Jianwei Wang1

1, Louisiana State University, Baton Rouge, Louisiana, United States

Uranium dioxide exists in hyperstoichiometric form, UO2+x. Structure and dynamics of the excess oxygen defects and their correlation with 5f electrons are fundamental to the understanding of mechanical, thermal, and electrical properties. Those excess oxygen atoms are not random but rather partially ordered. The widely-accepted model, the Willis cluster based on neutron diffraction, cannot be reconciled with the first-principles molecular dynamics simulations present here. We demonstrate that the Willis cluster is a fair representation of the numerical ratio of different interstitial O atoms; however, the model does not represent the actual local configuration. The simulations show that the average structure of UO2+x involves a combination of defect structures including split di-interstitial, di-interstitial, mono-interstitial, and the Willis cluster, and the latter is a transition state that provides for the rapid diffusion of the defect cluster. The 5f electrons are partially delocalized and the U5+ atoms are not spatially linked to the defect cluster. The mobility of the U5+ is thermally activated but decoupled temporally with the lattice defect. The observation can be explained by a collective, dynamical, charge transfer-coupled lattice distortion involving the U(V) ↔ U(IV) excitation, occurring coherently over an entire domain combining charge, spin, and crystal lattice. The results provide new insights in differentiating the average structure from the local configuration of defects in a solid, 5f electron – lattice defect coupling, and the transport properties of UO2+x.