Plane-wave density functional theory was used to study the properties of oxygen vacancies and interstitials, with different charge states, in MgO. The calculated properties were the relaxed configurations, the Frenkel defect formation energies and the energies of the migration barriers, and all properties were found to be strongly dependent on the defect charge state. The lowest energy configuration of the O 2− interstitial was found to be the cube centre; however, the O − and O 0 interstitials formed dumb-bell configurations. The Frenkel defect energies were also strongly dependent on the defect charge, with the neutral pair energy calculated to be 3 eV lower than the doubly charged Frenkel pair defect energy. The migration barriers of the oxygen vacancies were found to increase as the net charge of the oxygen vacancies decreased, which suggests that vacancies with trapped electrons are much less mobile than the F 2+ vacancies modelled by classical potentials. The migration of the oxygen interstitials showed particularly interesting behaviour. The O 0 interstitial was found to have a higher migration barrier than the O 2− interstitial but a very low barrier (0.06 eV) was found for the O − interstitial. The results have significant implications for the reliability of classical radiation damage simulations.

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Zirconolite, CaZrTi 2 O 7 , is a proposed ceramic for the use in disposal of highly active nuclear waste. Density functional theory (DFT) has been used in conjunction with a random search technique to identify the stable interstitial sites for the intrinsic defects in zirconolite. There is a significant dependence on charge states for the vacancy defect structures, with the formation of an O 2 molecule in certain charge states of Ti and Zr vacancies. The low coordination of the oxygen ions provided sites on which DFT localized holes associated with the cation vacancies. The Frenkel defect energies calculated from the combination of the vacancy and interstitial formation energies show that the oxygen defects have a strong dependence on the chemical environment with a range of 1.5 eV between the oxygen Frenkel pair energies.