Phonon mode and dielectric function dynamics of Bi_0.5Na_0.5TiO₃-xSm₂Ti₂O₇ ceramics during the phase transitions discovered by Raman scattering and spectroscopic ellipsometry

Recently, Bi_0.5Na_0.5TiO₃-based ceramics have drawn widespread attention due to their excellent energy storage properties. Here, a comprehensive analysis of the composition- and temperature-driven transition processes in (1 − x)Bi_0.5Na_0.5TiO₃-xSm₂Ti₂O₇ (BNST-x) polycrystals have been presented using Raman scattering and spectroscopic ellipsometry. With increasing Sm₂Ti₂O₇ content, BNST-x ceramic gradually becomes disordered and belongs to the superparaelectric state at x = 0.12 near room temperature. Moreover, the thermal evolution of the lattice kinetic behaviors shows two crucial temperatures: the depolarization temperature T_d and the temperature at the maximum dielectric permittivity T_m, which suggest the transitions from nonergodic relaxor ferroelectric—ergodic relaxor (ER) ferroelectric and ER ferroelectric—superparaelectric, respectively. It is noteworthy that the series of changes is closely related to the ordering degree of the B-site ion affected by the doping and the temperature. This work gives an insight into the connection among the phonon behavior, electronic transition, and lattice structure of BNST-x ceramics, which can further understand the phase transition mechanisms under the doping and thermal field.