High-pressure neutron study of the morphotropic PZT: phase transitions in a two-phase system

TL;DR

This study investigates how high pressure affects phase transitions in Zr-rich PZT ceramics, revealing pressure-induced phase stability changes, cation displacements, and the metastability of the monoclinic phase, which influence piezoelectric properties.

Contribution

It provides new insights into pressure-induced phase behavior and cation displacements in Zr-rich PZT near the morphotropic boundary, highlighting the metastability of the monoclinic phase.

Findings

Pressure favors the R3c phase over the Cm phase.

The Cm to R3c phase transition is discontinuous.

High pressure induces large displacements of Zr and Ti cations.

Abstract

In piezoelectric ceramics the changes in the phase stabilities versus stress and temperature in the vicinity of the phase boundary play a central role. The present study was dedicated to the classical piezoelectric, lead-zirconate-titanate (PZT) ceramic with composition Pb(Zr$_{0.54}$Ti$_{0.46}$)O$_3$ at the Zr-rich side of the morphotropic phase boundary at which both intrinsic and extrinsic contributions to piezoelectricity are significant. The pressure-induced changes in this two-phase (rhombohedral $R3c$+monoclinic $Cm$ at room temperature and $R3c+P4mm$ above 1 GPa pressures) system were studied by high-pressure neutron powder diffraction technique. The experiments show that applying pressure favors the $R3c$ phase, whereas the $Cm$ phase transforms continuously to the $P4mm$, which is favored at elevated temperatures due to the competing entropy term. The $Cm\rightarrow R3c$ phase transformation is discontinuous. The transformation contributes to the extrinsic piezoelectricity. An important contribution to the intrinsic piezoelectricity was revealed: a large displacement of the $B$ cations (Zr and Ti) with respect to the oxygen anions is induced by pressure. Above 600 K a phase transition to a cubic phase took place. Balance between the competing terms dictates the curvature of the phase boundary. After high-pressure experiments the amount of rhombohedral phase was larger than initially, suggesting that on the Zr-rich side of the phase boundary the monoclinic phase is metastable.