Jean Rouxel

Les intercalaires Ax Ti S2 et Ax Zr S2. Structure et liaisons. (A = Li, Na, K, Rb, Cs)

Abstract A general survey of alkali metals intercalation compounds with titanium and zirconium disulfides is given. A model A + x (Ti S 2 x− or A + (Zr S 2 ) x− with an electronic delocalization in the (Ti 2 S 2 ) or (Zr S 2 ) slabs, is suggested according to crystallographic and magnetic studies, N.M.R. measurements, and chemical behaviour. The alkali metal coordination is related to several factors: size of the A + ion, value of the x parameter, strength of the T.S. bonds.

Importance of short interlayer Te···Te contacts for the structural distortions and physical properties of CdI2-type layered transition-metal ditellurides

Abstract We examined how the short intra- and interlayer Te···Te contacts of layered transition-metal tellurides affect their structures and physical properties by carrying out tight-binding band electronic structure calculations for the CdI 2 -type layered transition-metal dichalcogenides Ti X 2 (X =S, Se, Te) and M Te 2 (M =V, Nb, Ta) on the basis of the extended Huockel method. In the CdI 2 -type tellurides, the top portion of the Te p -block bands overlaps significantly with the bottom portion of the metal d -block bands, thereby causing a substantial electron transfer from the p - to the d -block bands. For this p → d electron transfer, the interlayer Te···Te contacts are found to be essential because the overlap between the Te p z -orbitals (perpendicular to the layer) associated with the interlayer Te···Te contacts is most effective in raising the top portion of the Te p -block bands. As a consequence, layered transition-metal tellurides are likely to possess a three-dimensional metallic character, and a slight change in their interlayer Te···Te contacts significantly affects their electrical and other physical properties.

Sur un diagramme ionicité-structure pour les composes intercalaires alcalins des sulfures lamellaires

The structure of alkali-metal intercalation compounds in layer disulfide host lattices is discussed. The sulfur surrounding of the alkali-metal (octahedral or trigonal prismatic) is a function of the size of the A+ ion, the amount of intercalated atoms, and the nature of the TS bond. It is possible to study the relationship between the structural models and the ionicity of the bonds. Such a diagram could be used to predict the structures to be expected. A discussion of the second stage phases is given.

Etude structurale des composés MxTiSe2 (M = Fe, Co, Ni)

Abstract New compounds M x TiSe 2 have been prepared with M = Fe ( x ⩽ 0.66), M = Co or Ni ( x ⩽ 0.50). The metal M is located in vacant octahedral sites of the TiSe 2 host lattice (hexagonal unit cell a ′, c ′). An ordering of vacancies occurs if x ⩾ 0.20. With M = Co or Ni ( x = 0.50) and with M = Fe (0.25 ⩽ x ⩽ 0.66) isotypic compounds of Ti 3 Se 4 can be obtained ( M 3 □ X 4 type; monoclinic unit cell a ≈ a ′ √3, b ≈ a ′, c ≈ 2 c ′). The compounds Fe 0.38 TiSe 2 and Co 0.38 TiSe 2 (hexagonal unit cell a ≈ a ′ √3, c ≈ 2 c ′) are of the M 2 □ X 3 type, variety 2 c ′. The Fe 0.25 TiSe 2 and Co 0.25 TiSe 2 monoclinic unit cells ( a ≈ 2 a ′ √3, b ≈ 2 a ′, c ≈ 2 c ′) allow us to assume, for these two compounds, a structure of the M 5 □ 3 X 8 type, variety 2 c ′, identical to the Ti 5 Se 8 one. The compound Ni 0.25 TiSe 2 has an hexagonal unit cell ( a ≈ 2 a ′, c ≈ 3 c ′); it belongs to a so-called 3 c ′ variety of the M 5 □ 3 X 8 type.

