Impedance Spectroscopy of the Relaxor Behaviour of P.M.N. and La-Doped P.M.N.-P.T. Compositions

Abstract

AC-impedance spectroscopic studies are carried out on lead magnesium niobate (PMN) and La-modified PMN-PT (lead magnesium niobate-lead titanate), Pb 1−x La x [(Mg 1+x/3 Nb 2−x/3 ) 0.65 Ti 0.35(1−x/4) ]O 3 , ceramic, with x = 0 to 0.07. Complex modulus and impedance plots exhibit two depressed semicircles and one depressed semicircle, respectively for PMN. This is attributed to the nearly same capacitance of grain and grain boundaries and higher grain boundary resistance than the grain resistance for PMN. Un-doped MPB composition of PMN-PT (0/65/35) and La-doped PMN-PT composition (7/65/35) also show similar complex modulus and impedance plots of PMN but in these samples the grain boundary capacitance was found to be higher than the grain capacitance. Cole-Cole plot shows two depressed semicircles, exhibiting grain and grain-boundary contributions in all the samples. La substitution is found to result in a broad peak in the frequency dependence of the imaginary part of the impedance and the modulus, for the temperature range 300–600°C. The maxima in the impedance and the modulus are found to shift towards higher frequencies with increasing temperature for all La-doped PMN-PT compositions, indicating a deviation from Debye behaviour. La-substitution results in broadening of the ϵ(T) peak, and a marked frequency dependence of ϵ max (T max ), similar to that observed in PMN. It is believed that La-substitution induces the relaxor-like permittivity characteristics in MPB composition of PMN-PT by constructing higher-resistive and lower-capacitive grains, and grain boundaries.