Phase stability, electronic structures and elastic properties of (U,Np)O2 and (Th,Np)O2 mixed oxides

Abstract

Mixing enthalpies (ΔH_mix) of U_1−xNp_xO₂ and Th_1−xNp_xO₂ solid solutions are derived from atomic scale simulations based on density functional theory (DFT) employing the generalised gradient approximation corrected with an effective Hubbard parameter (U_eff). The variation of structural and electronic properties of UO₂ and NpO₂ with collinear ferromagnetic (FM), collinear anti-ferromagnetic (AFM) and non-collinear anti-ferromagnetic arrangements of the uranium and neptunium magnetic moments are investigated while ramping up U_eff from 0 eV to 4 eV (the U_eff-ramping method). A combination of the U_eff-ramping method to treat the presence of metastable magnetic states and special-quasirandom structures (SQS) for the random distribution of Np atoms in UO₂ and ThO₂ is employed to calculate ΔH_mix of U_1−xNp_xO₂ and Th_1−xNp_xO₂ mixed oxides (MOX). The effect of collinear FM and AFM ordering is also considered in determining the ΔH_mix. The calculated ΔH_mix of Th_1−xNp_xO₂ MOX were positive compared to the end members and nearly symmetric around x = 0.5 and ΔH_mix of the AFM configuration were higher compared to the FM configuration maximum by 0.19 kJ mol⁻¹. The ΔH_mix of U_1−xNp_xO₂ MOX were negative up to U_0.50Np_0.50O₂ with a maximum value of −1.21 kJ mol⁻¹ for U_0.4375Np_0.5625O₂ whereas Np-rich (U,Np)O₂ MOX compositions exhibited ΔH_mix close to zero. Values of ΔH_mix for (Th,Np)O₂ are consistent with a simple miscibility-gap phase diagram while those for (U,Np)O₂ suggest more complex behaviour. Nevertheless, lattice parameter variation with composition still follows a Vegard's law relationship. Finally, single crystal elastic constants of pure oxides and MOX are reported. The linear-elasticity models describe the mixing energies to within an accuracy of approximately 1 kJ mol⁻¹ for the U_1−xNp_xO₂ and Th_1−xNp_xO₂ MOX systems.