Phase Separation in Fluorite-Related U_1-yCe_yO2_-x: A Re-Examination by X-ray and Neutron Diffraction

The phase separation observed at low temperature (below circa 600 K) in the U_1-yCe_yO2_-x system and for values of y between roughly 0.34 and 0.5 purportedly involves fluorite structures only. However, for y values above 0.5, an oxygen-deficient C-type bixbyite is also reported. In this work, the phase separation in U_0.54Ce0_.46O_2-x has been reexamined using X-ray and neutron diffraction. 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 endmember 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. (authors)