Phase Separation in Fluorite-Related UCeO: A Re-Examination by X-ray and Neutron Diffraction.

The phase separation observed at low temperature (below 600 K) in the UCeO system and for values of between roughly 0.34 and 0.5 purportedly involves fluorite structures only. However, for values above 0.5, an oxygen-deficient C-type bixbyite is also reported. In this work, the phase separation in UCeO 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 UCeO phase and an oxygen-deficient UCeO 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.