A theoretical study of the stability and electronic structure of the polar {111} face of MgO

The stability of the polar {111} face of MgO is discussed on the basis of the results obtained by a total-energy semi-empirical Hartree-Fock method, and on general theoretical arguments relying on a perturbative approach of covalent effects in mixed iono-covalent solids. The ideal stoichiometric (1 x 1) surface is studied and compared to two (2 x 1) and (2 x 2) reconstructed surfaces, and to a fully hydroxylated (1 x 1) surface. On the ideal surface, a strong electron redistribution takes place in order to cancel the polarity, which is associated with a metallization of the surface layers and with a high surface energy. A spontaneous symmetry breaking of the Peierls type takes place on this surface, with an opening: of a gap at the Fermi level which induces a desorption of half of the surface atoms and a (2 x 1) reconstructed surface configuration. On the two reconstructed surfaces the modification of the stoichiometry in the surface layers provides the compensating charges. As a result, they are much more stable and insulating. The (2 x 2) reconstruction which involves nanopyramids with {001} facets is favored, but we predict a trend to form larger facets in (2n x 2n) reconstructions. (C) 1997 Elsevier Science B.V.