Atomistic determination of flexoelectric properties of crystalline dielectrics

TL;DR

This paper uses a microscopic lattice dynamics approach to estimate the flexoelectric tensor in various crystalline dielectrics, comparing results with existing data and examining empirical scaling laws.

Contribution

It provides new atomistic estimates of the flexoelectric tensor for multiple crystal types and critically evaluates existing experimental and theoretical values.

Findings

Estimated flexoelectric tensors for cubic ionic crystals, perovskites, and semiconductors.

Identified discrepancies between different experimental estimates.

Re-assessed the validity of empirical scaling relationships for flexoelectric coefficients.

Abstract

Upon application of a uniform strain, internal sub-lattice shifts within the unit cell of a non-centrosymmetric dielectric crystal result in the appearance of a net dipole moment: a phenomenon well known as piezoelectricity. A macroscopic strain gradient on the other hand can induce polarization in dielectrics of any crystal structure, even those which possess a centrosymmetric lattice. This phenomenon, called flexoelectricity, has both bulk and surface contributions: the strength of the bulk contribution can be characterized by means of a material property tensor called the bulk flexoelectric tensor. Several recent studies suggest that strain-gradient induced polarization may be responsible for a variety of interesting and anomalous electromechanical phenomena in materials including electromechanical coupling effects in non-uniformly strained nanostructures, dead layer effects in nanocapacitor systems, and giant piezoelectricity in perovskite nanostructures among others. In this work, adopting a lattice dynamics based microscopic approach we provide estimates of the flexoelectric tensor for certain cubic ionic crystals, perovskite dielectrics, III-V and II-VI semiconductors. We compare our estimates with experimental and theoretical values wherever available, address the discrepancy that exists between different experimental estimates and also re-visit the validity of an existing empirical scaling relationship for the magnitude of flexoelectric coefficients in terms of material parameters.