Observation of structural inhomogeneity at degraded Fe-doped SrTiO3 interfaces

We report on the detection of structural inhomogeneity across anode and cathode interfaces in electrically degraded reduced and oxidized Fe-doped SrTiO3 (Fe:STO) single crystals by optical second harmonic generation (SHG) spectroscopy. SHG spectra were collected from several regions across the anode and cathode interfaces in both degraded reduced and oxidized Fe: STO crystals. We identify the formation of defect concentration gradients along both degraded reduced and oxidized anode interfaces. While the broken symmetries decrease from the outer region towards the central region of the reduced anode, the opposite trend is seen in the degraded oxidized anode. These results are attributed to the formation of centrosymmetric Fe4+:Ti4+-O-6 octahedral structures in the central region of the reduced sample's degraded anode and non-centrosymmetric Jahn-Teller distortions in the central region of the oxidized sample's degraded anode. The observed changes in SHG intensity from the outer region towards the central region of the degraded cathode interfaces is accompanied by a structural phase transition in the inner and outer regions, marked by strong changes to the s-polarized intensity spectra. We explain the SHG intensity changes by the formation of lower order symmetry Fe3+:Ti3+-O-6 structures in the outer region and a modification of the second-order nonlinear susceptibility near the central regions due to the clustering of dissociated oxygen vacancies. These significant structural and spatial inhomogeneities are linked directly to the field-driven migration of oxygen ions and vacancies. Published by AIP Publishing.