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

摘要

Relaxor ferroelectric PbMg_1/3Ta_2/3O_3 ceramics and thin films wereinvestigated by means of broad-band dielectric, time-domain terahertz (THz) andFourier-transform infrared (IR) spectroscopy in the frequency range 100 Hz - 90THz at temperatures 100 - 490 K, the THz and IR spectra were studied from 20 to900K. Diffused and strongly temperature dependent peak in the complexpermittivity is caused by a dielectric relaxation due to the dynamics of polarclusters. The relaxation appears below Burns temperature T_d in the THz range,slows down on cooling through the microwave and MHz range and anomalouslybroadens. The shortest and longest relaxation times of the distribution ofrelaxation times follow Arrhenius and Vogel-Fulcher law, respectively. Thedegree of B-site order has only a small influence on the parameters of thedielectric relaxation and almost no influence on the phonon parameters. BelowT_m ~ 180K, the distribution of relaxation frequencies becomes broader than ourexperimental spectral range and frequency independent dielectric losses developbelow 100 GHz in the spectra. Although the macroscopic crystal structure iscubic, IR spectra give evidence about the lower local symmetry which can beassigned to the presence of polar clusters below T_d. Infrared spectra aboveT_d still reveal more modes than predicted by selection rules in theparaelectric phase of the Fm-3m space group so that we suggest selection ruleswhich take into account chemical inhomogeneity in the beta''-perovskitesublattice.