Probing Point Defects in Fluorite-Structured Actinide and Analogue Oxides

Lanthanide and actinide oxides, such as CeO₂ [cerium dioxide], ThO₂ [thorium dioxide], and UO₂ [uranium dioxide], are attractive candidates for various energy-related applications such as nuclear fuel and electrolytes for solid oxide fuel cells owing in part to the resiliency of their bulk structures at room temperature up to near-melting temperatures. These materials exhibit broad regimes of phase stability under various extreme conditions including high temperature, pressure, and/or energetic ion irradiation. Upon modification from external perturbation (e.g., ion irradiation) or chemical changes (e.g., doping or oxidation), these fluorite-structured oxides incorporate large concentrations of point defects, which can agglomerate and result in complex microstructures that can severely impact component performance. The final state of defect arrangements is governed by unique interactions among the various vacancies, lanthanides, actinides, oxygens, and dopant atoms. This work investigates short-range atomic disorder in swift heavy ion-irradiated CeO₂ [cerium dioxide] and ThO₂ [thorium dioxide], oxidized UO₂ [uranium dioxide], and lanthanide-doped UO₂ [uranium dioxide] systems in order to understand how changes in local atomic arrangements correlate to bulk structural modifications and degradation of key material properties. Detailed structural analyses revealed that defect complexes, mostly small oxygen clusters, form in all fluorite-structured oxides after high energy ion irradiation, oxidation, and chemical doping. A number of computational studies have shown that these types of defect agglomerates can exhibit diffusion pathways much faster than isolated point defects. Accurate characterization and understanding of defect cluster stability and migration mechanisms will therefore enable better bulk property predictions that are critical to engineering improved fluorite-structured materials for energy applications.

Portions of this dissertation were previously published by Raul I. Palomares, Jacob Shamblin, Cameron L. Tracy, Joerg Neuefeind, Rodney C. Ewing, Christina Trautmann, Fuxiang Zhang, Changyong Park, Dmitry Popov, and Maik Lang in the journals: Journal of Materials Chemistry A, Nuclear Instruments and Methods in Physics Research B, and Journal of Applied Crystallography: R.I. Palomares et al., “Defect accumulation in swift heavy ion-irradiated CeO₂ and ThO₂ .” J. Mater. Chem. A, 2017, 5, 12193 R.I. Palomares et al., “Thermal defect annealing of swift heavy ion irradiated ThO₂ .” Nucl. Instr. Meth. B 405 (2017) 15-21 R.I. Palomares et al., “In situ defect annealing of swift heavy ion irradiated CeO₂ and ThO₂ using synchrotron X-ray diffraction and a hydrothermal diamond anvil cell.” J. Appl. Cryst. (2015). 48, 711-717