Oxygen vacancies on MgO(100)

The electronic and atomic structures of nondashstoichiometric MgO {100} surfaces, with several densities of neutral oxygen vacancies, have been studied using a semi-empirical Hartree-Fock method associated with a geometry optimization code. The oxygen vacancies are periodically repeated in the surface top layer with equal spacings. Strong electron redistributions take place on the magnesiums which surround the vacancies and localized electron states appear in the gap below the conduction band minimum. Their positions shift towards lower and lower energies as the vacancy density increases. The energy of formation of a vacancy is found to be close to 10 eV at low densities. It strongly decreases as the oxygen deficiency grows, due to electrostatic interactions. The mechanisms responsible for the displacements of the magnesiums which surround the vacancy are analysed.