Structural and electrical properties interpretation through band structure calculations on the (MSe₄)_nI SERIES (M = Nb, Ta).

Abstract

Halogened transition metal tetrachalcogenides (MX₄)_nY [M = Nb, Ta ; X = S, Se ; Y = I, Br, C1 , n = 2, 3, 10/3, 4] provide us with a new series of pseudo-1D compounds with exciting properties. In these compounds infinite (MX₄) chains are well separated from each other by halogen chains. Metal ions are sandwiched by two rectangular chalcogen units with a dihedral angle of approximately 45°. According to n, different sequences of metal-metal distances can be observed in unit cells. There are also various possible arrangements of rectangular units and different halogen ions environments. Differences in structures are reflected in drastically different electronic properties such as semiconducting properties or occurrence of charge-density-waves (CDW) with related non-linear effects. A systematic study of the structure of (MSe₄)_n I compounds is presented along with tight binding band calculations. It is shown that Peierls distortion and CDW phenomena are strongly affected by dz² band filling. As n decreases, dz² band filling decreases from 1/2 leading to a decrease in the tendency to metal chain distortion. 3D interactions are also discussed in connection with CDW occurrence.