And Two-Dimensional Compounds J

IV. – ORDER IN ONE AND TWO-DIMENSIONAL SYSTEMS.SOME ONE- AND TWO-DIMENSIONAL COMPOUNDS J. Rouxel

To cite this version:

J. Rouxel. IV. – ORDER IN ONE AND TWO-DIMENSIONAL SYSTEMS.SOME ONE- AND TWO- DIMENSIONAL COMPOUNDS. Journal de Physique Colloques, 1977, 38 (C7), pp.C7-235-C7-242. 10.1051/jphyscol:1977745. jpa-00217246

HAL Id: jpa-00217246 https://hal.archives-ouvertes.fr/jpa-00217246 Submitted on 1 Jan 1977

HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. /V. - ORDER IN ONE AND TWO-DIMENSIONAL SYSTEMS.

SOME ONE AND TWO-DIMENSIONAL COMPOUNDS

J. ROUXEL Laboratoire de Chimie Minerale A, E.R.A. 472, U.E.R. de Chimie, BP 1044, 44037 Nantes Cedex, France

RbumC. - Le caractere bidimensionnel ou unidimensionnel d'une structure est une notion relative qui traduit en fait une tres forte anisotropie des liaisons chimiques. Dans une trb large mesure les solides a dimensionnalite restreinte peuvent itre considerks comme construits partir d'unitks structurales telles que des feuillets ou des fibres. A I'intkrieur de ces entitb les liaisons sont fortes, iono- covalentes ou metalliques ; entre fibres ou feuillets les liaisons sont faibles, le plus souvent de type Van der Waals (tres exceptionnellement elles peuvent cependant etre fortes : elles participent alors a des mkanismes de transferts de charge). Les consequences de ce mod6le sont triples : cristallographiques, chimiques et physiques. En pre- mier lieu I'aspect structural se manifeste par les multiples formes polytypiques likes aux glissements relatifs des entitks structurales, glissements autorises par les liaisons faibles qimaintiennent la cohesion des edifices. D'un point de vue chimique il est possible d'kcarter feuillets et fibres et ceci introduit directement les composks intercalaires. Enfin I'extrsme anisotropie gtomktrique des structures transparait directement dans une anisotropie tres grande des proprietb physiques, vibration- nelles, mtcaniques, electriques, etc. .. Par ailleurs I'existence de surfaces de Fermi A larges portions paralleles determine parfois I'apparition d'ondes de densite de charge. Les chalcogt?nures des elements de transition sont utilisk pour illustrer ces definttions. Les dichal- cogCnures bidimensionnels sont Cvoques d'abord : ils sont introduits du point de vue de I'ordre- desordre en considerant des entitks de plus en plus petites. Dans une deuxieme partie de nouveaux chalcogCnures de transition sont decrits et discutts du point de vue de leur dirnensionnalite vraie. I1 s'agit des pseudo-unidimensionnels NbSe, et XxNbSe4.

Abstract. - The two-dimensional or onedimensional character of a structure is a relative notion : it is indicative of a very strong anisotropy in the chemical bonding. To a large extend solids with low dimensionality can be regarded as built up of chains or layers inside of which there are strong iono- covalent or metallic bonds, whereas they are separated by rather large distances in agreement with weak interlayers or interchains bonding. The slabs or chains can behave as independant units. Gliding motions lead to polytypism. From a chemical point of view it is possible to pull them apart through various intercalations. On the other hand the structural anisotropy results in a very high anisotropy in the electronic, vibrational and mechanical properties. Largely two dimensional Fermi surfaces favour the formation of charge density waves. The chalcogenides of transition elements show some of the best examples of these definitions. Lamellar dichalcogenides will be introduced at first and from the point of view of ordering of smaller and smaller species. Then, new chalcogenides with low dimensionality will be considered. NbSe, and X,NbSe, compounds are described. Their real dimensionality is discussed through physical measurements and according to the chemical behaviour.

