Enthalpic Stabilization of CeTi2O6 by Ce(III)-VO Inclusion

Abstract Brannerite (UTi2O6) is a mineral analogue that is both flexible to a wide range of actinide/lanthanide substitutions and is also highly refractory; thus, it has generated interest as a waste form for used nuclear fuel (UNF). Given the wide range of elements in UNF, systematically tracking how compositional changes map onto changes in stability will guide the rational design of waste forms like brannerite. Changes in valence state, not just cation substitutions, are particularly important for actinide-containing materials, and the brannerite structure is flexible to a variety of charge-balancing substitutions. To isolate the impact of valence contributions to (de)stabilization of brannerite materials, a series of Ce(III/IV)Ti2O6–x ceramics were synthesized for study. Their compositions and structures were rigorously characterized by X-ray absorption spectroscopy, X-ray diffraction, and thermogravimetric analysis. High temperature oxide melt drop solution calorimetry was used to investigate thermodynamic properties: 20% coupled Ce(III)-VO resulted in increased stability driven by enthalpic contributions. These findings highlight that valence-driven changes in stability are factors that must be considered when designing waste forms or other ceramic materials with longevity in mind.