Occurrence and characterization of anionic bondings in transition metal dichalcogenides

Abstract Transition metal dichalcogenides MX2 crystallize in either two-dimensional or three-dimensional (3D) structures. This originates from the competition between cationic d levels and anionic sp levels. The occurrence of a chalcogen pairing may be obtained through oxidation of a ternary phase: Li2FeS2 leads to a metastable new binary compound Fe3+S2−(S2)2−. Such an electronic situation may also be found within the 3D family, the IrX2 (XS, Se) and RhSe2 compounds with quite elongated XX bonds attributed to strong strains. IrTe2 should confirm the structural type presented by sulphur and selenium derivatives. Its previously reported CdI2-like structure is in fact based on a polymeric network with multiple TeTe bonds (Irn3+(Te−1.5)2n) as confirmed by integrated overlap population calculations. This polymeric modification is presented by several other MTe2 phases and explains the very low c a value (1.38) of the hexagonal cell observed in this family. The polymerization phenomena must be generalized to most pyrite-like MTe2 with the noticeable exception of MnTe2. The layered binary Cr 2 3 □ 1 3 Te 2 is another example of tellurium polymeric bondings. Finally a classification of structures taking into account not only the dimensionality but also the polymerization degree of such materials is suggested. From many examples, it is shown that the polymerizing behaviour of the heavy chalcogen anion seems to be much more general than expected and should lead to many charge transfer studies.

Structure determination of a new misfit layer compound (PbS)1.18(TiS2)2

Abstract (PbS)1.18(TiS2)2 is a misfit layer compound in which two types of slabs, PbS and 2(TiS2), alternate along the c direction. The |TiS2| blocks (about 11.39 A thick) are interleaved by |PbS| layers, thus leading to a value for c of 17.46 A. The misfit between the two slab types occurs along the a direction; the parameter values being a( PbS )=5.761 A and a( TiS 2 )=3.390 A respectively. This yields a ratio of approximately 1.699, which is irrational but close to 5/3. The common in-plane b parameter is equal to 5.873 A. The PbS unit consists of a {001} slice (half an edge thick) of an NaCl-type f.c.c. cell.

The influence, on lithium electrochemical intercalation, of bond ionicity in layered chalcogenophosphates of transition metals

Abstract In the course of lithium electrochemical intercalation in the host structure of layered M∥PX 3 phases (M = V, Mn, Fe, Co, Ni, X = S, Se), it was shown that the best energy yield was obtained from low ionicity bond materials. The absorption edge energy, along with the free energy of the intercalation reactions have been correlated in a satisfactory way to the ionicity fi of the M-X bonds. These diagrams indicate which phases have to be looked at to obtain the maximum electrochemical yields.

Les conducteurs ioniques NaxInxZr1−xS2

Substituted and intercalated lamellar dichalcogenides have been prepared in the series Na/sub x/In/sub x/Zr/sub 1-x/S/sub 2/. According to the values of x, (0 less than x less than or equal to 1), three different phases have been characterized with trigonal prismatic or octahedral coordination of sodium and with different ways of stacking of the slabs of the host structure. Conductivity measurements have been performed and show these phases to be valuable ionic conductors, especially the trigonal prismatic compounds.

Proprietes electriques, magnetiques et structurales des phases mxtis2 (M = Fe, Co, Ni)

The structures of MxTiS2 phases are reported (M = Fe, Co, Ni; x = .25, .33, .40, .50, .75). Electric and magnetic measurements were carried out from 7 to 400°K. Iron compounds seem to show a competition between ferromagnetism and antiferromagnetism. Cobalt derivatives are mainly antiferromagnetic. For nickel compounds, no paramagnetic moment can be observed, in agreement with an electronic delocalization. Direct and super exchange interactions are discussed, so are energy levels diagrams.

Electronic structures and charge transfer in lithium and mercury intercalated titanium disulfides

Mercury can be intercalated into TiS2 by a reaction between elemental mercury and TiS2 at room temperature. The structure of the obtained compound Hg1.24TiS2 can be described as two non-commensurate monoclinic sublattices. The mercury atoms form metal chains inserted into trigonal prismatic channels created by the expanded TiS2 host lattice. The structural arrangement and interatomic distances for this compound indicate the presence of primarily neutral mercury, with very low charge transfer, and relatively weak HgS interactions. In order to understand this peculiar behaviour, electron band structure calculations have been made using the extended Huckel method and compared with experimental data from different spectroscopies: XAFS, EELS and XPS. Pristine TiS2 and its lithium and mercury intercalated compounds have been investigated. The experimental data are in good agreement with the calculated electronic structures. The main conclusion is that 0.24 electrons are transferred by lithium atom to the TiS2 host structure for the composition Li1TiS2. This transfer is almost equivalent on titanium (0.09 electrons) and sulphur (0.075 electrons per atom). For the mercury intercalated TiS2 phase, both calculations and experimental data show an electronic transfer from mercury to TiS2 very close to zero.