Mode-coupling and polar nanoregions in the relaxor ferroelectric Pb(Mg1/3Nb2/3)O3

TL;DR

This study analyzes phonon lineshapes in the relaxor ferroelectric PMN using neutron scattering, revealing mode coupling and the origin of polar nanoregions from soft mode condensation.

Contribution

It introduces a quantitative mode-coupling model explaining phonon behavior and polar nanoregion formation in PMN.

Findings

Mode coupling explains phonon lineshape differences.

Polar nanoregions result from soft mode condensation.

The model accounts for temperature and wave vector dependence.

Abstract

We present a quantitative analysis of the phonon lineshapes obtained by neutron inelastic scattering methods in the relaxor ferroelectric Pb(Mg1/3Nb2/3)O3 (PMN). Differences in the shapes and apparent positions of the transverse acoustic (TA) and transverse optic (TO) phonon peaks measured in the (300) and (200) Brillouin zones at 690 K are well described by a simple model that couples the TA and soft TO modes in which the primary parameter is the wave vector and temperature-dependent TO linewidth Gamma(q,T). This mode-coupling picture provides a natural explanation for the uniform displacements of the polar nanoregions, discovered by Hirota et al., as the PNR result from the condensation of a soft TO mode that also contains a large acoustic component.