Dielectric relaxation and polar phonon softening in relaxor ferroelectric PbMg_(1/3)Ta_(2/3)O_3

TL;DR

This study investigates the dielectric relaxation and phonon behavior in relaxor ferroelectric PbMg_1/3Ta_2/3O_3 ceramics and films across a broad frequency and temperature range, revealing complex dynamics of polar clusters and local symmetry deviations.

Contribution

It provides new insights into the dielectric relaxation processes and local symmetry breaking in relaxor ferroelectrics, considering the influence of chemical inhomogeneity and B-site order.

Findings

Dielectric relaxation appears below Burns temperature T_d in the THz range.

Relaxation times follow Arrhenius and Vogel-Fulcher laws.

Infrared spectra indicate lower local symmetry and additional modes beyond selection rules.

Abstract

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.