Ferroelectricity in crystals with non-polar point groups

TL;DR

This paper challenges the traditional view that ferroelectricity only occurs in polar point groups by showing that non-polar groups can also exhibit switchable polarization due to boundary effects and long ion displacements, expanding the scope of ferroelectric materials.

Contribution

It provides first-principles evidence that non-polar point groups can host ferroelectricity, contradicting classical symmetry-based rules and explaining previously unclarified phenomena.

Findings

Ferroelectric polarization can occur in some non-polar point groups.

Boundaries can break symmetry and induce ferroelectricity.

Switchable polarization is possible in systems with weak bonds.

Abstract

Ferroelectric crystals must adopt one of the 10 polar point groups according to the Neumann's principle. In this paper we propose that this conclusion is based on perfect bulk crystals without taking the boundaries into account, and we show first-principles evidence that ferroelectric polarizations may also be formed in some non-polar point groups as the edges generally break the crystal symmetry, which may even maintain at macroscale. They can be switchable in some systems with weak van der Waals bondings or covalent-like ionic bondings where long ion displacements with moderate barriers are possible. Such unconventional ferroelectricity violates the Neumann's principle and Abrahams' conditions respectively due to the boundaries and long ion displacements, which may explain some unclarified phenomena reported previously as well as significantly expand the scope of ferroelectrics.