Diffusion mechanism of bound Schottky defect in magnesium oxide

In simple ionic crystals, intrinsic point defects must satisfy electrical neutrality and exist as Schottky defects. \nIn magnesium oxide (MgO), a Schottky defect is then a combination of anionic and cationic vacancies. Since \nvacancies are charged, the stable configuration of the Schottky defect corresponds to a bound pair of vacancies \nof opposite signs. In this study, we investigate the kinetics of formation and migration of such a bound pair on \nlong timescales reaching in some cases thousands of seconds using the kinetic activation-relaxation technique, an \noff-lattice kinetic Monte Carlo method with an event catalog built on-the-fly during static molecular simulations. \nWe show that the diffusion of this bound Schottky defect involves the migration of vacancies bounded to the first \nand third neighbor sites of the crystal structure with an apparent migration energy which cannot be inferred from \nthe migration energies expected from isolated defects. Overall, this study gives insights and constraints on the \noxygen diffusion mechanism reported experimentally in high-purity MgO samples.