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

B Labonne and C Guéneau and M Bertolus, PHYSICAL REVIEW MATERIALS, 9, 123803 (2025).

DOI: 10.1103/qx3v-m636

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 AmOx with 1.5 x 2, i.e., from the sesquioxide C-Am2O3 to the dioxide AmO2, 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 AmO1.55 and AmO1.60 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 AmO2 in literature but for a much lower O/M ratio, approximately around 1.55-1.60. The reduction of AmO2 to AmO2-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 AmO2 in temperature up to around 3000 K using density functional theory in the 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 AmO2 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.

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