Macroscopic Polarization from Antiferrodistortive Cycloids in Ferroelastic SrTiO$_3$

TL;DR

This study reveals new mechanisms, including rotopolar and trilinear couplings, that induce macroscopic polarization in SrTiO3 twin walls, suggesting ways to engineer pyroelectric and piezoelectric properties in nonpolar ferroic materials.

Contribution

It identifies two novel mechanisms contributing to polarization in SrTiO3 twin walls, expanding understanding of ferroic behaviors beyond previous models.

Findings

Rotopolar coupling significantly contributes to polarization.

Macroscopic inversion symmetry can be broken in periodic twin structures.

New pathways for engineering pyroelectricity in nonpolar materials.

Abstract

Based on a first-principles based multiscale approach, we study the polarity (P) of ferroelastic twin walls in SrTiO$_3$. In addition to flexoelectricity, which was pointed out before, we identify two new mechanisms that crucially contribute to P: a direct "rotopolar" coupling to the gradients of the antiferrodistortive (AFD) oxygen tilts, and a trilinear coupling that is mediated by the antiferroelectric displacement of the Ti atoms. Remarkably, the rotopolar coupling presents a strong analogy to the mechanism that generates a spontaneous polarization in cycloidal magnets. We show how this similarity allows for a breakdown of macroscopic inversion symmetry (and therefore, a macroscopic polarization) in a periodic sequence of parallel twins. These results open new avenues towards engineering pyroelectricity or piezoelectricity in nominally nonpolar ferroic materials.