Surface Oxygen Diffusion into Neutral, Cationic, and Dicationic Oxygen Vacancies on MgO(100) Surfaces

First principles electronic structure investigations of the oxygen diffusion into neutral, cationic, and dicationic oxygen vacancies on MgO(100) surfaces have been carried out within a density functional theory cluster-embedding approach. The transition states and activation barriers for the oxygen diffusion were determined. It was found that the charge of the vacancy has a marked effect on the oxygen diffusion as the activation barrier decreases from 2.84 eV in the neutral, to 2.18 eV in the cationic, and to 0.94 eV in the dicationic oxygen vacancies. An analysis of the relaxation of the surface atoms around the vacancies, and the geometry of the transition states, leads us to conclude that the attractive interaction between the localized positive charge and the electron-rich oxygen atoms, and the relaxation of surface atoms, are responsible for lowering the activation barrier. In addition, our studies on layer relaxations indicate that the oxygen diffusion is strongly confined to the surface atoms. The analysis of the spin density along the reaction path of the oxygen diffusion into a cationic vacancy suggests that the oxygen movement is accompanied by an electron hopping process.