Correlation between low frequency dielectric dispersion (LFDD) and impedance relaxation in ferroelectric ceramic Pb 2KNb 4TaO 15

Abstract

An impedance spectroscopy experiment has been carried out on a Pb 2KNb 4TaO 15 ceramic sample of tetragonal tungsten bronze-type structure ( T c =643 K) from 300 to 900 K in the 20-3×10 5 Hz frequency range. A low frequency dielectric dispersion (LFDD) has been found. An impedance relaxation occurs simultaneously. A model is proposed to account for the observed phenomena. An empirical impedance expression has been deduced: Z ∗=R 0/(1+iω/ω 1+(iω/ω 2) n) where ω 1 and ω 2 characterize respectively the lattice response and the “universal behavior” of the charge carriers. This formula may be considered as generalizing the ideal Debye and non-ideal Cole-Cole expressions. Various dielectric dispersion relations are also obtained. An excellent fitting has been obtained for the dielectric constant spectrum in the 570–780 K temperature range. The ajustable parameters A and n of the “universal law” A(iω) 1− n are accurately determined and discussed. A shows a prominent peak in the paraelectric-ferroelectric transition region, while n exhibits a linear temperature dependence in both ferroelectric and paraelectric phases. At T c a minimum is observed for n.