Influence of sintering conditions on the structure and the redox speciation of homogenous (U,Ce)O2+δ ceramics : a synchrotron study

Although uranium-cerium dioxides are frequently used as surrogate material for (U,Pu)O2-δ, there is currently no reliable data regarding the oxygen stoichiometry and the redox speciation of the cations in such samples. In order to fill this gap, this manuscript proposes a synchrotron study of highly homogeneous (U,Ce)O2±δ sintered samples prepared by wet-chemistry route. HERFD-XANES spectroscopy led to determine accurately the O/M ratios (with M = U + Ce). Under reducing atmosphere (PO2 ~ 610-29 atm), the oxides were found close to O/M = 2.00 while the O/M ratio varied with the sintering conditions under argon (PO2 ~ 210-6 atm). They globally appear to be hyper-stoichiometric (i.e. O/M > 2.00), the departure from the dioxide stoichiometry decreasing with both the cerium content in the sample, and the sintering temperature. Nevertheless, such deviation from the ideal O/M = 2.00 ratio was found to generate only moderate structural disorder from EXAFS data at the U-L3 edge. Indeed, all the samples retained the fluorite-type structure of the UO2 and CeO2 parent compounds. The determination of accurate lattice parameters thanks to SPXRD measurements led to complete the data already reported in the literature, and to propose a mathematic expression linking the unit cell parameter, the chemical composition and the deviation from the stoichiometry. Such relation can now be used as a first approximation to estimate the O/M stoichiometry of uranium-cerium mixed oxides on a large composition range.