Clusters of Oxygen Interstitials in UO and α-UO: 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 UO. 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 UO. Considering different types of random arrangements of these clusters, we reproduce the fact that UO and UO are the only two stable phases at low temperature in their composition range. Focusing on UO at low temperature, we obtain a model for the α-UO phase, which exhibits the experimental 3 space group. We present the results of the original neutron diffraction experiments measured on UO at 1.5 K and perform a Rietveld refinement of the calculated UO structure, which enables us to propose a complete description of the α-UO phase.