In this project we investigate perovskite oxides, in particular (Sr,La)MeO3, Me= Ti, Ru, Mn. These materials play important roles as cathodes in SOFC's and represent promising low-cost alternatives for noble metals in catalysis. Because of their tunable functionalities they also represent an exciting class of materials in condensed matter physics. Despite the importance of surfaces and interfaces in these areas and many other emerging applications, little is known about the fundamental surface properties of these perovskite materials; their surface structure and reconstructions; composition and segregation phenomena; defects such as O vacancies and step edges; how these depend on environmental parameters such as the oxygen chemical potential and, ultimately, how these affect the local electronic structure and the adsorption of molecules. The research builds on our proven expertise in tackling metal oxide surfaces, but reaches far beyond the state-of-the-art in terms of structural and compositional complexity. Sample preparation is a key issue for meaningful surface science investigations, yet on perovskites conventional sputter-annealing cycles mostly result in B-site terminated surfaces with complicated, treatment-dependent reconstructions, and often a phase-segregated bulk. To avoid such a disruptive sample treatment, we will grow epitaxial thin film samples on appropriate substrates (e.g., SrTiO3, LaAlO3, Sr2RuO4), with laser-MBE. The surfaces will be investigated in-situ, i.e., in the same UHV set-up using surface science techniques (RHEED, STM, LEED, XPS, LEIS). In the first phase of the project, we will investigate SrTiO3 (001), in particular the SrO-terminated surface that is considered more relevant in the oxidizing environments present in an SOFC and in catalysis. In our surface investigations we will put special emphasis on achieving atomically-resolved STM images. This should give us valuable information of the most common surface defects, and how these affect the adsorption of oxygen and other adsorbates. We will use 18O LEIS and time-lapse STM images to study the diffusion of oxygen species across the surface and into the bulk of the material. The second system we will study are strontium ruthenates (001); as good electrical conductors they represent an excellent model system for SOFC cathodes. In the last phase of this project, we will turn our attention to (Sr, La)MnO3 and other mixed oxide perovskites. We will rely on a close collaboration with the theoretical group on data interpretation and in guiding the experiments. In turn, we will provide the theorists with high-quality experimental data as input to model surface structures, defect configurations, and adsorption processes. An important part of the project will be to provide to other experimental groups within the SFB with epitaxial thin film samples, and to develop procedures for preparation of surfaces with particular characteristics.Project part F4507 aims at understanding the surface properties of selected perovskite surfaces and on following chemical processes at these surfaces on the atomic scale.  Building on the our results in years 1-3, and on established collaborations within and beyond this SFB, we propose the following work plan: