Probing structural evolution in thorium–cerium mixed oxides thermally annealed in oxidizing and reducing conditions

Abstract ThO2 is a promising matrix either for incineration or direct disposal of plutonium and (Th, Pu)O2 is a potential advanced fuel. In this view, a ThO2–CeO2 mixed oxide system has been investigated under oxidizing as well as reducing conditions wherein CeO2 has been employed as a non-radioactive surrogate for PuO2. Selected compositions in Th1−xCexO2 (0 ≤ x ≤ 0.5) series were prepared by gelcombustion synthesis and thoroughly characterized by XRD and Raman spectroscopy. Both the studies confirm the formation of a fluorite-type (F-type) solid solution throughout the series after calcination of gelcombusted powders at 873 K in air. The F-type phase is retained even on annealing these solid solutions at an elevated temperature 1773 K in air. Lattice parameter of the F-type phase decreases as a function of ceria content in accordance with the Vegard’s law. On the contrary, the position of the F2g Raman band in these solid solutions exhibits non-ideal behavior. Annealing of calcined samples at 1873 K in a reducing atmosphere (Ar−8% H2) yields a single phase of F-type structure at x ≤ 0.2, while a biphasic phase field at x ≥ 0.3. Characteristic Raman vibration bands have been utilized to identify the presence of possible defects (due to Ce4+/Ce3+ substitution in ThO2) that are otherwise difficult to detect by the XRD studies. Detailed structure and defect analysis in ThO2–CeO2 mixed oxides will not only give valuable insight to the fuel designer but also bring out fundamental chemical aspects of such mixed oxides.