarajan

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

NSRDS

NSRDS-NBS 41

Nat. Stand. Ref. Data Ser., Nat. Bur. Stand. (U.S.), 41, 53 pages (July 1972) CODEN: NSRDAP

Issued July 1972

For sale by the Superintendent of Documents, U.S. Government Printing Office Washington, D.C., 20402 (Order by SD Catalog No. C 13.48:41)- Price 55 cents Stock Number 0303-0964 Library of Congress Catalog Card Number: 78-186213 Foreword

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hi Preface

Many solids undergo transformations from one crystal structure to another. Studies of such phase transformations are of great value in understanding the nature and properties of solid materials. Recent literature abounds in information on the phase transformations of solids and it is often difficult to obtain relevant data or references on the transformations of any specific solid. We, therefore, considered it worthwhile to collect the literature on the phase transformations of various types of inorganic solids and present them in an organized manner. In this and subsequent reviews are summarized the important data and conclusions regarding the crystal structure transformations of a few series of inorganic solids. In doing so, we have surveyed most of the material abstracted up to 1970 in Chemical Abstracts and Solid State Abstracts. We have, however, not listed references to the entire published literature on the transformations of a given solid, but have presented only the crucial ones. We believe that all the literature can be traced back through the references listed at the end of each section. In reporting the data on the transformations of a solid we have indicated the methods employed as well as the important data and conclusions from the study. For each solid we have given the crystal structure data for the stable phase around room temperature and at atmospheric pressure making use of the standard patterns of the NBS and ACA Crystal Data; we have indicated the temperature wherever data are available. The crystal structure transformations of binary halides form the subject matter of this review. This will be followed by reviews on the transformations of binary oxides and other systems.

IV Contents Page Foreword Ill Preface IV Symbols Employed 1

Introduction 1

1. Hydrogen halides 4

2. Alkali halides 8

3. Ammonium halides 15

4. Alkaline earth halides 24

5. Group IIIA halides 25

6. Group IVA halides 27

7. Group VA halides 30 8. 3 d Transition metal halides 31 9. 4d Transition metal halides 34 10. Sd Transition metal halides 40 11. Rare-earth fluorides. 41 12. Inert gas halides 42

13. Halogens ; 43

—

Crystal Structure Transformations in Binary Halides*

C. N. R. Rao and M. Natarajan

A critical survey of the data describing crystal structure transformations in binary halides is compiled. Data on thermodynamic, crystallographic, spectroscopic and electronic properties are given for each transformation. Experimental techniques used to obtain the data are named and comments on the data are included in the tables. The literature is surveyed up to 1970. References have been selected on the basis of their pertinence to the data which are cited and do not represent all the available literature.

Key words: Binary halides; crystal structure transformation; electronic data; phase transformation: spectroscopic data; thermodynamic data; x-ray diffraction data.

Symbols Employed transformations. Valuable information has been obtained on the crystallography, thermodynamics, P = Pressure and kinetics as well as on the changes in optical, T = Temperature in K dielectric, magnetic and electrical properties — Transformation temperature accompanying the transformations of a large Tt number AT = Thermal hysteresis in a reversible trans- of inorganic solids. The subject of phase transforma- formation tions in inorganic solids has been reviewed by 1 P T = Transformation pressure Staveley [l], Ubbelohde [2] and Rao [3, 4]. We shall

AH tr — Enthalpy change accompanying the now briefly describe the general features of crystal transformation structure transformations.

C p = Heat capacity at constant pressure Thermodynamics of Transformations. Classical AS — Entropy change thermodynamics gives a simple treatment of the AV = Change in volume accompanying a transi- equilibrium between two phases. The equilibrium tion properties of each phase are defined by the Gibbs p = Density free energy function, G, the pressure, P, and the a: = Volume thermal expansion coefficient temperature, T. The G-P—T surfaces of two phases — 7) Linear thermal expansion coefficient are considered to be independent of each other and e = Dielectric constant in a first order phase transformation the surfaces

Ps = Spontaneous polarization intersect at the transition point. If the transforma- E c = Coercive field tion takes place from a low-temperature to a high tan 8 = Dissipation factor temperature form, there will be an entropy increase cr = Electrical conductivity and an associated change in volume during the E n = Energy of activation transformation. The classical thermodynamic ap- E Cation migration energy proach in describing phase transformations is = Es Formation energy of Schottky defects inadequate since a variety of systems are known atm = Atmosphere to exhibit transformations occurring over a wide cal = Calorie range of temperatures. These systems show eV = Electron volt anomalous changes in specific heats and specific DTA = Differential thermal analysis volumes and show evidence of “premonitory” = IR Infrared phenomena. The transition temperature, Tt , in Z — Number of molecules per unit cell these transformations is taken to be that at which NMR = Nuclear magnetic resonance the heat capacity or the coefficient of expansion NQR = Nuclear quadrupole resonance shows a maximum variation. This class of trans- EPR = Electron paramagnetic resonance formations which is not predicted by classical thermodynamics has often been referred to as “gradual, smeared or diffuse” transformations. Introduction Essentially, the difference between these transformations and the first order transformations de- There has been increasing activity in recent years scribed by classical thermodynamics lies in the in the area of crystal structure transformations in nature or/and the magnitude of the discontinuity inorganic solids. A variety of experimental techniques has been employed to study these phase

Supported by Project G— 77 of the National Bureau of Standards under the Special 1 Figures in bracket indicate the literature references that appear at International Programs. the end of each section.

1 E

of the thermodynamic energy functions and their (if the two phases are not crystallographically re- derivatives. lated). While transformations classified as first In first order transformations, the difference in order are structurally discontinuous and involve the free energies of the two phases is given by drastic variations in energy, there are indications 0 = AC = G'z — Gi = A — TAS 4- PAV, where AE is of higher order effects such as premonitory phe- the difference in internal energies of the two phases; nomena and thermal hysteresis in many of these if PAV in a transformation is small, one can com- transformations. pare the stabilities of the two phases in terms of In continuous or second- (or higher) order trans- their Helmholtz free energies: 0 = AA = AE — TAS. formations, premonitory changes are often found. According to this equation, the transformation Even though in these transitions there should be results from the compensation of the lattice energy no discontinuity of structure, x-ray studies have difference by the entropy difference at Tt . At Tt , shown that the two structures coexist over a narrow the free-energy curves intersect each other at region of temperature. In some cases, transforma-

AG = 0 . These phase transformations involve tions are found to proceed through the formation step-wise increase in energy with increase in tem- of “hybrid crystals" where the two structures co- perature and show discontinuity in the energy as exist within a general pattern of orientation. well as in many other properties of the system There are many phase transformations which at T(. strictly belong to neither first order nor to second Many instances are known where phase trans- order. For example, the ferroelectric transformation formations are induced by application of pressure; of KH2PO4 should theoretically be first order but in these transformations the PAV term is very conforms more closely to second order. The trans- important. Wherever there is a decrease in volume formation of BaTiC>3 and related materials show during a transformation, pressure would favor such lambda-like changes in properties as well as small a transformation and the thermodynamic variables changes in latent heat. There is superposition of are simply correlated by the Clausius-Clapeyron second-order behavior on many first order trans- equation. Since the phase produced by the appli- formations as in the transformation of alkali cation of pressure will have a lower volume, the sulphates. It often happens that transformations decrease in volume is compensated by an increase have observable heat effects and yet the approach in the AE or in the coordination number. to the transformation is evidenced by a gradual In the second- (or higher) order transformations, change of properties. Even the order-disorder energy increases gradually with temperature until change in second order transformations may be the rate of increase of energy sharply falls. As a seen as an abrupt change towards the termination. result of this, discontinuity is found in specific Irreversible crystal structure transformations are heats or specific volumes. Since lambda-shaped found in many systems where one of the polymorphs specific heat curves are generally obtained in these is metastable in a particular temperature (or pres- transformations, these are also termed lambda- sure) range. In such transformations, a polymorph point transformations. Second- or higher-order transforms to the other form on heating above a transformations occur over a temperature interval particular temperature and remains in the new and are generally associated with increase in dis- transformed structure even after cooling. It is not order with rising temperature. If a solid has perfect possible to assign any critical transformation tem- order at absolute zero of temperature, a rise in perature in an irreversible transformation and the temperature disturbs the order of the structure transformation will be a function of time as well and with progressive increase in temperature, the as temperature. One can only define a temperature structure gets more and more distorted until the below which a transformation does not take place. transformation temperature Tt is reached. One Typical examples of irreversible transformations can introduce an order parameter £ which is equal are anatase- rutile (of Ti02), aragonite-calcite (of to unity at absolute zero of temperature and be- CaCO.3) and cubic-hexagonal or cubic-monoclinic comes zero at Tt . transitions of rare-earth sesquioxides. Many of Although for the sake of definition one can classify these systems showing irreversible transformations transformations as first-order and second-order are associated with small changes in internal energy, types, in reality 'it is difficult to find out exactly but large energies of activation may involve the whether a transformation belongs to either of these breaking of the primary coordination or change in two types. By definition first-order transformations bond type. should occur at one temperature and two phases Simple thermodynamic considerations indicate of the same substance should not coexist at tem- that one polymorph of a solid can be more stable the other. peratures other than Tt . If a single crystal of a under a specified set of conditions than substance is taken through a transition point, the It should be realized, however, that it is difficult crystal should break up into a random assembly of to strictly define the thermodynamic stability of a one or more crystals of the other phase at T, polymorph since a variety of physical factors as

(depending on the magnitude of AV ). If the tem- well as compositional differences affect the stability is any perature is then decreased below Tt , the first phase of polymorphs markedly. Thus, if there should reform randomly from the second phase difference in the stoichiometry between two poly-

2 morphs, one cannot really classify them as belong- mations of barium titanate and lead titanate are ing to the same polymorphic set. Similarly, if likely to belong to this category since the coordina- different samples of a solid contain different tion of titanium goes from 6 to 5 due to the shifting quantities of impurities, the regions of thermo- of the position of titanium from the center of the dynamic stability will vary. octahedron towards the vertex. A better insight into the nature of phase trans- Many of the transformations involving changes in formations can be obtained by correlating structural higher coordination may also have to proceed changes and the thermodynamics of transforma- through the breaking of the primary bonds and for tions. The thermodynamics of transformations gives this reason, the energy changes and other features some indication as to the nature of structural of the reconstructive transformations involving changes accompanying transformations, but a higher coordination may resemble those of the better understanding can be obtained by considering reconstructive first order transformations. In some the geometrical or crystallographic mechanisms. transformations changes in higher coordination can Since phase transformations in solids are associ- be effected by a distortion of the primary bond. Such ated with increase in entropy, one can visualize transformations may be called distortional or dis- the structural origin of the entropy increases. For placive transformations. These transformations example, in first-order transformations, the phases will involve considerably smaller changes in energy with high internal energy and low density will have and are usually fast. In the displacive transforma- high entropy as well because of the greater magni- tions, the high temperature form is usually more tude of voids in the structure. In second-order or open and has higher specific volume, specific heat continuous transformations, the entropy changes and symmetry. In addition to the transformations have been related in many systems with the poten- discussed above, Buerger also discusses disorder tial barrier to rotation. While going from the low- transformations of two types: rotational and to the high-temperature forms in these systems, substitutional transformations. there will be some randomization of the structure. Experimental Techniques Employed to Study Ubbelohde has discussed the structural origins [2] Phase Transformations. A variety of techniques increase in in of entropy phase transformations have been employed for the study of phase trans- solids. formations in addition to x-ray diffraction which Structural Changes Accompanying Transforma- is an essential part of the study of any crystal tions. Buerger [5] has classified various types of structure transformation. Once a phase transition thermal transformations based on structural is identified in a solid by any technique, one has changes involving the primary or higher coordina- to investigate the same by other methods depend- tion. Transformations where there are changes in ing on the particular properties of interest. For the primary coordination will involve a more example, the ferroelectric transformation of barium drastic change in energy and structure rather than titanate has been investigated by the anomalies those where there are changes only in the higher in heat capacity, breaks in thermal expansion coordination. This can visualized in the case of be curves and variations in optical properties (including ionic crystals, where the energies of the ionic bonds vibrational spectra) in addition to changes in crystal vary inversely as the distance the energies and structure and dielectric properties. The semicon- of the van der Waals bonds vary inversely as the ductor-to-metal transitions in VO 2 and V 2 O 3 have sixth power of distance. Accordingly, one can been studied by measurements of changes in mag- classify transformations of the first order into two netic susceptibility, heat capacity, electrical categories: first coordination transformations and conductivity in addition to methods like nuclear higher coordination transformations. magnetic resonance spectroscopy, Mbssbauer Changes in primary coordination can take place spectroscopy, neutron diffraction, and differential by a reconstructive transformation, where the first thermal analysis. The transformation of KNO 2 coordination bonds are broken and reformed. Such provides another interesting example where a transformations will involve high energies of variety of methods of relevance have been employed activation and will be sluggish. Further, there may to study the transformation. The reversible transi- be no symmetry relation between the two phases. tion of KNO 2 was established by differential thermal Reconstructive transformations give rise to large analysis; x-ray diffraction was utilized to prove discontinuities such as cell dimensions, symmetry, that the change was from monoclinic to cubic internal energy, specific heat, etc. Changes in structure. Based on the crystal structures and also primary coordination may also take place through a analogy with NaNC> 2 , it was suspected that the dilatational mechanism as in the thermal trans- transformation was associated with a change from formation of cesium chloride or (479 °C) ammonium a ferroelectric to a paraelectric state. In fact, the chloride (184 °C). Dilatational transformations are dielectric hysteresis loop disappeared around likely to be rapid compared to reconstructive 40 °C (Curie temperature, Tc the dielectric transformations. ); anomaly was also observed at this temperature. There are some transformations where only a A plot of electrical conductivity against temperature part of the structure may undergo a change in the indicated a break at 40 °C. A study of the tempera- primary coordination. The ferroelectric transfor- ture-dependent infrared spectrum of KNO 2 showed

456-089 0 - 72 -2 that the absorbance of the NO 2 deformation fre- by employing magnetic susceptibility measure- -1 ments. of magnetic quency (830 cm ) showed a marked increase in Changes anisotropy as a func- intensity at the Curie temperature. This is similar tion of temperature has also been used to study to the behavior observed in the ferroelectric-para- polymorphic changes. Mbssbauer spectroscopy electric transformation of KNO 3 . The literature containing (or doped with) the appropriate muclei has many such examples where several techniques provides valuable information on such transitions have been employed to examine phase transfor- if the spectra are recorded at different temperatures. mations in solids. Measurements of heat capacities as a function of X-ray diffraction methods are by far the most temperature are valuable in the study of phase important tools for the study of crystal structure transformations (particularly in the case of second transformations. Identification of structures of or higher order transitions). Differential thermal various phases by x-ray diffraction is very essential analysis is another powerful tool in the study of irrespective of what other techniques one employs phase transformations. This technique has been to identify or examine the changes in the system employed to study the enthalpy change, activation accompanying the transformation. Single crystal energy and thermal hysteresis in transformations. work with Laue and Weissenberg photographs are In principle, any technique that can measure a particularly useful for a detailed knowledge of the property of the substance which undergoes marked mechanism of a phase transformation. Neutron change during the transformation can be employed diffraction has been employed effectively as a tool for the study of phase transformations. Dilatometric to examine the positions of light atoms as well as to measurements, change in Young’s modulus, thermal study magnetic structures. expansion of solids are some of the classical Both electronic and infrared spectroscopy are methods for the study of phase transformations. useful tools, provided a characteristic absorption band of the substance shows variations during a transformation. Wide-line NMR spectroscopy has References been useful in the study of transformations containing suitable nuclei. [1] Staveley, L. A. K., Quart. Rev. (London) 3,64 (1949). Ubbelohde, A. R., Quart. Rev. (London) 1 1, 246 (1957). Measurements of (a.c. or d.c.) conductivity and [2] [3] Rao, C. N. R., and Rao, K. J., in Progress in Solid State dielectric are for constant important techniques the Chemistry. Ed. H. Reiss, Vol. 4, (Pergamon Press, Oxford, study of phase transformations. The transformations 1967). are generally indicated by a break in the con- [4] Rao, C. N. R., in Modern Aspects of Solid State Chemistry, Ed. C. N. R. Rao (Plenum Press, New York, 1970). ductivity versus temperature curves. Magnetic [5] Buerger, M. J., in Phase Transformations in Solids, Ed. transitions (paramagnetic ^ ferromagnetic or para- R. Smoluchowski (John Wiley, New York, 1957) also see magnetic-antiferromagnetic) are generally studied Fortschr. Mineral. 39,9 (1961).

1. Hydroge Halides

Phase transitions of solid hydrogen halides have recently found to be associated with a change from been investigated by many workers with a variety a ferroelectric to a paraelectric state. of experimental methods. The transitions generally The gradual transitions in hydrogen halides were take place at low temperatures. Dielectric relaxa- examined by Pauling [24] who attributed the transition, dielectric anomalies, specific heat, NMR and tions in HBr and HI to the incipient rotation about calorimetric measurements are some of the methods one crystal axis and the expansion of the crystal employed for these studies in addition to x-ray and only along this axis. The difference between the

(a:) neutron diffraction. While in HC1 there is only low and high (/3 ) forms of HC1 has been likened one transition at ~ 98.36 K, in HBr and HI there are by Hettner [13], to that between liquids and solids. two transitions. Molecules in the a-state vibrate about fixed and The transformation in HC1 involves a change from regularly oriented equilibrium positions while in the face centered cubic structure to a low symmetry the /3- state only the equilibrium positions of several structure (face centered orthorhombic); the transi- temporary groups are oriented with respect to one tion is of first order. HBr undergoes two transitions, another. On the basis of classical mechanics, one at ~ 89 K (III —^ I) and the other at — 1 14 K Kirkwood [17] accounts for the transitions in hydro- (II —^ I). Phases III, II, and I are encountered suc- gen halides, in terms of hindered rotation. The cessively when liquid HBr is cooled. The transition effects of dielectric relaxation during phase changes at 89 K is first order while the one at 114 K is a in hydrogen and deuterium halides have been dis- second order transition. Two transitions in HI are cussed by Bauer [ 2 ]. Powels [28] has discussed the known at —70.1 K (III —> I) and at —125.7 K mechanism of transitions from a knowledge of the (II —* I); these are similar to the transitions in experimental results including those from calori- HBr. A third transition at 25 K has been reported in metric, dielectric, structural and thermal expansion HI from infrared studies. The theory of phase tran- measurements. sitions in hydrogen halides has been studied by Krieger and James [19] proposed a model for a Pauling and others. Some of these transitions are crystal exhibiting successive orientational or rota- 4 >

tional transitions as in hydrogen halides. The model gen halides. Thus, DC1 exhibits the II —* I transition -1 consists of an array of classical rotors with next at ~ 105 K with an enthalpy of 320 cal • mol both ; neighbor coupling. The nature of transitions in DBr and DI exhibit the III — I transition at ~ 93.5 K -1 ~ HC1, HBr, HI, DC1, DBr and DI have been success- (A// tr ~ 169 cal -mol ) and 77.3 K (AHtr -175 -1 — fully explained with this model. Recently, cal mol ) respectively and the II > I transition at ~ -1 ~ Kobayashi and co-workers [18] have suggested that 120 K (A H tr ~ 303 cal mol ) and 128 K (A H tr -1 ~ • the phase transitions in hydrogen halides can be 386.0 cal mol ) respectively. The Tt and AH tr are understood qualitatively on the basis of a two higher compared to those in the respective hydrogen dimensional rotational transition model with two analogues. Just as in hydrogen halides, phase transi- sublattices. tions in the deuterium halides have been investi- The deuterium analogues of HC1, HBr and HI also gated by a variety of experimental methods. exhibit transitions corresponding to those in hydro-

Hydrogen Halides

Substances and Data Remarks References measurement techniques

Hydrogen Chloride, HC1

- Transition is indicated by anomalies in Calorimetric measurements like Tt 98.36 K 17,91. -1 • molar heat, specific heat etc., as a AH tr ~ 284.3 cal mol molar heat or in the Cp versus T function of temperature AS ~ 2.89 e.u. curves. The entropy of HC1 was calculated to verify the third law.

X-ray and neutron diffraction Cubic HC1 has a = 5.44 ± 0.01 A at The Bb2im-space group indicates that [22, 30], 98.5 K. At 85 K, solid HC1 has a HC1 structure is non-centro tetragonal form, with c/a = 1.10 symetric in this phase. The structure where a — 5.27 A, c — 5.797 A. At at 118.5 K is an average of disordered 92.4 K, the structure is face structures. centered orthorhombic with the space group, Bb2im. a = 5.082 A, b = 5.410 A, c = 5.826 A. At 118.5 K, the structure is FCC, with = ~ a 5.482 A. {Tt 98.4 K)

NMR spectroscopy Linewidth is measured as function of The transition is found to be an order- [1]- temperature. disorder type. No evidence was found for a rotational transition.

IR spectra of crystalline HC1 at Two fundamental vibrations at 2704 The basic structural unit in HC1 is a [15]. -1 68 K, in KBr solvent and 2746 cm , are observed. The zigzag hydrogen bonded chain; 107°. angle H — Cl . . . H is ~ the crystal appears to be built from anti-parallel pairs of such chains, but

the relative position of the chains is uncertain.

~~ Dielectric constant, e, and tan 8 Tt 98.4 K The dielectric behavior of solid HC1 is [5. 35], measurements, as a function of tem- predicted by Pauling’s theory of free perature in various frequency ranges rotation of molecules in crystals. The dielectric behavior of liquid and disordered phases of HC1 can be explained in terms of Onsager’s equation.

Dielectric hysteresis and ferro- Spontaneous polarization. Ps , in HC1 This observation seems to support [16]. -2 electric behavior of solid HC1 at low is 1.2 fxc cm at 90 K. The coercive Kobayashi’s two sublattice theory temperature -1 ~ . field. Ec , is 3.3 kV cm The of rotational transition in HC1. ferroelectric loop disappears at ~ 98.4 K.

