Relative energies of surface and defect states: ab initio calculations forthe MgO(001) surface

We present the results of calculations of the energy levels of defects at t he (001) surface of MgO relative to the top of the valence band and values of defect ionisation potentials and electron affinities. The calculations w ere made using an embedded cluster method in which a cluster of several ten s of ions treated quantum mechanically is embedded in a finite array of pol arisable and point ions modelling the crystalline potential and the classic al polarisation of the host lattice. The calculated ionisation potential of the ideal surface, which fixes the position of the top of the valence band with respect to the vacuum level, is about 6.7 eV. This value is used as a reference for positioning the energy levels of three charge states of a su rface anion vacancy, which are also calculated as ionisation energies with respect to the vacuum level. The surface and vacancy electron affinities ar e calculated using the same method. As a prototype low-coordinated surface site, we have considered a cube corner. Our calculations predict the splitt ing of the corner states from the top of the surface valence band by about 1.0 eV. Both unrelaxed and relaxed holes are strongly localised at the corn er oxygen ion. The ionisation energies and electron affinities of the corne r anion vacancy are calculated. The electrons in the F and F+ centres at th e corner are shown to be significantly delocalised over surrounding Mg ions . (C) 2000 Elsevier Science B.V. All rights reserved.