Abstract
Describes a calculation, using an atomistic computer simulation method, for the energies of vacancy formation near an (001) surface of magnesium oxide. The variation of energy with depth below the surface is presented and it is found that the energy has a maximum just below the surface. This could influence both the population and the kinetics of defects in the surface region. Since the maximum arises from the competition between the variation with depth in surface Madelung potential and polarisation energy, the shape of the depth profile may be similar in many ionic materials. The vacancy energies do not converge at large distances below the surface to the value calculated using a program designed for bulk defect calculations. This is because the relaxation of the surface structure introduces a potential into the bulk of the crystal that is neglected in most bulk calculations. Correction for this effect is essential in the calculation of segregation energies of charged impurities or defects.