Oxygen interstitial-assisted cation interstitial diffusion in fluorite oxides

Understanding how defects evolve under irradiation - where atomic displacements generate defects of vacancy and interstitial types - is vital for unlocking the full potential of fluorite oxides in nuclear energy applications. In fluorite oxides like UO₂ and ThO₂, interstitials often aggregate into dislocation loops, degrading thermal conductivity and promoting cavity swelling, which undermines fuel performance and safety. However, the mechanisms behind loop formation remain elusive due to challenges in scale, even for the state-of-the-art techniques: the relevant atomic processes are too small for direct observation and too complex for modeling. Notably, the low mobility of cation interstitials predicted by density functional theory (DFT) cannot account for the widespread experimental observations of loop formation. Using ThO₂ as a model system, we employed computationally efficient molecular dynamics (MD) simulations to capture the real-time evolution of initially isolated thorium and oxygen interstitials and identify stable and mobile interstitial clusters. The thermal stability and mobility of kinetically favorable interstitial clusters were further validated through high-fidelity DFT calculations. Interestingly, the results show that the Th interstitial decorated with O interstitials becomes increasingly stable and mobile until up to four O interstitials, with a decreasing barrier over the cluster size. Bader charge analysis suggests this enhanced mobility arises from O-mediated screening of the Coulomb repulsion between Th ions in the local environment. These insights were extended to UO₂ and CeO₂ via MD calculations, advancing the current understanding of ionic transport and resolving a critical knowledge gap in understanding interstitial loop formation in fluorite oxides.