Hydrothermal conversion of mixed uranium(iv)–cerium(iii) oxalates into U1−xCexO2+δ·nH2O solid solutions

• Published in: Dalton transactions : an international journal of inorganic chemistry Vol. 52; no. 31; pp. 10951 - 10968
• Main Authors: Benarib, S, Dacheux, N, Le Goff, X F, Lautru, J, L Di Mascio, Clavier, N
• Format: Journal Article
• Language: English
• Published: Cambridge Royal Society of Chemistry 08.08.2023
• Subjects: Cerium oxides; Conversion; Fluorite; Hydrothermal treatment; Liquid phases; Microspheres; Mixed oxides; Oxalates; Plutonium; Solid phases; Solid solutions; Speciation; Uranium
• ISSN: 1477-9226; 1477-9234
• DOI: 10.1039/d3dt01510f

Uranium–cerium oxide solid solutions, U1−xCexO2+δ·nH2O, were prepared through hydrothermal conversion of mixed U(iv)–Ce(iii) oxalate precursors, cerium being used as a surrogate for plutonium. Whatever the starting pH, the fluorite-type structure of AnO2 was obtained after heating at 250 °C for 24 h. The initial pH of the reaction media appeared to affect significantly the oxide morphology: for pH ≤ 2, the powder was found to be composed of microspheres, whereas for more alkaline pH values, agglomerates of nanocrystallites were found. Furthermore, a study of the hydrothermal treatment duration (T = 250 °C, pH = 8, t = 1–48 h) showed that fluorite-type mixed dioxides started to form after only 1 h, and then became single phase after 3 h. SEM and TEM/EDS analyses revealed that the cationic distribution narrowed with time to finally form highly homogeneous mixed oxides. Such a preparation route was then applied to various cerium incorporation rates and it was found that the formation of U1−xCexO2+δ·nH2O mixed oxides was possible for 0.1 ≤ x ≤ 0.75. In all the systems investigated, the speciation of uranium and cerium was questioned in both the solid and liquid phases. Thermodynamic calculations and evaluation of the O/M ratio in the final oxides led us to understand the complex redox behaviour of uranium and cerium in solution during hydrothermal processes and to propose a conversion mechanism.