  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.
