Phase Separation In Fluorite-Related U1-Y Ceyo2-X: A Re-Examination By X-Ray And Neutron Diffraction

The phase separation observed at low temperature (below circa 600 K) in the U1-y CeyO2-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 U0.54Ce0.46O2-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 U0.54Ce0.46O2 phase and an oxygen-deficient U(0.54)Ce(0.46)O(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.