Theory ofFcenters in the alkaline-earth oxides MgO and CaO

We present results of a theoretical study of the electronic structure of isolated anion vacancies (F-center-like defects) in the alkaline-earth oxides MgO and CaO using the multiple-scattering-theory–based muffin-tin Green’s-function method. These ab initio calculations are valence-charge self-consistent for the vacancy and first shell of cation neighbors but, at present, do not include effects due to atomic relaxation around the vacancy site. Different charge states are explored by varying the occupation of the s-like bound state from two (neutral F center), to one (positively charged $F^{+}$ center), including results for the Hubbard Coulomb localization parameter, U. Various ad hoc ‘‘fixes’’ to the underestimated local-density theory valence-conduction band gap for the perfect crystal are explored, with primary emphasis given to ‘‘scissor-operator’’ type of band-gap widening. Calculated defect charge and state densities are compared with experiment and with other calculations. Although good qualitative, or even semiquantitative, agreement with experiment is obtained as to the nature and locations of the defect-induced ground and excited states, it is concluded that atomic relaxation effects, and extensions to further shells of neighbors, need to be included before a full evaluation of the theory can be made.