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

Oxidation of UO(2) in the nuclear fuel cycle leads to formation of the layered uranium oxides. Here we present DFT simulations of U(2)O(5) and U(3)O(8) using the PBE + U functional to examine their structural, electronic and mechanical properties. We build on previous simulation studies of Amm2 α-U(3)O(8), P2(1)/m β-U(3)O(8) and P6[combining macron]2m γ-U(3)O(8) by including C222 α-U(3)O(8), Cmcm β-U(3)O(8) and Pnma δ-U(2)O(5). All materials are predicted to be insulators with no preference for ferromagnetic or antiferromagnetic ordering. We predict δ-U(2)O(5) contains exclusively U(5+) ions in an even mixture of distorted octahedral and pentagonal bipyramidal coordination sites. In each U(3)O(8) polymorph modelled we predict U(5+) ions in pentagonal bipyramidal coordination and U(6+) in octahedral coordination, with no U(4+) 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-U(3)O(8) are predicted to be stable at low temperatures but β-U(3)O(8) and γ-U(3)O(8) dominate at high temperature and high pressure respectively.