Electronic structures of M21S8 (M=Nb, Zr) and (M,M')(sub 21)S8 (M,M'=Hf, Ti; Nb, Ta) phases and reasons for variations in the metal site occupations

The electronic structures of binary M21S8 (M = Nb, Zr) and isostructural ternary (M,M')(sub 21)S8 (M,M' = Hf, Ti; Nb, Ta) phases are analyzed by means of extended Huckel tight-binding band structure calculations. The distribution of hafnium and titanium atoms on the six crystallographically independent metal sites in the ternary phase Hf(sub 7.5)Ti(sub 13.5) are rationalized using metal-metal bond strength, electronegatavity/charge, and site volume arguments.