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.