Novel approach in analyzing phase transitions in Na_0.5Bi_0.5TiO₃—Comparison with 0.95Na_0.5Bi_0.5TiO₃–0.05CaTiO₃

Recently, Na0.5Bi0.5TiO3 and its solid solutions are receiving intensive study as one of the most perspective lead-free ferroelectrics. Not only physical properties, but also the structure and nature of phase transitions of these compositions are of great interest, as their previous studies contain many uncertainties. In the present research, Na0.5Bi0.5TiO3 and 0.95Na0.5Bi0.5TiO3–0.05CaTiO3 solid solutions were thoroughly studied focusing on the elastic and thermal expansion characteristics, accompanying the obtained results by x-ray diffraction, scanning electron microscopy, differential calorimetry, and second harmonic generation measurements. Temperature-frequency dependences of dielectric permittivity were observed to be similar for both compositions. In spite of this, the experimentally obtained temperature dependences of thermal expansion and Young's modulus in Na0.5Bi0.5TiO3 and 0.95Na0.5Bi0.5TiO3–0.05CaTiO3 reveal unambiguous differences in the temperature range of the observed or expected (as in the case of Na0.5Bi0.5TiO3) phase transitions. X-ray diffraction patterns are fitted using Pnma symmetry. This allows us to distinguish the temperature regions with different behaviors of lattice parameters, which correlate with the observed behavior of thermal expansion and Young's modulus. A reduction in the intensity of second optical harmonic was observed upon increasing the temperature in the whole studied temperature range. This encourages us to reconsider the mechanism responsible for the temperature dependence of dielectric permittivity.