First-principles study of intrinsic defects in yttrium oxysulfide

Abstract

Atomic and electronic structures of intrinsic point defects in yttrium oxysulfides (Y2O2S) are studied by first-principles total-energy calculations based on density-functional theory combined with normconserving pseudopotentials. Energetics of all the intrinsic point defects are determined for a variety of charge states. From the energetics, the concentrations of the anion vacancies and the interstitial anions are found to be larger than those of the yttrium vacancy and the interstitial yttrium atom under practical conditions. It is also found that the oxygen vacancy, the sulfur vacancy, and the interstitial sulfur atom induce relatively deep levels in the energy gap, whereas the interstitial oxygen atom induces relatively shallow acceptor levels. These findings are consistent with observed broad-band blue luminescence in undoped yttrium oxysulfide, existence of shallow acceptor levels in oxysulfides, and are presumably related to persistent phosphorescence and energy storage phenomena in Eu-doped oxysulfides. Furthermore, negative-U characters are found in the oxygen vacancy and the interstitial sulfur. These behaviors of the defects can be explained from the viewpoint of the covalent bonds newly appearing around the defects in the ionic host material.