Assessment of Strain Relaxation and Oxygen Vacancy Migration near Grain Boundary in SrTiO₃Bicrystals by Second Harmonic Generation

Mesoscopic structural distortions that result from ionic space charge accumulation near a grain boundary have impacts on the electrical conductivity and optical properties of perovskite oxides. Here, we report on our study that address the correlation between strain distribution and oxygen vacancy migration and pile up in undoped SrTiO3 (STO) bicrystals using local probing via the optical second harmonic generation (SHG) under electrical degradation. Comprehensive SHG measurements were taken near the grain boundary in both reflection and transmission geometries. While we confirmed a 10°-tilted grain boundary and an anisotropic strain distribution in the pristine bicrystal, we also revealed oxygen vacancies are confined and accumulated near the strain-imposed boundary in the electrically degraded bicrystal. During the degradation process, oxygen vacancies migrate, and electronic charge can be injected. With the grain boundary acting as a barrier to the oxygen vacancy migration, there can be a strain gradient on both sides of the grain boundary, and also with the charge an internal electrical field is developed. Our results strongly indicate that oxygen vacancies are preferentially formed and are segregated at the grain boundary where the strain accumulates and impedes oxygen vacancy diffusion under a concentration gradient.