Abstract
The Ma{sub x}Te{sub 2} phase (M = Nb, Ta; A = Si, Ge; 1/3 {le} x {le} {1/2}) band structures are reexamined by means of ab initio LMTO-TB band structure calculations for two representative compounds, NbGe{sub {1/2}}Te{sub 2} and NbGe{sub 1/3}Te{sub 2}. 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 MT{sub 2} ribbons.
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