A statistical model approximation for perovskite solid-solutions: a Raman study of lead-zirconate-titanate single crystal

TL;DR

This study uses Raman spectroscopy on lead-zirconate-titanate single crystals to reveal complex local ordering and phase transition mechanisms that deviate from simple soft-mode theory, highlighting the influence of defects and compositional disorder.

Contribution

It introduces a statistical model explaining local order variations and phase transitions in PZT, contrasting with traditional soft-mode descriptions.

Findings

Mode splitting indicates local regions with different order.

Defects and compositional disorder influence phase transition.

Complex local order near the morphotropic phase boundary.

Abstract

Lead titanate (PbTiO3) is a classical example of a ferroelectric perovskite oxide illustrating a displacive phase transition accompanied by a softening of a symmetry-breaking mode. The underlying assumption justifying the soft-mode theory is that the crystal is macroscopically sufficiently uniform so that a meaningful free energy function can be formed. In contrast to PbTiO3, experimental studies show that the phase transition behaviour of lead-zirconate-titanate solid solution (PZT) is far more subtle. Most of the studies on the PZT system have been dedicated to ceramic or powder samples, in which case an unambiguous soft-mode study is not possible, as modes with different symmetries appear together. Our Raman scattering study on titanium-rich PZT single crystal shows that the phase transitions in PZT cannot be described by a simple soft-mode theory. In strong contrast to PbTiO3, splitting of transverse E-symmetry modes reveals that there are different locally-ordered regions. The role of crystal defects, random distribution of Ti and Zr at the B-cation site and Pb ions shifted away from their ideal positions, dictates the phase transition mechanism. A statistical model explaining the observed peak splitting and phase transformation to a complex state with spatially varying local order in the vicinity of the morphotropic phase boundary is given.