Review of the recent x-ray and neutron powder diffraction studies on lead zirconate titanate

Abstract

The issues related to the structure refinement of Pb(ZrxTi1-x)O3 (PZT) solid solutions are discussed. Particular attention is paid on the modelling of the co-existing phases in the vicinity of the morphotropic phase boundary (MPB), where local symmetry is often significantly lower than the average symmetry. According to recent studies, two-phase co-existence in the vicinity of the MPB is a thermodynamical necessity. Significantly different structural models for PZT with $x$ in the vicinity of the MPB have recently been published. Structural models, based on x-ray, neutron and electron diffraction studies, are reviewed and two essentially different approaches were identified: (i) a method where space group symmetry was decreased untill the features due to the local distortions were 'modelled' and (ii) a method where the highest space group symmetry compatible with the powder diffraction data was used together with a model for a local disorder. Related to the method (ii), the essential features of a model taking the hkl-dependent line broadening into account are summarized. The underlying theme is to consider how local regions, with symmetry lower than the average symmetry, should be modelled. Method (i) often introduces unjustified structural parameters and should be avoided. By studying the temperature and composition dependence of the structural parameters, including line shape parameters and phase fractions in the case of the two phase samples, one can further test the proposed models. The significance of the local structure for electrical properties (switchable polarization and electrical conductivity) is discussed by considering the local Pb ion displacements and their dependence on the neighbouring B-cation configuration.