OCCURRENCE AND CHARACTERIZATION OF ANIONIC BONDINGS IN TRANSITION-METAL DICHALCOGENIDES

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 (Ir(n)3+ (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 Cr2/3square1/3Te2 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.