Niobium and tantalum diselenides

Abstract

Abstract Niobium and tantalum diselenides have been found to exist in the following polymorphous forms: 2 s ‐NbSe 2 , 3 s ‐NbSe 2 , 4 s ‐NbSe 2 , 2 s ‐TaSe 2 , 3 s ‐TaSe 2 , 4 s ‐TaSe 2 , and 6 s ‐TaSe 2 . Unit‐cell dimensions are collected in Table I and the crystal structures of the various forms are schematically shown in Fig. 1. All the polymorphs have hexagonal or rhombohedral layer structures. In the 2 s ‐, 3 s ‐ and 4 s ‐ phases the metal atoms have a trigonal‐prismatic environment, but in 6 s ‐TaSe 2 half of the metal atoms have octahedral and half have trigonal‐prismatic coordination. The conditions of preparation of the various forms are discussed. Excess of selenium over the composition MSe 2 leads to mixtures of the MSe 3 and MSe 2 phases. Excess of metal results in the formation of phases of the type M 1+x Se 2 , where the additional metal atoms are accommodated in the octahedral interstices between the MSe 2 slabs.