Peculiar Thermal Behavior of UO Local Stucture.

Most materials expand with temperature because of the anharmonicity of lattice vibration, and only a few shrink with increasing temperature. UO, whose thermal properties are of significant importance for the safe use of nuclear energy, was considered for a long time to belong to the first group. This view was challenged by recent in situ synchrotron X-ray diffraction measurements, showing an unusual thermal decrease of the U-O distances. This thermal shrinkage was interpreted as a consequence of the splitting of the U-O distances due to a change in the U local order from Fm3̅ m to Pa3̅. In contrast to these previous investigations and using an element-specific synchrotron-based spectroscopic method, we show here that the U sublattice remains locally of the fluorite type from 50 to 1265 K, and that the decrease of the first U-O bond lengths is associated with an increase of the disorder.