Study of cation vacancies with localized hole states in MgAl2O4 crystals

Abstract

The density functional theory with the Hubbard U parameter method was used to evaluate the lattice structure, electronic structures, and optical spectra of MgAl2O4 crystals with cation vacancies. We analyzed the electronic structures of oxygen atoms around the cation vacancies. The holes prefers to distribute themselves on all the nearest oxygen atoms around the vacancy rather than to localize on a single oxygen atom. We propose the color center models VMg2−−[O23−]2+ and VAl3−−[O23−]3+ for the magnesium vacancy and the aluminum vacancy with localized hole states. Taking into account electron-phonon coupling, we obtained the optical spectra of the cation vacancies with localized hole states. The calculated absorption peaks of the cation vacancies with localized hole states are consistent with the experimental results. The emission peaks of the magnesium vacancy and the aluminum vacancy with localized hole states are at 2.63 eV and 2.62 eV, close to that of the F center at 2.69 eV. We predict that cation vacancies with localized hole states will act as a source of blue luminescence.