Solids with low dimensionality have recently arisen The corresponding materials are of considerable a great deal of interest. The two dimensional or one interest. The structural anisotropy results in a very dimensional character of a structure is of course high anisotropy in the electronic, vibrational and a relative notion : it is indicative of a very strong mechanical properties. The physical topics which anisotropy in the chemical bonding. To a large have been studied in these materials include metal- extend solids with low dimensionality can be regarded non metal transformations, Kohn anomalies and as built up of chains or layers inside of which there charge density waves. In particular, largely two are strong iono-covalent or metallic bonds, whereas dimensional Fermi surfaces favour the formation they are separated by rather large distances (generally of charge density waves. From a chemical point of of the order of the Van der Waals radii), in agreement view the most important aspect is that the slabs or with weak interlayers or interchains bonding. the chains can behave as independant units : it is

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1977745 C7-236 J. ROUXEL possible to intercalate and to deintercalate various chemicals such as metallic ions or Lewis bases. The chalcogenides of transition elements show some of the best examples of the above definitions. There exist two dimensional compounds corresponding to the formula MX, (X = S, Se, Te) and pseudo one dimensional compounds MX, and MX,. Those materials can be introduced from the point of view of' the order-disorder problems in solids. It seems possible to discuss the problem in three different ways if the ordering of smaller and smaller species is considered. First let us consider the two dimensional chalcogenides MX,. They are mainly to be found in the FIG. 3. - Some T>tSc2polytypes. titanium and vanadium groups and for the following elements : MO, W, Pt, Re, Sn, etc ... They are built up as atomic arrangements of slabs. These slabs complications may still arise from folding or dis- are stacked one on another and separated by a Van tortion of the slabs or from the occurrence of clusters der Waals gap (figure 1). Consequently the structures related to the setting of metal-metal bondings : can be classified in respect of : that is the case of the high temperature form of MoTe, and of rhenium chalcogenides. - the nature of the slabs, The weak interslab bondings allow the slabs to - the way they stack. be pulled apart through various chemical intercalations in the Van der Waals void. This is a straight forward introduction to intercalation compounds. An intercalated compound arises from the inter- -Van der Waols gop Tr~gonalPrlsmahc slab OcFahedral slob calation of molecules or ions in a host structure in such. a way that it is possible to retu? reversibly to the initial state through appropriate thermal or chemical actions. This definition assumes an idea FIG. 1. - Basic features for a representation of lamellar d~chal- of reversibility and distinguishes the true intercalated cogenides. compounds (formed mainly with alkali metals or Lewis bases) from the ternary sulfides (obtained The slabs are made of three atomic layers : two with transition metals for example). But in both cases, anionic layers framing a metallic one. The coordi- from a geometrical point of view, the two kinds of nation of the metal in the slabs can be either octahedral compounds may be regarded with the same concept or trigonal prismatic. The simplest stackings lead to which is again an order concept. But this time an CdI, or NbS, 2'.~structures (figure 2). MoS, shows atomic order problem is involved, namely an atomic the first difference in respect of stacking of trigonal order among the sites of each Van der Waals void prismatic slabs. More generally gliding of the slabs and an order among the Van der Waals voids. one over the other explains clearly the large number Let us consider at first the ternary sulfides MLMS, of polytypes that can be observed and are referred where M' is a 3d element and with 0 c x 4 1. The to in the Ramsdell notation. Also layers having M element can occupy either octahedral or tetra- different cation coordination may be interleaved in hedral voids of the host structure according to its many ways : different possibilities are shown in nature and to the MS, structure. TiS, and NbS, figure 3 in the case of tantalum diselenide. Other have given rise to nu.merous studies [l-61. The 3d elements from Ti to Ni occupy in those cases the octahedral voids of the Van der Waals gap. The onset of order occurs between the empty and the now occupied voids and this order is responsible for the particular values of x such as x = 0.25, 0.33, 0.50, 0.66, 0.75. The structures of the M,X,, M5X8, M,X8 ... types stem from the order and are very well known. Figure 4 shows the M5X8 structure. From recent results, three remarks can be put forward :