5 Hydrogen Halides — Continued

Substances and Data Remarks References measurement techniques

Hydrogen bromide , HBr

-» - — ~ Calorimetric measurements like T, (III I) 90 K; (II >1) 118 K; The A// tr values are not consistent [7.10]

molar heat, specific heat etc., as a The A// tr values are in the range with other experimental results. -1 • function of temperature 150-250 cal mol . Transitions The transitions are probably akin are indicated by anomalies in to those in NH^Cl. C,, versus T curves.

X-ray diffraction Existence of solid HBr in the (space The x-ray studies of Ruhemann and [22,23,29], • 7 33 7 33 group V or V i.e., D or D Simon [29] between 82-120 K, structure in the 85-90 K range is however, do not indicate any established beyond doubt. Below fundamental crystallographic - 100 K a= 5.76 A. Below 100 K, changes during the transition. HBr exhibits rhombic symmetry They propose a FCC structure with axial ratios, a:b:c=0. 985:1: throughout this temperature range. 1.075, where a=5.555 A, 6=5.64 A and c = 6.063 A; Z = 4; -3 p = 2.81 g cm .

NMR spectroscopy Line width is measured as a A basic factor affecting the line width [!] function of temperature. is the characteristic flipping time

i.e., the average time to change its orientation. The results suggest that

the phase transition in HBr is an order-disorder type.

Infrared of HBr crystals at 68 K. Two fundamental vibrations at 2404 The basic structural unit in HBr [15]. -1 and 2438 cm are observed. crystals is a zigzag hydrogen bonded chain.

Measurement of e as a function of e varies with oj only below 95 K. The In addition to the two dispersions [6, 25, 26, 27],

frequency, oj and temperature dispersion domains are displaced mentioned, a third one was indicated towards the high frequency with T. at higher frequencies. A rotation One of the dispersions is of tunnel effect is introduced to explain constant amplitude while the the variation of the critical frequency

other is temperature dependent, with temperature. Such an effect is

and exists even after T t . Definite shown to be present in HBr. hysteresis effects were observed in the transitions.

e and tan 8 measurements The sharp increase in e suggests a The complex e is ascribed to a skewed [12, 20] first order transition at —89.8 K arc function found for the low- while a sharp increase in tan 8 temperature phase. Deviations of the suggests a second order relaxation rates can be explained in transition at — 113.6 K. terms of cooperative interaction effects in the disordered phase.

Static dielectric constant and e stat above 117 K agrees well with Phase transitions are indicated by [3], dispersion loss measurements. Onsager’s equation. In phase anomalies in e. II, e rises to peak value of 200 at 89 K and falls to 32 at — 72 K in phase III.

Dielectric hysteresis in HBr single The polarization versus field curves The phase transition in HBr i.e., the [16].

crystals. show a ferroelectric hysteresis second order change is thus loop. The measured spontaneous possibly associated with a change

polarization, P.„ is about from ferroelectric to paraelectric 0.4 g.c cm'2 and the coercive field, state thus conforming to the two -1 E r is —4.3 KV cm . sublattice model of Kobayashi.

Effect of ultrasonic waves on thermal The hysteresis is not affected by The position of the equilibrium curve [«]• hysteresis of transition in HBr. ultrasonic waves or by in the transition interval could not inoculation. be determined.

6 Hydrogen Halides — Continued

Substances and Data Remarks References measurement techniques

Hydrogen iodide, HI

-> Calorimetric measurements like For III I, Tt , 72 K; AH tr ~ 150 The transitions are indicated by anom- 1 7 .11]. - cal 1 -124 alies in heat/C -rsus molar heat, specific heat etc., as a , K; the molar p v - • function of temperature A//, r ~ 395 cal mol h temperature curves.

X-ray diffraction HI has a face centered tetragonal The transitions in HI may be similar to 121,22,23.29], structure with c/a= 1.08 (a = 6.10 those in NH jCl. A) at ~ 100 K. This structure changes to a FCC structure above 127 K: a = 6.19 A at 125 K; -3 p=3.17 g cm .

NMR Spectroscopy The line width is measured as a The line width is affected by the IH function of temperature. average time the molecules take to change from one orientation to another. This indicates the nature of the transition to be an order-disorder type.

Infrared study of a new phase transi- At 25 K, HI undergoes a phase The transition at 25 K is readily re- [14].

tion in HI, in the range 6 to 79 K. change to a phase with more versible and rapid. No hysteresis is molecules in the unit cell and also evidenced in these studies.

the phase below 25 K is one of less symmetry than the high phase above 25 K. Deuterium chloride, DC1 ~ ~ Calorimetric measurement of molar Tt 105.03 K, A H tr 320.1 Both T, and AH lr are higher than in HC1. [4|.

' 1 heat of DC1. cal • mol The transition is probably a first order one.

X-ray and neutron diffraction (as well T, ~ 105 K. At 92.4 K, the structure is The FCC structure at 118.5 K is only an 130], as heat capacity measurement). face-centered orthorhombic, with average of disordered structures, a = 5.0685 A, b = 5.3990 A and similar to that in HC1. On the other

c — 5.828 A. At 1 18.5 K. The struc- hand the unit cell of DC1 shows an ture is face-centered cubic with anisotropic reduction in lattice con- a — 5.475 A. The space group at stants compared to HC1. 77 K is Bb2im (same as HC1). Deuterium bromide, DBr

Calorimetric T, -> measurement of molar ~ 93.5 K (III I) The T, and AH lr are higher than in [4J. -1 heat as a function of • temperature. AH iT ~ 196.7 cal mol . T, ~ HBr. The second transition 120.26 -1 K. AH lr ~ 303 cal mol . appears to go through a metastable

T, ~ 128.28 K. A// tr ~ state. -1 • 386.4 cal mol . X-ray and neutron diffraction (as well Two second order transitions at Below 93.5 K, DBr is isomorphous with [31] as heat capacity). ~ -» 93.5 K (III I) and at ~ 120.3 K DC1 (i.e.. it has an ordered crystal (II I) are noticed. At 84 K, the structure, composed of parallel zig- space group is Bb2im while at zag chains). Between 93.5 K and 107 K the space group is Bbcm. 120.3 K, DBr has a disordered

At 84 K, the structure is ortho- structure where the disorder is rhombic with a = 5.44 ± 0.02 A, orientational with two equilibrium b = 5.614 ±0.005 A, and c = 6.12 orientations for each molecule. ±0.005 A; z= 4. At 107 K, the structure is orthorhombic with a = 5.559 ± 0.005 A, b = 5.649 ± 0.005 A, and c = 6.095 ± 0.005 A.

Kinetics of 3 DBr transition at 93.5 K Tt~ 93.5 K AV~ 3.66 A per unit Kinetics and the nature of the transi- [32] by x-ray and ~ neutron diffraction cell. AV/Vo 1.94 percent. A tion are discussed in terms of these methods. hysteresis of about 0.60 was experimental results. observed.

7 Hydrogen Halides — Continued

Substances and Data Remarks References measurement techniques

Deuterium iodide, DI

-> Calorimetric measurement of molar T, ~ 77.3 K (III I); AH lr ~ 175.6 T, and AH {r are higher than for HI [4,34]. ~ • — — heat as a function of temperature. cal mol '. T, 128.28 K (II » I); transitions. ~ • AH lr ~ 386.4 cal mol '.

References

r 1 1 Alpert. N. L., Phys. Rev. 75, 398 (1949). [17] Kirkwood, J. G., J. Chem. Phys. 8, 205 (1940). [2] Bauer. E., compt. rend. 272 (1952). [18] Kobayashi, K. K., Hanamura, E., and Shishido, F., Phys. [3] Brown, N. L., and Cole, R. H., J. Chem. Phys. 21, 1920 Letters 28A, 718 (1968).

(1953). [19] Krieger, T. J., and James, H. M., J. Chem. Phys. 22 , 796 [4] Clusius, K., and Wolf. G., Z. Naturforsh 2a, 495 (1947). (1954). [5] Cone. R. M., Denison. G. H., and Kemp. J. D., J. Am. Chem. [20] Lacerda, L., and Brot, C., compt. rend. 236, 916 (1953). Soc. 53,1278(1931). [21] Mauer, F. A., Keffer, C. J., Reeves, R. B., and Robinson, [6] Damkiihler, G., Ann. Physik. 31, 76 (1938). D. W.,J. Chem. Phys. 42 (2), 1465(1965). [7] Eucken. A., and Karwat. E., Z. physik. Chem. 112, 467 [22] Natta,G„ Nature 127,235(1931). (1924). [23] Natta, G., Gazz. Chim. ital, 63, 425 (1933). [8] Eucken. A., and Giittner, W., Math, physik., Klasse, Fach- [24] Pauling, L., Phys. Rev. 36, 430 (1930). gruppe. II [N.F.] 2, 167 (1937). [25] Powles, J. G., compt. rend. 230, 836 (1950). [9] Giauque, W. F., and Wiebe. R., J. Am. Chem. Soc. 50, [26] Powles, J. G., Nature 165,686(1950). 101 (1928). [27] Powles, J. G., J. Phys. Radium 13,121(1952). [10] Giauque. W. F., and Wiebe, R., J. Am. Chem. Soc. 50, [28] Powles, J. G., Trans. Faraday Soc. 48, 430 (1952). 2193 (1928). [29] Ruhemann, B., and Simon, F., Z. physik. B15, 389 (1932). [11] Giauque, W. F., and Wiebe, R., J. Am. Chem. Soc. 51, [30] Sandor, E., and Farrow, R. F. C., Nature 213, 171 (1967). 1441 (1929). [31] Sandor, E., and Johnson, M. W., Nature 217, 541 (1968).

[12] Groenewegen. P. P. M., and Cole, R. H., J. Chem. Phys. [32] Sandor, E., and Johnson, M. W., Nature 2 2 3 , 730 (1969). 46 (3), 1069 (1967). [33] Smyth, C. P., and Hitchcock, C. S., J. Am. Chem. Soc. 55, [13] Hettner. G., Ann. physik 32, 141 (1938). 1830 (1933). [14] Hiebert, G. L., and Hornig, D. F., J. Chem. Phys. 26, [34] Staveley, L. A. K., Quartr. Rev. 3,64 (1949). 1762 (1957). [35] Swenson, R. W., and Cole, R. H., J. Chem. Phys. 22, Hornig, D. F., [15] and Osberg, W. E., J. Chem. Phys. 23 , 662 284 (1954). (1955). [16] Hoshino, S., Shimaoka, K., and Niimura, N., Phys. Rev. Letters 27, 1286 (1967).

2. Alkali Halides

All the alkali halides with the exception of CsCl, halides. Pressure transformation (from Fm3m to CsBr, and Csl crystallize in the NaCl structure Pm3m) data have been reported for sodium, potas- (Fm3m), the three cesium halides crystallizing in sium and rubidium halides. The thermal transforma- the CsCl structure (Pm3m). CsCl undergoes a tion data on CsCl as well as the pressure transition first order phase transformation at ~ 480 °C (at data on other alkali halides have been employed 1 atm pressure) from the Pm3m structure to the to evaluate the lattice energies of the halides in the Fm3m structure. The transformation in CsCl is Pm3m and Fm3m phases. The Born treatment of reversible and is associated with considerable ionic solids have been employed with fair success thermal hysteresis, A T, as well as an appreciable to explain the relative stabilities of the Pm3m and change in molar volume, AF. The transformation of Fm3m phases. CsCl is likely to proceed by a mechanism involving The phase transformation in CsCl at 753 K dilatation along the body diagonal; the role of lattice involves a change from the Pm3m structure to the vacancies in the mechanism of transformation is NaCl structure. Menary, Ubbelohde and Woodward not clearly established. The phase transition in [40] suggested that a large increase in vacancies CsCl has been studied with the aid of a variety of occured in the neighborhood of the transition tem- experimental tools. Fm3m structures for CsBr, perature. However, Hoodless and Morrison [21] and Csl have been reported in evaporated thin showed from electrical conductivity measurements films at low temperatures. No other transformation that the transitions may not involve a disruption data are available for CsBr, Csl, or CsF. of the lattice and that the mechanism is likely to No phase transformations are known in lithium be dilatational as suggested by Buerger [10].

8 The Born-Mayer model [23] has been widely em- structure (Pm3m) [7, 25, 47]. The product of the ployed to calculate the lattice energies of ionic volume change, AV, accompanying the transition solids especially of alkali halides. May [38] employed and the transition pressure, P, may be taken to be a three-parameter repulsive term and a higher van equal to the difference between the lattice energies der Waals contribution to account for the greater of the two structures at the transition pressure [25]. stability of the Pm3m phase of CsCl. The use of Several workers have attempted to calculate the a higher van der Waals term was criticized by lattice energy differences between the two phases Tosi and Fumi [68, 69] who employed structure- with the aid of the Born model of ionic solids. dependent repulsive parameters to account for the Structure-dependent [68, 69] as well as the four- phase transition in CsCl. Recently, Rao and parameter [62] repulsive terms in the Born-treatment co-workers [45, 60-62] have employed a four param- have been employed to explain these transforma- eter repulsive term in the Born treatment to explain tions. The need for a higher van der Waals term in the phase transition in CsCl. The model also ac- the Born model to account for the transition energies counts for the stabilization of the Fm3m phase by of KX and RbX (A = C1, Br or I) has been pointed added KC1 or RbCl as well as for the opposite be- out [62]. An atomistic formulation of compressi- havior of the CsCl + CsBr solid solutions [45, 60]. bility has also been used to discuss the pressure It is definitely established that all the potassium transitions of alkali halides [53]. Madan and Sharma and rubidium halides with the likely exception of [65] have calculated the lattice energies of all the the fluorides undergo pressure transformations alkali halides with the Lennard-Jones (12:6) from the NaCl structure (Fm3m) to the CsCl potential.

Alkali Halides

Substances and Data Remarks References measurement techniques

Sodium fluoride, NaF (Fm3m, a = 4.6344 A at 299 K)

Volumetric investigation of high Transition begins at ~ 18.3±1 kbar Transition is reversible and incomplete [48]. pressure transformation in NaF. and 430.5 K. AF~5%. even at 45 kbar.

Quantum mechanical calculation, P T — 280 kbar. The value is slightly different from the [37], (with nearest neighbor repulsion known experimental data. only) of transition pressures for NaF and NaCl for the Fm3m- Pm3m change.

Sodium chloride, NaCl (Fm3m, a=5.6402 A at 299 K)

X-ray investigation of the high Pt, 17.5-18 kbar An extreme value of 300 kbar is also [4. 17, 18, 28], pressure transformation employ- a (Pm3m)=3.36±0.04 A at 298 K reported for Pt- Experimental and ing a diamond anvil (up to ~ 18 kbar) and 1 atm. calculated transition pressures A F 1.0±0.05 cm3 mol -1 are compared for some more halides.

-1 • -1 AS— 1.5±0.3 cal • deg mol .

-1 ~ • Latent heat of the Fm3m-Pm3m AH tT 660±370 cal mol at 298 K. The AH tr is comparable to the A H tr [50], transformation from the temperature for the thermal transition of CsCl. dependence of Pt up to 473 K.

Dilatometric study on NaCl single Pt ~ 21 kbar at 613 K [12]. crystals with organosilicon liquids — 25 kbar at 803 K

as the dilatometric medium. ( dP/dT ) is positive.

Shear induced transition in shock Fm3m-Pm3m transition occurred NaCl is unreliable as a high-pressure [b 33]. loaded NaCl employing a piezo- and conformed to a first order. gauge. electric quartz (up to 30 kbar).

Equation of state for NaCl between Pressure as a function of lattice [15, 16]. 0-500 kbar and 273-1773 K. constant and temperature are obtained for the equation of state.

Dielectric constant and elastic No anomaly (characteristic of a [11]- constant measured as a function of structural change) was found in pressure up to ~ 25 kbar at room elastic or dielectric constants. temperature.

9 Alkali Halides — Continued

Substances and Data Remarks References measurement techniques

Sodium bromide, NaBr (Fm3m. a = 5.9772 A at 299 K)

High pressure transition by volume P T ~ 10-14 kbar at 473 K. The transition is very sluggish. [49, 52], discontinuity method.

Sodium iodide, Nal (Fm3m. «=6.4728 A at 299 K)

Temperature dependence of Pt~~ 10-14 kbar, at 473 K. There is a [52], transition pressure. transition to the Pm3m structure.

Volume discontinuity method up- P T ~ 10.00±2.5 kbar at 437 K; [49], to ~ 50 kbar and 437 K. AF~8%.

Potassiu m fluoride, KF (Fm3m, a — 5.347 A at 299 K)

High pressure transition investigated Pt ~ 88 kbar from theory No experimental P T is known for KF. [17]. by x-rays.

Lattice energies are evaluated for Pt ~ 20 kbar [73], both Pm3m and Fm3m structures. Pt is estimated from available pressure transition data (calcd).

Potassium chloride , KC1 (Fm3m, a — 6.2931 A at 298 K)

3 -1 High pressure studies on KC1 and Pt~ 19-20 kbar. A V~ 0.055 cm g Lattice energies are estimated for [7, 27, 30, 50, -1 10.0 • . x-ray investigation. AH tr ± 8 cal mol both Pm3m and Fm3m structures to 44, 73], a (Pm3m) = 3.61 A at 32 kbar. explain the stability etc.

Optical method — by noting the P T - 19-30 kbar Transition is indicated by a decrease [31,66]. shift in the vibrational frequency of in the transparency of the salt at Pt- - an added ion like CN in KC1 crystal subjected to pressure.

Calculation of P^from experimental PT ~ 22-36 kbar. AF/F- 0.08 [17, 64], data on coefficient of thermal expansion and compressibility.

Potassium bromide, KBr (Fm3m, a = 6.60 A at 298 K)

Pressure transition in KBr by PT ~ 17-19.3 kbar. AV ~ 0.0325 Transition pressure is independent of [7, 30, 52]. 3 -1 volume discontinuity method. cm g . temperature. Transition is sluggish at room temperature. Around 323 K,

hysteresis is about 30 bar. The experimental data are in agreement

with Bridgman’s reports [7|.

Optical study of polymorphic change Pt ~ 18 kbar. Transition was ob- [31,66]. under pressure. served by noticing an abrupt change in the transmission of the

material as it passes through the phase transition. Also, the transition was indicated by a discon tinuity in the shift of the vibrational frequency of the added CN“ ion.

Calculation of Pt- Pt ~ 18 kbar [73],

10 Alkali Halides — Continued

Substances and Data Remarks References measurement techniques

Potassium iodide, KI (F m3m, a = 7.0655 A at 298 K)

High pressure studies on KI. Pt 18-21 kbar. Transition is almost The value of Pt agrees well with [7, 26, 17, 30, Diamond squeezers are usually complete around 24 kbar. Internal calculated value of 21 kbar. Pt 52, 73], employed in addition to piston pressure in KI after the transition appears to be independent of tem- displacement and volume discon- is ~ 20.8 kbar as estimated from perature. Transition is sluggish at but faster at 423 K. tinuity method etc., and examined AF/Fu value. a Pm . ?m = 4.13 ±0.02 298 K by x-ray. A at 298 K.

Rubidium fluoride , RbF (Fm3m, a =5.64 A at 298 K; a lower value of a has been reported by Lew [33a]).

Pressure temperature curves for Pt~ 22 kbar at -973 K. Transition takes place to the Pm3m [51]. RbF. structure.

Estimation of Pt Pt~ 11.76 kbar. [73],

Rubidium chloride, RbCl (Fm3m, a =6. 58 10 A at 300 K). — Thin layers of RbCl evaporated a Pm 3 m 3.97 A at 193 K. The Pm3m phase slowly disappears as [5]. onto amorphous bases at low the temperature is increased. temperatures and examined by electron diffraction.

High pressure transition in RbCl Pt —31 kbar (calculated) The volume contraction in RbCl is [6, 17, 24, 47, and x-ray investigation. — 5 kbar at 133 K (observed) comparable to that in CsCl for the 50,51,68, -1 • A// tr 130±20 cal mol . Pm3m-Fm3m phase. Thus the high 69], P T is - 7.5 kbar at 973 K. pressure phase of RbCl is CsCl type only.

Investigation of pressure effects on Pt ~ 7.5 kbar. A simultaneous The simultaneous deformation and [34]. the compressive strength and deformation and polymorphic transformation are attributed to the stress-strain behavior of RbCl. transformation were observed. crack-healing effect. Cracks appear at about 1% deformation and vanish in large deformations, especially at the buckled surface of the sample. At

pressures above Pt , the cracks do not disappear but transform to a system of slanted fissures.

Rubidium bromide, RbBr (Fm3m, a=6.889A at 298 K) = Thin layers of RbBr evaporated «pm3m 4.24 A at 230 K. The intensity of the Pm3m lines [5], onto amorphous bases and ex- disappear with rise in temperature. amined by electron diffraction at low temperatures.

Pressure transition in RbBr by Pt ~ 25 kbar (theory) The experimental observations can be [6. 17, 47,51. x-ray investigation, in the range — 4.6 kbar (observed). fitted to Simon equation. 52], 0-40 kbar and 273-873 K. — 5.5 kbar at 873 K.

RbBr grown from aqueous solution The films have RbBr with Pm3m Vapor grown RbBr films change to [64], on oriented silver films and x-ray structure where a=4.06 A. If the Pm3m if dissolved in water and investigation. film is grown from vapor, the grown from solution on silver structure is Fm3m. films.

456-089 0 - 72 -3 Alkali Halides — Continued

Substances and Data Remarks References measurement techniques

Rubidium iodide , Rbl (Fm3m, a = 7.342 A at 300 K)

== Thin layers of Rbl evaporated onto «i>m3 m 4.59 A at 193 K. The intensity of the Pm3m lines [5]. amorphous bases and examined by slowly disappears as the tempera- electron diffraction. ture is increased. ~ Pressure-temperature curves for Pt 4.8 kbar at 873 K, above The experimental points fit to Simon [51]. Rbl from 0-40 kbar and 273 to 873 K. which the Pm3m phase is equation. possible

Optical investigation of the Fm3m- Pt ~ 3.5 kbar. The transition was affected by the [13, 14]. Pm3m pressure transition in Rbl nature of the crystal surfaces. The single crystals. A high pressure lattice dynamics of the transition has vessel with a modified Vickers been discussed in terms of the shifts metallographic microscope and in the optical and acoustical mode time lapse photographic technique frequencies. Results of this discus- are employed. sion are not compatible with the experimental thermal expansion data on Rbl. ~ Dilatometric studies of the Fm3m- Pt 4.3 kbar at 298 K. (dT/dP) Hysteresis effects are observed in the [54], -1 Pm3m transition in Rbl; construc- = 1.4° bar for the equilibrium transition. tion of the P—T diagram. curve.

