  Relaxor ferroelectric PbMg_1/3Ta_2/3O_3 ceramics and thin films were
investigated by means of broad-band dielectric, time-domain terahertz (THz) and
Fourier-transform infrared (IR) spectroscopy in the frequency range 100 Hz - 90
THz at temperatures 100 - 490 K, the THz and IR spectra were studied from 20 to
900K. Diffused and strongly temperature dependent peak in the complex
permittivity is caused by a dielectric relaxation due to the dynamics of polar
clusters. The relaxation appears below Burns temperature T_d in the THz range,
slows down on cooling through the microwave and MHz range and anomalously
broadens. The shortest and longest relaxation times of the distribution of
relaxation times follow Arrhenius and Vogel-Fulcher law, respectively. The
degree of B-site order has only a small influence on the parameters of the
dielectric relaxation and almost no influence on the phonon parameters. Below
T_m ~ 180K, the distribution of relaxation frequencies becomes broader than our
experimental spectral range and frequency independent dielectric losses develop
below 100 GHz in the spectra. Although the macroscopic crystal structure is
cubic, IR spectra give evidence about the lower local symmetry which can be
assigned to the presence of polar clusters below T_d. Infrared spectra above
T_d still reveal more modes than predicted by selection rules in the
paraelectric phase of the Fm-3m space group so that we suggest selection rules
which take into account chemical inhomogeneity in the beta''-perovskite
sublattice.
