Nonstoichiometry and the Electrical Activity of Grain Boundaries in ${\mathrm{SrTiO}}_3$

A combination of experiments and first-principles calculations is used to show that grain boundaries in $\mathrm{SrTiO}{}_3$ are intrinsically nonstoichiometric. Total-energy calculations reveal that the introduction of nonstoichiometry into the grain boundaries is energetically favorable and results in structures that are consistent with atomic-resolution $Z$-contrast micrographs. Electron energy-loss spectra provide direct evidence of nonstoichiometry. These results and calculations for nonstoichiometric grain boundaries provide an explanation of the microscopic origin of the “double Schottky barriers” that dominate the electrical behavior of polycrystalline oxides.