Correlating structure and energy landscape of perovskite oxide grain boundaries

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The development of a fundamental understanding of how crystal structure, point defects and extended defects influence the energy landscape for ion migration is of great importance for many applications of perovskite oxides. However, the relationship between the atomic structure of grain boundaries, their space charge regions, the site energies for occupation and migration of ions, and their changes with concentration is still not very well understood. In this project of the planned research group, therefore, anisotropic diffusion along and across grain boundaries as well as in the volume of bicrystals with selected tilt grain boundaries will be investigated by means of analytical and high-resolution transmission (scanning) electron microscopy (HR(S)TEM). Initially, we select as a model system the alkali ion diffusion in the cubic perovskite SrTiO3 with well-defined small and large angle grain boundaries of different tilt angles. In the course of the project development, the investigations will be extended to bicrystalline epitaxial manganate perovskite films that are relevant for solid oxide fuel cells. The atomic and chemical structure of the grain boundaries will be investigated by using HR(S)TEM in combination with spatially resolved spectroscopic techniques (EDX, EELS). This also allows the determination of local diffusion profiles and their comparison with the results of secondary ion mass spectroscopy (SIMS, AG Weitzel, P1) and 3D concentration distributions from tomographic atom probe (APT, AG Volkert, P3). A further central goal is the determination of the electric potentials as well as the space charge regions of the grain boundaries by off-axis electron holography. Spectroscopy and diffuse electron scattering methods are used to detect the influence of point defects in the volume and in the surrounding of grain boundaries. The results on the atomic, chemical and electrical structure of grain boundaries are central for the theoretical determination of the site energies (ab initio methods, AG Jacob, P6) as well as for the determination of the energy landscape from diffusion experiments. In the research group, the proposed project with its combination of very advanced TEM methods can make a central contribution to the understanding of the landscape of site and populated site energies and their dependence on local electrical potentials .