Ti S2 Nb S2 2H MoS2 2H - this model is only a general frame. It can accommodate some non stoechiometry around the critical X FIG. 2. - TiS,, NbS,, 2 H, MoS, structural types (l l20 sections). value. An excess metal content can statistically SOME ONE- AND TWO-DIMENSIONAL COMPOUNDS C7-237

two kinds of positions. This remark is of some importance if we consider the type of magnetic ordering that occurs at low temperature in the MLMS, compounds ; - In a MLMS, phase, we can imagine the occurrence of a disorder between the M and M' atoms, namely at high temperature and when the M and M' elements show the same coordination. Mossbauer studies have been carried on stoechiometricFe0.,,TiS2 samples [g]. Both slowly cooled and quenched samples show one iron site only with 'a 8 line pattern below approximately 150 K (Neel point) and a paramagnetic doublet above this temperature (E = 0.22 mmjs). This result of only one iron-site gives some experi- mental evidence that iron atoms are present only in the vacancy layers with the ordered distribution (Fe + 0)Ti2S4 meaning that full titanium layers alternate with well ordered Fe + layers. Any disordering between Fe and Ti would lead to at least two different sites for Fe corresponding to the two sites of the^ Cr,S, structure. Quenched samples show however a line broadening that suggests the existence of slightly unequivalent iron sites due to a smaller degree of vacancy ordering as compared to the high order (Fe + U) Ti2S4 of the slowly cooled material. The situation of non stoechiometric Fe,+,Ti2+,S2 samples is much more complicated in particular when considering the distribution of the (1 + X) Fe and y Ti atoms in the Van der Waals void.

Finally, in the perspective of these ternary sulfides, it is worth noticing that the first instances of such compounds are the non stoechiometric sulfides MS, themselves which are formulated M, +,S,. Various non stoechiometric Nb, +,S2 structures have been studied [9, 101 and exhibit several common features with the M,NbS2 series. It has also been shown recently that in non stoechiometric TiS2(Ti, +,S,) there are layers fully occupied by titanium alternating with partially occupied ones, as found for M,TiS2 compounds [Ill. But for both examples (Nb, +,S2 and Ti, +,S,), the observed order occurs not only for the occupancy FIG. 4. - The Co,,,,TiS, structure : a) general view ; b) and c) of the empty layer, it happens also on the way of occupancy of octahedral voids by cobalt in the z = 112 (b) and stacking the slabs. There is a relationship .between z = 0 (c) slabs. the occupancy of the gap and the way the layers are stacked. In the case of Ti, +,S, it has been noticed occupy the empty voids. In the same way, a metal that around X = 0.20, the 4 H and 12 R polytypes deficiency can occur on the superstructure sites ; were appearing [10]. It is known that impurities -the temperature plays an important role. For consisting of a 3d element lead inevitably to NbS,3 R CO,,,~T~S,it has been shown that the superstructure and not to NbS22 H. The relationship between non is not related to an ordering between positions comple- stoechiometry and polytypism is an open question. tely filled or completely empty [7]. One must distin- The compounds taking place between layered guish between three different occupancy ratio among chalcogenides and alkaline metal are genuine inter- tke sixteen possible positions : two positions with calates as opposed to ternary sulfides. They can be a ratio of 0.1, two of 0.40 and twelve with a ratio described as two dimensional compounds. The alkali of 0.25 corresponding to the statistic disorder. The metal can occupy either all the Van der Waals gap superstructure is related to ordering among the first (stage I compounds) or only one out of two (stage I1 C7-238 J. ROUXEL compounds) or one out of four (stage IV compounds). 4 Its sulfur coordination can be either octahedral 132- or trigonal prismatic (figure 5). It stems from three factors [l21 : the size of the alkali metal, the amount 1 - of it, the nature of the MS, slab of the host structure. =S-

R b. K -

Na- 005-

Li -

F[(, 5 5t11lctl1t.1ltypes for A,TiS, intercalation compounds.

