Crystal chemistry of dichalcogenides MX2

Crystal-chemical analysis of the structures of metal (other than REE) dichalcogenides of composition X:M = 2:1 has been carried out. It is shown that there are two types of structure depending on the formal degree of metal oxidation. The first group are compounds M(X-2) containing metals in the oxidation state 2+ and covalently bonded pairs of chalcogen elements; the main structural types here are cubic (pyrite) and orthorhombic (marcasite) FeS2. The second group involves the compounds MX2 with metals in the formal oxidation state 4+ and chalcogenide ions X-2(-); the main structural types in this group are Cdl(2), CdCl2, MoS2. The bond lengths M-X, X-X, and X...X are analyzed; it is shown that the high polarizability of the chalcogenide ions is reflected in shortened X...X distances. In the covalently bonded pairs X-X, the bond lengths change within the following limits: 2.03-2.30, 2.35-2.55, and 2.70-2.83 Angstrom; for X...X ion contacts, the minimal values are 3.07, 3.10, and 3.20 Angstrom (X = S, Se, and Te, respectively).