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

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IV. – ORDER IN ONE AND TWO-DIMENSIONAL

SYSTEMS.SOME ONE- AND TWO-DIMENSIONAL

COMPOUNDS

J. Rouxel

To cite this version:

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SOME O N E 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 t r b large mesure les solides a dimensionnalite restreinte peuvent i t r e considerks comme construits partir d'unitks structurales telles que des feuillets ou des fibres. A I'intkrieur de ces e n t i t b 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 q i maintiennent 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 struc- tures 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 d u 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 a great deal of interest. The two dimensional or one dimensional character of a structure is of course 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 (generally of the order of the Van der Waals radii), in agreement with weak interlayers or interchains bonding.

The corresponding materials are of considerable interest. The structural anisotropy results in a very high anisotropy in the electronic, vibrational and mechanical properties. The physical topics which have been studied in these materials include metal- non metal transformations, Kohn anomalies and charge density waves. In particular, largely two dimensional Fermi surfaces favour the formation of charge density waves. From a chemical point of view the most important aspect is that the slabs or the chains can behave as independant units : it is

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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 o f ' 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 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 are stacked one on another and separated by a Van der Waals gap (figure 1). Consequently the structures can be classified in respect of :

-

the nature of the slabs,

- the way they stack.

-Van der W a o l s gop Tr~gonal Prlsmahc slab OcFahedral slob

FIG. 1. - Basic features for a representation of lamellar d~chalcogenides.

The slabs are made of three atomic layers : two anionic layers framing a metallic one. The coordination of the metal in the slabs can be either octahedral or trigonal prismatic. The simplest stackings lead to CdI, or NbS,

2 ' . ~

structures (figure 2). MoS, shows the first difference in respect of stacking of trigonal prismatic slabs. More generally gliding of the slabs one over the other explains clearly the large number of polytypes that can be observed and are referred to in the Ramsdell notation. Also layers having different cation coordination may be interleaved in many ways : different possibilities are shown in figure 3 in the case of tantalum diselenide. Other

Ti S2 N b S2 2H MoS2 2H

FIG. 2. - TiS,, NbS,, 2 H , MoS, structural types ( l l20 sections).

FIG. 3. - Some T>tSc2 polytypes.

complications may still arise from folding or dis- tortion of the slabs or from the occurrence of clusters

related to the setting of metal-metal bondings :

that is the case of the high temperature form of MoTe, and of rhenium chalcogenides.

The weak interslab bondings allow the slabs to 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 intercalation 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 of reversibility and distinguishes the true intercalated compounds (formed mainly with alkali metals or Lewis bases) from the ternary sulfides (obtained with transition metals for example). But in both cases, from a geometrical point of view, the two kinds of compounds may be regarded with the same concept which is again an order concept. But this time an atomic order problem is involved, namely an atomic order among the sites of each Van der Waals void and an order among the Van der Waals voids.

Let us consider at first the ternary sulfides MLMS, where M' is a 3d element and with 0 c x 4 1. The M element can occupy either octahedral or tetra- hedral voids of the host structure according to its nature and to the MS, structure. TiS, and NbS, 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 :

- this model is only a general frame. It can accommodate some non stoechiometry around the critical X

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FIG. 4. - The Co,,,,TiS, structure : a) general view ; b) and c)

occupancy of octahedral voids by cobalt in the z = 112 (b) and z = 0 (c) slabs.

occupy the empty voids. In the same way, a metal deficiency can occur on the superstructure sites ;

-the temperature plays an important role. For CO,,,~T~S, it has been shown that the superstructure is not related to an ordering between positions comple- tely filled or completely empty [7]. One must distin- guish between three different occupancy ratio among tke sixteen possible positions : two positions with a ratio of 0.1, two of 0.40 and twelve with a ratio of 0.25 corresponding to the statistic disorder. The superstructure is related to ordering among the first

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 stoechiometric Fe0.,,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 of the empty layer, it happens also on the way of stacking the slabs. There is a relationship .between the occupancy of the gap and the way the layers are stacked. In the case of Ti, +,S, it has been noticed that around X = 0.20, the 4 H and 12 R polytypes

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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-

L i

-

F[(, 5 5t11lctl1t.1l types 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 and the trigonal prismatic structure is favored. 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

may be taken in account in order to explain the existence of I1 stage phases. Possibly the A+ ions occupy at first more distant Van der Waals layers thus lowering the repulsion. Towards these repulsion~, the MS, slab of the host structure behaves as a screen. From this point of view the A,Ta,S,C phases are of some importance [15]. The fact that no second stage phases were observed i n that case is probably related to the bigger screen effect due to a five layers slab (S-Ta-C-Ta-S).

