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

Abstract

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.

Structure and Properties of Some Layered U2O5 Phases: A Density Functional Theory Study

“…The values of the U and J parameters were 4.5 eV and 0.54 eV respectively, which were taken from the XPS experiments of Yamazaki and Kotani on UO2 21 . No such parameters have been derived for U2O5 (or any other uranium oxides) and so in the interest of transferability of results we have used these values, as in our previous works 3,10 . Work in the literature has demonstrated that changing the effective U parameter mainly affects the predicted band gap, and has considerably less impact on the other calculated properties 22 .…”

“…U2O5 marks an important, yet underexplored, area of the uranium-oxygen phase diagram. Although its' practical applications have received little attention, owing to its instability relative to both higher and lower oxides [1][2][3][4][5][6][7][8] . One of its intriguing features is that it represents the composition of the transition point between the fluorite and layered uranium oxides, with reports of it forming in both types of structure 1,9 .…”

“…Early XPS studies have suggested a mixture of U 4+ and U 6+ ions 14 , but more recent experiments have proposed U2O5 is composed entirely of U 5+ ions 15 . Brincat et al showed that the DFT + U methodology was able to reproduce the structure of δ-U2O5 3…”

Direct observation of pure pentavalent uranium in U2O5 thin films by high resolution photoemission spectroscopy

“…On the other hand, using the Perdew-Burke-Ernzerhof exchange-correlation functional with on-site Coulomb correlations (PBE + U) within the Generalized Gradient Approximation (GGA), Brincat et al . 46 predict a stable δ-U 2 O 5 structure containing exclusively U(V) ions in mixture of distorted octahedral and pentagonal bipyramidal coordination sites. A similar approach has been used by Molinari et al .…”

Theoretical prediction of some layered Pa2O5 phases: structure and properties

“…In the actinide elements, uranium (U) exhibits rich oxide phases, such as UO 2 , U 2 O 3 , U 2 O 5 , and U 3 O 8 , [20][21][22][23][24][25][26][27] and its two prominent oxidation states are U 4+ and U 6+ . Plutonium (Pu) has stable Pu 4+ , Pu 5+ , and Pu 6+ oxidation states in aqueous solutions.…”