Cesium chloride, CsCl (Pm3m, a = 4.123 A at 298 K)

Calorimetric methods Tt , 743 K AHlT , 580-707 cal mol-' [2, 29, 32].

Application of the Clausius- The relation is applicable with [36], Clapeyron relation to the transition. limited utility.

DTA of pure.CsCl and solid solutions Transition ( T„ 745-753 K) is of first The Fm3m phase of CsCl gets stabi- [45, 56, 59, -1 x-ray • with other alkali halides and order AH , 600-880 cal mol lized at ordinary temperatures in 60, 73], lr ; ~ 3 -1 diffraction studies of CsCl transi- A T ~ 33°; A V 7.34 cm mol . solid solutions with 30% KC1 or 35% tion at various temperatures. With incorporation of KC1 or RbCl, RbCl. With KClor RbCl solid solu-

the Tt and AH lr are decreased tions, first order characteristics of the with increase in KC1 or RbCl. With CsCl transition are still preserved. In

CsBr, Tt and AH tr are increased. CsBr solid solutions higher order components are present in the transition and the Pm3m structure is retained at ordinary temperatures.

Temperature variable x-ray diffrac- T, , 742-753 K. The lattice const, The lattice constants of the Pm3m as [22, 23, 40, 45, tion studies on CsCl powder as extrapolated to 298 K is, a = 6.91 A well as the Fm3m phases decrease 57,58,71, well as single crystals. for the Fm3m phase (at 753 K, with increasing percent of KC1 or 74, 75], a = 7.02 A). In CsCl + CsBr solid RbCl. On the other hand the lattice solutions coexistence of Pm3m and constant increases with CsBr content. Fm3m phases is observed in the The molar volume change, AV, x-ray patterns in the neighborhood slightly increases in KC1 solid solu- of the transition temperature. tions indicating that the transitions

are first order. In CsBr solid solutions, AV decreases indicating the presence of higher order components in these transitions.

Electron diffraction studies of CsCl T,, 753 K; a Fm 3 m = 6.923 A at 227 K The transformation is believed to [5,41,63], transition. and 6.940 A at 298 K (extrapolated proceed through a defect mechanism. values). Hybridization and mixed crystal (of both Pm3m and Fm3m structures)

formation were noticed just below Tt .

12 Alkali Halides — Continued

Substances and Data Remarks References measurement techniques

Ionic conductivity measurements of The Tt is indicated by a break in the The conductivity experiments on single [3,42,43,46, CsCl and its solid solutions with log cr versus 1/T plots. The activa- crystals as well as polycrystalline 46a, 72].

KC1, RbCl, CsBr and SrCl 2 . tion energies from the plot give pellets give results which do not values of cation migration energy, vary much. The role of vancancies in

£7" ~ 0.6 eV and Schottky defect the mechanism of the transition is

formation energies, Es . Es — 1.4- not definitive. Though vancancies 1.8 eV in pure CsCl in Pm3m phase. may aid the transition by decreasing in the Fm3m phase, The value of Ex the Tt , the mechanism would still is — 2.0 eV. The cation migration involve dilatation along the body energy decreases slightly with K + diagonal. or Cs + content and increases with - Br content. Addition of Sr +2

slightly decreases the Tt of CsCl.

Self-diffusion and electrical En for conduction — 1.26 eV; E„ for Conduction is only ionic. Schottky 20,21]. conductivity measurements. diffusion: 1.53 eV for Cs 127 defects are predominant. The 36 1.27 eV for Cl . transformation does not involve an extensive disruption of lattice. The transformation is essentially dilatational with an increase in lattice vacancies around 7Y

Electronographic investigation of Both Pm3m and Fm3m structures [19]. evaporated thin films. were noticed.

Study of the phase transition by T, is indicated by the increased [35]. S :!5 emanation from CsCl35 and radioactivity. 77 is increased its solid solution with CsBr. by CsBr addition.

Kinetics of the CsCl transition above The kinetics become difficult to Both Pm3m and Fm3m phases coexist [70], 733 K, by electron diffraction studies. follow because CsCl starts to near T,. sublime at — 730 K and is extremely sensitive to moisture.

Cesium bromide, CsBr, (Pm3m, a=4.2953 A at 298 K)

X-ray diffraction up to melting Some extra weak lines are observed The weak lines are with 2 to 4 times [55,71], point. in the x-ray spectra, which do not the basic lattice period of the stable belong to the Pm3m structure. Pm3m structure.

Thin films of CsBr evaporated on to Fm3m structure of CsBr was noticed The Fm3m lines are of weak intensity [5,63], surface of mica, CaCo.j or amorphous <2=7.2300 A — 7.253 A at - 155 K. and disappear with time. bases at low temperatures and examined by electron diffraction.

Evaluation of Tt and AH tr from ex- For CsBr, Tt — 1153 K for Pm3m_ The transition temperature is higher [72],

perimental data from CsCl+CsBr Fm3m transition; A// tr (calcd)=300 than the melting point of CsBr. -1 solid solutions and thermodynamic cal mol . considerations.

Cesium iodide, Csl (Pm3m, a=4.5679 A at 299 K)

X-ray diffraction of Csl up to the No transition was observed. [71]. melting point.

Thin films of Csl evaporated on to Both Pm3m and Fm3m structures The intensity of the lines due to the [5,63], amorphous bases or mica or CaCO :i were observed with the intensity of Fm3m structure weakens slowly with at very low temperatures and ex- Fm3m structures becoming weak increase of temperature and amined by electron diffraction. =7.631 at with time. a Fm :s m A 148 K. completely disappears at room temperature.

13 Alkali Halides — Continued

Substances and Data Remarks References measurement techniques

Csl evaporated on to K and Rb Microcrystals of NaCl and CsCl type Microcrystals of Fm3m structure were [19]. halides (as thin films) and examined structures were observed. With unstable in air and recrystallized by electronograph. increasing thickness of the Csl into Pm3m structure. Exchange of films the lines due to Pm3m Cs and Rb cations was observed to structure increase in intensity. take place in the presence of moisture.

References

[38] May, A., phys. Rev. 339 (1937); ibid. [1] Al’tshuler, L. V., Brazhnik, M. I., German, V. N., and 52, 54, 629 (1938). [39] Mayer, J. E., J. Chem. Phys. 1, 270 (1933). Mirkin, L. I., Sov. Phys. Solid State 9, 2417 (1968). [40] Menary, J. W., Ubbelohde, A. R., and I., [2] Arell, A., Roiha, M., and Aaltonen, M., Phys. Kondens. Woodward, Materie. 6, 140 (1967). Proc. Roy. Soc. (London) A208, 158 (1951). [9] [41] Morlin, Z., and Tremmel, J., Acta. Phys. 21, 129 (1966). [3] Arends, J., and Nijboer, H., physica stat. sol. 26, 537 (1968). and Weaver, [42] Morlin, Z., Acta. Phys. 21, 137 (1966). [4] Bassett, W. A., Takahashi, T., Mao, H., J. S., Morlin, Z., J. Appl. Phys. 39, 319 (1968). [43J Acta. Phys. 24, 277 (1968). [44] Nagasaki, H., and Minomura, S., [5] Blackman, M., and Khan, I. H., Proc. Phys. Soc. (London) J. Phys. Soc. Japan 19, 77,471 (1961). 1496 (1964). [45] Natarajan, M., Rao, K. and [6] Bridgman. P. W., Z. Krist. 67, 363 (1928). J., Rao, C. N. R., Trans. Faraday Soc. (1970). [7] Bridgman, P. W., Phys. Rev. 48, 893 (1935). 66, 2497 Natarajan, M., Ph.D., Thesis, [8[ Bridgman, P. W., Proc. Am. Acad. Arts Sci. 76, 1 (1945) [46] 1970, Indian Institute of Tech- Buckle. E. R., Diss. Faraday Soc. 30, 46 (1960). nology, Kanpur, India. [46a] Natarajan, M., Prakash, B., and [10] Buerger, M. J., in Phase Transformations in Solids, Ed. Rao, C. N. R., J. Chem. R. Smoluchowski (John Wiley & Sons. Inc., New York, Soc. A, 2293 (1971). 1951). [47] Pauling, L., Z. Krist 69, 35 (1928). Pistorius, C. W. F. T., and Admiraal, L. J., Nature 201, [11] Corll, J. A., and Samara, G. A., Solid State Communications [48] 4,283(1966). 1321 (1964). Pistorius, C. F. T., Nature 2 (1964). [12] Davis, L. A., and Gordon, R. B., phys. stat. sol. 36, K133 [49] W. 04, 467 (1969). [50] Pistorius, C. W. F. T„ J. Phys. Chem. Solids 25, 1477 (1964). [13] Daniels, W. B., and Skoultchi, A. I., J. Phys. Chem. Solids. [51] Pistorius, C. W. F. T., J. Chem. Phys. 43, 1557 (1965). 27,1247(1966). [52] Pistorius, C. W. F. T„ J. Phys. Chem. Solids 26, 1003 (1965). stat. sol. [14] Daniels, W. B., J. Phys. Chem. Solids. 28, 350 (1967). [53] Plendl, J. N., and Gielisse, P. J. M., physica 35, [15] Decker, D. L., J. Appl. Phys. 36, 157 (1965). K151 (1969). [16] Decker, D. L., J. Appl. Phys. 37, 5012 (1966). [54] Petrunina, T. I., and Estrin, E. I., Dokl. Acad. Nauk. S.S.S.R. [17] Evdokimora, V. V., and Vereschagin, L. F., Sov. Phys. 183, 817 (1968). JETP 16,855 (1963). [55] Popov, M. M., Simanov, V. P., Skuratov, S. M., and SuzdaL- [18] Evdokimora, V. V., and Vereschagin, L. F., Zh. Eksperim. tseva, M. N., Ann. Sect. anal. phys. chim. Inst. Chim.

i. Teor. Fiz. 43,1208(1962). gen. (U.S.S.R.), 16,111(1943). [56] Piiyhonen, J., and Mansikka, K., phys. Kondens. Materie 3, [19] Haav, A.. Trudy. Inst. Fiz. i Astron. Akad. Nauk. Eston. S. S.R., page 123 (1961). 218 (1965). Piiyhonen, Jaakkola, S., and Rasanen, V., Ann. Univ. [20] Harvey, P. J., and Hoodless, I. M., Phil Mag. 16, 543 (1967). [57] J., Ser. (1964). [21] Hoodless, I. M., and Morrison, J. A., J. Phys. Chem. 66, Turku, AI 80, 12 557(1962). [58] Piiyhonen, J., and Ruuskanen, A., Ann. Acad. Sci. Fennicae, Ser. VI (1964). [22] Hovi, V., Ann. Univ. Turkuensis, Ser. A I 26, 8 (1957); A 146, 12 Acta. Met. 6,254(1958). [59] Rao, K. J., and Rao, C. N. R., J. Materials Sci. 1, 238 (1966). Rao, K. Rao, G. V. S., and Rao, C. N. R., Trans. Faraday [23] Huggins, M. L., and Mayer, J. E., J. Chem. Phys. 1, 637 [60] J., (1933). Soc. 63, 1013 (1967). K. C. N. R., Proc. Phys. Soc. (London) 91, [24] Jacobs. R. B., Phys. Rev. 54, 325 (1938). [61] Rao, J., and Rao, [25] Jacobs. R. B., Phys. Rev. 54, 468 (1938). 754 (1967). Rao, K. J., Rao, G. V. S., and Rao, C. N. R., J. Phys. C. (Proc. [26] Jamieson, J. C., J. Geol. 65, 334 (1957). [62] Phys. Soc.) 1, 1134 (1968). [27] Jamieson, J. C., and Lawson, A. W., J. Appl. Phys. 33, 776(1962). [63] Schulz, L. G„ J. Chem. Phys. 18, 996 (1950); ibid. 19, 504 [28] Johnson, Q., Science 153, 419 (1966). d951). D. P., Phys. Rev. 1679 (1962). [29] Kaylor, C. E., Walden, G. E., and Smith, D. F., J. Phys. [64] Schumacher, 126, Chem. 64,276(1960). [65] Sharma, M. N., and Madan, M. P., Ind. J. Phys. 38, 231 [30] Kennedy, G. C., and La Mori, P. N., J. Geophys. Res. (1964). 67,851 (1962). [66] Slykhouse, T. E., and Drickamer, H. G., Coll, intern, centre. [31] Klynev, Yu. A., Dokl. Acad. Nauk. S.S.S.R. 144,538(1962). natl. recherche, Sci. 77, 163 (1959). [32] Krogh-Moe, J.. J. Am. Chem. Soc. 82, 2399 (1960). [67] Smits, A., Z. physik. Chem. B51, 1 (1941). Solids [33] Larson. D. B., Keeler, R. N., Kusubov, A., and Hord, [68] Tosi, M. P„ and Fumi, F. G., J. Phys. Chem. 23,359 B. L., J. Phys. Chem. Solids 27, 476 (1966). (1962). [33a] Lew, H., Can. J. Phys. 42, 1004 (1964). [69] Tosi, M. P., J. Phys. Chem. Solids 24, 1067 (1963). [34] Livshits, L. D.. Beresnev, B. I., Genshaft, Yu. S., and [70] Tremmel, J., and Morlin, Z., Radex Rundsch., 449 (1967). Rvabinin, Yu. N., Dokl. Acad. Nauk. S.S.S.R. 164, [71] Wagner, G., and Lippert, L., Z. Physik chem. B31, 263 541 (1965). (1936). [35] Makarov, L. L., Aloi, A. S., and Stupin, D. Yu., Radio- [72] Weijma, H., and Arends, J., phys. stat. solidi 35, 205 (1969). khimiva 11, 116 (1969). [73] Weir, C. E., and Piermarini, G. J., J. Res. Nat. Bur. Stand. [36] Majumdar, A. J., and Roy, R., J. Inorg. Nucl. Chem. 27, (U.S.) 68A (Phys. and Chem.), No. 1, 105 (1964). 1961 (1965). [74] Wood, L. J., Secunda, W., and McBride, H., J. Am. Chem. [37] Mansikka, K., Ann. Univ. Turku. Ser. A I 121, 11 (1968). Soc.- 80, 307 (1958). [75] Wood, L. J., Sweeney, Ch., and Derbes, M. T., J. Am. Chem. 14 Soc. 81, 6148 (1959). 3. Ammonium Halides

Phase transitions in ammonium halides as well as based on available data show that the behavior is their deuterium analogues are well documented in described by the Clausius-Clapeyron equation for the literature. A wide variety of experimental meth- the first order transitions rather than Ehrenfest’s ods have been employed to study these transitions. equation for the second order transitions. An While a higher order X-transition at low temperatures abrupt change in dielectric constant at the transi- is common to all the halides, a Pm3m(CsCl)-Fm3m tion points also indicate a first order transition. A (NaCl) transition is exhibited by ammonium chlo- compressible Ising model has also been employed rides, bromides and iodides and their deuterated to show that the anomalous AF due to ordering is analogues at higher temperatures. The latter is a essentially identical with the anomalous changes in first order transition and is accompanied by con- the elastic constant, C 44 [9, 9a]. siderable thermal hysteresis. In addition to the X- Thermal Transitions. Ammonium fluoride is and Pm3m-Fm3m transitions, a third transition is hexagonal at room temperature and belongs to reported in NtUBr at ~ 100 K. The X-transitions of B4 group (similar to hexagonal Agl or wurtzite ammonium halides are associated with a change in type); this is the only ammonium halide which the rotational motion of the NHf ion about the ter- exists in the hexagonal structure. No transition is nary axis. Pressure transitions in NH4X have been known in NH4 F above 298 K. However, a X-transition reported. Lattice energies of all the four ammonium at 242 K and a pressure transition around 4000 atm halides have been estimated theoretically [38] and have been reported. the calculated values are in good agreement with the Below 450 K, ammonium chloride possesses the observed values. B2 structure with the Pm3m (CsCl type) space

Much of the theoretical work has been devoted group. Phase transitions in NH4 C1 have been the to the X-transitions in ammonium halides. These X- subject of detailed investigation by many workers. transitions are believed to be due to the onset of NH^ The following is the sequence of phase transitions ion rotation by Pauling [28a]. While the X-changes' in ammonium chloride. in NH 4CI and NtLBr have been attributed to an ~ 240 K ~ 456 K fusion order-disorder change in the NHJ ion orientation ^ ^ ^ -liquid (Pm3m) (Pm3m) (Fm3m) [8 a, 25], a temperature-dependent one-dimensional NH4CI free rotation has been suggested in the case of NHJ Forms III and II are both CsCl type structures, the [30]. The low temperature phase of NH 4CI consists transition between them being of X-type. Form I of parallel oriented NH| groups while the high tem- is of NaCl type (Fm3m) with <2 = 6.520 A at 523 K. perature phase is characterized by randomly orient- Thus the phase transition at 456 Kis a Pm3m-Fm3m ed NH| groups [27]. In NHUBr, the low temperature similar + transformation to the transition in CsCl. phase consists of an antiparallel orientation of NH 4 The Pm3m-Fm3m transition is of first order and is ions and the high temperature phase is similar to associated with considerable thermal hysteresis. that of NH 4CI [26]. Neutron diffraction studies IR, Raman and NMR spectroscopy, x-ray diffrac- [22, 25] have confirmed the ion orientations in NH j tion, DTA as well as measurements of piezoelec- these halides. polarization effect The of the halide tricity, dielectric constant, heat capacity and heat of ions is considered to be responsible for the differ- solution have been employed for the study of the ence in the NHJ ion orientation in the low tempera- transitions. ture phases of NH 4CI and NH 4Br [26]. NMR [10a, Ammonium bromide is body centered cubic 15] and IR [36, 54] studies have been employed to (Pm3m, CsCl type) below 411 K. Above this temper- investigate the problem of NH| ion rotation. Sonin ature, the structure is face-centered cubic (Fm3m,

[40] has discussed the possible antiferroelectricity in NaCl type). Just as in NH4 CI, there are two transi-

NH 4I and NH 4Br from a correlation of physical prop- tions in NH4 Br. The Pm3m-Fm3m transition takes erties with structure. place at —411 K with appreciable volume change The Pippard equations and the phase diagram for and thermal hysteresis. The transition has been NH4CI have been analyzed in terms of a compressi- found to be first order. The X-change takes place ble Ising model; the Pippard relations are completely around 235 K. In addition to these two transitions a consistent with the occurrence of a first-order transi- third transition at — 102 K has also been reported. tion in the immediate vicinity of a X-point [33]. Anom- The transitions at 235 K and 102 K are also accom- alous heat capacity variations in ammonium halides panied by hysteresis effects; these transitions have at low temperatures have been ascribed to second been attributed to changes in the NHJ ion orienta- order phase transitions [53]. However, calculations tion in III and IV phases.

T„ 102 K 235 K TT 411 K T fusion IV< > III > IF » I < > liquid

(Pm3m) Tetragonal X-change (Pm3m) (Fm3m) NH 4 Br (nearly CsCl type)

15 393 fusion The room temperature structure of NH 4 I is K < « Fm3m. This transforms to the Pm3m structure II > I > liquid (Fm3m) Br (CsCl type) at about 257 K. A ^-transition in NHJ (Pm3m) ND 4 takes place at -231.3 K. It is interesting to note

that the stability range of phase II (CsCl-type) is The III— IV transition is faster than in NH 4 Br; narrowed down to 25°. T, is also higher in the former case. The AV in

ND 4 Br transitions are comparable to those in

231.3 K 257 K fusion NH 4 Br. > < » IIP >II< I No literature data seem to be available on Tetragonal (X-change) (Pm3m) (Fm3m) ND 4 I after Staveley’s report [47]. The III— II liquid (nearly transition takes place at ~ 229 K, with no thermal NH4I CsCl type) hysteresis and negligible volume change (~ 0.1 3 -1 cm mol ). This is likely to be a higher order transformation. Pressure Transitions. Pressure transitions in ammonium halides are not reported extensively in the literature, compared to thermal transitions. halides, X, exhibit the Deuteroammonium ND 4 The earliest work on pressure-temperature relation- with the exception Pm3m-Fm3m phase transitions ships in ammonium halides seems to be that of I. a X-transition is common to all of them of ND 4 But Bridgman [7 a] who reported data for NH4CI, just as in their hydrogen counterparts. The Pm3m- NFCBr and NH 4I between 273 K and 473 K and Fm3m transitions retain their first-order character- -2 12,000 kg cm . The only reports on the transi- transition istics in the deuterated solids. The tion of NH4F (at ~ 3800 atm and liquid nitrogen lower than in temperatures of ND 4X are generally temperatures) are that of Stevenson [48], and III— transi- the corresponding NH 4 X, except the IV Swenson and Tedeschi [50a]. Stevenson [49] tion of ND 4 Br. also reports data on NH4CI, NFCBr, NH4I and The Pm3 m-Fm3 m transition in ND 4 C1 takes ND 4 Br. Adams and Davis [1] report four polymor- place at 448 while the X-change occurs at ~ 250 K phic forms for NH 4 I. Pressure effects on the K. These transition temperatures are lower than X-transition of NH 4 C1 have been investigated by corresponding values in NH 4 C1. While the the Klynev [17] as well as Trappeniers and co-workers transition is accompanied by con- Pm3m-Fm3m [52]. A systematic investigation of pressure transi- volume change, the siderable hysteresis and tions in ammonium halides has been carried out X-change has no hysteresis effects. by Pistorius [29] recently. A brief account of in Br corre- There are three transitions ND 4 phase transitions in ammonium halides has been to those in 4 Br: sponding NH a]. reported recently by Hovi [ 11 This review 164 K 215 K summarizes the experimental results from di- IV < > HI < > II alatometric, x-ray, calorimetric and NMR studies; (Pm3m) (nearly CsCl type; X- change (Pm3m) the effect of deuteration of ammonium halides is tetragonal) also included.

