Study of Tetragonal Phase in 0.4Na_1/2Bi_1/2TiO₃-(0.6-x)SrTiO₃-xPbTiO₃ Solid Solutions by Second-Harmonic Generation

Description of the local symmetry of relaxor ferroelectrics, especially in connection with nature of polar nanoregions, is one of the key questions in research of relaxor ferroelectrics. Unlike such well-known relaxor ferroelectrics as PbMg_1/3Nb_2/3O₃ and (Pb,La)(Zr_0.65Ti_0.35)O₃, ternary solid solutions 0.4Na_1/2Bi_1/2TiO₃-(0.6-x)SrTiO₃-xPbTiO₃ show clear coexistence of phases of tetragonal and cubic symmetry, which is established by x-ray diffractometry. Since the second-harmonic generation spectroscopy is a sensitive tool in determination of deviation from symmetry, containing a centre of inversion, it is used as an additional method in characterization of structure in 0.4Na_1/2Bi_1/2TiO₃-(0.6-x)SrTiO₃-xPbTiO₃. Measurements indicate low, increasing intensity of the second-harmonic generation upon decreasing of temperature, for the samples with low concentration of PbTiO₃. Whereas an abrupt change is observed at the phase transition between ferroelectric and relaxor states for the samples with higher concentrations of PbTiO₃.