Phase coexistence in PZT ceramics

In lead zirconate titanate ceramics, the low temperature tetragonal and rho mbohedral ferroelectric phases coexist for compositions near the morphotrop ic phase boundary (MPB), and the explanation of this fact remains a controv ersial issue. A model of a statistical distribution of the phases can descr ibe the coexistence of the phases, assuming that it results from the therma l fluctuation quenching during the cooling from the paraelectric to the fer roelectric phases. In this model the width of the phase coexistence region (Delta x) is related to the grain size, G, and it should be observed that D elta x proportional to G(-3). In this work we checked the applicability of this relation on PZT powers and ceramics. Chemical equilibrium is not attai ned in calcined powders at 900, 1000 and 1100 degrees C, even after long ti me heat treatments, and therefore we considered the results from sintered s amples. In these samples, instead of the Delta x proportional to G(-3) pred icted relation, we found that Delta x proportional to G(-3/2). To explain t his relation, we assumed that the elementary volume in the model is the fer roelectric domain instead of the grain of the material, based in a paraboli c relation between the domain size and the grain size. Calculation of the e lementary volumes using thermodynamic data is in good agreement with the vo lumes of the domains. (C) 1999 Elsevier Science Limited. All rights reserve d.