Phase coexistence region and dielectric properties of PZT ceramics

In PZT ceramics it is commonly observed that the tetragonal and the rhomboh edral phases may coexist around the morphotropic phase boundary (MPB). Some controversy still exists concerning the causes of the real occurrence of t he phase coexistence, the distribution of the coexisting phases and their c hemical and structural properties. In a previous work we found a relation b etween the width of the coexistence region and the grain size of the cerami c that could be explained by the statistical distribution model, as long as the elementary phase volumes were considered as the ferroelectric domains inside the grain. In the present work the structural parameters of the phas es and the dielectric permittivity of PZT in a compositional range covering the phase coexistence region are determined and analysed. It is observed t hat in both tetragonal and rhombohedral phases the permittivity increases a s the lattice distortion relative to the cubic symmetry decreases. The diel ectric permittivities of PZT inside the phase coexistence region were calcu lated considering that the phase coexistence corresponds to a statistical d istribution of phases with the same composition. This model provides dielec tric results consistent with the experimental ones. It was also shown that the maximum of the dielectric properties in the MPB does not result from th e phase coexistence, but it is a consequence of the approach to a minimum s tructure distortion. (C) 2000 Elsevier Science Ltd. All rights reserved.