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

Mixing enthalpies (Delta H-mix) of U1-xNpxO2 and Th1-xNpxO2 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 antiferromagnetic 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 Delta H-mix of U1-xNpxO2 and Th1-xNpxO2 mixed oxides (MOX). The effect of collinear FM and AFM ordering is also considered in determining the Delta H-mix. The calculated Delta H-mix of Th1-xNpxO2 MOX were positive compared to the end members and nearly symmetric around x = 0.5 and Delta H-mix of the AFM configuration were higher compared to the FM configuration maximum by 0.19 kJ mol(-1). The Delta H-mix of U1-xNpxO2 MOX were negative up to U0.50Np0.50O2 with a maximum value of 1.21 kJ mol(-1) for U0.4375Np0.5625O2 whereas Np-rich (U,Np) O-2 MOX compositions exhibited Delta H-mix close to zero. Values of Delta H-mix for (Th,Np)O-2 are consistent with a simple miscibility-gap phase diagram while those for (U,Np)O-2 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 U1-xNpxO2 and Th1-xNpxO2 MOX systems.