Phase transitions in (Pb, La)Zr0.53Ti0.47O3 ferroelectric ceramics

Perovskite ferroelectric systems where A-site and/or B-site ions are substituted by elements of different valencies can exhibit diffuse phase transitions (DPTs) which are observed in materials often called relaxer ferroelectrics. The lanthanum lead zirconate titanate (PLZT) system shows some unusual and interesting electrical and optical properties, especially the composition ranges, in which a strong broadening of the permittivity-temperature dependence around the transition temperature occurs. These are examples of material with a diffuse phase transition where a nonlinear Curie-Weiss law could be observed for the low frequency dielectric constant. In this work PLZT compositions with a Zr/Ti ratio of 53/47 and variable lanthanum contents (0.5 - 14 at %) have been investigated. The dielectric permittivity (epsilon) as a function of the temperature (T) can be described by a quadratic law, epsilon(-1) = A + B(T-T-m)(2) above T-m (the temperature where epsilon has a maximum value) for PLZT x/53/47 (x greater than or equal to 8 at%) DPTs. Below that lanthanum content, a Curie-Weiss behaviour was obtained (normal phase transition). Ifa distribution of local Curie temperatures is assumed, the dielectric behaviour could be correlated with the lanthanum concentration. On the other hand, by analysing the grain size and the vacancy concentration, and its possible influence on the domain size morphology, the appearance of a DPT with the lanthanum concentration could be explained.