Combined interatomic potential and electronic structure investigation of the cubic phases of the Am-O system

The knowledge of the properties of americium oxides is crucial for nuclear fuel optimization and potential future use in radioisotope thermoelectric generators. In this paper, we used a combination of state-of-the-art complementary atomic scale modeling methods to determine the properties of the cubic phases of americium oxides. We used the Cooper-Rushton-Grimes (CRG) interatomic potential updated for the study of americium-bearing oxides to study the structural and thermodynamic properties of the cubic phases of americium oxides ${\mathrm{AmO}}_{x}$ with $1.5\ensuremath{\le}x\ensuremath{\le}2$, i.e., from the sesquioxide C-${\text{Am}}_{\text{2}}{\text{O}}_{3}$ to the dioxide ${\text{AmO}}_{\text{2}}$, between 250 K and 4500 K. A large number of properties were calculated as a function of composition and temperature: cell parameter, thermal expansion coefficient, enthalpy increment and specific heat capacity. The results reproduce almost exactly the melting temperature of the sesquioxide and show a phase transition at high temperature for the ${\mathrm{AmO}}_{1.55}$ and ${\mathrm{AmO}}_{1.60}$ compounds, which could result in a phase observed experimentally for these O/M ratios. They also allow us to recover the only melting temperature reported for ${\text{AmO}}_{\text{2}}$ in literature but for a much lower O/M ratio, approximately around 1.55--1.60. The reduction of ${\text{AmO}}_{\text{2}}$ to ${\text{AmO}}_{\text{2-x}}$ in temperature cannot, however, be obtained using the CRG empirical potential formalism due to the fixed charge formalism. We therefore performed electronic structure calculations to determine the properties of ${\text{AmO}}_{\text{2}}$ in temperature up to around 3000 K using density functional theory in the $\mathrm{DFT}+U$ approximation and using a functional taking into account van der Waals interactions. The enthalpy increment and specific heat capacity obtained are in excellent agreement with the available measurements and calphad modeling. The formation of oxygen dimers separated from the crystal and the reduction of ${\text{AmO}}_{\text{2}}$ is observed, with the variation in O/M ratio as a function of temperature agreeing with the results of calphad calculations. The results presented in this paper provide significant insight into the properties of the cubic phases of americium oxides and complement the little experimental data on this system, especially at high temperature, and are now available to refine phase diagram calculations and fuel performance codes simulations.