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Phase Separation in Fluorite-Related U1–yCeyO2–x: A Re-Examination by X‑ray and Neutron Diffraction

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journal contribution
posted on 19.08.2019, 11:43 by David Simeone, Philippe Garcia, Audrey Miard, Gianguido Baldinozzi, Florence Porcher, Jean-Francois Berar
The phase separation observed at low temperature (below circa 600 K) in the U1–yCeyO2–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 U0.54Ce0.46O2–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.

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