It was found, from first-principles calculations, that an O (anion) vacancy in WO3 not only generates a donor-like state near the fundamental band gap, derived from the top valence bands, but also gives rise to an additional pair of defect states: a hyper-deep resonant state in the valence band and a high-lying resonant state in the conduction band, derived from s-like bonding and antibonding bands, respectively. These states show distinctively different properties from their counterparts in other conventional semiconductors. With a change in the charge state of the vacancy, a strong lattice relaxation was found for the W ions nearest to the vacancy, accompanied by large changes in the energies of all the defect states.

Resonant Defect States and Strong Lattice Relaxation of Oxygen Vacancies in WO3. S.Z.Karazhanov, Y.Zhang, L.W.Wang, A.Mascarenhas, S.Deb: Physical Review B, 2003, 68[23], 233204 (4pp)