Nonlinear optical detections of structural distortions at degraded Fe-doped SrTiO₃ interfaces

We investigate structural deformations at the interfaces of degraded Fe-doped SrTiO₃ (Fe:STO) single crystals in both oxidized and reduced forms by using optical second harmonic generation (SHG) with the implementation of Raman spectroscopy in reflection geometry. SHG spectra were acquired from three different regions, namely, central, inner, and outer, across the interfaces. Structural inhomogeneity was verified across the sample interfaces by variations in SHG intensity. The SHG intensity decreased when moving towards the inner region of the degraded reduced anode, and the intensity increased as moving towards the central region of the degraded oxidized anode. These observations are associated with the formation of centrosymmetric Fe⁴⁺:Ti⁴⁺-O₆ octahedral structures and non-centrosymmetric Jahn-Teller distortions in the inner region of the reduced anode and the central region of the oxidized anode, respectively. These results are consistent with Raman measurements taken from the anode interfaces of both crystals, which verify concentration changes of Fe⁴⁺ and octahedral distortions across the probed regions. The accumulation of oxygen vacancies is identified across the cathode interfaces of both crystals. These structural distortions and Fe⁴⁺ concentration changes are well correlated with the migration of oxygen ions and vacancies.