Ammonium Halides

Substances and Data Remarks References measurement techniques

Ammonium fluoride, NH 4 F (P6 mc, a = 4.44 A and c — 7.17 A at 298 K)

~ Dielectric constant as a function Anomaly in e at 242 K. T t The anomaly in e is due to the A-tran- [16].

of temperature. ~ 241.5 JK. sition in NH 4 F.

Ammonium chloride, NH4CI (Pm3m, a — 3.8756 A at 299 K)

Pm3m-Fm3m transition

X-ray diffraction = 3.868 A at 291 K Changes of volume, densities ac- [46a].

a Fm'Sm — 6.52 A at 523 K companying the transition are

Tt ~ 457.3 K. listed. ~ Specific heat as a function of T, -456.1 K, A// tr 1030 Transition is indicated by an anomaly [6 , 37, -1 • 55a]. temperature cal mol in the C lt (T) curves.

16 Ammonium Halides — Continued

Substances and Data Remarks References measurement techniques

• Thermal analysis Tt ~ 458 K; AHn ~ 1073 cal Hysteresis is shown as a difference in [32a, 37, 1 mol- . T 8 13.25,; AC 8 7.14 the Tt in heating and cooling 42, 57], 3 -1 curves. cm mol .

~ • detailed discussion of the possible 2 Differential Calorimetry Tt 456.1 K; A7/tr ~ 1073 cal A [ ]- -1 mol . sources of error and their correc-

tions in evaluating the AH tT is furnished. The data are likely to be the

best for NH 4 C1 transition and very reliable.

Dielectric constant as a T, ~ 458 K. For single crystals e is 7.08 at 293 K [16]. function of temperature and 2 MHz.

~ 456.1 K; ~ 19.31% at These results compare well with the Dilatometric studies T t AC [31].

1 = 0.0724° . T t ( dT,ldP ) atm" results of other investigations. The transition conforms to first order as indicated by Clausius-Clapeyron equation.

Review articles T t ~ 456 K [11a, 24, 47, -1 • -1 AS tr ~ 2.2 cal deg mol 52a],

3 • -1 AC 8 5.26 cm mol .

\-transition

X-ray diffraction down to 80 K T k ~ 243 K [46, 46a], a = 3.82 A at 88 K.

Thermal analysis Tk ~ 242.5 K. The transition is due to a change in [18. 19]. the rotational motion of NH| ion. Hysteresis effects are observed in the transition.

Differential calorimetry 7\ ~ 242.1 K [4], A T ~ 0.3° -1 • AH ~ 109 cal mol .

Dielectric constant as a function T k ~ 242.5 K. Transition is indicated by an anomaly [16]. of temperature in e (T) curve.

dV - 1° Dilatometric studies; with CBr 4 7= is maximum at 7\ ~ 242.4 K Hysteresis was about if CBr 4 was [43, 51], d 1 as the dilatometric liquid. used. AT ~ 0.30°.

Thermal expansion of NH 4 CI and a is maximum between (7\), 242 Transition is believed to be due to an [7]- x-ray investigation. and 242.5 K. order-disorder change.

Temperature dependence studies T\ ~ 242.8 K; the temperature There appears to be an indirect re- [58a], of proton and deuteron spin- trend of the nuclear correlation lation between the temperature lattice relaxation in NH 4 C1. time tc is in qualitative agree- dependence of tc and the degree of ment with volume-temperature order in the crystal lattice in the dependence. region of the A-transition. ESR and +2 DTA studies on doped Tk is indicated by a peak in DTA; With increasing Cu ion, Tk de- [20. 28. 35]. NH 4 CI samples containing a in ESR, Tk is indicated where creases. The intensity of the signal paramagnetic ion, like 2 Cu+ , the first derivative of the hyper- in VO.f doped NH 4 C1 vanishes at Ni +2 or VO £ fine constant with respect to Tk ~ 241 K. In the electronic ab- temperature is a maximum. sorption spectrum of Ni +2 doped

NH 4 CI, a blue shift of 100-150 -1 cm observed at ~ 235 K in the positions of the band maxima is attributed to the anomalous lattice

contraction of the NH 4 C1 lattice.

17 Ammonium Halides —Continued

Substances and Data Remarks References measurement techniques

Vibrational spectra of thin non- No indication of fine structure due Low temperature modification belongs [56], scattering films of NH 4 C1 at to free rotation of the NH| ion to T' sym. is which the NH| ion (l 301, 195 and 83 K. was found. Evidence for the symmetry is 7V Transition is prob- presence of a torsional lattice ably of the order-disorder type. mode involving the NH^ ions both above and below 7\ was found.

Total scattering cross section per An apparent change in cross sec- The transition involves a change in [23], H atom measured as a function tion is observed at T the rotational motion of the NH| of temperature by using sub- ion from torsional oscillation to thermal neutrons. relatively free rotation.

Review articles T„ 247.2 K; AS ~ 0.82 eu. [11a, 24, 47, 3 -1 AV~~ 0.16 cm mol . 52a],

Ammonium bromide, NH 4 Br (Pm3m, a — 4.059 A at 299 K)

Pm3m-Fm3m transition

X-ray diffraction of NH 4 Br from The lattice constants are: AV calculated from x-ray data shows [32, 46a], -5 293 to 467 K. a, = 6.85187 + 1.38 x 10 r + good agreement with the value -7 2 5.385 x 10 t from dilatometric studies. -5 a„ = 4.5409 + 2.1 16 X 10 * + : 7 2 3.29 X 10" t

where t is in °C. = 1 AFj n 0.0767 cmV .

Solubility as a function of T, ~ 410 K The T t is indicated by the point of [41]. temperature by a close tube cross section of the solubility curves method. of I and II phases.

Dielectric constant as a function T, ~ 410 K For single crystal NH 4 Br, e — 7.24 [16]. of temperature. at 293 K and 2 MHz.

Thermal analysis T,~ 410.4 K; AT -24° [33a, 37, 57]. -1 ~ • AH tT 882 cal mol 3 -1 AV ~ 9.5 cm mol .

(Kinetics of ~ variation of 31], Dilatometric method T t 410.2 K Total volume changes, [8, the transition, of NH 4 Br at AV ~ 18.85% density and thermal expansion

410.2 K). -1 coefficients are discussed. A dis- 0.0878° atm . ( cussion of thermal expansion be- ~ E„ for the transition is 15000 havior is given. -1 • cal mol .

Rate of transition of super- The transition rate is very high and It is inferred that the transition has [21]. cooled high-temperature modifi- comparable to the speed of a martensite character. cation (NaCl type) of NH 4 Br sound. crystallizing from highly super- saturated aqueous solution into the low temperature CsCl phase (by oscillographic recording of the photocurrent change accompanying the I —» II transition).

18 Ammonium Halides — Continued

Substances and Data Remarks References measurement techniques

~ 3 -1 Measurement of specific volumes AC 7.4866 cm mol at Tt for Effect of deuteration of NH 4 Br is [15a],

by hydrostatic weighing in NH 4 Br discussed; AV values from the lit- ~ 3 -1 293-413 K range. AV 7.4704 cm mol at Tt erature are compared and

(392.5 K) for ND 4 Br. discussed.

IR spectra of NH 4 Br thin and The spectra in phase II is in [56], nonscattering films at 301, 105 agreement with the symmetry,

and 83 K. D 4h . No evidence for free rotation at any temperature is found.

Review articles T, ~ 414 K Review of phase changes in NH 4 Br [11a, 24, 47, 3 -1 AV ~ 6.34 cm mol . upto 1964. 52a].

\-transition

X-ray diffraction between 295 and For phase II, a.i = a 2 = a 3 for phase II [12, 46a], -4 aj = 4.0534+ 1.6 X 10 t (where = 9^ 148 K. a i a 2 a 3 for phase III £= 1, 2, 3) -4 cii= 4.0539 + 1.34 X 10 t (where £=1, 2)

where t is in °C. For Phase III, -4 a 3 = 4.0537 — 2.72 x 10 t 3 -1 V, the molar volume (in cm mol ! is given by -3 F=40.113 +4.82 x 10 t for phase II V= 40.042 — 2.34 x 10 -3 t- -5 2 3.64 x 10 t for phase III. Thermal expansion coefficient, -4 ' a = 1.24 X 10 for phase II -5 -6 2 a = — 6.0 X 10 — 1.8 X 10 t for phase III.

DTA 7\ ~ 234.9 K. Transition is believed to be due to [19]. the change in the rotational motion of NH| ion. ~ Dielectric constant as a function Tk 236 K. [16]. of temperature.

Dilatometric methods with 7\ ~ 234 K. Transformation is heterogeneous and [44], toluene as the dilatometric is accompanied by a small liquid. hysteresis.

Total scattering cross section per T\ is indicated by a change in Transition is due to a change in mo- [23], H-atom measured as a function the cross section. tion of the NH^ ion from tor- employing subthermal sional oscillation to relatively free neutrons. rotation.

ESR Investigation of NH 4 Br The hyperfine constant shows a The maximum change at ~ 238 K is [34]. single crystals doped with maximum at ~238 K. due to the \-change which occurs +2 Cu ion through \-point. at ~ 235 K. ~ Review articles Tt 234.4 K Reviews on phase changes in NH 4 Br [11a, 24, 47, ~ AS 0.34 e.u upto 1964. 52 a]. AV ~ — 0.16 cm mol -1

Ammonium iodide, NH 4 I (Fm3m, a = 7.2613 A at 299 K)

456-089 0 - 72 -4 Ammonium Halides — Continued

Substances and Data Remarks References measurement techniques

Pm3m-Fm3m transition

X-ray diffraction. T t - 257 K Effect of particle size on the time [12a, 13, AV — 16.96% for half- conversion in the transi- 46a], a = 7.259 A at 295 (Fm3m) tion is investigated. K Below T t , the a = 4.334 A at 252 K (Pm3m) needed super-cooling increases with decreasing particle size.

Differential calorimetry T t - 257 K The method is applied for the first [3]. -1 • AH tr — 809 cal mol time for the study of a transition below 273 K.

analysis - to K. affects the Thermal T t 253 259 Moisture Tt. [37],

IR investigation of NH^I in Tt ~ 255.4 K. In phase I, one hydrogen bond is [30], - phase I. formed with I and the NH| ion rotates freely about this bond.

Review articles T t - 258 K Reviews upto 1964. [11a, 24, 47, 3 -1 AV — 8.14 cm mol . 52a], k-transition

-4 X-ray diffraction a, =4. 3387+2. 41 X 10 f (£=1,2,3) Molar volume changes are given as a [13].

(ai=a 2 —a 3 for phase II) function of temperature. -4 a;=4.3439+2.28X 10 t (i = l,2); -3 a3 = 4.2999-1.071 X 10 t-6.41 6 2 XlO~ t (ai = «2 + a 3 for phase III).

Differential thermal analysis. 7\~ 231.35 K. Transition is likely to be associated [19]. with a change in the NH| ion rotation.

Molar volume as a function of T\ — 233 to 213 K. Above the T\ the structure is regular [44], temperature. and below T\ the structure is tetragonal.

Dielectric constant (e' and e") Two maxima between 253 and 233 K The rotation of NHj ion along the [16, 24]. measurements. are observed. N—H axis cannot explain these maxima.

Review articles. 7\ — 233 K [11a, 24, 47, AS — 0.3 eu AT~0. 52a], 3 -1 AV 0.1 cm mol .

Deuteroammonium chloride, ND 4 CI (Pm3m. a — 3.8682 A at 291 K)

X-ray diffraction. a Pm3m = 3.8190 A at 88 K; [7, 46a]. apm3 m = 3.8682 A at 291 K. — (for II-I transition). T t 448 K 7\~ 249.4 K.

Neutron diffraction. + The nature of the NH ion rotation The neutron diffraction results are in [ 22 ], is examined. agreement with spectroscopic results of Wagner and Hornig.

IR spectra of their non-scattering The evidence for the order-disorder No evidence of fine structure due to [56], films at 301.295 and 83 K. nature of the second order free rotation of NH^ ion was found. transition is confirmed. Evidence for the presence of a torsional lattice mode involving the NH^ ion both above and below A-point was available. Low temperature T' modification belongs to the d symmetry in which the NH| ion

symmetry is Tfj . A-point transitions are probably order-disorder type.

20 Ammonium Halides — Continued

Substances and Data Remarks References measurement techniques

Review article. C, — 448 K for Pm3m-Fm3m transi- [11a, 47]. tion. -1 T\ — 249.3 K: AF— 0.13 cm mol .

Deuteroammonium bromide, ND+Br (Pm3m, a=4.06 A at 299 K)

X-ray diffraction. T„ I —»• II — 393 K; AV - 7.61 cm Replacement of H by D lowers the T t [14. 46a]. -1 mol at 398 K: while the volume changes, AV, are -> 7\, II III - 215 K; AF- -0.293 not very much affected. In ND 4Br -1 cm mol at 215 K. some metastable states (with BCC Ill -> IV - 164 K; AF- 1.08 structure) were observed in the range -1 cm mol at 164 K. 408-399 K.

Lattice constants

a=6.889 A at 469 K (phase I) a=4.084 A at 385 K (phase II) a=4.047 A, c=4.062 A at 195 K (phase III); a=4.016 A at 152 K (phase IV).

Neutron diffraction. The nature of rotation of NH-f ion The freely rotating model is not [22], is examined. tenable with the experimental results.

Heat capacity in the range 17 to Two maxima in the Cp versus T curve [50], 300 K. are noted. The first maxima is associated with the polymorphic transition in the crystal lattice and the second one with the ordering of the NH^ ion tetrahedra with respect to the two equilibrium positions in the unit cell. 7\ — 215 -1 • K; AH\ — 307 cal mol . Tt — 166 -1 K; AHtr — 340.5 cal mol . AH values are obtained from the inte- grated areas under the peaks.

IR spectra of thin nonscattering films The symmetry of ND 4Br in Phase III No evidence for free rotation at any [56].

of ND 4Br at 301. 95 and 83 K. is DIh . in accordance with x-ray temperature studied, was found.

data. Below 173 K, ND 4Br transforms to phase IV where the symmetry is Ta

just as in NH 4 C1.

Spin-lattice relaxation and phase In 99 percent of the sample, a hyster- Assuming the activation energy for the [58b].

transition in ND 4 Br, at 163.9 K. esis effect was observed. With a transition to be a function only of the

94 percent ND 4 Br no such hys- lattice type, the behavior of 99%

teresis effect was observed. ND 4 Br can be explained in terms of coexistence of two phases at 163.9 K.

Neutron diffraction study of the Four different models are considered The freely rotating ND^ ion model is [22]. NaCl phase. to explain the experimentally not tenable, with the observed exper- observed phenomena of transition. imental situation. IR absorption data on thin films by Wagner and Hornig support a model involving a single- + axis rotation of NH 4 ion.

Review articles. Tt — 405 K (Pm3m-Fm3m) [11a, 24,47]. — — -1 7\ 215 K; AF —0.17 cm mol , AT — 0.06°.

21 Ammonium Halides — Continued

Substances and Data Remarks References measurement techniques

Pressure Transitions ofAmmonium Halides Pressure transition in NH+F. Pt ~ 3800 atmospheres at room The phase change occurs at a very high [48, 50a], temperature; volume change is pressure at liquid nitrogen tempera- 28%. The high pressure phase is ture. The recovery begins only FCC at this pressure. With still when the pressure is completely high pressures of ~ 11 kbar a removed. The phase change involves BCC phase is obtained where the a collapse of the wurtzite structure volume change is ~ 11 % of the into a cubic modification. zero pressure volume.

X-ray investigation of high pressure The high pressure phase in both the [24a]. phases of NH+F and ND 4F, under cases is likely to be tetragonal. The purely hydrostatic conditions. transitions are observed at 3.64 ± 0.02 kbar in both fluorides.

DTA study of pressure effects on A- Transition entropy and enthalpy The fact that the entropy of transition is [52], transition in NH 4 CI and ND 4 CI up decrease up to 1500 atmospheres reduced by increased pressure, to 2700 atmospheres. and remain unchanged thereafter. cannot be explained on the basis of an order-disorder theory which pictures the transition as due to a reorienta- tion of NHi" ion over two sites

(AS = 7? In 2 ).

Spectral investigation of the A- The A-transition takes place around The change is associated with the [17]. transition in NH 4 CI up to 23 kbar 13-15 kbar at room temperature. suppression of the random orienta- pressure in the range 3000-7000 The transition was followed by tion of the NH

Ultrasonic investigation of the The adiabatic elastic constants of The behavior of NH 4Br is compared [ 10 ]. high pressure transition in NH 4Br crystals have been with that of NH 4 C1 near its order-

NH 4 Br single crystals. measured at 200 MHz as a disorder phase transition. function of temperature and pressure in the ranges 180-240 K and 0—6 kbar. A new high pressure ordered phase desig-

nated as 0 n was noticed in this range.

DTA of ammonium halides up to NH 4F has a triple point at 16.8 The A-changes in NH 4CI and ND 4 CI [29], 40 kbar; the pressure-temperature kbar and 582 K. The melting possibly change to first order curves of the halides are given. curve of NH 4 F shows an increase transitions above 30 kbar. up to 706 K at 40 kbar.

_, • CI is independent Volumetric study of the pressure A// tr ~100 cal mol (NH 4 Cl). The AH lr in NH 4 [7a], - • 1 transitions in NH4CI and NH 4 Br A7/ tr ~ 160 cal mol (NH 4 Br). of pressure. These anomalies -2 up to 12000 Kg cm between 273 K may in part be explained with and 473 K. Pauling’s suggestion that there is a change from torsional oscillation to rotational motion of NH^ ions; further considerations may be necessary to account for the different

behaviors of NH 4 CI and NH 4 Br.

22 Ammonium Halides — Continued

Substances and Data Remarks References measurement techniques

Piston displacement technique Transition temperature, K The Ammonium tetrahedra are [49]. down to liquid nitrogen tempera- placed on the face of the cube as in tures, on NH-iCl, NH 4 Br, NH 4I a—/3 /3-y £-8 y—8 the a-phase or at the center of the and ND_iCl, ND 4 Br and ND 4 I. cube for all the other phases. In * * NH 4 C1 458 243 /3-, y-, and 8-phases the NH^" * * ND 4 C1 249 tetrahedra are located at the * NH 4 Br 411 235 100 center of the cube formed by the * ND 4Br 398 215 169 halide ions. The tetrahedra are * * NH 4 I 257 231 oriented in a parallel sense in * * ND 4 I 254 227 8-phase from cell to cell; in y-phase there is antiparallel *No transition takes place at orientation while in the /3-phase the atmospheric pressures. orientation is randomly.

Modified Bridgman’s device with NH 4I which exists in Fm3m The transition conforms to first [1]. diamond and beryllium anvils structure at room temperature order at the start but second order employed to study pressure changes to Pm3m at —500 bars. effects are prominent during the changes up to 20 kbars, in NH 4 I. major interval of the change.

References

HI Adams. L. H., and Davis, B. L.. Am. J. Sci. 263, 359 (1965). [20] Kuroda, N., Kawamori. A., and Mito, E., J. Phys. Soc. [2] Arell, A., Ann. Acad. Fennicae, Ser. A VI 57 (1960). (Japan). 26, 868 (1969). [3] Arell, A., and Alare, O., Phys. Kondens. Materie 2, 423 [21] Kuzmin. S. V., Rab. Fiz. Tverd. Tela 2, 155 (1967). (1964). [22] Levy. H. A., and Peterson. L. W., J. Chem. Phys. 21, 366 [4] Arell, A., Ann. Acad. Fennicae, Ser. A VI 2 04,5(1966). (1953); Phys. Rev. 83, 1270 (1951); 86, 766 (1952). [5] Ayant, Y.. and Soutif, M., Compt. rend. 232, 639 (1951). [23] Leung. P. S.. Taylor, T. I.. and Havens. W. W., Jr.. J. Chem. [6] Birge. P., and G. Blanc. J. Phys. Radium 21, Suppl. 141A- Phys. 48,4912(1968). 145A (1960). [24] Meinnel. J.. Bull. Soc. Sci. Bretagne 39, 31 (1964). [7] Boiko. A. A.. Kristallografiya 14, 639 (1969). [24a] Morosin, B., and Shirber. J. E., J. Chem. Phys. 42, 1389 [7a] Bridgman. P. W.. Phys. Rev. 38, 182 (1931). (1965). [7b] Crystal Data, ACA Monograph. No. 5. Eds. J. D. H. Donnay [25] Nagamiya. T.. Proc. Phys. Math. Soc. Japan 24, 137 (1942). and G. Donnay, American Crystallographic Association [26] Nagamiya, T.. Proc. Phys. Math. Soc. Japan 25, 540 (1943). (1963). [27] Nagamiya. T., compt. rend. 251 (1952). [8] Erofeev, B. V.. and Mendeleev, L. T., Vesti. Akad. Navuk. [28] Narayana, P. A., and Venkateswarlu, P., J. Chem. Phys. Belarus. S.S.R. Ser. Fiz. Tech. Navuk 57 (1957). 52,5159(1970). [8a] Frenkel, J., Acta. Physicochim, U.R.S.S. 3,23 (1935). [28a] Pauling, L., Phys. Rev. 36, 430 (1930). [9] Garland. C. W.. and Young. R. A., J. Chem. Phys. 48, 146 [29] Pistorius. C. W. F. T., J. Chem. Phys. 50, 1436 (1969). (1968). [30] Plumb, R. C., and Hornig, D. F., J. Chem. Phys. 21, 366 (1953). [9a] Garland, C. W., and Jones. J. S., Bull. Am. Phys. Soc. Ser. 2,8,354(1963). [31] Poyhiinen. J.. Ann. Acad. Sci. Fennicae. Ser. A. 58, 52 (1960). [10] Garland, C. W., and Robert. R. A., J. Chem. Phys. 49, Piiyhonen. Mansikka. K., and Heiskanen. K.. Ann. Acad. 5282 (1968). [32] J., Sci. Fennicae, Ser. A., 168, 8 (1964). [10a] Gutoswsky, H. S., Pake, G. E., and Bersohn, R.. J. Chem. [32a] Rao. K. and Rao, C. N. R.. Materials. Sci. , Phys. 22,643(1954). J., J. 1 268 (1966). [33] Renard, R., and Garland, C. W., J. Chem. Phys. 45 763 [11] Hovi. V., and Varteva. M., Phys. Kondens. Materie 3, 305 , (1966). (1964). Sastry, M. D., and Venkateswarlu, P., Proc. Ind. Acad. [11a] Hovi, V.. Proc. Intern Conf. Sci & Tech, of Non-metallic [34] Sci. A66, 208 (1967). Crystals (New Delhi India) (1969). [35] Sastry, M. D., and Venkateswarlu. P., Molecular Physics [12] Hovi. V., Heiskanen. K.. and Varteva, M., Ann. Acad. Sci. 13, 161(1967). Fennicae, Ser. A. 144, 12 (1964). Sato, Y., Phys. Soc., Japan 2304 (1965). [12a] Hovi. V., Paavola. K., and Nurmi, E., Ann. Acad. Sci. [36] J. 20(12), Scheffer, F. E. C., Proc. Akad. Wetenschappen 446 Fennicae A, 328 (1969). [37] 18, Proc. Akad. Sci. Amsterdam, 19, [13] Hovi, V., and Lainio. J., Ann. Acad. Sci. Fennicae. Ser. A, (1915); 1498 (1916); 2 1 5,1 (1966). 798(1917). [14] Hovi. V.. Paavola, K.. and Urvas, O.. Helv. Phys. Acta. 41, [38] Sharma, M. N., and Madan, M. P., Ind. J. Phys. 38, 305 938 (1968): Ann. Acad. Sci. Fennicae, Ser. A. 291 (1968). (1964).

