Rate Theory Modeling of Defect Evolution in Fluorite Oxides

A rate theory (RT) model of microstructure evolution in ion beam irradiated fluorite oxides is presented. The model considers independent evolution of point defects on cation and anion sublattices. It includes Frenkel pair generation, mutual recombination of interstitial and vacancies, clustering of interstitials into stoichiometric dislocation loops, and assumes cation vacancies are immobile under temperature ranges of interest. RT model is applied to interpret the results of microstructure characterization in ion irradiated cerium dioxide, thorium dioxide, and uranium dioxide. Dislocation loops revealed from transmission electron microscopy characterization are used to parametrize the migration barriers of individual defects utilized as input parameters to the model. Extracted migration barriers when compared to values from ab-initio calculations suggests that dislocation loop evolution is governed by mechanisms other than just the migration of the slower ion interstitial. Additionally, RT model is used to interpret the results of Raman and optical spectroscopy characterization each probing vibrational and electronic properties of point defects, respectively. This analysis, complemented by first-principles calculations of defects’ electronic and vibrational structure, allow us to identify signatures of oxygen vacancies in the optical spectra, while the assignment of Raman peaks to point defects requires further investigations.