Solid solutions of (Na,Bi)TiO₃ (NBT) and SrTiO₃ (ST) are materials of interest for high‐strain or high‐energy density capacitor applications. Often, they exhibit chemical heterogeneity and develop core‐shell structures during regular solid‐state synthesis with an NBT‐rich core. In this case, the NBT forms first so that the strontium needs to diffuse into the material to reach chemical homogeneity. Depending on the presence of core‐shell structures, the electrical properties can vary drastically. In this work, we rationalize the effect of variations in oxygen vacancy concentration by Fe‐acceptor and Nb‐donor doping. It can be shown that a diffusion couple of strontium and oxygen is responsible for chemical homogenization and that the oxygen vacancy content can control the formation of a core‐shell structure.