[15] Itoh. J., Kusaka. R., and Saito, Y., J. Phys. Soc. (Japan), 17, [39] Shustin, O. A., Yakovlev, I. A., and Velichkina, T. S., 463(1962). JETP Letters 5,3 (1967).

[15a] Jaakkola, S., Piiyhonen. J.. and Simola, K: Ann. Acad. [40] Sonin, A. S.. Kristallografiya 6(1), 137 (1961). Sci. Fennicae. A, 295 (i%8). [41] Smits, A., and Eastlack, H. E., J. Am. Chem. Soc. 38, 1261 [16] Kamiyoshi. K: Sci. Repts. A8, 252 (1956). (1916). [17] Klynev, Yu. A.. Soviet Physics — Solid State 8, 322 (1966). [42] Smits, A., Eastlack, H. E., and Scatchard, G., J. Am. Chem. [18] Klug, H. P., Northwest Sci. 10. 13 (1936); 11, 36 (1937). Soc. 41, 1961 (1919). [19] Klug, H. P., and Johnson, W. W., J. Am. Chem. Soc. 59, [43] Smits, A., and Mac Gillavry, C. H., Z. physik. Chem. A166, 2061 (1937). 97 (1933). 23 , '

[44] Smits, A., Ketelaar. J. A. A., and Muller, G. J., Z. physik. [52] Trappeniers, N. J., and Van der Molen, Th. J., Physica 32,

Chem. A175 , 359 (1936). 1161(1966). [45] Smits, A., and Muller, G. J., Z. physik. Chem. B36, 140 [52a] Ubbelohde, A. R., Quart. Rev. 1 1, 246 (1957). (1937). [53] Urbakh, V. Yu., Zhur. Fiz. Khim. 30, 217 (1956). [46] Stammler, M., Orcutt, D., and Colondy, P. C., Advan. [54] Vedder, W., U.S. Dept, of Com. Office Tech. Serv. PB Rept., X-ray Anal. 6,202 (1962). 143-474, 96 p. (1958). [46a] Stammler. M., J. Inorg. Nucl. Chem. 29, 2203 (1967). [55] Venkataraman, G., Deniz, K. U., Iyengar, P. K., Roy, A. P.,

[47] Staveley, L. A. K., Quart. Rev. 3 , 64 (1949). and Vijayaraghavan, P. R., J. Phys. Chem. Solids. 27, [48] Stevenson. R.. J. Chem. Phys. 34, 346 (1961). 1103(1966). [49] Stevenson. R., J. Chem. Phys. 34, 1757 (1961). [55a] Voronel, A. V., and Garber, S. R., Zh. Eksp. Teor. Fiz. [50] Stephenson. C. C., and Karo, A. M., J. Chem. Phys. 48, 104 52(6), 1464(1967) (1968). [56] Wagner, E. L., and Hornig, D. F., J. Chem. Phys. 18, 296, [50a] Swenson. C. A., and Tedeschi, J. R., J. Chem. Phys. 40, 305 (1950). 1141(1964). [57] Wallace, R. C„ Cent. Min. 33 (1910). [51] Thomas. D. G., Staveley, L. A. K., and Cullis, A. F., J. Chem. [58] Woessner, D. E., and Snowden, B. S., J. Phys. Chem. Soc., 1727(1952). 71(4), 952 (1967); J. Chem. Phys. 47(7), 2361 (1967).

4. Alkaline Earth (Group IIA) Halides

Substances and Data Remarks Reterences measurement techniques

Beryllium fluoride BeF 2 (Tetragonal, «=6.60A and c = 6.74 A)

X-ray diffraction and DTA. Three DTA peaks are observed The peak at 673 K does not produce [5, 8a], at 308, 673 and 818 K. The transi- any new form. The products after tion at 308 K is a change from heating in the range 673 to 803 K tetragonal to cubic structure, showed x-ray patterns of cubic with a=6.78A; this is an edge form only. centered cubic structure. (8 mole-

cules in the unit cell]. .

High pressure transitions up to At 773°, BeF-i transforms from the Some high pressure phases are [1 to 4]. 873 K and 60 kbar. quartz form to the coesite form, also produced by grinding. under pressure. At room temperature transition occurs at ~ 15.5 kbar.

Beryllium chloride, BeCl 2 (orthorhombic, Ibam, a=5.36A, 6=9.86 A and c=5.26A)

X-ray diffraction and DTA. On cooling the melt rapidly, the a [6,9]. phase is obtained. The structure

of ct'-BeCl 2 is similar to SiS. The a'-phase on heating to 523 K gives a cubic modification Q3'). on further The f3' form heating to 613 K produces the

stable (3 form. If, however, the melt is allowed to slow equilibrium cooling the a form is encountered

at ~ 698 K and further to f3 form at -678 K.

to is — for the oc — transi- for Heat capacity in the range 13 Tt 676 K (3 Thermodynamic properties BeCl2 [8]. -1 715 K. tion; AH lr — 1495±50 cal mol . are tabulated for the range 0-750 K.

Beryllium iodide, Bel> (Tetragonal, P4/nmm. a=6.12±0.01 A and

c=10.63±0.04 A )

Two forms of Bel 2 are known with a Form I is tetragonal; this transforms The number of molecules per unit [7]- transition at 623 K. to Form II (orthorhombic) above cell is 4 in the low temperature phase, 623 K. The lattice constants of while there are 32 molecules in the Form II are: a= 16.48±0.02 A, high temperature phase. 6=16.702±0.01 A and c=11.629± ±0.01 A. AF = 0. 24 Alkaline Earth (Group IIA) Halides — Continued

Substances and Data Remarks References measurement techniques

Fluorides of calcium. Magnesium Strontium and Barium

Changes in lattice spaeings of Transitions to structures of lower The dependence of the repulsive [10]. —30 CaF 2 , SrF 2 . and BaF 2 investigated by symmetry take place at kbar potential on interionie distances is

x-ray diffraction at pressures up to in BaF 2 and at — 60 kbar in SrF2 . estimated. V arious alternative 65 kbar in a tetrahedral anvil press. assignments of repulsive parameters 6 and p for the individual ion-ion interactions are discussed in the light of the Born model. The repulsive parameters appropriate to alkali halides are insufficient to provide the observed repulsion energy.

X-ray studies up to 160 kbar. CaF2 transforms reversibly to MgF2 reveals no transformation up to [2a].

a-PbCl 2 structure (space group 160 kbar. V16 Pbnm) around 80 kbars; a=3.50A. 6=5.64.4. and

c=6.96 A (AV/V~10%). In SrF2

and BaF2 . the high pressure

a-PbCl 2 phases could be re-

covered: in BaF2 . the transition is around 20 kbar. The lattice param-

eters of the a-PbCl2 phases are:

SrF2 : g=3.804-A: 6= 6.287 A: c=7.469 A; AF/F~ 8%

BaF2 : a=4. 035.4: 6=6.676 A: c=7.879 A; AV/V ~ 11%.

References

[10]

Staritzky. [1] Dachille, F., and Roy, R., Z. Krist. Ill, 451 (1959). [7] Johnson. R. E., E., and Douglass, R. M., J. Am.

Chem. Soc. 79 , 2037 H957). [2] Dachille. F.. and Roy, R.. Nature 186 , 34 (1960). McDonald. R. A., and Oetting. F. L., Phys. Chem. 69 [2a] Dandekar, D. P., and Jamieson. J. C., personal com- [8] J. munication (1971). (11), 3839 (1965). [8a] Novoselova, A. V., and Simanov. Yu. P,, Uchenye Zapiski. [3] Douglas. T. B., STAR. 2 , 1348A (1964). Moskov Gosudarst. Univ. im. M. V. Lomonosova No. [4] Kirkina. D. F.. Novoselova. A. V., and Simanov. Yu. P.. 174,7(1955). Proc. Akad. Sci., U.S.S.R.. Sect. Chem. 107 , 201 (1956). [9] Rundle. R. E., and Lewis, P. H., J. Chem. Phys. 20. 132 [5] Kirkina. D. F.. Novoselova. A. V'., and Simanov, Yu. P., (1952). Doklady Akad. Nauk. S.S S.R. 107 , 837 (1956). Smith. H. I., and Chen, H.. Bull. Am. Phys. Soc. [6] Kuvyrkin. O. N., Breusov, O. N., Novoselova. A. V., and J. 11, 414 (1966). Semenenko. K. N.. Zhur. Fiz. Khim. 34 , 343 (1960).

5. Group IIIA Halides

There are no data available regarding the phase transform to the NaCl structure when evaporated transitions of the halides of boron and gallium. into thin films on suitable substrates at low tempera- A1F3 is known to exist in two polymorphic forms, tures [3, 7, 12, 13]. The abnormal NaCl structures

a and /3 [7a]. NQR investigations have shown that have been examined by electron diffraction as well

Inl3 undergoes a sluggish transformation under as x-ray diffraction techniques. Lattice energies pressure at low temperatures [4a]. There are, how- of the thallium halides have been calculated by

ever, a few reports on the phase transitions of Mayer [8a], Altshuller [1] as well as Sharma and thallium halides. T1F is orthorhombic [8] and under- Madan [14]. No pressure transformation is known goes a phase change at ~354 K to a tetragonal in T1C1 or TIBr up to 500 kbar [11a]; three high structure [10]. T1C1 [5, 9, 15], TIBr [15, 16] and pressure polymorphs of T1F are known [10]. Til [11] which crystallize in the CsCl structure

25 ,

Group III A Halides

Substances and Data Remarks References measurement techniques

Aluminum fluoride A1F 3 (Rhombohedral, R32-D^; u=5.039 A, a=58°31' at 298 K)

Heat capacity as a function of T, -718 K [7a]. -1 • temperature. AH lr — 160 cal mol .

Indium triiodide, Inl 3

NQR investigations and pressure- The sample changes from normal Extrapolation of data seems to suggest [4a, 13a], dependence studies. yellow to brick red color at low that the transition may be feasible temperatures, indicating a around 200 K at atmospheric sluggish transition, under pressure. Earlier observations indi- pressure. cated a transition around 77 K, at atmospheric pressures.

Thallium fluoride, T1F (orthorhombic Dfj]-Fmmm. a=5.495 A, 6=6.08 A and c=5.18A at 298 K)

X-ray and DTA investigation of T t —354 K. The space group of the The orthorhombic to tetragonal [10]. 7 ~ 14 high temperature and high pressure tetragonal structure (I) is D \ h l transition is accompanied by a transitions. mmm; o=3.771 A and c=6.115 A decrease in density. Pressure- at 408 K, z= 2. A high pressure temperature curves of T1C1, TlBr

transition is observed at — 12.6 and Til are also presented. kbars and 128.5 K. TIF(II) is orthorhombic with D^-Fmmm; a=5.18 A, 6=5.495 A, c=6.08 A; 2=4.

Thallium chloride, T1C1 (Pm3m, a=3.8427 A at 298 K)

Thin films of T1C1 evaporated on to Abnormal NaCl structure was The stability of the NaCl phase is [3, 7, 12, 13]. amorphous bases at low observed with a=6.30-6.37A influenced by the base and the temperatures and investigated by around 208 K. temperature. electron and x-ray diffraction.

Thallium bromide, TlBr (Pm3m, a=3.9588 A at 298 K)

Thin films of TlBr evaporated on to a Fm3m =6.58— 6.63 A around 213 K. The NaCl structure disappears with [3, 7, 12, 13]. amorphous bases and investigated increase in temperature. by electron and x-ray diffraction.

Thallium iodide. Til (orthorhombic, Djj-Cmcm, a=4.582 A, 6= 12.92 A and c=2.251 A at 298 K)

Thin layers of Til evaporated on to The CsCl type structure of Til has The CsCl structure passes onto the [3, 5a, 7, 12 = near room 13]. amorphous bases (or ionic crystals) ®pm3 m 4. 205 A at 288 K. orthorhombic structure = that Til and examined by electron and x-ray a Fm3m 6.94 A (on LiF substrate). temperature. It is interesting diffraction. deposited on LiF has the NaCl structure. 26 Group IIIA Halides — Continued

Substances and Data Remarks References measurement techniques

X-ray and DTA investigation; Til transforms from a layer ortho- The transition at 430 K is also accom- [4, 11, 17]. pressure transition in TIL rhombic to cubic structure at 430 K. panied by about 35% change in -1 • A// tr ~ 293 ±38 cal mol (from dielectric constant e. Structural -1 DTA) ~ 321 cal • mol (from and thermodynamic aspects of the Clausius-Clapeyron equation), transition are discussed. ao ~ 4.2099 A at 293 K for the high temperature form (extrapolated) Pt 4.7 ±0.1 kbar at 298 K. AF ~ 3.3% at Pt. a shows in the range 296-443 K. 6 -1 aa = 15.90± 0.5 X 10~ (°C) 10-6 o a6 = 48.7 ±0.3 x ( C)-* 10" 6 a r = 60.4±0.6X (°C)-'. For the cubic phase, a„ = 51.0 ±0.3 X 10“ 6 (°C)-\ between 443-573 K.

Dilatometric study. T, ~ 423-447 K The transition is autocatalytic. [2,6]. 3 _1 AF~ 0.003 cm . Presence of gases like g [9] CO 2 , nitrogen or air does not affect the rate of transition.

References

[1] Altshuller, A. P., J. Chem. Phys. 22, 1136 (1954). Meyerhoff, V. K., Acta. Cryst. 12 , 330 (1959).'). [2] Benton, A. F., and Cool, R. D., J. Phys. Chem. 35, 1762 [10] Pistorius, C. W. F. T., and Clark, J. B., Phys. Rev. 173, (193D. 692 (1968). [3] Blackman, M., and Kahn. I. H., Proc. Phys. Soc. 77, 471 [11] Samara, G. A., Walters, L. C., and Northrop, D. A., J. (1961). Phys. Chem. Solids 28, 1875 (1967). Krist. [4] Bradley, R. S., Grace. J. D., and Munro, D. C., Z. [11a] Samara, G. A., and Drickamer, H. G., J. Chem. Phys. 37, 120,349(1964). 408 (1962). [4a] Brooker, H. R., and Scott, T. A., J. Chem. Phys. 41, 475 [12] Schulz, L. G., J. Chem. Phys. 18, 996 (1950). (1964). [13] Schulz, L. G., Acta Cryst. 4, 487 (1951). [5] Hambling, P. G., Acta. Cryst. 6, 98 (1953). [13a] Segel, S. E., and Barnes, R. G., J. Chem. Phys. 25, 578 [5a] Helmholtz, L., Z. Krist. 95, 129 (1936). (1956). [6] Ishikawa, F., and Sato, Y., J. Chem. Soc. Japan 55, 930 N., P., Ind. Phys. 305 (1934). [14] Sharma, M. and Madan, M. J. 38, (1964). [7] Kahn, I. H., Proc. Phys. Soc. (London), 76, 507 (1960). [7a] Kirk-Othmer, Encyclopedia'of Chemical Technology, 2nd [15] Smakula, A., and Kalanajs, J., Phys. Rev. 99, 1737 (1955). [16] Westman, S., and Magneli, A., Acta. Chem. Scand. 11, Edition, Vol. 9, p. 529 (John Wiley & Sons., New York, 1966). 1587 (1957). [8] Ketelaar, J. A. A., Z. Krist. 92, 30 (1935). [17] Zahner, J. C., and Drickamer, H. G., J. Phys. Chem. Solids [8a] Mayer, J. E., J. Chem. Ph^s. 1, 327 (1933). 11, 92 (1959).

6. Group IVA Halides

Carbon tetrafluoride exhibits a first order thermal monoclinic structure. The large enthalpy (— 1080 ~ -1 transformation at 76.2 K and atmospheric pres- cal mol ) suggests the transition to be of first order. sure [6]; the transformation shows no hysteresis. CBr4 exhibits a similar transformation from a cubic Stewart [21] observed a volume change of about 1.8 to a monoclinic structure [10]. 3 -1 cm mol at ~ 89 K. The transition temperature for Bridgman [3] observed three solid forms of CC14 CF 4 observed by Stewart is, however, higher than in the range 253-473 K and up to 12 kbar; Kleiss that mentioned in the previous report. CC1 4 when and co-workers [12] have reported two pressure cooled below its freezing point undergoes a trans- transitions in CClj at — 4 kbar (I— II) and at — 7 kbar formation at —225 K [11, 14]; the transition is (II—III). Bridgman [4, 5] reports pressure transi- accompanied by a change from the cubic to the tions in CBr4 . SiF 4 undergoes a pressure transition

27 , S

from a BCC structure to a close-packed structure [7, 9]. An x-ray study of over 600 crystals of Pbl 2 [8] in the region 5—20 kbar [21]. A pressure transition has revealed some important facts for the formula- occurs at —94.3 K in SiF 6 [6]. SiCl 4 is reported to tion of a satisfactory theory of polytypism [9].

undergo a pressure transition by Bridgman [3]. Phase changes in Pbl 2 have been investigated by

Gel 2 exists in two hexagonal modifications as evi- the Hahn emanation method of following the emana- 135 135 denced by electron diffraction studies on thin tion of the Xe from Pb I 2 [15]. The theory of films of Gel2 [1]. Electrical resistance measurements polytypism has been discussed by Schneer [19]; a under pressure indicate a polymorphic change in systematic account of polytypism with literature

Snl 4 to a metallic state [17]. PbF 2 exists in two references is furnished by Verma and Krishna

modifications [2] while Pbl 2 exhibits polytypism [ 22 ].

Group IVA Halides

Substances and Data Remarks References measurement techniques

Carbon tetrafiuoride , CF 4 ~ Calorimetric study T, 76.2 K The transition is reversible with [ 6 ]. negligible hysteresis.

Piston displacement method for Isothermal pressure — volume The transition is a first order one. [ 21 ]. studying high pressure transition curves were examined for solid No crystal structure data are

in CF 4 CF4 . A volume change of ~ 1.8 available for the low and high 3 -1 cm mol is consistent with cal- pressure phases of CF 4 . -1 culated of ~ 2.0 cm3 mol for the transition.

Carbon tetrachloride CC1 4 (cubic, a = 8.34 ±0.03 A at 238 K)

X-ray diffraction Below 225.3 K an ordered mono- High pressure studies by Weir et al. [17a, 23]. clinic phase of CC1 is reported. furnish — 4 [23] the following crystal a 20.3 A, 6 = 11.6 A, c = 19.9 A; structure data: CC1 4 (I) rhombo- 3 0=111°; Z = 32; p= 1.868 gem- . hedral, a = 14.27 A, a = 90°;

Above 225.3 K this phase trans- Z = 21 . CCUdl) monoclinic, forms to a hitherto unknown (C 2/C or Cc), a = 22.1 A, 6= 11.05 rhombohedral phase stable up to A,c = 25.0 A, = 114°; Z = 32. the melting point. 0 CCLi(III) orthorhombic, (C222i), a = 11.16 A, 6 = 14.32 A, c = 5.74 A,

Z = 8 . ~ • Calorimetric study employing a Tt 224.4 K; A//tr ~ 1080 cal A suspension of solid CC1 4 in [11, 14, 16, -1 modified vacuum calorim- . Nernst mol methanol was employed. The Tt of 20],

eter. CCI 4 is very sensitive to impurities and hence not useful as a thermo- metric standard.

High pressure changes in CC1 4 Two transformations at ~ 4 and Observations of the phase changes [12]. investigated by employing a dia- ~ 7 kbar are observed. were made by using a transmitted mond high pressure cell. light polarizing microscope.

Carbon tetrabromide, CBr 4 (Mono- r clinic, a = 21.12 A, b— 12.26 A, c = 24.14 A; 0 = 125° 3' also a = 21.12 A, b— 12.26 A, c = 21.05 A; 0=110° 10')

Heat of in ~ capacity CBr 4 the range Tt 222.13 K. Ml lT ~ 1594.5 cal Heat capacity data from experiment [13]. -1 -1 273-333 • K. mol . A tr ~ 1.87 cal mol and theory are employed to show 1 deg- . the absence of free rotation in

CBr4 above Tt . There is essentially no change in the molecular motion at the transition point.

