Dynamics and Structure of PMN and PZN

TL;DR

This review discusses recent neutron and x-ray scattering studies of lead-oxide relaxor systems, revealing complex phase behaviors including surface-limited rhombohedral distortions and a bulk phase X with nearly cubic symmetry.

Contribution

It introduces a new understanding of the structural phases in relaxor ferroelectrics, highlighting the existence of a bulk phase X distinct from surface distortions.

Findings

Rhombohedral-monoclinic-tetragonal phase sequence with increasing PbTiO3.

Surface rhombohedral distortions only in outer crystal layers.

Presence of a bulk phase X with nearly cubic symmetry.

Abstract

A review is given of recent neutron and x-ray scattering studies of the lead-oxide perovskite relaxor systems Pb(Zn1/3Nb2/3)O3-xPbTiO3 and Pb(Mg1/3Nb2/3)O3-xPbTiO3. X-ray measurements by Noheda et al. have established that these two systems exhibit nearly identical phase diagrams in which a rhombohedral-monoclinic-tetragonal structural sequence takes place with increasing PbTiO3 concentration. Recent high-energy x-ray and neutron measurements on single crystals of PZN and PMN-10PT, however, show that the rhombohedral distortions occur only in the outermost 20 - 40 microns, while the bulk of each crystal transforms into a new phase X, which has a nearly cubic unit cell. This situation is very similar to the structural behavior of pure PMN at Tc = 220 K. A simple model has been suggested that correlates phase X to the unique atomic displacements of the polar nanoregions, which are created at the Burns temperature.