Prediction of intrinsic ferroelectricity and large piezoelectricity in monolayer arsenic chalcogenides

TL;DR

This paper predicts that monolayer arsenic chalcogenides are intrinsic ferroelectrics with strong in-plane piezoelectricity, supported by theoretical calculations and phonon mode analysis, expanding the family of 2D ferroelectric materials.

Contribution

It demonstrates that arsenic chalcogenides in the Pmn2$_1$ phase are intrinsic ferroelectrics with large piezoelectricity, and identifies potential new metastable polymorphs with functional properties.

Findings

Pmn2$_1$ phase is an intrinsic ferroelectric with reasonable energy barriers.

Soft phonon modes induce phase transformation from high-symmetry to ferroelectric phase.

Discovered metastable polar phase with sizable piezoelectricity.

Abstract

Two-dimensional materials that exhibit spontaneous electric polarization are of notable interest for functional materials. However, despite many two-dimensional polar materials are predicted in theory, the number of experimentally confirmed two-dimensional ferroelectrics are still far less than bulk ferroelectrics. We provide strong evidence that the Pmn2$_1$ phase of arsenic chalcogenides As$_2$X$_3$ (X=S, Se, and Te), which include the recently isolated monolayer orpiment, are intrinsic ferroelectrics and demonstrate strong in-plane piezoelectricity. We found the calculated energy barriers for collectively reversing the electric polarization or moving a 180$^\circ$ domain wall are reasonable compared to previously reported ferroelectrics. We propose a high-symmetry structure (with Pmmn space group) transforms into the ferroelectric Pmn2$_1$ phase by a soft B$_{2u}$ phonon mode. By studying other soft modes of the high-symmetry Pmmn structure, we identify several undiscovered metastable polymorphs, including a polar phase (with a P2$_1$ space group) with sizable piezoelectricity.