The influence and mutual relationship of these factors can be discussed according to the fact that an octahedron can accommodate higher charges on the anions than a trigonal prism does [13]. With a bigger alkali metal atom the sulfur layers are more distant FIG. 6. - Ionicity-Structure diagram for intercalates in CdI, like and the trigonal prismatic structure is favored. structures. The general ionization scheme xA+, MS$- explains why for a given alkali metal the octahedral forms may appear for the higher values of X whereas the trigonal prismatic forms are ,obtained for the lower values of X. The last factor involves the covalency of the slabs of the host structure : ZrS,, more ionic, favours the formation of the octahedral species, as compared to TiS,. A general diagram concerning ionicity- structure in the intercalation compounds has been proposed [14]. By plotting the r,+/r,-- ratio versus a function related to the ionicity it is possible to draw an unambiguous limit between octahedral and trigonal prismatic domains (figure 6). The repulsion between successive A+ positive layers FIG. 7. - Band structures for lamellar chalcogenides. may be taken in account in order to explain the existence of I1 stage phases. Possibly the A+ ions intercalates are more stable and less sensitive to occupy at first more distant Van der Waals layers moisture for the latter than for the former. Inter- thus lowering the repulsion. Towards these repul- calation compounds formed with molybdenum dichal- sion~,the MS, slab of the host structure behaves cogenides appear to be the most unstable : this can as a screen. From this point of view the A,Ta,S,C be related to the fact that the dz2 .band of the host phases are of some importance [15]. The fact that structure is filled up and the electron lost by the no second stage phases were observed in that case alkali metal has to be accommodated in a higher level. is probably related to the bigger screen effect due to The alkali metal intercalates are good ionic conduc- a five layers slab (S-Ta-C-Ta-S). tors in agreement with a high mobility of the A+ The band structure of the host chalcogenide ions between the slabs of the host structure. Most (figure 7) seems to play an important role in the of them are also good electronic conductors. These kinetics of intercalation and in the stability of the remarks along with a charge-discharge mechanism products. Intercalation has been often found to be involving intercalation and deinterca1atio.n of the easier in chalcogenides with a broad conduction A+ ions make it possible to use lamellar chalcogenides band (TiS,, ZrS,, TiSe,) than in a chalcogenide in batteries [16]. A lithium-TiS, battery is based on with a narrow band (NbSe, for example) and alkali a lithium negative and a TiS, positive electrode. SOME ONE- AND TWO-DIMENSIONAL COMPOUNDS C7-239

One can remark also that it is possible to suppress as neither the way the molecules are linked to the the electronic contribution induced by intercalation slabs nor the electronic transfers along the molecular through substitutions in the slabs performed along bridges are known. with intercalation between them : that is the case The use of pressure that brings the slabs closer of the A+M!'Zr, -,S, ionic conductors with M = In, to one another corresponds to the reverse mechanism Ga, Y [17]. Extensive NMR studies have been per- and lead to consistent and well explained results [24- formed [18-211. Nevertheless, according to the great 261. It may be noticed that intercalation of alkali number of host structures, to the possibility of metals offers at first an easy way to achieve high intercalating all alkali metals in various amounts carrier densities. It also transforms the two dimen- and with different coordinations, alkali intercalates sional host structure in a three dimensional model have provided an opportunity to study factors and a major effect of intercalation is therefore sup- affecting ionic conductivity. Lithium has the lowest pression of the charge density wave instability. activation energy in sulfides : 0.11 eV for LiTiS, But I wish now to introduce new low dimensional instead of 0.18 eV for Na,TiS, [21]. It depends on compounds in the chalcogenides series and especially the amount of intercalation through the A+/O ratio, in the Nb-Se system. New materials have been it depends also of the covalency of the host structure : recently prepared that provide new interesting possi- 0.11 eV for Li,TiS, and 0.22 eV for Li,ZrS, [21]. bilities for the future : NbSe, and X,NbSe, A third factor is the kind of coordination around the compounds (X = I, Br, Se). alkali metal : a lower activation energy is observed In figure 8 the structure of NbSe, [27] is compared for Na,,,,TiS, (trigonal prismatic) than for Na,TiS, to the two dimensional model of NbSe,. From a (octahedral). Towards selenides the polarisation power geometrical point of view in both cases the structure of lithium causes a levelling off of the activation can be regarded as built up with MX, structural energy. No ordering has been found for the A+ ions in the Van der Waals gap. However it doesn't seem possible for the ions to occupy completely disordered positions at least when short distances are involved. The electrostatic repulsion tends to push further away ions having the same charge. This should lead to the occurrence of domains inside of which all the cells contain Na' ions in identical positions. These positions are different from a domain to another and this explains the structure statistical disorder. The moving of the ions would then imply a changing in the distribution of the domains through a trans- FIG.8. NbSe3 and NbSe, schematic models. port of their edges. This can only happen through a mechanism of cooperative diffusion with simul- taneous moving of all the ions of the edges. Electron units. In the case of NbSe,, irregular I NbSe, l microscopy studies and electron diffraction patters trigonal prisms (with one Se-Se pair in each base) of Na-TaS, intercalates have been performed at low are stacked in order to form (NbSe,) chains, in the temperatures [22a]. Interesting transitions were observ- case of NbSe, regular I NbSe, I prisms are arranged ed. However a comparison of the spectra obtained in (NbSe,) infinite layers. The questions arises thus with various alkali or organic intercalates suggests to know to what extend NbSe, can be considered that the observed diffraction effects are in that case to be a one dimensional compound as compared probably intrinsic of the host structure. Nevertheless to the NbSe, two dimensional description. An cluster ordering as in P alumina seems to be highly answer was given through appropriate physical and probable at low temperature 12261. chemical experiments. Lamellar dichalcogenides and their intercalation The electrical resistivity along the chains (b axis) products have arisen a great deal of interest for was found to be of the order of 600 ~Rcm.This value physical measurements. They offer an opportunity is very similar to the longitudinal resistivity of TTF- for studying physics in two dimensional systems. TCNQ or the polymeric sulfur-nitride (SN), but Fermi surfaces with large parallel portions favor much larger than the resistivity along the planes structural distortions in relation with charge density of NbSe, (100 mcm). More significant is the tempe- waves. Such perturbations in electronic metallic rature variation of p [28]. It is shown in figure 9. models could represent a third approach of these Above 145 K, p decreases with a slight curvature materials as far as ordered-disordered problems are when T is lowered, showing a metallic behaviour. concerned [23]. Intercalation leads to possible un- Below 10 K, p appears to saturate to a value limited coupling of the slabs, namely through organic mole- by the defects. The most interesting features are the cules, although one has to be prudent in this field two strong anomalies which appear respectively at C7-240 , . J. ROUXEL