The band structure of the host chalcogenide (figure 7) seems to play an important role in the kinetics of intercalation and in the stability of the products. Intercalation has been often found to be easier in chalcogenides with a broad conduction band (TiS,, ZrS,, TiSe,) than in a chalcogenide with a narrow band (NbSe, for example) and alkali

FIG. 6 . - Ionicity-Structure diagram for intercalates in CdI, like

structures.

FIG. 7. - Band structures for lamellar chalcogenides.

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One can remark also that it is possible to suppress the electronic contribution induced by intercalation through substitutions in the slabs performed along with intercalation between them : that is the case of the A+M!'Zr, -,S, ionic conductors with M = In, Ga, Y [17]. Extensive NMR studies have been performed [18-211. Nevertheless, according to the great number of host structures, to the possibility of intercalating all alkali metals in various amounts and with different coordinations, alkali intercalates have provided an opportunity to study factors affecting ionic conductivity. Lithium has the lowest activation energy in sulfides : 0.11 eV for LiTiS, instead of 0.18 eV for Na,TiS, [21]. It depends on the amount of intercalation through the A + / O ratio, it depends also of the covalency of the host structure :

0.11 eV for Li,TiS, and 0.22 eV for Li,ZrS, [21]. A third factor is the kind of coordination around the alkali metal : a lower activation energy is observed for Na,,,,TiS, (trigonal prismatic) than for Na,TiS, (octahedral). Towards selenides the polarisation power of lithium causes a levelling off of the activation energy.

N o 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- 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 microscopy studies and electron diffraction patters of Na-TaS, intercalates have been performed at low temperatures [22a]. Interesting transitions were observed. However a comparison of the spectra obtained with various alkali or organic intercalates suggests that the observed diffraction effects are in that case probably intrinsic of the host structure. Nevertheless cluster ordering as in

P

alumina seems to be highly probable a t low temperature 12261.

Lamellar dichalcogenides and their intercalation products have arisen a great deal of interest for physical measurements. They offer an opportunity for studying physics in two dimensional systems. Fermi surfaces with large parallel portions favor structural distortions in relation with charge density waves. Such perturbations in electronic metallic models could represent a third approach of these materials as far as ordered-disordered problems are concerned [23]. Intercalation leads to possible un- coupling of the slabs, namely through organic molecules, although one has to be prudent in this field

as neither the way the molecules are linked to the slabs nor the electronic transfers along the molecular bridges are known.

The use of pressure that brings the slabs closer to one another corresponds to the reverse mechanism and lead to consistent and well explained results [24- 261. It may be noticed that intercalation of alkali metals offers at first an easy way to achieve high carrier densities. It also transforms the two dimensional host structure in a three dimensional model and a major effect of intercalation is therefore sup- pression of the charge density wave instability.

But I wish now to introduce new low dimensional compounds in the chalcogenides series and especially in the Nb-Se system. New materials have been recently prepared that provide new interesting possibilities for the future : NbSe, and X,NbSe, compounds (X = I, Br, Se).

In figure 8 the structure of NbSe, [27] is compared to the two dimensional model of NbSe,. From a geometrical point of view in both cases the structure can be regarded as built up with MX, structural

FIG. 8. NbSe3 and NbSe, schematic models.

units. In the case of NbSe,, irregular

I

NbSe,

l

trigonal prisms (with one Se-Se pair in each base) are stacked in order to form (NbSe,) chains, in the case of NbSe, regular

I

NbSe,

I

prisms are arranged in (NbSe,) infinite layers. The questions arises thus to know to what extend NbSe, can be considered to be a one dimensional compound as compared to the NbSe, two dimensional description. An answer was given through appropriate physical and chemical experiments.

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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 was detected when the temperature was varied across the transitions.

The heat capacity shows an anomaly at the same initial temperature

T,,

where p increases sharply. At

T,,

it does not present a pronounced anomaly. The magnetic susceptibility without any correction of core diamagnetism is found to be diamagnetic and depends on the orientation of the fibers in the field as in the case of 2 H-TaSe,. After correction a slight Pauli Paramagnetism remains.

