Charge deformation and geometric relaxation at the (001) MgO surface

Abstract

The relaxation of the (001) surface of a semi-infinite MgO crystal is calculated using a shell model. The model is borrowed from existing bulk studies but is modified so as to include three effects which arise from the presence of the surface and the considerable deformability of the oxygen ion: (a) environment-dependent variations in the anion polarizability, (b) quadrupole contributions to the total energy of the crystal, (c) overlap charge adjustments which alter the phenomenological model parameters near the surface. The results are compared with a recent photoemission experiment as well as with LEED, EELS, and specific surface energy measurements. An enhancement of the static effective ionic charge at the top layer relative to the bulk suggests itself as a dominant feature, the variable polarizability and quadrupole effects being largely suppressed in the static problem, However, all three effects may become important in the study of surface vibrations.