Novel potentials for modelling defect formation and oxygen vacancy migration in Gd2Ti2O7 and Gd2Zr2O7 pyrochlores

Abstract

Ceramic materials play a critical role in the safe long-term immobilisation of nuclear waste and fundamental understanding of both radiation damage and healing processes are required in order to determine the suitability of proposed storage materials, such as pyrochlores. Here, new interatomic potentials are derived for the pyrochlores Gd2Ti2O7 and Gd2Zr2O7. Their accuracy and robustness is demonstrated by calculating and comparing values for a selection of point defects with those calculated using a selection of other published potentials. Frenkel pair defect formation energies are substantially lowered in the presence of a small amount of local cation disorder. The activation energy for oxygen vacancy migration between adjacent O48f sites is calculated for Ti and Zr pyrochlores using an improved tangent nudged elastic band method. This energy is lower for the non-defective Ti than for the Zr pyrochlore by ∼0.1 eV, consistent with the majority of the potentials tested. The effect of local cation disorder on the VO48f → VO48f migration energy is minimal for Gd2Ti2O7, while in contrast the migration energy is lowered typically by ∼43% for Gd2Zr2O7. Since the healing mechanisms of these pyrochlores are likely to rely upon the availability of oxygen vacancies, the healing of a defective Zr pyrochlore is predicted to be faster than for the equivalent Ti pyrochlore.