Regularities of the formation of polytype structures in layered metal dichalcogenides

Abstract

Possible sequences of alternating triple layers X-M-X in MX ₂ dichalcogenides (where M is a metal and X is a chalcogen) that are consistent with the close packing conditions are considered, and the structure and symmetry of the unit cells of the main polytypic modifications of these compounds are determined. Reasoning from the molecular structure of the crystals and the fact that their smallest structural unit is an MX ₂ molecule, the notation of each triple layer can be represented in a reduced form in which only the position of the central metal atom and the orientation of the molecule are specified. In the proposed notation, allowances are made for two types of coordination of the metal atom in the molecule, namely, the octahedral coordination, in which the nearest X layers occupy different positions and form close packing, and the trigonal prismatic coordination, in which both X layers occupy identical positions. The use of the reduced (molecular) form of the notation significantly simplifies the analysis of the polytype structures and makes it possible to distinguish several structural groups differing in structure and symmetry among the crystals with unit cells including two, three, and four layers.