Optical second-harmonic generation microscopy as a tool for ferroelastic domain wall exploration

Domain walls (DWs) of four typical ferroelastics, CaTiO3 (CTO), LaAlO3 (LAO), Pb3(PO4)2 (PPO), and BiVO4 (BVO), were observed by the second harmonic generation (SHG) microscopy, and the results were compared. The DWs of the examined ferroelastics are all polar. This fact does not depend on whether the crystal is an insulator (CTO, LAO, and PPO) or a semiconductor (BVO), on whether it has ferroelectric instability (CTO) or not (LAO, PPO, and BVO), or on the DW state, namely, whether the DW is crystallographically prominent or non-prominent. The symmetry of these DWs was determined from the SHG anisotropy, and it was clarified that they are a subgroup of the high temperature phase. In the cases of CTO and PPO, stress was applied to move the DW, and its polarity and symmetry were examined. The main characteristics did not change even when stress was applied. In all samples, uniform SHG was observed for each DW. This indicates that the DWs of the examined ferroelastics seem to be in a ferroelectric single domain state.