Intrinsic oxygen defects in UO2 (1 1 1) and PuO2 (1 1 1) surfaces

• Our work for the first time reveals different oxygen stoichiometries in their most commonly seen (111) surfaces, which is important for their safely applications under various environments. • We find that surface strains modulate the surface oxygen stoichiometries of UO 2 and PuO 2 in a different way with environmental pressure and temperature. • The strain-modulated formation energy phase diagrams of the two types of O defects have been established over a wide range of temperature and pressure. Density functional theory (DFT) + U combined with the first principle atomistic thermodynamic method was employed to study the physical properties of intrinsic oxygen defects in (111) surfaces of UO 2 and PuO 2 . The calculated formation energy indicates that the oxygen interstitial (O i ) defects are dominant in UO 2 , while the oxygen vacancy (O v ) and O i defects can coexist in PuO 2 . Such a difference can be attributed to the distinct electronic properties of U and Pu elements. Moreover, the formation energy of O v is found to be sensitive to the surface depth, and O v prefer to form on the sub-surface layer for both UO 2 and PuO 2 . Comparatively, the formation of O i is insensitive to the incorporation depth. The strain effect of formation energy is mainly contributed by the local structural distortion, rather than the electronic hybridization. Through thermodynamic calculation, the strain-modulated formation energy phase diagrams of the oxygen defects have been established over a wide range of temperature and pressure, providing the potential strategy for controlling the type and concentration of intrinsic oxygen defects in UO 2 and PuO 2 . Our results shed some light on the defect behaviors of actinide dioxide, and thereby the oxidation properties of actinide metals.