Analysis on dipole polarization of BaTiO₃-based ferroelectric ceramics by Raman spectroscopy

Analysis of asymmetric Raman line shape disclosed the variation of phonon correlation length in A₁(3TO) mode, L_A1(3TO), with temperature in the ceramics of BaTiO₃ (BT), Ba_0.6Sr _0.4TiO₃ (BST-0.6) and BaZr_0.25Ti _0.75O₃ (BZT-0.25), namely normal ferroelectrics, ferroelectrics with diffuse phase transition (DPT ferroelectrics) and relaxor ferroelectrics, respectively. In BT, L_A1(3TO) exhibited steep increase at the Curie temperature (T_c) on cooling. This is attributed to the formation of the ferroelectric domains at the T_c. Both BST-0.6 and BZT-0.25 showed gradual increase in L_A1(3TO) on cooling across the dielectric maximum temperature (T_m), indicating the continuous increase in the average size of the polar nanoregions (PNRs). Normal ferroelectrics can be distinguished from DPT and relaxor ferroelectrics in this point. L_A1(3TO) of BZT-0.25 was longer than that of BST-0.6 near the T_m. This suggests the size of PNRs in BZT-0.25 is larger than that in BST-0.6. Huge dipole polarization of BZT-0.25, giving rise to the strong relaxor behavior, could be originated from the contribution of the large PNRs near the T_m. DPT ferroelectrics can be also differentiated from relaxor ferroelectrics in terms of the average size of PNRs.