Broadband Dielectric Spectroscopy of 0.2PMN-0.4PSN-0.4PZN Relaxors Ceramics
Keywords:ceramics, relaxors, dielectric dispersion, distribution of relaxation times, soft mode, Burns temperature.
Results of broadband dielectric spectroscopy of 0.2PbMg1/3Nb2/3O3-0.4PbSc1/2Nb1/2O3-0.4PbZn1/3Nb2/3O3 ceramics are reported for 20 ≤ T ≤ 500 K and 20 Hz ≤ ν ≤ 100 THz. Dielectric constant is very high (more 14000) in the vicinity of the peak. Anomalous broad dielectric relaxation has been observed near the temperature of the maximum permittivity, Tm (at 1 kHz). The distribution of relaxation times has been calculated directly from the dielectric spectra. Over the temperature of the maximum permittivity, Tm the distribution of the relaxation times is symmetrically shaped (Cole-Cole function is satisfactory to describe the dielectric response). At lower temperatures, the distribution of relaxation times becomes asymmetrically shaped. On further cooling the second maximum appears. Above 1 THz, the dispersion is determined by polar phonons active in infrared spectra. Their contribution to total static permittivity is substantial above the Burns temperature Td and softening of the TO1 optic mode accounts for the Curie-Weiss behaviour near Td. Below Td the ferroelectric soft mode hardens on cooling exhibiting no anomaly near temperature Tm of permittivity maximum. Around Tm the polar phonon contribution to permittivity is small compared to low-frequency permittivity due a strong dielectric dispersion which appears below Td in the microwave and submilimetre region.
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