The environmental dependence of redox energetics of PuO2 and α-Pu2O3: A quantitative solution from DFT+U calculations

Abstract: We report a comprehensive density functional theory (DFT) + $U$ study of the energetics of charged and neutral oxygen defects in both PuO${2}$ and $\alpha$-Pu${2}$O${3}$, and present a quantitative determination of the equilibrium compositions of reduced PuO${2}$ (PuO${2-x}$) as functions of environmental temperature and partial pressure of oxygen, which shows fairly agreement with corresponding high-temperature experiments. Under ambient conditions, the endothermic reduction of PuO${2}$ to $\alpha$-Pu${2}$O${3} $ is found to be facilitated by accompanying volume expansion of PuO${2-x}$ and the possible migration of O-vacancy, whereas further reduction of $\alpha $-Pu${2}$O${3}$ is predicted to be much more difficult. In contrast to the endothermic oxidation of PuO${2}$,\ the oxidation of $\alpha$-Pu${2} $O${3}$ is a stable exothermic process.