GRAIN BOUNDARY SEGREGATION IN STRONTIUM TITANATE AND ITS RELATION TO SPACE CHARGE LAYERS

O. KIENZLE AND M. RÜHLE

Max-Planck-Institut f¸r Metallforschung, Seestraße 92, D-70174 Stuttgart, Germany

The macroscopic electronic and ionic conductivities of electroceramics are strongly influenced or even dictated by changes of the chemical composition at grain boundaries. Differences of the composition of the boundary cores as compared to the bulk are caused by interfacial films, solute segregation, and differing formation energies of vacancies introducing space charge regions extending several nanometer into the adjacent grains. A fundamental understanding of these interface phenomena is of major importance for tailoring properties of electroceramics by optimizing doping and processing. Strontium titanate (SrTiO3) serves as a model system for the defect chemistry of several titanate ceramics, which are of great technical importance for sensors, actuators, and capacitors. We have analyzed the atomistic structure and the chemical composition of several grain boundaries in acceptor (Fe) doped SrTiO3 bicrystals and polycrystalline ceramics by sub-nanometer resolution techniques of transmission electron microscopy (TEM). The combined analysis of the atomistic structure of the grain boundaries by high-resolution TEM and the analysis of the same interfaces by energy dispersive X-ray spectroscopy (XEDS) and electron energy-loss spectroscopy (EELS) on a dedicated scanning TEM enables us to study the influence of the atomistic structure on the composition of the grain boundaries. A general trend observed in bicrystals and ceramics is that boundaries consisting of repeating structural units do not show segregation of the acceptor dopand Fe above the detection limit (0.2 at. / nm2), whereas Fe segregates to more strongly distorted ?general" grain boundaries. Furthermore, general boundaries in ceramics are decorated with 0.8 nm thin amorphous grain boundary films with compositions corresponding to Fe-rich titania. These investigations demonstrate, that segregation to grain boundaries in SrTiO3 is a rather complex phenomenon which cannot be ascribed solely to the formation of space charge layers.