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 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.