Piezoelectric mimicry of flexoelectricity

TL;DR

This paper demonstrates that asymmetric distribution of piezoelectricity within a material can mimic true flexoelectric effects, explaining the observed giant flexoelectricity without requiring intrinsic flexoelectricity.

Contribution

It reveals how non-uniform piezoelectricity can produce flexoelectric-like signals, challenging the interpretation of giant flexoelectricity in experiments.

Findings

Asymmetric piezoelectricity distribution can generate flexoelectric-like polarization.

Giant effective flexoelectric coefficients can arise from tiny piezoelectric gradients.

This mechanism explains the discrepancy between observed and predicted flexoelectricity.

Abstract

The origin of "giant" flexoelectricity, orders of magnitude larger than theoretically predicted, yet frequently observed, is under intense scrutiny. There is mounting evidence correlating giant flexoelectric-like effects with parasitic piezoelectricity, but it is not clear how piezoelectricity (polarization generated by strain) manages to imitate flexoelectricity (polarization generated by strain gradient) in typical beam-bending experiments, since in a bent beam the net strain is zero. In addition, and contrary to flexoelectricity, piezoelectricity changes sign under space inversion, and this criterion should be able to distinguish the two effects and yet "giant" flexoelectricity is insensitive to space inversion, seemingly contradicting a piezoelectric origin. Here we show that, if a piezoelectric material has its piezoelectric coefficient be asymmetrically distributed across the sample, it will generate a bending-induced polarization impossible to distinguish from true flexoelectricity even by inverting the sample. The effective flexoelectric coefficient caused by piezoelectricity is functionally identical to, and often larger than, intrinsic flexoelectricity: the calculations show that, for standard perovskite ferroelectrics, even a tiny gradient of piezoelectricity (1% variation of piezoelectric coefficient across 1 mm) is sufficient to yield a giant effective flexoelectric coefficient of 1 $\mu$C/m, three orders of magnitude larger than the intrinsic expectation value.