The electrical resistivity along the chains have been measured under hydrostatic pressure [28]. Figures 10 and 11 show the variation of the resistivity with pressure for the two anomalies. The

T,

temperatures appear' to vary linearly with pressure and the slope is the same for the two transitions :

The amplitude of the higher anomaly decreases with pressure and is reduced by 30

%

at 4 kbar. The reduc-

FIG. 10. - Variation of the resistivity of thc first anomaly under pressure.

FIG. 11. -Variation of the resistivity of the second anomaly under pressure.

tion of the lower anomaly is much more important :

the amplitude of the anomaly is more than 95

%

suppressed at 6 kbar.

The physical properties of NbSe,, can be consistent- ly explained by assuming the formation of charge density waves. When a CDW forms, gaps open at the Fermi surface at those portions that satisfy the nesting condition. The increase in resistivity in NbSe, has been attributed to the decrease in area of the Fermi surface resulting from the opening of gaps. The formation of a CDW is determined by the competition between two terms in the free energy of the system :

the strain energy, which increases with the formation of superlattice distortions, and the gain in electronic energy resulting from the opening of the gaps. The gain in electronic energy increases with decreasing temperature because the Fermi surface is sharper 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,.

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lies are suppressed by an electric field [29] : the non linearity of the conductivity with electric field agrees with a Zener breakdown analysis across extremely small gaps induced by the CDW. Also NbSe, is found superconductor under pressure.

The chemical behaviour of NbSe, appears to be similar to the behaviour of lamellar chalcogenides :

it is possible to intercalate and to deintercalate lithium between the fibers. The experiments have been made 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 ( l 6) 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, ~iiobium can be considered as surrounded by eight Se atoms forming a bicappest trigonal prism. According to this point,

FIG. 12. - Keal~ty o f the two d~rnensloii,llir) ot' NbSc,.

of view we have to consider genuine slabs made of coupled fibers. Between them lies a Van der Waals gap with dimensions close to those of NbSe,. Figure 12 shows three of these ,slabs separated by two Van der Waals gaps. The conclusion may be that NbSe, is a two dimensional compound with a great anisotropy within the slabs. Each slab is built up with linear (NbSe,) fibers bonded in order to form a zig-zag arrangement. The two charge density waves could

FIG. 13. - - Thc ~rl-uccu~~c 01' I,, ,,NbSc, . i r ) pl.o~cctions on the

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C7-242 J. ROUXEL

be related, one to the existence of two dimensional slabs. the other to the pseudo one-dimensional character within the slabs.

TaSe, with a related structure was found to present a classical metallic resistivity curve [32]. NbS, is a semiconductor with Nb-Nb pairing along the chains and a 2 b superstructure [33].

I,,,,NbSe, presents a quadratic unit cell with

a = 9.489(1)

di

and c = 19.13(3)

A.

The structure was refined to a R value of 0.030 [34]. It can be described as (NbSe,) chains developing along the c axis, the iodine atoms lying between these chains. The niobium coordination is a rectangular antiprism built of (Se-Se) pairs. The Se-Se distance in these pairs is 2.34

di

in good agreement with the value found in other polyselenides. The structure is shown in figure 13.

It is possible to undergo a substitution of iodine by bromine. Other kinds of substitution (and inter-

calation) are possible between the NbSe, chains [35]. The physical .properties of these compounds, now under investigation should be of great interest according to the structural model. However many problems concerning the real oxydation states of niobium and the role played by iodine, bromine or extra selenium have yet to be solved and make it difficult to give completely satisfying explanations at this moment.

Lamellar transition metal dichalcogenides have provided a very stimulating field for research. The chemical and physical results, already very interesting by themselves, have in addition open new ways for research in other scientific. domains. ,Tri and tetra- chalcogenides of transition elements provide new interesting possibilities. However their real dimensionality remains an open question, although in the NbSe, example it is possible to suggest a two dimensional model with strong anisotropy within the slabs themselves.

References

[ l ] DANOT, M,, ROUXEL, J., C . R. Hebd. Shun. Acad. Sci. 271 (1970) 998.

[2] ANZENHOFER. K., VAN DEN BERG, J. M., COSSEE, P., HELLE, J. N., J. Phys. & Chem. Solids 31 (1970) 1057.

