Role of the TiO6 octahedra on the ferroelectric and piezoelectric behaviour of the poled PbMg1/3Nb2/3O3–xPbTiO3 (PMN–PT) single crystal and textured ceramic Original Research Article

Field cooling (FC) poled/unpoled PMN–29%PT single crystal and room temperature (RT) poled/unpoled PMN–34.5%PT textured ceramic were investigated between ∼0 and 300 °C by thermal expansion, dielectric and Raman spectroscopy. New phase transitions are evidenced at 40, 91 and 180 °C in the case of FC PMN–29%PT as well as at 70 and 200 °C for RT PMN–34.5%PT and their order is discussed. The physical properties of the textured ceramics are rather similar to the ones observed for the single crystals that make them low-cost alternative for a wide range of applications. However, the temperatures and character of the phase transitions strongly depend on the kind of the poling conditions. Temperature dependences of the Raman line parameters show that the NbO6 octahedra remain stable during temperature increase, while TiO6 ones evolve quasi-continuously. The step transitions of the Pb2+ ion sublattice are evidenced. This suggests that the TiO6 and Pb2+ sublattices are especially coupled. The role of the TiO6 clusters on the structural phase transitions and dielectric properties of the PbMg1/3Nb2/3O3–xPbTiO3 (PMN–PT) system is discussed. The presence of the Raman modes above the maximum dielectric permittivity reveals that the local symmetry is lower than the cubic one (Pm3m). The decrease of the Raman line intensities vs. temperature indicates precisely the continuous evolution of the local symmetry towards the cubic one. The temperature evolution of the Rayleigh wing parameters appears sensitive to the phase transitions’ presence.