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 struc- turales 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 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 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 corres- ponding 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 chalco- genides 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 inter- calations 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 for- ward : Ti S2 Nb S2 2H MoS2 2H - this model is only a general frame. It can accom- modate 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 impor- tance if we consider the type of magnetic ordering that occurs at low temperature in the MLMS, com- pounds ; - In a MLMS, phase, we can imagine the occur- rence of a disorder between the M and M' atoms, namely at high temperature and when the M and M' elements show the same coordination.