Dilatometric study The change is from cubic to mono- [18]. clinic structure. — Optical investigation of the kinetics Tt 226.1 K. The activation Optical crystallographic data for [10].

of CBr4 transition in polycrystalline energy is of the same order of monoclinic CBr 4 are obtained. samples. magnitude as the sublimation energy.

28 , ,

Group IVA Halides — Continued

Substances and Data Remarks References measurement techniques

Silicon tetrafluoride, SiF 4 (BCC, I43m or 123. a = 5.42 ±0.01 A)

Pressure changes in the range 97- SiF 4 changes to a close-packed The overall lattice of the low- [21]. 200 K by x-ray diffraction. structure at high pressures. Two pressure phase is molecular rather transitions are known, one at than ionic. ~ 8.2 kbar and another at ~ 10.2 kbar.

Siliconhexafluoride SiFe

Calorimetry. T, ~ 94.3 K. Transition is reversible and discontin- [6], uous; likely to be a first order transition.

Germanium diiodide Gels (hexagonal, P3ml, a = 4.13 A and c = 6.79 A) Two hexagonal forms have been Electron diffraction of thin films. The structure of the I and II Forms identified. Form I w-ith space [1]- comprise of four and six-layer group. ; a=4.17 A and c=33 Q6v stackings of iodine atoms respec- 0 A. Form II with space group, tively, the germanium atoms lying Dg a = 4.17 and c = 20.16 A. in the octahedral spaces of these d ; A Z is 2 and 3 in I and II respec- layers. tively.

Stannic iodide, Snl 4 (cubic, T£-Pa3. a = 12.273 A at 299 K)

Electrical conductivity measure- Snl 4 becomes metallic at about 185 The results are in reasonable agree- [17].

ments on Snl 4 . kbar as indicated by a sharp ment with extrapolated optical-gap decrease in resistance. data.

Lead fluoride , PbF 2 (a-PbF 2 , orthorhombic. D|£-Pnmb. a = 3.899 A. 6 = 6.4421 A, c= 7.6514 A at 298 K).

Thermodynamic properties of PbF 2 PbF 2 exists in two modifications a The T, was determined thermo- [2. 5a].

at high temperatures and x-ray in- and /3. a transforms to /3 at ~ 473 graphically. The high temperature

vestigation of phase transition. K. /3-PbF 2 is cubic with a = 5.940 cubic form can also be prepared by A at 299 K. the action of HF on PbCOs.

Lead iodide Pbl (hexagonal , 2 D|d P3ml, a = 4.557 A and c — 6.979 A at 298 K).

Polytypism in gelgrown Pbl 2 . Growth conditions and various poly- The small extent to which polytypism [7. 5a, 22], typic structures are linked to- has been observed earlier has gether. Besides the 2H, 4H, 6R. been interpreted as lending support 6H. and 12R polytypes already to Frank’s screw dislocation known, twenty new polytypes theory. However, the experimental have been found. results prove the importance of growth conditions. For various crystal structure data, reference [5a] may be consulted.

X-ray study of about 600 crystals A satisfactory epitaxial theory of [8].

of Pbl 2 . polytypism has been formu- lated. A new polytype of the largest size, 48R is described.

References

[1] Avilov, A. S., and Imanov, R. M., Sov. Phys-Cryst. 13. 52 [4] Bridgman. P. W., Proc. Am. Acad. Arts Sci. 52. 57 (1916). (1968): [5] Bridgman. P. W., Proc. Am. Acad. Arts Sci. 72, 227 (1938). [5a] Crystal Data, Monograph, No. 5, Ed. J. D. H. Donnay, [2] Banashek. E. I., Patsukova, N. N., and Rassonskaya. I. S., ACA and Donnay, G., American Crystallographic Association Neorg. Khim Acad. Nauk. S.S.S.R. 27, 223 (1956). (1963). [3] Bridgman. P. W., Phys. Rev. 3. 153 (1914); ibid., 6. 1 [6] Eucken, A., and Schroder, E., Math-Physik. Klasse, (1915). and Donnay. G., Fachgruppe II. 3, 65 (1938). 29 ,

[7] Hanoka, J. I., Vedam, K., and Henisch. H. K.. J. Phys. [15] Narang, V. P., and Yaffe, L., Canadian J. Chem. 46, 491 Chem. Solids, Suppl. 1, 369 (1967). (1968). [8] Hanoka, J. I., and Vand, V., J. Appl. Phys. 39, 5288 [16] Phipps, H. E., and Reedy, J. H., J. Phys. Chem. 40, 89 (1968). (1936). [9] Hanoka. J. I., Vedam. K., and Henisch, H. K., Crystal [16a] Post, B., Acta. Cryst. 12, 349 (1959). Growth. Ed. H. S. Peiser (Pergamon Press. Inc. Oxford. [17] Rigglemann, B. M., and Drickamer, H. G., J. Chem. Phys. 1967) p. 369. 38, 2721 (1963). [10] Hartshorne. N. H., and Swift, P. McL., J. Chem. Soc. [17a] Rudman, R., and Post, B., Science 154, 1009 (1966). 3705 (1955). [18] Sakovich, G. V., Zhur. Fiz. Khim. 34, 2808 (1960). [11] Johnston. H. L., and Long. E. A., J. Am. Chem. Soc. 56, [19] Schneer, C. J., Acta. Cryst. 8, 279 (1955). 31 (1934). [20] Skau. E. L., and Meier. H. F., J. Am. Chem. Soc., 51, 3517 [12] Kleiss. R. M., Nourbakhsh, M., and Alder, P. N., J. Mate- (1929). rials Sci. 3, 657 (1968). [21] Stewart, J. W„ J. Chem. Phys. 33, 128 (1960). [13] Marshall. J. G., Staveley, L. A. K., and Hart, K. R., Trans. [22] Verma, A. R., and Krishna, P., in Polymorphism and Poly- Faraday Soc. 52, 19 (1956). typism in Crystals (John Wiley, New York, 1966). [14] McCullough. J. C., and Phipps, H. E., J. Am. Chem. Soc. [23] Weir, C. E., Piermarini, G. J., and Block, S., J. Chem. 50. 2213 (1928). Phys. 50, 2089 (1969).

7. Group VA Halides

Substances and Data Remarks References measurement techniques

Arsenic triiodide AsL (Trigonal C2.-R3, a = 7.208 A, c = 21.45 A at 298 K).

Polymorphism in Asl 3 . Two polymorphic forms are known. No conversion was observed when the [3, 4], An yellow (metastable accicular) samples were carefully dried and form is obtained by rapid cooling stored at room temperature in the of a hot benzene or toluene solu- absence of air, even after a period tion. On standing, the yellow of one month. crystals change to a stable red form. The conversion becomes faster on heating.

Antimony trichloride, SbCl 3 and

Antimony tribromide, SbBr3 .

Investigation of phase changes in Two new forms for each of the Study on Sbl 3 and Bil 3 showed no [la]. the range 273 to 450 K and up to halides were noticed. Their phase transitions in either of them. 30 kbar. diagrams are similar.

Bismuth trifluoride, BiF 3 orthorhombic, Pnma, a = 6.563 A, 6 = 7.021 A, c = 4.845 A at 298 K).

Polymorphism in BiF3 . Two other modifications are re- For various crystal structure data [1, 4a, 5]. ported in the literature. An a- reference 2 must be consulted. form stable up to 473 K is cubic (Fm3m), with a = 5.865 ±0.06 A. a-changes to /3-form above 473 K.

Bismuth triiodide, Bil 3 (Trigonal, C§j-R3, a = 7.522 A, c = 20.73 A at 298 K).

— Two new forms of Bil 3 . Form I, hexagonal, R3, with a 7.5 [2]. A and c — 20.65 A. Form II cubic, with a = 20.676 A.

References

[1] Aurivillius, B., and Lindqvist, T., Acta. Chem. Scand. 9, [3] Hass, D., Z. anorg. allgem. chem. 327, 84 (1964). 1209 (1955). [4] Heyworth, D., Phys. Rev. 38, 351 (1931). [la] Bowles, B. F., Scott. G. J., and Bable, S. E., Jr., J. Chem. [4a] Kirk-Othmer, Encyclopedia of Chemical Technology, 2nd Phys. 39, 831 (1963). Edition, Vol. 3, p. 539 (John Wiley & Sons., New York, [2] Crystal Data, ACA Monograph, No. 5, Ed. J. D. H. Donnay, 1964). anu Donnay, G., American Crystallographic Association [5] Zalkin, A., and Templeton, D. H., J. Am. Chem. Soc. 75, (1963). 2453 (1953).

30 8. 3d Transition Metal Halides

Substances and Data Remarks References measurement techniques

Titanium trichloride, TiCl3 (ct- — TiCl3 , rhombohedral, R3 , a 6 = 6.12 A, c= 17.50 A; y= 120°)

X-ray diffraction TiCR on reduction with hydrogen [3a, 14].

at 1070 to 1173 K gives a-TiCl3 ; this is bidimensional and belongs

to rhombohedral symmetry, R 3 . The coordinates of Ti are: 0, 0

V3; chlorine atoms are at, V3, 0 , 0.079. TiCR on reduction with aluminum alkyls give /3-form of

TiCl3 ; this is a linear polymer. This has a hexagonal structure

with a = b = 6.27 A, c=5.82 A. (3- form on heating to about ~ 573 K

passes on to a new ( 7 ) modification.

Chromium chloride, CrCl3 (Mono- clinic, C2/m; a = 5.959 A, 6 = 10.321 A, c = 6.114 A; 0 = 108.49°)

X-ray diffraction and NQR Tt ~ 240 K. The monoclinic struc- The transformation seems to be a [13]. 35 ( C1) with CrCl 3 single ture transforms to a rhombohed- first order one. crystals. ral structure with R3 symmetry; a = 5.942 A, c= 17.333 A.

Ferrous bromide, FeBr2 (hexagonal, - P3ml, a 3.74 ± 0.05 A, c = 6.171 A.)

X-ray diffraction FeBr 3 undergoes a cooperative T t is markedly affected by the pres- [5]. second-order transformation at ence of moisture in the sample. ~ 673 K. The high-temperature phase is different from hep or random-packed-layer structure of

the Cd(OH )2 type.

Ferric fluoride, FeF3 (hexagonal, R3c, a — 5.20 A, c= 13.40 A at 291 K)

X-ray diffraction FeF3 grown by sublimation of For other data on the hexagonal [4]. anhydrous crystals has a cubic crystal structure, reference should symmetry. At 683 K, transition be made to [4a]. to a rhombohedral structure takes place.

31 3d Transition Metal Halides — Continued.

Substances and Data Remarks References measurement techniques

Manganese fluoride, MnF 2 (Rutile, P4/mnm. a = 13.59 ± 0.05 A, c = 3.3099 ± 0.05 A at 298 K)

X-ray investigation of high At 33 ± 4 kbars, MnF 2 (I) (rutile A triclinic form of MnF 2 with 1 [1,4b, 7, 17],

pressure transitions up to 160 kbar type) transforms to MnF 2 (II) (a point group has been mentioned in — and at 298 K. distorted CaF 2 type) with a = b the literature and referred to in 5.03 A, c=5.28 A; density, p = “Crystal Data” [4a]. The lattice -3 4.62 g cm at 80 kbar. At constants are: higher pressures (150 kbar) a a = 5.13 A, b = 5.31 A, c = 5.07 A,

third phase, III, is formed (a = a = 180° 30', /3 = 119° 35' and 3.25 A, 6 = 5.54 A, c = 6.88 A; y = 88° 33', Dandekar and Jamieson -3 p = 4.98 g cm ). A metastable [4b] suggest that the rutile phase phase, IV, is obtained on re- transforms to a new tetragonal moving the pressure. phase which then transforms re-

versibly to the a-PbCl2 form (a = 3.323 A, 6 = 5.560 A, c = 6.454 A; AF/F = 10%).

Pressure transitions of CoF 2 ,

NiF 2 , and ZnF 2

X-ray investigation of high The phase changes occur from the These data have been obtained in [4b, 7], 130-150 pressure changes in CoF-2 , low pressure rutile phase to kbar region at 298 K. X-

NiF 2 and ZnF 2 upto 160 kbar at the distorted fluorite structure. ray diffraction patterns of these 298 K. The lattice constants and den- fluorides indicate the presence of a sities are given below: mixture of the high and low pressure phases under these conditions. Dandekar and Jamieson [4b] sug- High pressure phase gest that the initial rutile structure transforms to another tetrag- -3 a, A p, g cm onal structure (starting around 30 kbar) with a = 4.908 A and c =

CoF 2 4.91 5.44 4.658 A (at 25 kbar); release of

NiF 2 4.84 5.66 pressure yields a mixture of the ZnF> 4.82 6.13 two phases. There is no further change in structure up to 120 Low pressure phase kbar.

-3 a, A c, A p, g cm

CoF 2 4.54 3.32 4.70

NiF 2 4.51 3.30 4.78

ZnF 2 4.41 3.27 5.40

Copper (/ ) Chloride CuCI (Tjj—F43m;

a = 5.416 A at 299 K)

X-ray powder pattern of CuCI at — 10 Tt 683 K. The low temperature The high temperature phase may be [ ]. high temperatures. phase of CuCI is FCC (ZnS) with similar to wurtzite a = 5.4057 A. The high temperature form has the hexagonal structure (a = 3.91 A and c = 6.42 A).

32 3 d Transition Metal Halides — Continued

Substances and Data Remarks References measurement techniques

High pressure phases in CuCl by A phase diagram has been con- Phase I is wurtzite type and phase II [ 2 , 16]. studying the P—T curves; conduc- structed indicating the stability is zinc blende type structure. DTA tivity and TEP measurements in range of various high pressure diagrams are given for various the 370-670 K and 10-55 kbar. phases. The stability of phase I is phases and their mutual transitions. in a small range of 673-683 K and below 5 kbar. Phase II is stable in the range up to 600 K and 40 kbar. Between 40 and 60 kbar a new phase Ha is reported. IV and V are stable above 60 kbar up to 140 kbar and below 673 K.

Copper (II) chloride, CuCl 2 (Mono- clinic, I2/m, a = 6.70 A, 6 = 3.30 A, c-6.67 A; 0=118° 23')

X-ray investigation of polymorphisn There are two polymorphic forms [ 8 ].

in CuCl . 2 a and 0 with a Tt ~ 761 K.

Copper (I) bromide, CuBr (T^-F43m, a = 5.6905 A)

Calorimetric and x-ray investigation. y-CuBr is FCC with a = 5.6794 A [6].

at 293 K. This is ZnS type and is like Cul. j3-CuBr is hexagonal, C| v a = 4.04 A, c = 6.58 A at -703 K.' a-CuBr is BCC and is similar to a-Agl. a = 4.53 A at ~ 753 K. A//y-0=14OO cal -mol -1 -1 ASy -0 = 2.1 e.u. mol -1 AH 0 -a = 700 cal • mol -1 AS/3 _a = 0.9 e.u. mol 0 and a are only average structures.

Microcalorimetric studies (employ- CuBr (obtained from CuBr 2 dis- Immediately after the a-form is [9. 18]. ing DTA) sociation at ~ 453 K) exhibits the formed, CuBr starts melting at

— —» ~ y > 0 a transitions by DTA 770 K. anomalies.

Copper (/) iodide, Cul (cubic T|-

F43m, y-form, a = 6.0507 A)

DTA and TGA studies on Cul 2 and Cul 2 readily loses iodine and trans- [9, 15]. Cul. forms to the 0 form at ~ 648 K and finally to the a form at ~ 785 K. The solid melts at ~ 873 K. a is cubic, 0 is hexagonal (ZnO type) and y is of ZnS type structure.

Investigation of Cul x-ray -* high temperature forms and 12 ]. by Tt , y 0 , 642 K The (0 [ diffraction, heat capacity, etc. 0 -> a, 680 K. a) are average structures.

High pressure polymorphism in In Cul, six phases have been re- For both CuBr and Cul phase dia- [3. 11, 12, CuBr and Cul, by DTA and x-rays. ported: a (FCC phase) 0 (wurtz- grams are given. 16 ]- ite) y (zinc blende) as well as hi,

6 2 , and h 3 phases are known, — y » hi transition is at 4 kbar, —* hi —» 6 2 at 5 kbar and 6 2 h 3 at ~ 15 kbar. In the case of CuBr, a (BCC), 0 (wurtzite) and y (zinc blende) are all stable below 40 kbar and ~ 800 K.

33 .

3 d Transition Metal Halides — Continued

Substances and Data Remarks measurement techniques

Zinc chloride, ZnCL

Dilatometric study The glass transition temperature The rate of crystallization was also was determined by dilatometric measured dilatometrically and and linear expansion coefficient results are presented in detail. methods. There are three forms of ZnCL

known, a, (3, and y. For the various structural data reference should be made to [4a]

Polymorphic Phases of Copper Halides

Designation Structure Range of stability

CuCl I wurtzite Between 650-700 K and below 5 kbar. II zinc blende Below 650 K and ~ 30 kbar. III BCC (similar to a-Agl) Above 700 K and 10 kbar.

CuBr I BCC (similar to a-Agl) Above 800 K and ~10 kbar. II wurtzite Between 600-700 K and below — 8 kbar. III zinc blende Below 600 K and up to — 30 kbar.

Cul I FCC (disordered zinc blende) [9] Between 675-1000 K and up to 30 kbar. II wurtzite Between 575—675 K and below — 5 kbar. III zinc blende Below 575 K and — 12 kbar. IV Below —600 K and above — 10 kbar. V Above 20 kbar and — 600 K. VI Above 30 kbar and around 800 K. VII Above 900 K and — 10 kbar.

References

r [1] Azzaria, L. M., and Dachille, F., J. Phys. Chem. 65, 88 Le-Van-My, Perinet, G., and Bianco, P., Bull. Soc. Chim (1961). France 12, 3651 (1965). [2] Bradley, R. S., Munro, D. C., and Spencer, P. N., Trans. [10] Lorenz, M. R., and Prener, J. S., Acta. Cryst. 9, 538 Faraday Soc., 65, 1912 (1969). (1956). [3] Bridgman, P. W., Proc. Am. Acad. Arts Sci. 52, 91 (1916); Proc. Nat. Acad. Sci. 1 , 513 (1915). [11] Mao-Chia, Y. W., Schwartz, L. H., and La Mori, P. N., [3a] Cras, J. A., Nature 194, 678 (1962). J. Phys. Chem. Solids 29, 1633 (1968). [4] Croft, W. J., and Kestigian, M., Electrochem. J. Soc. [12] Miyake, S., Hoshino, S., and Takenaka, T., J. Phys. Soc. 115, 674 (1968). Japan 7, 19 (1952). [4a] Crystal Data, ACA Monograph, No. 5, Ed. I. D. H. Donnay, Morrison, B., and Narath, A., J. Chem. Phys. 40, 1958 and Donnay G., American Crystallographic Association [13] (1963). (1964).

[4b] Dandekar, D. P., and Jamieson, J. C., personal com- [14] Natta, G., Corrodini, P., Bassi, I. W., and Porri, L., Atti. munication (1971). 'acad. nazl. Lincei, Rend. 24, 121 (1951). [4c] Eisenberg, A., and Hilemen, B. U.S. Gov. J., Res. Rep. [15] Pechkovskii, V. V., and Sofronova, A. V., Zh. Neorgan. 39, 108A (1964). Khim. 10, 1513 (1965). Gregory, N. W., and O’Neal, H. E., Am. Soc. [5] J. Chem. [16] Rapoport, E., and Pistorius, C. W. F. T., Phys. Rev. 172, 2649 (1959). 81, 838 (1968). [6] Hoshino, S., J. Phys. Soc. Japan 7, 560 (1952). Vereshchagin, L. F Kabalkina, S. S., and Kotilevets, [7] Kabalkina, S. S., Vereshchagin, L. F., and Lityagina, [17] , A. A., Sov. Phys. JETP 22, 1181 (1966). L. M., Sov. Phys. Solid State 11, 847 (1969). [8] Korzhukov, N. G., and Psalidas. V. S., Vestn. Mork. Univ. [18] Winkler, H. G. F., Z. anorg. u. allgem. chem. 276, 169 Khim. 23, 54 (1968). (1954).

9. 4 d Transition Metal Halides

ZrF4 exists in two polymorphic forms [13a, 42a]. AgF undergoes a pressure transformation from

A transition in MoF6 has been reported from NMR NaCl to the CsCl structure [18b]. There is no thermal studies [la]. transition in AgCl or AgBr. Agl, which usually

34 *

exists in the B4 or wurtzite. form (hexagonal P63 percent of AgBr the NaCl type of structure (B1 me; a= 4.592 A, c=7.510 A) and in the B3 or phase) is formed [16, 39]. About 25 percent of Agl sphalerite form 1 (low cubic, F43m, a=6.495 A) seems to enter into solid solution with AgBr in the both of which undergo a thermal transformation B1 -phase. However, the literature data available at 418 K to the B23 (high cubic) phase. The trans- on the solid solution limits and the range of stability formation is reversible and is associated with of specific phases are widely different. large A H as well as considerable thermal hysteresis, Pressure transitions of silver halides with Bl- AT and volume change, A V; the transformation is structure have been studied in detail, but there likely to be of first order [39]. appears to be some uncertainty regarding the The existence of the low-cubic (sphalerite or B3-form) Agl was questioned by Majumdar and exact structures. The most commonly postulated high pressure structure is the CsCl type (B2) Roy [29]. Burley [12], however, reports the kinetics structure [2, 9], essentially because of its higher of the B3 ~ B4 transition in Agl by x-ray diffrac- anion-cation coordination. X-ray studies re- tometry at high temperature. The structure of the have vealed this to be the case with ionic solids B23 (high-cubic) Agl has also been a subject of like interest [22]. Statistical models have been em- rubidium and potassium halides. However, with ployed to explain the type of one-dimensional more covalent solids this is not the case. It has polymorphism (polytypism) in Agl [44]. Accordingly, been shown by x-ray studies that AgCl and AgBr

B3 and B4 are polytypes of Agl. Cdl 2 is another under pressure transform to a cinnabar type of example of polytypism. structure (B9) i.e., a tetragonal one [26. 47]. The AgBr enters into solid solution with Agl and the different phases of Agl encountered in the high structures of the specific phases depend on the pressure transitions have different structures composition and temperature [3, 4]. The solid and ranges of stability [1, 5]. In the following solutions of Agl and AgBr have the B3 and/or B4 table the high pressure phases of Agl are listed structure up to ~ 10 percent AgBr, and with higher along with their structures and stability ranges.

