| In this thesis, novel techniques have been developed in scanning transmission electron microscopy that can be used to analyze the atomic scale structure-property relationships in oxides, both at room and elevated temperatures. In particular, by using correlated Z-contrast imaging and electron energy loss spectroscopy, the structure, composition and bonding can all be characterized directly. In this thesis, it will be demonstrated that these techniques can be used to analyze a wide range of materials, and examples from dielectrics, electronic- and ionic conductors and catalysts will be shown. The majority of this thesis will address the atomic scale characterization of oxygen vacancy structures and point defect diffusion in ionic and electronic conducting ceramic membranes and at grain boundaries in model perovskite oxides. It is found that at high oxygen vacancy concentration the individual vacancies start to interact and associate to form arbitrarily oriented domains of Brownmillerite structures inside the bulk material. These domains continuously grow with increasing reducing conditions and ultimately, the structure collapses due to the high oxygen deficiency. Throughout the reduction, adjacent atomic planes exhibit a drastic difference in the oxygen vacancy formation energy, which causes a selective reduction of bulk. Further, oxygen vacancy and acceptor segregation towards the homo-interfaces was observed, that appears to be responsible for the formation of the widely observed grain boundary potential. This potential can be explained by the inherently larger oxygen vacancy segregation energy of the interfacial structural units rather than the phenomenological Schottky-barrier picture. At the metal-oxide interfaces in model catalysts, metal surface oxidation layers at higher calcination temperatures and reduction of the oxide support at the hetero-interfaces at lower calcination temperature was found. The effects of this oxygen vacancy segregation and surface oxidation on the catalysts performance will be the aim of future experiments. |
