Evidence of a phase transition from UO2-x to UO2+x

Uranium dioxide has been extensively studied for more than 70 years with regards to its importance as main nuclear fuel used worldwide. From a thermodynamic point of view, UO 2 was considered up to now as a single phase, in which point defects would explain its sharp change of oxygen potential when going from hypo to hyper-stoichiometry. Here, we demonstrate using Raman spectroscopy, that the change from hypo to hyper-stoichiometry should be considered as a phase transition. This leads us to a change of paradigm about UO 2 that is now a border between UO 2-x and UO 2+x .