[3] VAN LAAR, B., RIETVELD, H. M., LIDO, D. J. W., J. Solid Stule

Chem. 3 (1971) 154.

[4] ROYER, A., LE BLANC, A., ROUXEL, J., C . K. Hebd. Sdan.

Acad. Sci. 276 (1973) 102 1.

[5] MEERSCHAUT, A., ROUXEL, J., C . R. Hebd. SJan. Acad. Sci.

277 (1973) 163.

(61 LE BLANC, A., ROUXEL, J., GARDETTE, M. F., GOROCHOV, O.,

Mat. Res. Bull. 11 (1976) 106.

[q DANOT, M., BREC, R., Acta Crystallogr. B 31 (1975) 1647.

[8] FATSEAS, G. A., DORMAKN, J. L., DANOT, M., J. Physique Colloq. 37 (1976) C6-579.

[9] JELLINEK, F., Arkiv Kemi 20 (1963) 447.

[l01 KADJIK, F., JELLINEK, F., J. Less. Common metals 19 (1969)

421.

[l l] MORET, R., HUBER, M,, C O M ~ , A., Phys. Stut. Sol. 38 (1976) 695.

[l21 LE BLANC, A., DANOT, M., TRICHIT, L., ROUXEL, J., Mat.

Res. BUN. 9 (1974) 191.

[l31 HUISMAN, R., DE JONGE, R., HAAS, C., JELLINEK, F.. J. Solid

Stare Chem. 3 (1971) 56.

(141 ROUXEL, J., J. Solid State Chem. 9 (1974) 358.

[l51 BKEC, R., RITSMA, J., OUVRARD, G., ROUXEL, J., Inorg. Chem.

16 (1977) 660.

[l61 WHITTINC;HAM, S., Science 192 (1976) 1126.

(171 T R I C H ~ , L., ROUXEL, J., Mat. Res. Bull. 12 (1977) 345. [l81 SILBERNAGEL, B. G.. Solid State Commun. 17 (1975) 361. 1191 SII.BERNAGEL. B. G.. WHITTINGHAM, M. S., J. Chem. Phys.

[20] SILBERNAGEL, B. G., WHITTINGHAM. M. S., Mat. Res. BUN.

11 (1976) 29.

[21] TRICHET, L., BERHTIER, C., ROUXEL, J., to be published. [22] a ) WILLIAMS, P. M., SCRUBY, C. B., CLARK, W. B., PARRY,

G. W., J. Physique Colloq. 37 (1976) C4-139.

b ) BOILOT, J. P., ZUPPIROLI, L., DELPLANQUE, G., J ~ R ~ M E , L.,

Phil. Mag. 322 (1975) 343.

[23] WILSON, J. A., DI SALVO, F. J., MAHAJAN, S., Adv. Phys. 24 (1975) 117.

[24] DELAPLACE, R., M O L I N I ~ , P,, J ~ R ~ M E , D., J . Phj~sique Left.

37 (1976) L-13.

(251 B E R ~ I E R , G., M O L I N ~ , P., J E R ~ M E , D., Solid State Commun. 18 (1976) 1393.

[26] M O L I N ~ , P,, ROUXEL, J., BERHTIER, C., J ~ R ~ M E , D., Solid

State Commun. l 2 (1976) 131.

1271 MEERSCHAUT, A., ROUXEL, J., J. Less. common Metals 39 (1975) 197.

[28] MONCEAU, P., HAEN, P., WAINTHAL, A., MEERSCHAUT, A., ROUXEL, J., Solid State Commun. 20 (1976) 759. (291 MONCEAU, P., ONC, N. P., PORTIS, A. M., MEERSCHAUT, A.,

ROLIXEL, J., Phys. Rev. Left. 37 (1976) 602.

[30] CHIANELLI, R., DINES, M. B., Inorg. Chem. 14 (1975) 2417. 1311 MIJRPHY, D. W., TRUMRORE, F. A., J. Electrochenz. Soc. 124 . -

(1977) 325.

[32] HAEN, P,, MONCEAU, P., TISSIER, B., WAYSAND, G., MEERS- CHAUT, A., M O L I N I ~ , P,, ROUXEL, J., Proc. Low Temp.

Conf. L.T. 14 (1975) 445.

(331 KADUK, F., Thesis Groningen.

[34] PALVADEAU, P., MEERSCHAUT, A., ROUXEL, J., J. Solid. State

Chem. 20 (1977) 21.