Density Functional Theory Calculations of UO₂ Oxidation: Evolution of UO_2+x, U₄O_9–y, U₃O₇, and U₃O₈

Formation of hyperstoichiometric uranium dioxide, UO_2+x, derived from the fluorite structure was investigated by means of density functional theory (DFT) calculations. Oxidation was modeled by adding oxygen atoms to UO₂ fluorite supercells. For each compound ab initio molecular dynamics simulations were performed to allow the ions to optimize their local geometry. A similar approach was used for studying the reduction of U₃O₈. In agreement with the experimental phase diagram we identify stable line compounds at the U₄O_9–y and U₃O₇ stoichiometries. Although the transition from fluorite to the layered U₃O₈ structure occurs at U₃O₇ (UO_2.333) or U₃O_7.333 (UO_2.444), our calculated low temperature phase diagram indicates that the fluorite derived compounds are favored up to UO_2.5, that is, as long as the charge-compensation for adding oxygen atoms occurs via formation of U⁵⁺ ions, after which the U₃O_8–y phase becomes more stable. The most stable fluorite UO_2+x phases at low temperature (0 K) are based on ordering of split quad-interstitial oxygen clusters. Most existing crystallographic models of U₄O₉ and U₃O₇, however, apply the cuboctahedral cluster. To better understand these discrepancies, the new structural models are analyzed in terms of existing neutron diffraction data. DFT calculations were also performed on the experimental cuboctahedral based U₄O_9–y structure, which enable comparisons between the properties of this phase with the quad-interstitial ones in detail.