Space‐Charge Distribution across Internal Interfaces in Electroceramics Using Electron Holography: II, Doped Grain Boundaries

Abstract

Transmission high‐energy electron holography has been utilized to quantitatively determine the magnitude, sign, and spatial extent of the electrical potential, electric field, as well as the charge and stress field distributions across electrically active grain boundaries (GBs) in polycrystalline SrTiO3. The polycrystalline compacts utilized were bulk‐doped with Fe and GB‐doped with Mn, which is diffused into them under a nonequilibrium situation. The holography results reveal negatively charged GBs with associated positive space‐charge, indicating that Mn resides as an acceptor dopant on the Ti site, creating a negative bound charge at the GBs. A large heterogeneity in the electrical activity is observed among various GBs; while some GBs show high Schottky barriers, others have very small barriers. The distribution of the bound charge and the compensating free charge, which together contribute to the formation of GB charge and space‐charge, has been estimated. The bound charge experiences an outward stress field due to the force of the electric field on either side of the GB. The height of the double Schottky barrier, the Debye length, as well as the spatial extent of the bound charge and space‐charge at the GB are derived from these holography results. The results also provide a quantitative measure of the defect densities as well as of the contribution of the electrostatic stress field to the lattice relaxation at the GBs and yield a point defect model for the GB.