Ab initio study of the cation vacancy at the surface and in bulk MgO

The neutral V center in MgO bulk and at its (001) surface has been characte rized theoretically using a periodic supercell model and an embedded-cluste r approach, both based on the Hartree-Fock approximation. The two methods g ive essentially the same description of the electronic structure and energe tics of the defect. The charge which compensates for the missing Mg+2 ion i s localized on two oxygen ions (O') facing each other at the opposite sides of the V center, both in the case of the bulk and of the surface defect; t he holes occupy a p(z) oxygen orbital, z being the O'-V-O' direction. The f ormation energy of the defect is higher in the bulk (approximate to 350 kca l mol(-1)) than at the surface (approximate to 310 kcal mol(-1)), which mea ns that migration to the surface should be thermodynamically favored. Oxyge n ions around the defect relax outwards by 0.05 to 0.15 Angstrom, with appr eciable energy gain; relaxation of next nearest neighbors has not been syst ematically investigated. This description of the defect coincides qualitati vely with that provided by experiment. Substantial disagreement is found, h owever, between the calculated data concerning the hyperfine splitting cons tants related to the presence of Mg-25 nuclei in front of the O' oxygens an d those obtained using EPR spectroscopy. The reasons for this disagreement are discussed.