Interaction between oxygen vacancies on MgO(100)

Oxygen-deficient MgO(100) surfaces are studied by means of total-energy calculations, within the density-functional theory. Selected surface configurations presenting increasing densities of neutral oxygen vacancies, ranging from 12.5% to 100%, are considered, the missing oxygen being periodically distributed in the surface layer either homogeneously or in a close-packed fashion. We show that the electronic structure is characterized by occupied states in the gap which result either from the hybridization of atomiclike orbitals localized on vacancy sites, or involve surface magnesium orbitals. The latter case occurs at large vacancy concentrations, and the conditions for the apparition of metallicity in the surface layer are derived. The formation energy of oxygen vacancies is calculated as a function of the vacancy concentration and of the particular configuration. A two-body model potential is proposed to account for their interaction. The activation energy for vacancy diffusion at the surface is also computed, thus providing a complete set of parameters for a description of the thermodynamics as well as of the kinetics of oxygen vacancies on a MgO(100) surface. [S0163-1829(99)05407-7].