Cation oxidation states in sub-stoichiometric fluorite oxides: application to (U1-yCey)O2-x

Modelling the thermodynamic behaviour of sub-stoichiometric, ternary oxides such as (U1–yPuy)O2–x or its less hazardous analog (U1-yCey)O2–x, constitutes a scientific and industrial challenge in the context of optimizing these fuels for fast neutron and pressurized water reactor applications. Past XRD studies report a phase separation in oxygen deficient U-Ce-O and U-Pu-O systems (e.g. U0.5Ce0.5O1.95 compounds), in which the high temperature disordered fluorite phase gives way to two fluorite phases, stoichiometric and sub-stoichiometric, at some point below 400°C. Neutron diffraction experiments suggest however that the low temperature sub-stoichiometric phase is probably not a fluorite. The exact nature of the phase transition mechanism, has not, as yet, been clearly identified and it is thought the key to this, lies in understanding in situ, the interplay between mobile anion vacancies and polaronic disorder involving Ce3+, Ce4+, U4+, U5+ ions.