An ab initio study of intrinsic defects in zirconolite

Zirconolite, CaZrTi2O7, 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 O2 molecule in certain charge states of Ti and Zr vacancies. The low coordination of the oxygen ions provided sites on which DFT localised 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.