Resonant defect states and strong lattice relaxation of oxygen vacancies in WO₃

We find, from first principles calculations, that an oxygen (anion) vacancy in WO₃ not only generates a donorlike 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 is found for the W ions nearest to the vacancy, accompanied by large changes in the energies of all the defect states.