Phase coexistence region and dielectric properties of PZT ceramics

Abstract

In PZT ceramics it is commonly observed that the tetragonal and the rhombohedral phases may coexist around the morphotropic phase boundary (MPB). Some controversy still exists concerning the causes of the real occurrence of the phase coexistence, the distribution of the coexisting phases and their chemical and structural properties. In a previous work we found a relation between the width of the coexistence region and the grain size of the ceramic 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 phases and the dielectric permittivity of PZT in a compositional range covering the phase coexistence region are determined and analysed. It is observed that in both tetragonal and rhombohedral phases the permittivity increases as the lattice distortion relative to the cubic symmetry decreases. The dielectric permittivities of PZT inside the phase coexistence region were calculated considering that the phase coexistence corresponds to a statistical distribution of phases with the same composition. This model provides dielectric results consistent with the experimental ones. It was also shown that the maximum of the dielectric properties in the MPB does not result from the phase coexistence, but it is a consequence of the approach to a minimum structure distortion.