Structure and Stability of Silver Iodide in the High Pressure Area

Designation Structure Lattice constants Range of stability

I B23. high cubic a = 5.074 A at 443 K Stable above ~ 419 K and up to ~ 80 kbar.

II B4. wurtzite 0 = 4.591 A, c= 7.498 A Stable up to ~ 419 K and below ~ 2 kbar. at 298 K.

II' B3. sphalerite 0 = 6.495 A at 298 K Stable below ~ 413 K and below ~ 2 kbar.

III Bl, NaCl type a = 6.07 at 298 K Stable up to ~ 419 K and above 3-4 kbar.

IV Orthorhombic Stable below ~ 320 K and between 2—4 kbar.

V B2. CsCl type Stable above 95 kbar.

Lattice energies of silver halides have been There are as many as 130 structures for the CdBr2 calculated by Mayer [31], Huggins [23], Ladd and polytypes [36]; 92 of these belong to the 6R type;

Lee [27], Sharma and Madan [45] and recently sublimation of CdBr2 also yields the 6R type. by Natarajan and Rao [39]. Most of these workers Polytypism in Cdl 2 has been found to be due have employed the Born model. Lattice energy to screw dislocations [32]. X-ray diffraction studies calculations have enabled a study of the relative show the existence of many polytypes with dif- stabilities of the various structures of silver halides ferent stacking sequences [31]. The entire 11" and some of their solid solutions [39]. Born model “22 . . . series of structures could be gen- has been employed to explain the pressure tran- erated by spiral growth from single nonintegral sitions in silver halides as well as the thermal screw dislocations in an “ideal" 4H structure transition in silver iodide [39]. T ransition pres- [33, 34]. Other known structures not belonging “22 11" sures have been theoretically determined by Jacob to the . . . series could be generated [24] employing the Born model. by assuming the presence of two or more coopera-

Polytypism of Cadmium halides. Both CdBr 2 tive dislocations. These considerations suggest

[8] and Cdl 2 [18a] are known to exhibit polytypism that polytypism is not expected in certain sub- [32, 35, 37, 44, 57a]. Single crystal x-ray studies stances because of the nature of their ideal struc- on CdBr2 grown from solution have shown that tures. A detailed discussion on polytypism is fur- 6R is far by the most common polytype of CdBr2 . nished by Verma and Krishna [57a].

35 *>

4d Transition Metal Halides

Substances and Data Remarks References measurement techniques

Zirconium tetrafluoride, ZrF 4 (Mono- clinic, C!j a = 11.69 A, b = h ,

9.87 A, c= 7.64 A; /3 = 126° 9' at 298 K)

DTA and heat capacity ZrF 4 undergoes a phase transition Heat capacity measurements in the [13a, 42a, at ~ 680 K. The high tempera- range 5—307 K do not indicate any 58a]. ture form is cubic (Fm3m) with anomaly. DTA indicates a tran- a — 7.88 A. sition at ~ 680 K.

Molybdenum hexafluoride, MoFe

NMR investigations of phase Two jumps in half-line-width at The spectra of the two modifications [4

change in MoF 6 (temperature ~ 263 K and at ~ 219 K are ob- of MoFe are discussed in relation dependence of half width of the served. The jump at 263 K is to the different types of Mo-F bonds 19 F line). due to the phase transition in and other symmetry considerations.

MoF6 at ~ 263 K.

Silver Fluoride, AgF (Fm3m, a = 4.932 ± 0.003 A at 295 K)

X-ray studies at different Transforms to a new structure The lattice parameter of the B2 [18b].

pressures. (probably B2 or CsCl) at ~ 26 phase (extrapolated to 1 bar) is kbar with AF=1.64 cm 3 mol; 2.99 A.

AS ~ 4.2-5. 1 Joule/mole-deg.

Silver iodide, Agl (B3-Agl, F43m, a = 6.495 A at 298 K;

B4-Agl, P6 3 mc, a = 4.592 A, c= 7.510 A at 298 K)

Structure of various polymorphs Three modifications of Agl are The structure of B23-Agl was sug- [6, 7, 10„ 19, of Agl by x-ray diffraction. known; a low cubic-sphalerite gested to be tetragonal, but later 21,22,28, (B3) phase stable upto 410 K, a work confirmed that the structure 52, 60]. hexagonal-wurtzite (B4 phase) is an average between two struc-

which is formed when the Agl tures of space groups Im3m and melt is cooled and a hot cubic I4m2. The structure of Agl pre- (B23) form stable above 420 K. cipitated from solution depends on - The hot cubic phase has a space the presence of excess of I or + centred lattice of I atoms, with Ag as well as the rate of a = 5.04 A; Ag ions are dis- precipitation. tributed over the 30 largest gaps

in the I structure.

Agl polymorphism with Sphalerite to wurtzite transforma- The sphalerite-wurtzite transition is [11, 12,22, DTA, X-ray etc. tion probably takes place over a more likely to be an athermal 29,30,39, wide temperature interval. The higher order transition. Burley’s 43,58,59 transition is associated with a [12] observation, however, sug- 54], -1 • A Htr ~ 100 cal mol . The gests that the transition is a first- energy of activation is ~ 10.3 ± order one. Presence of iodine vapor -1 • 0.8 kcal mol . The low tem- seems to facilitate the B3~»B4 ~ perature phases of Agl trans- change at 393 K. The T t of B3/ form to the B23 phase at ~ 420 B4 — B23 transition also depends

K with a AH tr of about 1600 on the partial pressure of iodine. -1 • — cal mol . B3 B4 transition is not seen by x-ray diffraction or DTA when small heating rates are employed. The —* B3/B4 B23 transition is a well defined first order transition.

36 4d Transition Metal Halides — Continued

Substances and Data Remarks References measurement techniques

B3/B4—» B23 transition inves- The emf of the cell, [14, 15].

tigated by emf method. Ag(s) Agl(s) CaCl2 • 6H 2 0 Pb(s) Agl(Ci)

PbI(C 2 ) was studied as a function of tem- ~ perature. T,~ 419.6 K; AHtT | -1 • 1270 cal mol .

Electrical Conductivity The high ionic conductivity of the The cr versus l/T plots seem to sug- [20.38.

hot cubic (B23) phase above (420 gest the transition of B3—» B4 to 39, 53].

K) of Agl is in accordance with a occur at —410 K. random orientation of silver ions in the lattice. E„ for conduction in B3 and B4 phases are ~0.2 and —0.6 eV respectively. — X-ray and DTA study of the effect AgBr lowers the Tt to 493 K at AgBr and Agl form an eutectic at [3, 4. 16, of AgBr on the Agl transition. — 10% AgBr. This seems to be — 68% AgBr with a eutectic tem- 39, 51, 55], the solid solution limit in the low perature of — 613 K. and high temperature phases. The specific phases of the solid solutions depend on the temperature as well as composition.

Silver chloride, AgCl (Fm3m, a =

5.5491 A at 299 K ) and Silver bromide AgBr (Fm3m. a = 5.7745 A at 299 K).

X-ray diffraction of AgCl supported At — 25 kbar, AC is 0.9 V0 This is the first experimental evidence [26], at 298 is in amorphous boron vessel and where V, t is the volume K where a tetragonal structure subjected to quasi-hydrostatic pres- and 1 atm. At higher pressures, assigned to the AgCl high pressure sures up to 150 kbar. the structure is shown to be that phase. = 3.92 A, A of Hg2 Cl 2 ; a c=9.03 and c/a = 2.30.

X-ray diffraction Both AgCl and AgBr do not trans- The tetragonal structure is stabilized [47]. form to a B2 or CsCl-type struc- due to overlap of electron orbitals ture. The high pressure phase in resulting in the formation of two both the cases is of B9 type. For highly covalent collinear bonds. The

AgCl, a = 4.06 A, c= 7.02 A; Hg2 Cl2 type structure assigned to

c/a= 1.73. AV/K ~ 8% at the AgCl at high pressure is ques- transition. For AgBr, a = 4.0 A, tioned. c=7.15 A; c/a— 1.77.

V olumetric method Pt ~ 88 kbar for AgCl; — 84 The high pressure phases were con- [9]- kbar for AgBr. sidered to be of B2 structure.

Optical observation on single Pt ~ 90 kbar for AgCl, Pt ~ 86 [2]. crystals. kbar for AgBr.

Pressure-temperature curves for No change was observed in both the The pressure was not enough to [18]. AgCl and AgBr upto 60 kbar and halides. detect the appearance of the high 1073 K. pressure phases.

Pressure transitions in Agl:

High pressure transition — in Agl At 3 kbar the cell symmetry Stability range of six polymorphs is [1, 17,37]. investigated by x-rays. seem to belong to orthorhombic indicated in a P~T diagram. The one. Moore and Kasper [37], how- phase at 3 kbar is likely to be re- ever, consider the 3 kbar phase lated to the NaCl structure, accord- as tetragonal or pseudotetragonal. ing to Moore and Kasper.

High pressure transition of Agl up- At — 60 kbar, Agl has the NaCl(Bl) [40], to 60 kbar and x-ray investigation. structure with a = 6.067 A.

37 .

4d Transition Metal Halides — Continued

Substances and Data Remarks References measurement techniques

High pressure transition in Agl up Microscopic examination indicates a Electrical conductivity measure- [2.47], to~ 120 kbar; examination by transition at ~ 115 kbar. X-ray ments suggest that the transition microscopic and x-ray diffraction studies reveal that at ~ 97 kbar, involves a significant change to techniques. the Agl transforms to a tetragonal more covalent bonding.

structure with a = 5.61 1 A, c= 5.02

A and volume change is ~ 13%.

High pressure transformation in As many as six polymorphs have The 3 kbar phase is yet to be identi- [5]. Agl up to 100 kbar. and x-ray been reported: B3, B4, B23, a 3 fied exactly. There is a possible investigation. kbar phase, B1 structure and a mutual transformation between B3, sixth one. B4 and the ‘3 kbar’ phases by a displacive mechanism rather than a reconstructive mechanism. This immediately suggests that B3, B4 and the 3 kbar phases are polytypes of Agl and should be very similar to each other energywise and struc- turewise.

High pressure transition in Agl by Conductivity increases with pressure Riggleman and Drickamer [42] [42, 46], electrical conductivity measure- in the low pressure phase (stable however find a large increase in ments as a function of pressure up upto 2.5 kbar) and then decreases conductivity at ~ 97 kbar indicating to 100 kbar. in the two high pressure phases. a transformation. The increase in the low pressure phase is in accordance with — ve thermal expansion. A reversal of the slope above 45 kbar in the (NaCl type) cubic phase is explained in

terms of the formation of I- ions.

Cadmium fluoride, CdF2

X-ray study up to 160 kbar CdF2 transforms to the o:-PbCl2 [15a], structure reversibly around 60 kbar; a=3.368 A; 6=5.690 A, c = 6.730 A (at 120 kbar?).

Cadmium bromide, CdBr2 (hexagonal, R3m, a = 3.957 A, c= 18.668 A)

X-ray study of CdBr2 polytypic An x-ray study of solution grown Sublimation of CdBr2 does not produce [7, 36].

structures. CdBr 2 crystals has revealed that any polytype other than 6R. The 6R is by far the most common relation between screw dislocations

polytype. Other structures en- and polytypism is discussed. countered include, 4H, 12H (or 12R), 40H (or ~ 120R) and some disordered structures.

Cadmium iodide, Cdl 2 (hexagonal, P3ml, a = 4.24 A, c = 6.835 A)

X-ray and interferometric studies A combined optical and x-ray dif- Thirteen new polytypes of Cdl 2 are [56, 57]. probably of Cdl 2 polytypism in relation to fraction study was made of poly- described. This number

crystal growth. typism of Cdl 2 which grow by a may be eighteen. In addition, dislocation mechanism. For the several crystals resembling 2H and 2H and 4H structure types, a cor- 4H types are also reported. relation between spiral step height and x-ray unit cell is found; with larger unit-cells no such correlation was observed. This is contrary to expectations on the basis of Frank’s theory and can not be fully explained. 38 4d Transition Metal Halides — Continued

Substances and Data Remarks References measurement techniques

Investigation of the rhombohedral Two rhombohedral polytypes of Each of these polytypes occurs in [13].

polytypes of Cdl 2 and the trans- Cdl 2 , 30R and 42R, grown from syntactic coalescence with a hexa- formation to hexagonal polytypes. vapour phase are reported. Their gonal polytype having the same cell

complete structures are (221212)3 dimensions as the rhombohedral

and (22221212) 3 respectively. one showing that the lattice was transformed from hexagonal to rhombohedral or vice versa during crystal growth.

New polytypes of Cdl2 A 28 layered new polytype of Cdl 2 , The screw dislocation theory does not [48, 49, 50], designated as 28 He with unit fully explain the growth of this cell dimensions, a = 4.24 A, polytype 28 He which is probably

c — 95.69 A has been described. the largest polytype of Cdl2 known, The detailed atomic structure has with an atomic structure. 26 He been worked out and its zig zag also does not conform to the Frank

sequence is found to be screw dislocation theory. So the 2222222222221111. Another structure has been solved by a

anomalous polytype of Cdl2 is Fourier transform method as a 26 He. special case using calculated and observed x-ray intensities distribution. Other anomalous polytypes of

Cdl2 discussed, include 26 Hd, 50 He etc. Factors affecting the phenomenon of polytypism are enumerated.

X-ray investigation of polytypism as Interaction energies for the known This aspect was confirmed, experimen- [41]. a higher order transformation. polytypic structures are calculated tally by x-rays, which suggests a

for Cdl2 on the basis of Schneer’s gradual transition from 3C to 2H theory of polytypism. Almost structures. all polytypes are characterised by a maximum number of unlike interaction contacts.

X-ray study of the phenomenon of X-ray oscillation photographs of Prolonged microscopic observations [25], polytypism in Cdl2 . Cdl2 crystals reveal the existence have shown that 3-dimensional of two different structures in nuclei keep on forming at preferred parallel orientation on the same sites during the growth of crystals. face of a crystal. This helped in Subsequent x-ray investigation shows

understanding the frequent obser- that it leads to the formation of a new vation of the syntactic coalescence polytypic structure of lower free- of two more polytypes in the same energy. crystal as a phenomenon of epitaxial growth.

References

[1] Adams. L. H., and Davis. B. L., Am. J. Sci. 263, 359 (1965). [12] Burley, G., J. Phys. Chem. 68, 1111 (1964). [11] [la] Afanasev, M. L.. Gabuda. S. P., Lundin. A. G.. Opalovskii, [13] Chada, G. K., and Trigunayat, G. C., Acta. Cryst. 22, A. A., and Khaldoyanidi. K. A., Izv. Sib. Otd. Akad. 573(1967). Nauk. SSSR, Ser. khim. Nauk. 4, 18 (1968). [13a] Chretien, A., and Gaudreau, B., Compt. rend. 246, 2266 121 van Valkenberg, Alvin, Jr., Rev. Sci, Instruments. 33, (1958). 1462 (1962). [14] Cohen, E.. and Joss, E. J., J. Am. Chem. Soc. 50, 727 (1928). [3] Barth, T., and Lunde, G., Norsk. Geol. Tids. 8, 293 (1925). [15] Cohen, E., and Joss, E. J., Verslag Akad. Wetenschappen [4] Barth. T., and Lunde. G., Z. Physik. Chem. 122, 293 (1926). Amsterdam, 36, 980 (1927). [5] Bassett, W. A., and Takahashi, T.. Am. Mineralogist 50 [15a] Dandekar. D. P., and Jamieson. J. C., personal communi- (10). 1576(1965). cation (1971).

[6] Berry, C. R.. Phys. Rev. 161 , 848 (1967). [16] de Cugnac, A., Chateau, H., and Pouradier, J., Compt. Bijvoet, [7] J. M., and Nieuwenkamp, W., Z. Krist. 86 , 466 rend. Acad. Sci., Paris, Ser. C 14, 264 (1967). (1933). [17] Davis, B. L., and Adams, L. H., Science 146, 519 (1964). [8] Blocw, R., and Mbller, H., Z. Physik. Chem. Vol A, 152, [18] Deaton. D. C.. J. Appl. Phys. 36 (4), 1500 (1965). 245(1931). [18a] Forty, A. J., Phil. Mag. 43,377(1952). [9] Bridgman. P. W., Proc. Am. Acad. Arts. Sci. 1, 76 (1945). [18b] Halleck, P. M., Jamieson, J. C.. and Pistorius, C. W. F. T. 110] Burley G.. J. Chem. Phvs. 38, 2807 (1963). personal communication (1971); J. Phys. Chem. Solids Burley, G., Am. Mineralogist 48, 1266 (1963). (in print).

39 [19] Helmholtz. L.. J. Chem. Phys. 3, 740 (1935). [42a] Robbins, G. D., Thoma, R. E., and Insley, H., J. Inorg. [20] Hevesy, G. V.. Z. Physik. Chem. 127, 401 (1927). Nucl. Chem. 27, 559 (1965). [21] Hoshino. S., and Miyake, S., Science et. inds. phot. 25, [43] Schneer, C. J., and Whiting, W., Jr., Am. Mineralogist 154(1954). 48,737(1963). [221 Hoshino. S.. J. Phys. Soe. Japan 12, 315 (1957). [44] Schneer, C. J., Acta. Cryst. 8,279 (1955). [23] Huggins, M. L., in Phase Transformations in Solids, Ed. [45] Sharma, M. N., and Madan, M. P.. Indian J. Physics 38, R. Smoluchovski (John Wiley Sons., Inc., New York, & 305 (1964). 1951). [46] Shock R. N., and Katz, S., J. Chem. Phys. 2094 [24] Jacobs, R; B., Phys. Rev. 54, 468 (1938). 48 , (1968). [25] Jain. R. K., and Trigunayat. G. C., J. Cryst. Growth 2 (5), [47] Shock, R. N., and Jamieson, J. C., J. Phys. Chem. Solids 267(1968). 30, 1527 (1969). [26] Jamieson. J. C., and Lawson. A. W., J. Appl. Phys. 33, [48] Srivastava, O. N., and Verma, A. R., Proc. Nucl. Phys. 776(1962). Solid State Symp., Chandigarh, India, Part B, 362 (1964). [27] Ladd. M. F. C., and Lee. W. H., J. Inorg. and Nuclear Chem. 11,264(1959). [49] Srivastava, O. N., and Verma, A. R., Acta. Cryst. 17, 260 (1964). [28] Livitzkii. V.. Ukrain. Khem. Zhur 10, 283 (1935). [50] Srivastava, O. N., and Verma, A. R., Acta. Cryst. [29] Majumdar, A. J.. and Roy, R., J. Phys. Chem. 63, 1858 19, (1959). 56 (1965).

[30] Manson, J. E.. J. Phys. Chem. 60, 806 (1956). [51] Stasiw O., and Teltow, J., Z. anorg. Chem. 259, 143 (1949). [31] Mayer. J. E.. J. Chem. Phys. 1,327 (1933). [52] Strock, L. W., Z. physik. chem. B25, 441 (1934). [32] Mitchell, R. S., Phil. Mag. 45, 1093 (1954); ibid. 46, 1141 [53] Takahashi, T., Katusmi, K., and Osamu, Y., J. Electrochem. (1955). Soc., 116, 357 (1969). [33] Mitchell. R. S., Z. Krist. 108, 296 (1956). [54] Tamman, G., Z. Physik. chem. 75, 733 (through Chemical [34] Mitchell. R. S.,Z. Krist. 108,341 (1957). Abstracts, 5, 1219). [35] Mitchell. R.S., Nature 182,337(1958). [55] Tamman, G., Z. anorg. chem. 91, 263 (1915). [36] Mitchell. R. S.. Z. Krist. 117,309(1962). [56] Trigunayat, G. C., and Verma, A. R., Acta. Cryst. 15, [37] Moore, M. J., and Kasper. J. S.. J. Chem. Phys. 2446 48 , 499 (1962). (1968). [38] Mrgudich, J. N., J. Electrochem. Soc. 107, 475 (1960). [57] Verma, A. R., J. Sci. Ind. Res. 25, 487 (1966). [39] Natarajan. M., and Rao, C. N. R., J. Chem. Soc. (London), [57a] Verma, A. R., and Krishna, P., Polymorphism and Poly- A, 3087 (1970). typism in Crystals (John Wiley, 1966).

[40] Piermarini, G. J., and Weir, C. E., J. Res. Nat. Bur. Stand. [58] Weiss, K., and Vermaas, P. A., Z. physik. chem. 44, (U.S.) A66, (Phys. and Chem.), No. 4, 325 (1962). 372 (1965). [41] Rai. K. N.. and Krishna, P., Indian J. Pure and Appl. Phys. [58a] Westrum, E. F., Jr., J. Chem. Eng. Data 10, 140 (1965). 6, 118(1968). [59] Winkler, H. G. F., Z. anorg. u. allgem. chem. 276, 169 [42] Riggleman, B. M., and Drickamer. H. G., J. Chem. Phys. (1954). 38,2721 (1963). [60] Yamada, K., Bull. Soc. Sci. Phot. Japan No. 11,1 (1961).

10. 5 d Transition Metal Halides

OsCU is known to exist in two polymorphic The kinetics of the tetragonal-orthorhombic

forms [15]. Transitions in mercury halides are transition in Hgl 2 has been studied by Baram [2] also known. Both mercuric chloride and mercuric and others [3, 6]. The mechanism of the transition bromide are orthorhombic at room temperature. [14] has been discussed by Johansson [7], from No detailed phase transformation data are available considerations of the structures of the two modifica-

for these two halides. Some information regarding tions. The Hgl 2 transition has been considered as the isomorphic relations of these halides as well an example of a topochemical reaction which as their mixed salts are available [11, 12], takes place in the inner or outer fields of force of Red mercuric iodide is tetragonal and is the the crystalline matter [8, 9]. The high temperature

stable phase at room temperature. The change of yellow Hgl 2 could be stabilized on A12 C>3 base

red Hgl 2 to the orthorhombic yellow variety takes provided the sample is free from moisture [17].

place reversibly at ~403 K. The effect of state of Recently, the Hgl 2 transition has been studied aggregation [1], heating rate [3, 5] and the pres- with low frequency laser excited Raman spectros- reported ence of other halides [10] on the T t have been copy [13]. A third variety of Hgl 2 has been

studied. The theory of dynamic allotropy has been [16]. A pressure transition in HgL2 was reported by

found to be applicable to the Hgl 2 transition [5]. Bridgman [4].

