ELECTRONIC-STRUCTURE OF F AND V CENTERS ON THE MGO SURFACE

The electronic structure of neutral and charged O and Mg vacancies at the surface of MgO (F, F+, F2+, V, V-, and V2- centers) has been inves tigated by means of cluster models and ab initio wave functions. For e ach center we have determined the formation energy and the local geome trical relaxation. The distribution of the electrons in the vacancy ha s been analyzed through the calculation of charge density plots and, f or paramagnetic centers, isotropic hyperfine coupling constants. The l atter are in good agreement with the EPR data. On F+ surface centers t he unpaired electron is largely localized in the vacancy; the correspo nding eigenstate is in the band gap. On V- centers, on the contrary, t he unpaired electron is delocalized over the surface O neighbors and t he eigenstate is in the valence band. For the first time we have consi dered also O vacancies at edge sites. The energetics of the formation of these sites and their electronic structure is not too different fro m that of the corresponding centers on the terraces of MEG, thus reinf orcing the hypothesis that edge F centers can exist on the surface of thermally activated MgO.