Pseudorotation and conformational transitions in crystal structures having trigonal prismatic coordination

Abstract

Kinematic regularities have been examined for the conformational rearrangement of clusters showing trigonal prismatic coordination. Pseudorotation is postulated in an MX/sub 6/ coordination polyhedron, and it has been found that conformation transitions can occur without disrupting most of the bonds. A kinematic cell has been identified in the transition-metal dichalcogenides MX/sub 2/ as a pair of trigonal prisms linked by lateral edges, whose conformation features are similar to those of boat-shaped rings. It is found that the conformation changes in parts of the MX/sub 2/ structure can occur in two ways: by matched rearrangements of chains linking flexible rings or by local transformation of the ring segments. The equilibrium conformations of structurally flexible fragments have been examined: linear, clump-convoluted, and spiral. There is a discussion of the scope for a rotational isomeric mechanism for rearrangement involving trigonal prismatic coordination in glide and in polytype transitions.