Consistency in the chemical expansion of fluorites: A thermal revision of the doped ceria

The chemical expansion coefficient of doped ceria has been hitherto assumed to be temperature-independent. In this work, X-ray diffraction techniques are used to determine the effect of doping on the chemical expansion of ceria as a function of temperature in air. At room temperature, it is observed that the chemical expansion in ceria is a combination of two effects: the change in the effective cation radius and the formation of oxygen vacancies. In this study, the chemical expansion coefficient is found to be temperature-dependent, which stands in contrast to previous results that use high-temperature elastic strain data. Specifically, a 22% increase in the chemical expansion coefficient is observed with increasing temperature from room temperature up to 600 °C. This increase is attributed to the dissociation of local defect structures at higher temperatures. The contribution to the increase stemming from oxygen vacancy formation is calculated at various temperatures, and it is assumed that the effective size of anions and cations changes minimally with the temperature changes. More significantly, the effective radius for an oxygen vacancy is seen to be independent of trivalent dopant type and composition. The values determined in this work, as well as the methodology demonstrated, can be used broadly in the prediction of the chemical expansion coefficient at various temperatures for a given doped ceria system.