Prediction of scale-free ferroelectricity in elemental ferroelectric compound of M2X5 with distinctive structural prototype

Abstract

The combination between two-dimensional (2D) ferroelectrics (FEs) without dangling bonds and scale-free ferroelectricity with sharp domain walls provides unexpected opportunities for achieving reliable and high-density FE devices. Here we propose a distinctive structural prototype of displacive FEs from experimentally prepared In₂Te₅ compound, and find ferroelectricity can be triggered in In₂Te₅ monolayer by applying tensile strain to soften the transverse optical modes localized at planar-coordinated Te chains, where the shielding effect of wrinkled spacer chains formed by vertex-sharing In-Te tetragons leads to scale-free ferroelectricity with independently switchable FE polarizations. Furthermore, elemental substitutions with experimental feasibility enable us enriching 2D scale-free FEs of this prototype by revealing the stability of M₂S₅ (M=Al, Ga, In, Tl) and M₂Se₅ (M= In, Tl) with intrinsic ferroelectricity. The FE polarizations stem from tailoring chemical bonds of chalcogen elements by using tensile strains or ionic radius differences, which present definite feature of elemental ferroelectricity but are immersed in compounds. We believe the M₂X₅ monolayers with distinctive structural prototype represent the unique elemental ferroelectric compounds and will promote development of 2D scale-free ferroelectricity once realized in experiments.