MA(x)Te(2) phases (M=Nb, Ta; A=Si, Ge; 1/3<=x<=1/2): An electronic band structure calculation analysis

The MA(x)Te(2) phase (M = Nb, Ta; A = Si, Ge; 1/3 less than or equal to x less than or equal to 1/2) band structures are reexamined by means of ab initio LMTO-TB band structure calculations for two representative compounds, NbGe1/2Te2 and NbGe1/3Te2. Despite the high covalency of the phases, the band structures can be analyzed in terms of M d-like blocks and Te and Ge s- and p-like states. Formal oxidation states can therefore be assigned and a general formulation established. The existence of the phases is shown to emanate from a strong bonding between the A elements and their four metal neighbors. The bonding not only assures the stability of the A element in its tellurium square-like environment but also provides the stability of the lone metals in their trigonal prismatic sites, which in turn allows the commensurate or incommensurate in-plane insertion of otherwise unstable trigonal prismatic MTe2 ribbons.