Anisotropic dielectric function in polar nano-regions of relaxor ferroelectrics

TL;DR

This paper models the infrared reflectivity spectra of relaxor ferroelectrics by considering local anisotropy and inhomogeneous depolarization fields, successfully explaining spectral features through an effective medium approximation.

Contribution

It introduces a simple effective medium model to account for nanoscale polarization inhomogeneity and anisotropy in relaxor ferroelectrics, aiding spectral analysis.

Findings

Reproduces principal features of room temperature reflectivity of PMN

Identifies a geometrical resonance linked to nanoscale polarization

Provides a method to determine polar mode frequencies split by inhomogeneity

Abstract

The paper suggests to treat the infrared reflectivity spectra of single crystal perovskite relaxors as fine-grained ferroelectric ceramics: locally frozen polarization makes the dielectric function strongly anisotropic in the phonon frequency range and the random orientation of the polarization at nano-scopic scale requires to take into account the inhomogeneous depolarization field. Employing a simple effective medium approximation (Bruggeman symmetrical formula) to dielectric function describing the polar optic modes as damped harmonic oscillators turns out to be sufficient for reproducing all principal features of room temperature reflectivity of PMN. One of the reflectivity bands is identified as a geometrical resonance entirely related to the nanoscale polarization inhomogeneity. The approach provides a general guide for systematic determination of the polar mode frequencies split by the inhomogeneous polarization at nanometer scale.