Breaking of macroscopic centric symmetry in paraelectric phases of ferroelectric materials and implications for flexoelectricity

TL;DR

This paper investigates how inhomogeneity-induced symmetry breaking in paraelectric phases of centrosymmetric ferroelectric materials leads to unexpectedly large flexoelectric responses, challenging existing theoretical predictions.

Contribution

It reveals that macroscopic symmetry breaking due to inhomogeneity explains the large flexoelectric coefficients observed experimentally.

Findings

Large flexoelectric response linked to symmetry breaking

Inhomogeneity from high-temperature processing causes macroscopic symmetry loss

Potential resolution of the flexoelectric coefficient controversy

Abstract

A centrosymmetric stress cannot induce a polar response in centric materials, piezoelectricity is, for example, possible only in non-centrosymmetric structures. An exception is meta-materials with shape asymmetry, which may be polarized by stress even when the material is centric. In this case the mechanism is flexoelectricity, which relates polarization to a strain gradient. The flexoelectric response scales inversely with size, thus a large effect is expected in nanoscale materials. Recent experiments in polycrystalline, centrosymmetric perovskites [e.g., (Ba,Sr)TiO3] have indicated values of flexoelectric coefficients that are orders of magnitude higher than theoretically predicted, promising practical applications based on bulk materials. We show that materials with unexpectedly large flexoelectric response exhibit breaking of the macroscopic centric symmetry through inhomogeneity induced by the high temperature processing. The emerging electro-mechanical coupling is significant and may help to resolve the controversy surrounding the large apparent flexoelectric coefficients in this class of materials.