CHALCOGENIDE MISFIT LAYER COMPOUNDS

Misfit layer chalcogenides (MX)(1+x)(TX(2))(m) (M=Sn, Pb, Sb, Bi, rare earth; T=Ti, V, Cr, Nb, Ta; X=S, Se; 0.08 < x < 0.28; m = 1, 2, 3) result from an alternate stacking of MX and TX, slabs. Both sublattices have two a and b in-plane parameters; the ratio of the a parameters is irrational. The structural features associated with these planar intergrowth compounds are described. Multilayered misfit compounds (m = 2, 3) show various polytypes. Non-stoichiometry can occur in MX layers related to vacancies of M atoms (MX sublattice). The physical properties are discussed in detail. Superconductivity is observed in some series and can be studied as a function of m, the number of superconducting TX(2) slabs, such as NbSe2, that can stack one on top of the other. In addition to interesting magnetic properties, transport measurements allow us to reach a better understanding of the electronic structure of the phases which appear, in many cases, as an infinite two-dimensional intercalation of MX slabs between TX(2) slabs with electron exchange between them. This leads to the introduction of a critical discussion of chemical bonds.