Phase Separation in Fluorite-Related U_1-yCe_yO_2-x: New Insights via Variable Temperature Neutron Diffraction

The phase separation in the U_1-yCe_yO_2-x system for values of y between approximately 0.34 and 0.5 observed at low temperatures (below circa 600 K) purportedly involves only fluorite structures. Therefore, to confirm this assumption it is logical to employ high resolution diffraction techniques that can track the progression of the oxygen-sub lattice and resolve peaks that may be overlapping. In this study, the phase separation in the U_0.54Ce_0.46O_2-x system has been revisited using variable temperature high resolution neutron diffraction on samples that are sealed under Ar to prevent a change in oxidation state when heated. As neutron scattering lengths for U_nat and O do not differ to a large degree, U coherent cross section 8.903 barn vs O coherent cross section 4.232 barn, information about the oxygen sub lattice can be obtained from neutron diffraction patterns. This is not the case for X-ray diffraction which has been used for the bulk of the studies on this system. Below a critical temperature, the existence of two fluorite related structures in the miscibility gap is confirmed: a stoichiometric U_0.54Ce_0.46O₂ phase and an oxygen-deficient U_0.54Ce_0.46O_2-x phase. Although the former is indeed a fluorite, we show that the other end-member phase has a C-type bixbyite structure. This would suggest that the oxygen-deficient phase can be described as a bixbyite over the entire cerium composition range. In this work, we present some recent developments executed to understand the phase transformation in this material and try to relate this with the defects produced by the oxygen deficiency in the anionic sub lattice. The ultimate goal of this approach is to simulate this phase transformation within the Phase Field as it approaches phase transition