The role of local non-tetragonal polar displacements in the temperature- and pressure-induced phase transitions in PbTiO₃-BiMeO₃ ferroelectrics

In situ high-pressure/high-temperature Raman-scattering analyses on PbTiO \(_3\) $_3^{}$ , 0.92PbTiO \(_3-\) $_3^{}-$ 0.08Bi(Zn \(_{0.5}\) $_{0.5}^{}$ Ti \(_{0.5}\) $_{0.5}^{}$ )O \(_3\) $_3^{}$ and 0.83PbTiO \(_3-\) $_3^{}-$ 0.17Bi(Mg \(_{0.5}\) $_{0.5}^{}$ Ti \(_{0.5}\) $_{0.5}^{}$ )O \(_3\) $_3^{}$ single crystals reveal an intensity transfer between the fine-structure components of the A \(_1\) $_1^{}$ (TO) soft mode. The enhancement of the lowest-energy subpeak, which stems from intrinsic local non-tetragonal polar distortions, along with the suppression of the tetragonal A \(_1\) $_1^{}$ (1TO) fundamental mode with increasing pressure and temperature indicates the key role of the local polarization fluctuations in transformation processes and emphasizes the significance of the order-disorder phenomena in both the pressure- and temperature-induced phase transitions of pure PbTiO \(_3\) $_3^{}$ and its solid solutions with complex perovskites. Moreover, the temperature and pressure evolution of the fraction of the local non-tetragonal polar distortions is highly sensitive to the type of B-site substituent.