Evolution of intrinsic point defects in fluorite-based materials: Insights from atomic-level simulation

Point defects play a significant role in materials properties. On the one hand, point defects are exploited in applications such as fuel cells; on the other hand, they are deleterious to performance of in such applications as nuclear fuels. To tailor materials with desired properties, it is thus important to understand point defect behavior in materials under specific environments/applications. This thesis is focused on understanding the evolution of point defects in materials, primarily fluorite-related, used in solid oxide fuel cell (SOFC) and nuclear applications. Molecular-dynamics simulation (MD) and density functional theory (DFT) are used as materials modeling tools.;Cubic bismuth oxide (delta-Bi2O3) is a promising SOFC electrolyte and a fluorite-based model material, in which, it is found that while intrinsic oxygen vacancies contribute in high oxygen diffusion, under certain circumstances, they themselves act as limiting factors by forming a <110>-<111> vacancy-ordered system. It is found that high cationic polarizability plays a significant role in achieving high oxygen diffusivity. It is also found that the oxygen diffusivity may be limited due to the formation of covalent character bonds by some Bi ions.;Fluorite-structured UO2 is the fuel in almost all operating nuclear reactors. Due to irradiation, UO2 undergoes damage resulting in the formation of Frenkel pairs (FPs) on both uranium and oxygen sub-lattices. It is found that, while damage on oxygen sub-lattice alone is not deleterious, in that oxygen FPs annihilate by mutual recombination, presence of FPs on uranium sub-lattice cause long-lasting damage and make UO2 less radiation tolerant. Regardless of FPs on oxygen sub-lattice, FPs on uranium sub-lattice nucleate new O FPs, which form clusters. The oxygen vacancies' sequestration by uranium vacancies is found to be the mechanism of cluster formation.;The point-defect evolution is rocksalt MgO is also elucidated. MgO is an important engineering material and has nuclear applications. As in UO 2, it is found that when defects are present on both Mg and O sub-lattice, new FPs, in this case, both on Mg and O are formed. Defects present only on either Mg or O sub-lattice annihilate by mutual recombination.;While UO2 and MgO studies are done on single crystals to elucidate FPs elimination by mutual recombination only, to understand their evolution in the presence of GBs, similar studies are done on nanocrystalline Mo. This allows elucidation of dominant mechanism of point-defect elimination. It is found that for the grain size and temperature under study, mutual recombination of FPs dominates and GBs have little effect on the point-defect elimination.