Nonlinear optical studies of the temperature evolution of local asymmetric regions in solid solutions (1-x) PbMg_1/3Nb_2/3O₃-xPbSc_1/2Nb_1/2O₃

The temperature dependencies of second harmonic generation (SHG) in (1-x)PMN–xPSN solid solutions (x = 1, 0.35, 0.23, 0.1, 0) are measured. Presence of SHG signal in centrosymmetric phase in all samples studied is demonstrated. It indicates asymmetric regions in the centrosymmetric phase. In PMN crystal (x = 0) in the centrosymmetric phase, SHG temperature dependence follows deviations of strain and refractive index from their high-temperature dependencies, which are determined by random electric fields in local polar regions. In the intermediate temperature range, where macroscopic spontaneous polarization disappears, SHG signal is caused by relatively large and long-lived polar asymmetric regions.