FIG. 9. - Temperature variations of p for NbSe,.

T,, = 145 K with a maximum at 125 K and at T,, = 59 K with a maximum at 49 K before resuming a metallic-type temperature variation. No hysteresis FIG. 11. -Variation of the resistivity of the second anomaly was detected when the temperature was varied under pressure. across the transitions. The heat capacity shows an anomaly at the same tion of the lower anomaly is much more important : initial temperature T,, where p increases sharply. the amplitude of the anomaly is more than 95 % At T,, it does not present a pronounced anomaly. suppressed at 6 kbar. The magnetic susceptibility without any correction The physical properties of NbSe,, can be consistent- of core diamagnetism is found to be diamagnetic ly explained by assuming the formation of charge and depends on the orientation of the fibers in the density waves. When a CDW forms, gaps open at the field as in the case of 2 H-TaSe,. After correction Fermi surface at those portions that satisfy the nesting a slight Pauli Paramagnetism remains. condition. The increase in resistivity in NbSe, has The electrical resistivity along the chains have been attributed to the decrease in area of the Fermi been measured under hydrostatic pressure [28]. surface resulting from the opening of gaps. The Figures 10 and 11 show the variation of the resistivity formation of a CDW is determined by the competition with pressure for the two anomalies. The T, tempe- between two terms in the free energy of the system : ratures appear' to vary linearly with pressure and the the strain energy, which increases with the formation slope is the same for the two transitions : of superlattice distortions, and the gain in electronic energy resulting from the opening of the gaps. The gain in electronic energy increases with decreasing The amplitude of the higher anomaly decreases with temperature because the Fermi surface is sharper pressure and is reduced by 30 % at 4 kbar. The reduc- at lower temperatures. By applying pressure a stif- fening of the lattice is expected with a resultant increase of the strain energy. To offset this increase in energy the electronic energy gain must be larger for the CDW state to be stable. Consequently the critical temperature is lowered. The two critical temperatures of NbSe, have been shown to decrease with the application of pressure. This is very similar to the pressure dependance of the CDW critical temperature in the layered compounds. But for NbSe, the rate of decrease is much larger :4 K/kbar compared to 0.2 K/kbar for 2 H-TaS, [25] and 0.35 K/kbar for 2 H-NbSe, [25]. Two different CDW may exist in NbSe,. However low temperature structural studies and particularly diffuse X-ray measurements are needed to provide crucial evidence of a superlattice in NbSe,. FIG. 10. - Variation of the resistivity of thc first anomaly under New evidence for the formation of gaps in NbSe, pressure. is provided by the fact that the two resistivity anoma- SOlME ONE- AND 'I'WO-DIMENSIONAL COMPOUNDS C7-24 1 lies are suppressed by an electric field [29] : the non of view we have to consider genuine slabs made of linearity of the conductivity with electric field agrees coupled fibers. Between them lies a Van der Waals with a Zener breakdown analysis across extremely gap with dimensions close to those of NbSe,. Figure 12 small gaps induced by the CDW. Also NbSe, is shows three of these ,slabs separated by two Van found superconductor under pressure. der Waals gaps. The conclusion may be that NbSe, The chemical behaviour of NbSe, appears to be is a two dimensional compound with a great aniso- similar to the behaviour of lamellar chalcogenides : tropy within the slabs. Each slab is built up with it is possible to intercalate and to deintercalate lithium linear (NbSe,) fibers bonded in order to form a zig-zag between the fibers. The experiments have been made arrangement. The two charge density waves could through the n-butyllithium technique [30, 311. Three lithium were found to intercalate. The b parameter (related to the Nb-Nb distance in the (NbSe,) chains) remains unchanged. The a, c and /l parameters are slightly changed. This can be taken as an indication of the localization of the Li' ions between the one dimensional (NbSe,) chains. It has been suggested that the.first two alkali- metals reduce the dichalcogenide bond without dis- rupting the structure. The third alkali-metal atom is supposed to give its electron to the Nb-Nb chain. Through intercalation the (NbSe,) chains would then be separated from each other by the alkali- metals. This can account for the fact that relatively weak interchain bondings have been loosened. Lii ions have a high mobility between the chains. So the chemical behaviour could agree with a one dimensional model for NbSe,. The existence of CDW can be also explained by a structure with low dimensionality. Recent measurements of the trans- verse resistivity have shown that the anisotropy ratio p(// h)/p(l6) in NbSe, is much bigger than in NbSe,. However the transversal Nb-Se distances between niobium and selenium atoms of the neighbouring chains are not long enough to exclude bonding. Taking these bonds into account, ~iiobiumcan be considered as surrounded by eight Se atoms forming a bicappest trigonal prism. According to this point,

