Density functional theory investigation of the layered uranium oxides U3O8 and U2O5

Abstract

Oxidation of UO₂ in the nuclear fuel cycle leads to formation of the layered uranium oxides. Here we present DFT simulations of U₂O₅ and U₃O₈ using the PBE + U functional to examine their structural, electronic and mechanical properties. We build on previous simulation studies of Amm2 α-U₃O₈, P2₁/m β-U₃O₈ and P2m γ-U₃O₈ by including C222 α-U₃O₈, Cmcm β-U₃O₈ and Pnma δ-U₂O₅. All materials are predicted to be insulators with no preference for ferromagnetic or antiferromagnetic ordering. We predict δ-U₂O₅ contains exclusively U⁵⁺ ions in an even mixture of distorted octahedral and pentagonal bipyramidal coordination sites. In each U₃O₈ polymorph modelled we predict U⁵⁺ ions in pentagonal bipyramidal coordination and U⁶⁺ in octahedral coordination, with no U⁴⁺ present. The elastic constants of each phase have been calculated and the bulk modulus is found to be inversely proportional to the volume per uranium ion. Finally, a number of thermodynamic properties are estimated, showing general agreement with available experiments; for example α- and β-U₃O₈ are predicted to be stable at low temperatures but β-U₃O₈ and γ-U₃O₈ dominate at high temperature and high pressure respectively.