Evolution of the Uranium Chemical State in Mixed-Valence Oxides

Abstract

A fundamental question concerning the chemical state of uranium in the binary oxides UO, UO, UO, UO, and UO is addressed. By utilizing high energy resolution fluorescence detection X-ray absorption near edge spectroscopy (HERFD-XANES) at the uranium M edge, a novel technique in the tender X-ray region, we obtain the distribution of formal oxidation states in the mixed-valence oxides UO, UO, and UO. Moreover, we clearly identify a pivot from U(IV)-U(V) to U(V)-U(VI) charge compensation, corresponding with transition from a fluorite-type structure (UO) to a layered structure (UO). Such physicochemical properties are of interest to a broad audience of researchers and engineers active in domains ranging from fundamental physics to nuclear industry and environmental science.

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

“…Hexavalent uranium U 6þ is not observed in our case below y ¼ 0.2, confirming that U 6þ formation in a fluorite structure is probably energetically too expensive compared to the formation of two U 5þ cations. This is in good agreement with the absence of U 6þ in the early oxidation stages of UO 2 where U 4þ /U 5þ mixtures are reported in U 4 O 9 and U 3 O 7 [32], or in the cases of U 1Ày Bi y O 2±x and U 1Ày La y O 2±x where only U 5þ formation is reported up to y ¼ 0.5 [15] and y ¼ 0.4, respectively [35]. These upper limits are about two times the value found for Nd doped UO 2 and suggest that the dominating mechanism depends on the dopant nature.…”

“…The increasing intensity of the shoulder at the high energy side of the U 4þ WL indicates the formation of U 5þ and/or U 6þ valence states [14,31,32]. The U 0.38 Nd 0.62 O 2±x sample does not show any U 4þ feature and is most probably composed by a mixture of U 5þ and U 6þ only, even if its spectrum is dominated by U 6þ WL.…”

Trends in the valence band electronic structures of mixed uranium oxides

“…Up to now it was expected that the occupied O 2p level stays at constant energy with respect to the Fermi level while the U 5f states move up in energy upon the changes of the oxidation state of U from U(IV) → U(V) → U(VI). Our recent studies of the evolution of the U chemical state in a series of U oxides confirms a changeover of the oxidation states U(IV) -U(V)-U(VI) through the charge compensation mechanisms 5,19,22 . The established formal oxidation states for U in mixed U oxides are included in brackets: UO2(IV), U4O9(IV-V), U3O7(IV-V), U3O8(V-VI), -UO3(VI).…”

A Novel Metastable Pentavalent Plutonium Solid Phase on the Pathway from Aqueous Plutonium(VI) to PuO2 Nanoparticles

“…The chemical shift of the intermediate Pu phase is clearly resolved in the HERFD data reported in Figure and indicates the presence of the Pu V oxidation state, in agreement with the Pu M 4 HERFD results. However, for actinide systems, HERFD at the L 3 edge is not as sensitive as the M 4 edge HERFD to the presence of minor contributions (<10 %) from different oxidation states . HERFD at the L 3 edge is, however, extremely sensitive to the local structure around the absorber, which results in specific post‐edge features …”