FIG. 13. - - Thc ~rl-uccu~~c01' I,, ,,NbSc, . ir) pl.o~cctions on the FIG. 12. - Keal~tyof the two d~rnensloii,llir) ot' NbSc,. xoy plane and b) NbSe, anriprisms. C7-242 J. ROUXEL be related, one to the existence of two dimensional calation) are possible between the NbSe, chains [35]. slabs. the other to the pseudo one-dimensional The physical .properties of these compounds, now character within the slabs. under investigation should be of great interest accord- TaSe, with a related structure was found to present ing to the structural model. However many problems a classical metallic resistivity curve [32]. NbS, is a concerning the real oxydation states of niobium and semiconductor with Nb-Nb pairing along the chains the role played by iodine, bromine or extra selenium and a 2 b superstructure [33]. have yet to be solved and make it difficult to give I,,,,NbSe, presents a quadratic unit cell with completely satisfying explanations at this moment. a = 9.489(1) di and c = 19.13(3) A. The structure Lamellar transition metal dichalcogenides have was refined to a R value of 0.030 [34]. It can be des- provided a very stimulating field for research. The cribed as (NbSe,) chains developing along the chemical and physical results, already very interesting c axis, the iodine atoms lying between these chains. by themselves, have in addition open new ways for The niobium coordination is a rectangular antiprism research in other scientific. domains. ,Tri and tetra- built of (Se-Se) pairs. The Se-Se distance in these chalcogenides of transition elements provide new pairs is 2.34 di in good agreement with the value found interesting possibilities. However their real dimen- in other polyselenides. The structure is shown in sionality remains an open question, although in the figure 13. NbSe, example it is possible to suggest a two dimen- It is possible to undergo a substitution of iodine sional model with strong anisotropy within the slabs by bromine. Other kinds of substitution (and inter- themselves.

