An ab initio study of intrinsic defects in zirconolite

TL;DR

This study uses density functional theory to investigate intrinsic defects in zirconolite, revealing charge state dependencies and environmental influences on defect formation energies relevant for nuclear waste containment.

Contribution

It provides the first ab initio analysis of defect structures and energies in zirconolite, highlighting the role of charge states and chemical environment effects.

Findings

Oxygen Frenkel pair energies vary by 1.5 eV depending on environment

Charge states significantly influence vacancy defect structures

Formation of O2 molecules occurs in specific charge states of vacancies

Abstract

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.