The role of local non-tetragonal polar displacements in the temperature- and pressure-induced phase transitions in PbTiO 3 -Bi Me O 3 ferroelectrics

In situ high-pressure/high-temperature Raman-scattering analyses on PbTiO _3 , 0.92PbTiO _3- 0.08Bi(Zn _0.5 Ti _0.5 )O _3 and 0.83PbTiO _3- 0.17Bi(Mg _0.5 Ti _0.5 )O _3 single crystals reveal an intensity transfer between the fine-structure components of the A _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 (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 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.