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Dielectric response e*(f,T) and polar phonon spectra of coarse grain (grainsize ~ 4 mkm) and fine grain (grain size ~ 150 nm) ceramics ofPbMg_(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 temperaturesand a sharp anomaly at the ferroelectric phase transition. The fine-grainceramics exhibit mainly relaxor ferroelectric behavior with a smallerdielectric constant. The difference is explained by different relaxationaldynamics of polar nanoclusters, which appear to be more stabilized at hightemperatures in the fine-grain ceramics by pinning at grain boundaries. BelowTc, the growth of ferroelectric domains is suppressed in fine-grain ceramics assupported also by a second harmonic generation. On the other hand, polar phononfrequencies and their temperature dependences are almost independent of thegrain size, but the selection rules for the cubic symmetry are not obeyed andall phonons are split due to a locally broken symmetry by polar nanoregions andchemical disorder. The lowest-frequency polar phonon undergoes partialsoftening down to ~ 0.1 THz near Tc = 440 K in both ceramics, but thedielectric anomaly is caused predominantly by flipping and breathing of polarnanoclusters. Due to contribution of both the soft phonon mode and dielectricrelaxations into the dielectric constant, the ferroelectric phase transition,which corresponds to the percolation threshold of the polar nanoregions intomacroscopic domains, can be considered as a special case of crossover betweenthe displacive and order-disorder type.

Grain Size Influence on Dynamics of Polar Nanoclusters in PMN-35%PT Ceramics: Broadband Dielectric and Infrared Spectra