Noncollinear ferrielectricity and morphotropic phase boundary in GeS monolayer

TL;DR

This study explores the structural phases of GeS monolayer, revealing a stable noncollinear ferrielectric phase induced by phonon frustration, and suggests a morphotropic phase boundary that could enable high piezoelectricity.

Contribution

It establishes a structural evolution diagram for GeS monolayer, identifying a stable ferrielectric phase and proposing the potential for a morphotropic phase boundary.

Findings

Stable noncollinear ferrielectric δ-phase identified

Phonon frustration induces ferrielectricity

Potential for giant piezoelectricity at phase boundary

Abstract

Two-dimensional polarity is intriguing but remains in the early stage. Here a structural evolution diagram is established for GeS monolayer, which leads a noncollinear ferrielectric $\delta$-phase energetically as stable as the ferroelectric $\alpha$-phase. Its ferrielectricity is induced by the phonon frustration, i.e., the competition between ferroelectric and antiferroelectric modes, providing more routes to tune its polarity. Besides its prominent properties like large band gap, large polarization, and high Curie temperature, more interestingly, the morphotropic phase boundary between $\alpha$- and $\delta$-phases is highly possible, which is crucial to obtain giant piezoelectricity for lead-free applications.