Structural, thermodynamic, electronic and elastic properties of Th1−xUxO2 and Th1−xPuxO2 mixed oxides

Abstract

The structural, thermodynamic, electronic, and elastic properties of Th_1−xU_xO₂ and Th_1−xPu_xO₂ mixed oxides (MOX) have been calculated with Hubbard corrected density functional theory (DFT+U) to account for the strong 5f electron correlations. The ideal solid solution is approximated by special quasi-random structures and the U-ramping method is used to account for the presence of metastable states in the self-consistent field solution of the DFT+U approach. The mixing enthalpy (ΔH_mix) is positive throughout the composition range of the Th_1−xU_xO₂ MOX, consistent with a simple miscibility gap (at low temperature) phase diagram. The behavior of the Th_1−xPu_xO₂ MOX is more complex, where ΔH_mix is positive in the ThO₂-rich region and negative in the PuO₂-rich region. Electronic structure analysis shows that substitution of Th by U/Pu in ThO₂ leads to a reduction of the average Th–O bond lengths, causing distortion in the crystal structure. The distortion in the crystal structure results in an increase in the conduction bandwidth and a reduction of the band-gap in the MOX. Good agreement of our DFT+U calculated elastic properties of ThO₂, UO₂ and PuO₂ compounds with experiments leads to convincing prediction of these properties for Th_1−xU_xO₂ and Th_1−xPu_xO₂ MOX.