New insight in the uranium valence state determination in U Nd1−O2±

Abstract

The charge compensation mechanisms in U y Nd 1Ày O 2±x and its consequence on the overall O stoichiometry (or O/M ratio where M ¼ Nd þ U) have been studied through the uranium valence state mixture evolution as a function of Nd content up to y ¼ 0.62 by means of high energy resolution fluorescence detection X-ray absorption spectroscopy (HERFD-XAS) at the U M 4-edge. Our results clearly demonstrate the formation of U 5þ at low Nd content (y < 0.15). Upon increasing the Nd content, oxygen vacancies and the formation of U 6þ appear as competing mechanisms for intermediate Nd concentrations, leading to the coexistence of U 4þ /U 5þ /U 6þ mixed valence and an overall hypostoichiometry (O/M < 2.00). Finally, the formation of U 6þ associated with strongly distorted U local environment is observed for high Nd concentrations (y ¼ 0.62), leading to an overall hyperstoichiometry (O/M < 2.00).

Charge compensation mechanisms in Nd-doped UO2 samples for stoichiometric and hypo-stoichiometric conditions: Lack of miscibility gap

“…Those findings are in good agreement with the results reported in [17] where the experimental value matches the theoretical curve for concentrations of Nd lower than 9%, and where a mixed charge compensation between Model 1 and Model 3 is observed for higher Nd concentration. According to the work reported by Bès [25], it is only for a greater value of y=30% that the system exhibits the presence of U 6+ . This value deviates significantly from the Model 1, since it shows a mixed charge compensation mechanism combining all three models.…”

“…To visualize more clearly the dominant charge compensation mechanisms, Bès et al schematized the concentration of U 5+ and U 6+ in comparison with Models 1, 2, and 3 [25]. Following the same reasoning, figure 8 displays the percentage of U 5+ for each valence state in comparison to Model 1, the curve that expresses Model 2 was not introduced in the graphic as no U 6+ is seen in our samples.…”

“…As expected, they saw only the formation of U 5+ predominantly as the main factor for charge compensation. Finally, Bès et al [25] calculated the concentration of U 5+ and U 6+ on the system U-Nd-O. These measurements showed the presence of U 6+ on the samples starting from 29% of Nd.…”

“…For this purpose, three references were used: UO 2 for U 4+ , NaUO 3 for U 5+ and β-UO 3 for U 6+ . The UO 2 spectrum was obtained during this experiment, however, the NaUO 3 and β-UO 3 references were obtained in a different analysis carried on at the ID26 beamline of the European Synchrotron Facility (ESRF) in Grenoble, France, and reported in [25]. Special attention was given on the possible effect on results arising from beamline energy resolution differences when dealing with such references.…”

“…Position of the E 0 and the white-line for each sample at the U M 4 -edge XANES spectra. From a strictly electrical-balance standpoint, three models, as already mentioned by Bès et al[25], can be defined to describe the charge compensation mechanisms. These models are as follows, Model 1:…”