Dielectric response e*(f,T) and polar phonon spectra of coarse grain (grain size ~ 4 mkm) and fine grain (grain size ~ 150 nm) ceramics of PbMg_(1/3)Nb_(2/3)O3-35%PbTiO3 were investigated at temperatures 10 - 900 K. e*(f,T) in coarse-grain ceramics exhibits relaxor behavior at high temperatures and a sharp anomaly at the ferroelectric phase transition. The fine-grain ceramics exhibit mainly relaxor ferroelectric behavior with a smaller dielectric constant. The difference is explained by different relaxational dynamics of polar nanoclusters, which appear to be more stabilized at high temperatures in the fine-grain ceramics by pinning at grain boundaries. Below Tc, the growth of ferroelectric domains is suppressed in fine-grain ceramics as supported also by a second harmonic generation. On the other hand, polar phonon frequencies and their temperature dependences are almost independent of the grain size, but the selection rules for the cubic symmetry are not obeyed and all phonons are split due to a locally broken symmetry by polar nanoregions and chemical disorder. The lowest-frequency polar phonon undergoes partial softening down to ~ 0.1 THz near Tc = 440 K in both ceramics, but the dielectric anomaly is caused predominantly by flipping and breathing of polar nanoclusters. Due to contribution of both the soft phonon mode and dielectric relaxations into the dielectric constant, the ferroelectric phase transition, which corresponds to the percolation threshold of the polar nanoregions into macroscopic domains, can be considered as a special case of crossover between the displacive and order-disorder type.
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