40 5 cl Transition Metal Halides

Substances and measurement Data Remarks References techniques 7

Osmium tetrachloride. OsCfi (cubic, a = 9.5 A at 298 K)

X-ray and magnetic measurements A solution of OsCL when refluxed Both the low and high temperature [15]. with SOCL yields the cubic form. forms are paramagnetic and display This is a dark-brown solid. It magnetic susceptibilities as a result belongs to the space-group of strong spin-orbit coupling; the

fi O -P43 [32] or O — P4i[32]. high temperature form exhibits the

Os0 4 and CC14 when reacted in a temperature dependent magnetic sealed tube at ~ 743 K. yield the susceptibility. high temperature orthorhombic form (o= 12.08 A. b= 11.96 A and c= 11.68 A).

Mercuric iodide. HgL (Tetragonal, o = 4.39 A. c= 12.38 A at 296 K).

7 Factors affecting the T, of Hgl 2 A freshly prepared sol. of Hgl 2 The yellow sol. when dried is covered [1, 10], transforms at a lower tempera- with a red layer which disappears ture. The T, increases as precipit- slowly. 9 ation continues. Tt is also affected

by other halides such as HgCl2 , HgBr etc.

Dilatometry and kinetics The transition from red to yellow Transition rates in the range 400- [2, 3, 5, 6, 8, 9], variety takes place at ~ 400 K. 406 K are almost the same in the

The dilatometric studies indicate presence of air, C0 2 or nitrogen. that the transition is autocata- However, presence of hydrogen or lytic in both the directions. oxygen has a definite effect on the Quenching experiments indicate speed of the conversion of yellow to that the theory of dynamic allo- red form. An inconclusive evidence

tropy is applicable to Hgl 2 for a white modification of Hgl 2 is transition. also mentioned. 3 1 AF~ 0.0028 cm g- .

Low frequency laser Raman Raman bands for red Hgl 2 are at Marked changes in the spectra were [13]. -1 spectra. 17.5, 29 and 114 cm ; in addition, attributed to changes in crystal weaker bands are seen at 46, 142, structure. -1 and 246 cm . For yellow Hgl2 , stronger bands are at 37, 41. and -1 138 cm ; a weaker band is seen -1 at ~ 278 cm .

[10] References

[1] Balarev, D., Chemie 47 , 27 (1952). [11] Luczizkii, W., Mem. Soc. Nat. Kiew. 20, 191 (through 2 [2] Baram. O. M., ukrain. khim. Zhur. 22, 137 (1956). Chemical Abstracts, 1, 2849 ). [3] Benton, A. F., and Cool, R. D., Luczizkii, V. I., Z. Krist. Min. 297 (through Chemical J. Phys. Chem. 35 , 1762 [12] 46, (1931). Abstracts, 6, 57 ). [4] Bridgman, P. W., Proc. Am. Acad. Arts Sci. 51, 55 (1915). [13] Melveger, A. J., Khanna, R. K.. Guscott, B. R., and Lip- [5] Damiens, A., compt. rend. 177,816(1923). pincott, E. R., Inorg. Chem. 7, 1630 (1968). [6] Eade, D. G., and Hartshorne, N. H., J. Chem. Soc. 1636 [14] Newkirk, J. B., Acta. Met. 4 , 316 (1956). (1938). Paul, M., Z. Naturforsch 200 (1969). [7] Johansson, G., Arkiv Kemi 8. 153 (1955). [15] 24 , [8] Kohlsciiutter, H. W., kolloid Seiskind, M., Michel, P., and Grun, B., Chim. Phys. chem. Beihefte 24 , 319(1927). [16] J. J. [9] Kohlsciiutter, V.. kolloid-Z 42 , 254 (1927). 56,858(1959). Losana, L., Gazz. Chim. ital. 56 301 , (1926). [17] Zeitlin, H., and Goya. H., Nature 183, 1041 (1959).

1 1 . Rare-Earth Trifluorides

The crystal structures of some of the rare-earth can thus exist in a hexagonal structure or an ortho-

fluorides like SmF EuF HoF and TmF depend rhombic one, YF3 is orthorhombic at room tempera- 3 , 3 , 3 , 3 on the method of preparation [2], The fluorides ture. A cubic modification of YF3 reported by

41 Nowachi [1] has not been reproducible. The prepara- elements and also yttrium, although this has not fluorides their crystal dimen- tion of the various and been achieved in several cases. The data on LnF3 sions have been reported by Zalkin and Templeton structures are best understood if only the hexagonal 2 [ ]- form is stable at elevated temperatures even The hexagonal trifluoride seems to be the only stable form for elements from lanthanum to neody- though it may be precipitated from solution under mium. It is likely that it can be produced under certain conditions because of greater ease of crystal proper conditions for the rest of the rare-earth nucleation and growth. [1] References

Zalkin, Nowacki. W., Z. krist. 100 , 242 (1938). [2] A., and Templeton, D. H., J. Am. Chem. Soc. 75, 2453 (1953). [3] 12. Inert Gas Halides

Xenon fluorides (XeF2 , XeF4 , XeFe, etc.) have properties [3]. Heat capacity measurements have been well characterized in the literature. Two indicated the existence of two phase transitions forms of XeF have been reported from x-ray studies. in the range 200-300 K. The crystal structures 4 ,

XeFe has been found to exhibit interesting thermal of two XeFe phases have been reported [1].

Inert Gas Halides

measurement Substances and Data Remarks References techniques

Xenon tetrafluoride, XeF4

X-ray studies Two monoclinic forms I and II are The greater density of form I suggests [2. 4. 5],

reported for XeF4 . Form I belongs that this is the lower temperature

to P2i/C space group with polymorph of XeF4 . «=6.64±0.01 A. 6=7.33±0.01 A, c=6.40±0.01 A; /3=92°40'±5'. -3 p = 4.42 g cm . Form II belongs to P2j/n space group with 0 = 5.03 A. 6=5.92 A, c=5.79 A; 0=99° 27'. -3 p = 4.04 g cm .

Xenon hexafluoride, XeF 6

X-ray and neutron diffraction Single crystals of XeFe were grown The crystal structure of the form III is [!]• by sublimation in a thermal still to be worked out. gradient near room temperature. The crystal structure belongs to P2i/m or P2i space group (monoclinic) with o=9.33±0.03 A, 6=10.36±0.03 A. c=8.95±0.03 A; /3=91.9±0.2°. A second modification of XeFe, belongs to F43c or Fm3c, with a=25.34±0.05 A. This (FCC) XeFe transforms to the third form around 295 K with a hysteresis of ~ 8°.

Calorimetry Very large specific heat anomalies The anomalies are probably due to [3]. are observed in the regions slow phase transitions. 241-261 K and 288-297 K.

References

[1] Agron, P. A., Johnson, C. K., and Levy. H. A.. Inorg. Nucl. [4J Siegel, S., and Gerbert, E., J. Am. Chem. Soc. 85,240(1963). Chem. Letters 1. 145(1965). [5J Templeton, D. H.. Zalkin, A., Forrester, J. D.. and Willian- [2] Burns. J. H., J. Phys. Chem. 67, 536 (1963). son. S. M., J. Am. Chem. Soc. 85, 242 (1963). [3] Malm. J. G.. Schreiner. F., and Osborne, D. W., Inorg.

Nucl. Chem. Letters 1 , 97 (1965).

42 13. Halogens

Substances and measurement Data Remarks References techniques

Fluorine

Heat capacity as a function of T,~45.55±0.02 K The transition is indicated by an IH.

temperature AH tr . 173.9±0.004cal-mol-' anomaly in C,,(T) curves. The Entropy of fluorine at 85.02 K transition had been suspected by

= 39.58 ± 0. 16 e.u. Murphy and Rubin [3j from an analysis of the heat capacity

data of Kanda |2|.

Bromine

P—T behavior of single crystals A new solid phase at high pres- |4|. up to 273 K. sures (~35 kbar) and at —273 K is reported with the space group Cmca (orthorhombic) «=4.79 A,

5=6.75 A and l c=8.63A.

References

|lj Hu. J. H., White. D., and Johnston, H. L., J. Am. Chem. [3] Murphy, G. M., and Rubin, E., J. Chem. Phys. 20, 1179 Soc. 75,5642(1953). (1952).

|2] Kanda. E.. Bull. Chem. Soe. Japan, 12,473 (1937). |4] Wier, C. E.. Piermarini. G. J., and Block. S., J. Chem. Phys. 50.2089(1969).

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NSRDS-NBS 1 , National Standard Reference C. E. Moore, 1971 (55 cents), SD Catalog No. Data System— Plan of Operation, by E. L. C13.48:3/Sec.4. Brady and M. B. Wallenstein, 1964 (15 cents), SD NSRDS—NBS 3, Sec. 6, Selected Tables of Catalog No. C13.48:l. Atomic Spectra, Atomic Energy Levels and

NSRDS-NBS 2, Thermal Properties of Aque- Multiplet Tables, Hi , D, T, by C. E. Moore, ous Uni-univalent Electrolytes, by V. B. 1971 (In press), SD Catalog No. C13.48:3/Sec. 6.

Parker, 1965 (45 cents), SD Catalog No. 03.48:2. NSRDS-NBS 4, Atomic Transition Probabili-

NSRDS-NBS 3, Sec. 1, Selected Tables of ties, Vol. 1, Hydrogen Through Neon, by Atomic Spectra, Atomic Energy Levels and W. L. Wiese, M. W. Smith, and B. M. Glennon,

Multiplet Tables, Si II, Si III, Si IV, by C. E. 1966 ($2.50), SD Catalog No. C13.48:4/Vol. I.

Moore, 1965 (35 cents), SD Catalog No. NSRDS-NBS 5, The Band Spectrum of Car- Cl3.48:3/Sec.l. bon Monoxide, by P. H. Krupenie, 1966 (70

NSRDS-NBS 3, Sec. 2, Selected Tables of cents), SD Catalog No. 03.48:5.

Atomic Spectra, Atomic Energy Levels and NSRDS-NBS 6, Tables of Molecular Vibra- Multiplet Tables, Si I, by C. E. Moore, 1967 (20 tional Frequencies, Part 1, by T. cents), SD Catalog No. C13.48:3/Sec.2. Shimanouchi, 1967 (40 cents), SD Catalog No.

NSRDS-NBS 3, Sec. 3, Selected Tables of Cl3.48:6/Pt.l. Superseded by NSRDS-NBS 39.

Atomic Spectra, Atomic Energy Levels and NSRDS-NBS 7, High Temperature Proper- Multiplet Tables, C I, C II, C III, C IV, C V, ties and Decomposition of Inorganic Salts, C VI, by C. E. Moore, 1970 ($1), SD Catalog No. Part 1. Sulfates, by K. H. Stern and E. L. C13.48:3/Sec.3. Weise, 1966 (35 cents), SD Catalog No. NSRDS-NBS 3, Sec. 4, Selected Tables of C13.48:7/Pt.l.

Atomic Spectra, Atomic Energy Levels and NSRDS-NBS 8, Thermal Conductivity of Multiplet Tables, N iv, N v, N VI, N VII, by Selected Materials, by R. W. Powell, C. Y. Ho, and P. E. Liley, 1966 ($3). PB189698* Johnston, 1968 (45 cents), SD Catalog No.

NSRDS-NBS 9, Tables of Bimolecular Gas 03.48:20. Reactions, by A. F. Trotman-Dickenson and NSRDS-NBS 21, Kinetic Data on Gas Phase

G. S. Milne, 1967 ($2), SD Catalog No. C13.48:9. Unimolecular Reactions, by S. W. Benson

NSRDS-NBS 10, Selected Values of Electric and H. E. O’Neal, 1970 ($7), SD Catalog No. Dipole Moments for Molecules in the Gas 03.48:21. Phase, by R. D. Nelson, Jr., D. R. Lide, Jr., and NSRDS-NBS 22, Atomic Transition Proba- A. A. Maryott, (40 cents), Catalog No. bilities, II, 1967 SD Vol. Sodium Through Calcium, j 03.48:10. A Critical Data Compilation, by W. L. Wiese, NSRDS-NBS 11, Tables of Molecular Vibra- M. W. Smith, and B. M. Miles, 1969 ($4.50), SD tional Frequencies, Part 2, by T. Catalog No. Cl3.48:22/Vol.II. Shimanouchi, 1967 (30 cents), SD Catalog No. NSRDS-NBS 23, Partial Grotrian Diagrams Cl3.48:ll/Pt.2. Superseded by NSRDS-NBS 39. of Astrophysical Interest, by C. E. Moore and NSRDS-NBS 12, Tables for the Rigid Asym- P. W. Merrill, 1968 (55 cents), SD Catalog No. metric Rotor: Transformation Coefficients 03.48:23. from Symmetric to Asymmetric Bases and NSRDS-NBS 24, Theoretical Mean Activity 4 Expectation Values of Pf, P Z and P®, by Coefficients of Strong Electrolytes in Aque-

R. H. Schwendeman, 1968 (60 cents), SD Cata- ous Solutions from 0 to 100 °C, by Walter J. log No. 03.48:12. Hamer, 1968 ($4.25), SD Catalog No. 03.48:24. NSRDS-NBS 13, Hydrogenation of Ethylene NSRDS-NBS 25, Electron Impact Excitation

on Metallic Catalysts, by J. Horiuti and K. of Atoms, by B. L. Moiseiwitsch and S. J. Smith,

Miyahara, 1968 ($1), SD Catalog No. 03.48:13. 1968 ($2), SD Catalog No. 03.48:25. NSRDS-NBS 14, X-Ray Wavelengths and X- NSRDS-NBS 26, Ionization Potentials, Ap-

Ray Atomic Energy Levels, by J. A. Bearden, pearance Potentials, and Heats of Forma-

1967 (40 cents), SD Catalog No. 03.48:14. tion of Gaseous Positive Ions, by J. L.

NSRDS-NBS 15, Molten Salts: Vol. 1, Electri- Franklin, J. G. Dillard, H. M. Rosenstock, J. T.

cal Conductance, Density, and Viscosity Herron, K. Draxl, and F. H. Field, 1969 ($4), SD

Data, by G. J. Janz, F. W. Dampier, G. R. Laksh- Catalog No. 03.48:26. minarayanan, P. K. Lorenz, and R. P. T. Tom- NSRDS-NBS 27, Thermodynamic Properties kins, 1968 ($3), SD Catalog No. C13.48:15/Vol.l. of Argon from the Triple Point to 300 K at NSRDS-NBS 16, Thermal Conductivity of Pressures to 1000 Atmospheres, by A. L.

Selected Materials, Part 2, by C. Y. Ho, R. W. Gosman, R. D. McCarty, and J. G. Hust, 1969 Powell, and P. E. Liley, 1968 ($2), SD Catalog No. ($1.25), SD Catalog No. 03.48:27. C13.48:16/Pt.2. NSRDS-NBS 28, Molten Salts: Vol. 2, Section NSRDS-NBS 17, Tables of Molecular Vibra- 1. Electrochemistry of Molten Salts: Gibbs tional Frequencies, Part 3, by T. Free Energies and Excess Free Energies

Shimanouchi, 1968 (30 cents), SD Catalog No. from Equilibrium-Type Cells, by G. J. Janz Cl3.48:17/Pt.3. Superseded by NSRDS-NBS 39. and C. G. M. Dijkhuis; Section 2. Surface Ten-

NSRDS-NBS 18, Critical Analysis of the Heat- sion Data, by G. J. Janz, G. R. Laksh-

Capacity Data of the Literature and Evalua- minarayanan, R. P. T. Tomkins, and J. Wong, tion of Thermodynamic Properties of 1969 ($2.75), SD Catalog No. C13.48:28/Vol.2. Copper, Silver, and Gold from 0 to 300 K, NSRDS-NBS 29, Photon Cross Sections, At- by G. T. Furukawa, W. G. Saba, and M. L. Reilly, tenuation Coefficients, and Energy Absorp- 1968 (40 cents), SD Catalog No. 03.48:18. tion Coefficients from 10 keV to 100 GeV,

NSRDS-NBS 19, Thermodynamic Properties by J. H. Hubbell, 1969 (75 cents), SD Catalog No. of Ammonia as an Ideal Gas, by L. Haar, 1968 C13. 48:29. (20 cents), SD Catalog No. 03.48:19. NSRDS-NBS 30, High Temperature Proper- NSRDS-NBS 20, Gas Phase Reaction ties and Decomposition of Inorganic Salts, Kinetics of Neutral Oxygen Species, by H. S. Part 2. Carbonates, by K. H. Stern and E. L. Weise, 1969 (45 cents), SD Catalog No.

^Available from National Technical Information Service, Spring- C13.48:30/Pt.2. field, Virginia 22151. NSRDS-NBS 31, Bond Dissociation Energies in Simple Molecules, by B. deB. Darwent, 1970 NSRDS-NBS 38, Critical Review of Ul- (55 cents), SD Catalog No. C13. 48:31. traviolet Photoabsorption Cross Sections NSRDS-NBS 32, Phase Behavior in Binary for Molecules of Astrophysical and and Multicomponent Systems at Elevated Aeronomic Interest, by R. D. Hudson, 1971 Pressures: rc-Pentane and Methane-n-Pen- ($1), SD Catalog No. C13.48:38. tane, by V. M. Berry and B. H. Sage, 1970 (70 NSRDS-NBS 39, Tables of Molecular Vibra- cents), SD Catalog No. C13. 48:32. tional Frequencies, Consolidated Tables, NSRDS-NBS 33, Electrolytic Conductance by T. Shimanouchi, 1972 (In press), SD Catalog and the Conductances of the Halogen Acids No. 03.48:39. in Water, by W. J. Hamer and H. J. DeWane, NSRDS-NBS 40, A Multiplet Table of Astro- 1970 (50 cents), SD Catalog No. C13. 48:33. physical Interest (Reprint of 1945 Edition), by

NSRDS-NBS 34, Ionization Potentials and C. E. Moore, 1972 ($2), SD Catalog No. 03.48:40. Ionization Limits Derived from the NSRDS-NBS 41, Crystal Structure Trans- Analyses of Optical Spectra, by C. E. Moore, formations in Binary Halides, by C. N. R. 1970 (75 cents), SD Catalog No. C13. 48:34. Rao, 1972 (In press), SD Catalog No. 03.48:41. NSRDS-NBS 35, Atomic Energy Levels as NSRDS-NBS 42, Selected Specific Rates of Derived from the Analyses of Optical Spec- Reactions of the Solvated Electron in

5 23 24 41 tra, Vol. I, H to V; Vol. II, Cr to Nb; Alcohols, by E. Watson, Jr., and S. Roy, 1972 Vol. Ill, 42Mo to 57 La, 72 Hf to 89Ac, by C. E. (In press), SD Catalog No. 03.48:42. Moore, 1971 (Vol. I, $5; Vol. II, $4.25; Vol. Ill, NSRDS-NBS 43, Selected Specific Rates of

$4.50), SD Catalog No. C13.48:35/Vols. I, II, Reactions of Transients from Water in and III. Aqueous Solution, by M. Anbar, M. Bambenek, NSRDS-NBS 36, Critical Micelle Concentra- and A. B. Ross, 1972 (In press), SD Catalog No. tions of Aqueous Surfactant Systems, by P. 03.48:43. Mukerjee and K. J. Mysels, 1971 ($3.75), SD NSRDS-NBS 44, The Radiation Chemistry Catalog No. C13. 48:36. of Gaseous Ammonia, by D. B. Peterson, 1972 NSRDS-NBS 37, JANAF Thermochemical (In press), SD Catalog No. 03.48:44. Tables, 2d Edition, by D. R. Stull, H. Prophet, et al., 1971 ($9.75), SD Catalog No. C13. 48:37. formNBS-114A (l-7i) u.s. DEPT, of COMM. 1. PUBLICATION OR REPORT NO. 2. Gov t Accession 3. Recipient’s Accession No. BIBLIOGR^APHJC DATA NBS -NSRDS-41

4. TITLE AND SUBTITLE 5. Publication Date July 1972 Crystal Structure Transformations in Binary Halides 6. Performing Organization Code

7. AUTHOR! s) 8. Performing Organization C.HoR. Rao and M. Ha taraj an 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 Address 13. Type of Report & Period Covered M/A

Same as No. 9 14. Sponsoring Agency Code mm1 15. SUPPLEMENTARY NOTES

16. ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant bibliography or literature survey, mention it here.)

A critical survey of the data describing crystal structure transformations in binary halides is compiled. Data on thermodynamic, crystallographic, spectroscopic and electronic properties are given for each transformation. Experimental techniques used to obtain the data are named and comments on the data are included in the tables The literature is surveyed up to 1970. References have been selected on the basis of their pertinence to the data which are cited and do not represent all the available literature.

17. KEY WORDS (Alphabetical order, separated by semicolons) Binary halides; Crystal structure transformation; Electronic data; Phase transformation; Spectroscopic data; Thermodynamic data; X-ra.y diffraction data. 18. AVAILABILITY STATEMENT 19. SECURITY CLASS 21. NO. OF PAGES (THIS REPORT)

rX\ UNLIMITED. 53 UNCL ASSIFIED

20. SECURITY CLASS 22. Price 1 1 FOR OFFICIAL DISTRIBUTION. DO NOT RELEASE TO NTIS. (THIS PAGE) 55 Cents UNCLASSIFIED USCOMM-DC 66244-P'

U. S. GOVERNMENT PRINTING OFFICE - 1972 O - 456-089 NBS TECHNICAL PUBLICATIONS

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