Probing the temperature-induced phase transition mechanism in PbTiO3-BiM0.5Ti0.5O3 ferroelectrics across the morphotropic phase boundary

The fundamental understanding of the driving mechanism of dipolar ordering in complex perovskite oxides (ABO3) and its dependence on the degree of doping as well as on the type of dopant is critical for the effective design of novel functional materials with desired properties. To elucidate the temperature-induced transformations in mixed lead-bismuth systems, we have studied two solid solutions with a morphotropic phase boundary (MPB), (1 - x)PbTiO3-xBiNi0.5Ti0.5O3 (PT-xBNT) and (1 - x)PbTiO3-xBiMg0.5Ti0.5O3 (PT-xBMT), by Raman spectroscopy and second-harmonic generation (SHG). The temperature evolution of atomic dynamics in both PT-xBNT and PT-xBMT shows that the paraelectric-to-ferroelectric phase transition at the MPB does not involve any dynamical instability, indicating the dominance of order-disorder transformation processes. The temperature dependence of the SHG intensity reveals that the MPB can be identified by the smallest size of the preexisting intrinsic ferroic entities that on cooling merge to form a ferroelectric state with a macroscopic polarization. The difference between Ni- and Mg-containing solid solutions is merely in the correlation length of local structural distortions.