Clusters of Oxygen Interstitials in UO₂₊ and α-U₄O₉: Structure and Arrangements

We use the Second Moment Tight Binding with charge eQuilibration (SMTB-Q) advanced empirical potential to study clusters of oxygen interstitials in hyperstoichiometric UO₂. This potential reproduces the trends obtained from density functional theory calculations. We exhibit a new form of cuboctahedron that proves to be the most stable oxygen cluster in UO₂₊. Considering different types of random arrangements of these clusters, we reproduce the fact that UO₂ and U₄O₉ are the only two stable phases at low temperature in their composition range. Focusing on U₄O₉ at low temperature, we obtain a model for the α-U₄O₉ phase, which exhibits the experimental R3c space group. We present the results of the original neutron diffraction experiments measured on U₄O₉ at 1.5 K and perform a Rietveld refinement of the calculated U₄O₉ structure, which enables us to propose a complete description of the α-U₄O₉ phase.