References

[l] DANOT,M,, ROUXEL,J., C. R. Hebd. Shun. Acad. Sci. 271 [20] SILBERNAGEL,B. G., WHITTINGHAM.M. S., Mat. Res. BUN. (1970) 998. 11 (1976) 29. [2] ANZENHOFER.K., VAN DEN BERG,J. M., COSSEE, P., HELLE, [21] TRICHET,L., BERHTIER,C., ROUXEL,J., to be published. J. N., J. Phys. & Chem. Solids 31 (1970) 1057. [22] a) WILLIAMS,P. M., SCRUBY,C. B., CLARK,W. B., PARRY, [3] VANLAAR, B., RIETVELD,H. M., LIDO, D. J. W., J. Solid Stule G. W., J. Physique Colloq. 37 (1976) C4-139. Chem. 3 (1971) 154. b) BOILOT,J. P.,ZUPPIROLI, L., DELPLANQUE,G., J~R~ME,L., [4] ROYER,A., LE BLANC,A., ROUXEL,J., C. K. Hebd. Sdan. Phil. Mag. 322 (1975) 343. Acad. Sci. 276 (1973) 102 1. [23] WILSON,J. A., DI SALVO,F. J., MAHAJAN,S., Adv. Phys. 24 [5] MEERSCHAUT,A., ROUXEL,J., C. R. Hebd. SJan. Acad. Sci. (1975) 117. 277 (1973) 163. [24] DELAPLACE,R., MOLINI~,P,, J~R~ME,D., J. Phj~sique Left. (61 LE BLANC,A., ROUXEL,J., GARDETTE,M. F., GOROCHOV,O., 37 (1976) L-13. Mat. Res. Bull. 11 (1976) 106. (251 BER~IER,G., MOLIN~,P., JER~ME,D., Solid State Commun. [q DANOT,M., BREC, R., Acta Crystallogr. B 31 (1975) 1647. 18 (1976) 1393. [8] FATSEAS,G. A., DORMAKN,J. L., DANOT,M., J. Physique [26] MOLIN~,P,, ROUXEL,J., BERHTIER,C., J~R~ME,D., Solid Colloq. 37 (1976) C6-579. State Commun. l2 (1976) 131. [9] JELLINEK,F., Arkiv Kemi 20 (1963) 447. 1271 MEERSCHAUT,A., ROUXEL,J., J. Less. common Metals 39 [l01 KADJIK,F., JELLINEK,F., J. Less. Common metals 19 (1969) (1975) 197. 421. [28] MONCEAU,P., HAEN,P., WAINTHAL,A., MEERSCHAUT,A., [l l] MORET,R., HUBER,M,, COM~,A., Phys. Stut. Sol. 38 (1976) ROUXEL,J., Solid State Commun. 20 (1976) 759. 695. (291 MONCEAU,P., ONC, N. P., PORTIS,A. M., MEERSCHAUT,A., [l21 LE BLANC,A., DANOT,M., TRICHIT,L., ROUXEL,J., Mat. ROLIXEL,J., Phys. Rev. Left. 37 (1976) 602. Res. BUN. 9 (1974) 191. [30] CHIANELLI,R., DINES, M. B., Inorg. Chem. 14 (1975) 2417. [l31 HUISMAN,R., DE JONGE,R., HAAS,C., JELLINEK,F.. J. Solid 1311. - MIJRPHY,D. W., TRUMRORE,F. A., J. Electrochenz. Soc. 124 Stare Chem. 3 (1971) 56. (1977) 325. (141 ROUXEL,J., J. Solid State Chem. 9 (1974) 358. [32] HAEN,P,, MONCEAU,P., TISSIER,B., WAYSAND,G., MEERS- [l51 BKEC,R., RITSMA,J., OUVRARD,G., ROUXEL,J., Inorg. Chem. CHAUT,A., MOLINI~,P,, ROUXEL,J., Proc. Low Temp. 16 (1977) 660. Conf. L.T. 14 (1975) 445. [l61 WHITTINC;HAM,S., Science 192 (1976) 1126. (331 KADUK,F., Thesis Groningen. (171 TRICH~,L., ROUXEL,J., Mat. Res. Bull. 12 (1977) 345. [l81 SILBERNAGEL,B. G.. Solid State Commun. 17 (1975) 361. [34] PALVADEAU,P., MEERSCHAUT,A., ROUXEL,J., J. Solid. State 1191. . SII.BERNAGEL.B. G.. WHITTINGHAM,M. S., J. Chem. Phys. Chem. 20 (1977) 21. 64 (1976) 3670. (351 GU~MAS,L.. M~~KSCHAUT,A., ROLIXEL, J., to be published.