Grain boundary segregation in iron doped strontium titanate: From dilute to concentrated solid solutions

Strontium titanate, a perovskite oxide, is a frequently studied material for a large variety of applications. When acceptor-doped (with Fe, for example), the material is useful for its mixed oxygen and electronic conductivity, with potential use in oxygen transport membranes or as a cathode for solid oxide fuel cells. A barrier to conductivity in perovskites is the presence of space charge regions at the grain boundaries, which form due to the segregation of charged point defects. Typically, space charge theory assumes bulk dopant concentrations beneath the dilute limit, however concentrated solid-solutions are often utilized in applications. The current work aims to address this disparity: grain boundary segregation in strontium ferrite-strontium titanate solid-solutions is analyzed at three compositions, with Fe contents ranging from near the dilute limit to well above the dilute limit (Fe contents of 2%, 5%, and 25% on the B-site of the perovskite). Electrochemical impedance spectroscopy shows an increase in material conductivity as Fe is added. High-resolution STEM imaging and spectral mapping is utilized, showing that Fe segregates to the grain boundary core, contrary to what is expected from space charge theory. As the Fe content is increased, the amount of Fe segregated to the boundary increases significantly, but the segregation width of Fe at the boundary remains consistent.