The local-density approximation is used to investigate the electronic and structural properties of Nb3O3 and the reasons for its unusual defect rocksalt structure with 25% ordered vacancies. A tight-binding Hamiltonian is fitted to the energy bands which are compared to those of the hypothetical rocksalt phase. This analysis indicates that the ions in the ordered vacancy structure are nearly neutral, the Madelung energy is small, and electronic band effects contribute decisively to its stability. The tight-binding results are consistent with the interpretation that this structure favors a special hybridization between Nb d orbitals and their direct interaction gives rise to strong and nearly covalent metal-metal bonds. The formation of metallic bonds mainly between Nb p and Nb d orbitals around the O-vacancy site gives rise to the so-called vacancy band.
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