Quasi-Elastic Scattering, Random Fields and phonon-coupling effects in PbMg1/3Nb2/3O3

TL;DR

This study investigates the low-energy vibrational spectrum of the relaxor ferroelectric PbMg1/3Nb2/3O3 using neutron scattering, revealing phonon coupling, quasi-elastic scattering near the Burns temperature, and implications for random field models.

Contribution

The paper presents a model for phonon coupling in PbMg1/3Nb2/3O3 and analyzes the origin of quasi-elastic scattering in relation to the Burns temperature and random fields.

Findings

Transverse acoustic and optic phonons are strongly coupled.

The lowest optic phonon remains underdamped.

Quasi-elastic scattering peaks near the Burns temperature.

Abstract

The low-energy part of the vibration spectrum in PbMg$_{1/3}$Nb$_{2/3}$O$_3$ (PMN) relaxor ferroelectric has been studied by neutron scattering above and below the Burns temperature, T$_d$. The transverse acoustic and the lowest transverse optic phonons are strongly coupled and we have obtained a model for this coupling. We observe that the lowest optic branch is always underdamped. A resolution-limited central peak and quasi-elastic scattering appear in the vicinity of the Burns temperature. It is shown that it is unlikely that the quasi-elastic scattering originates from the combined effects of coupling between TA and TO phonons with an increase of the damping of the TO phonon below T$_d$. The quasi-elastic scattering has a peak as a function of temperature close to the peak in the dielectric constant while the intensity of the central peak scattering increases strongly below this temperature. These results are discussed in terms of a random field model for relaxors.