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

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 UO2 and PuO2. The calculated formation energy indicates that the oxygen interstitial (Oi) defects are dominant in UO2, while the oxygen vacancy (Ov) and Oi defects can coexist in PuO2. Such a difference can be attributed to the distinct electronic properties of U and Pu elements. Moreover, the formation energy of Ov is found to be sensitive to the surface depth, and Ov prefer to form on the sub-surface layer for both UO2 and PuO2. Comparatively, the formation of Oi 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 UO2 and PuO2. Our results shed some light on the defect behaviors of actinide dioxide, and thereby the oxidation properties of actinide metals.