Electronic structure of vacancy defects in MgO crystals

The electronic structure of oxygen-vacancy defects (F, F⁺ and F²⁺ centers) in MgO crystals has been studied within local-density theory, using the self-consistent mixed-basis pseudopotential techniques. The defects were modeled within a supercell having a volume 8 times that of the perfect MgO crystal. The band structure, density of states, charge-density contours, and total energy were calculated as a function of lattice relaxation. The partial density of states shows that each of the F-type centers introduces impurity states into the band gap as well as near the conduction-band edge of MgO. The total energy was calculated as a function of relaxation of the nearest-neighbor shell of Mg²⁺ ions. For F centers, the lowest-energy configuration was found to be a small inward relaxation of the nearest Mg²⁺ ions toward the vacancy site. For F⁺ and F²⁺ centers, the lowest total energies correspond to a small outward relaxation of the Mg²⁺ ions away from the vacancy site. The electronic structure of hydrogen impurities (H^- and H^2- substitutional defects) in MgO was also investigated using the same approach. These impurities contribute defect states within and below the oxygen p bands as well as near the conduction-band edge of MgO. The lowest-total-energy configurations of both H^- and H^2- substitutional defects correspond to a slight outward relaxation of the nearest Mg²⁺-ion shell.