Point defects and impurities in fluorite PuO 2

The native surface oxide of plutonium plays a critical role in ensuring the stability and safe storage of the underlying metal; consequently, understanding the role of defects and impurities in determining the properties of the oxide layer is critical. Here, we use hybrid density-functional theory calculations to evaluate the electronic structure and defect chemistry of PuO₂, the most stable of the native oxide phases, including both native and extrinsic defects. We find that oxygen vacancies (VO) form readily in PuO₂, as do polarons. Electron polarons (η^-) are the lowest-energy acceptor species in PuO₂, while the charge compensating donor species will shift from VO under O-poor conditions to hole polarons (η⁺) under O-rich conditions. Nitrogen and fluorine can substitute readily for oxygen atoms under O-poor conditions, while fluorine can also incorporate in an interstitial configuration (Fᵢ^-) under more O-rich conditions. Carbon and chlorine incorporation in PuO₂ will be very limited. We also evaluate the kinetic barriers for oxygen-related defects, which we find to diffuse readily when present. Our results provide valuable insights into the critical role and variable chemistry of point defects and impurities in PuO₂, which in turn have important implications for the safe storage of the underlying metal layer. In short, exposure of freshly prepared plutonium to reactive nitrogen- and fluorine-containing contaminants should be avoided, while carbon- or chlorine-containing contaminants are less likely to incorporate readily into the oxide.