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

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.