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 characterized theoretically using a periodic supercell model and an embedded-cluster approach, both based on the Hartree-Fock approximation. The two methods give essentially the same description of the electronic structure and energetics of the defect. The charge which compensates for the missing Mg+2 ion is 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; the holes occupy a p(z) oxygen orbital, z being the O'-V-O' direction. The formation energy of the defect is higher in the bulk (approximate to 350 kcal mol(-1)) than at the surface (approximate to 310 kcal mol(-1)), which means that migration to the surface should be thermodynamically favored. Oxygen ions around the defect relax outwards by 0.05 to 0.15 Angstrom, with appreciable energy gain; relaxation of next nearest neighbors has not been systematically investigated. This description of the defect coincides qualitatively with that provided by experiment. Substantial disagreement is found, however, between the calculated data concerning the hyperfine splitting constants related to the presence of Mg-25 nuclei in front of the O' oxygens and those obtained using EPR spectroscopy. The reasons for this disagreement are discussed.