LOCAL DISORDER IN DOPED CERIA: A CRYSTALLOGRAPHIC STUDY

In recent years, doped ceria compounds have attracted increasing attention as electrolytes for application in Solid Oxide Fuel Cell devices, thanks to their high performance at intermediate temperature. In cerium oxide, ionic conductivity is driven by the presence of oxygen vacancies, introduced for charge balance after doping with a lower valent cation. With increasing doping concentration the oxygen diffusion is impeded, probably because of the formation of aggregates that trap oxygen vacancies. Their nature is though still under debate. Owing to the close interplay between transport properties and local structure, an accurate description of the defects on the atomic scale is of the utmost importance for understanding the mechanisms at work in doped ceria. In this study we propose to unveil the complex disorder in doped ceria with a combined powder diffraction study. On the one hand, X-ray and Neutron powder diffraction are joint to couple their potentialities; on the other hand, two different crystallographic approaches are exploited. The conventional crystallographic methods allow to study the long range structure modifications that occur with doping. The Pair Distribution Function is instead employed to study the local structure, in terms of deviations from the long range atomic ordering. The goal of the thesis is to provide a crystallographic description of the atomic rearrangements induced by doping, looking for a relationship between the local structure and the transport properties.