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

The combination of 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 an experimentally prepared In₂Te₅ compound, and find ferroelectricity can be triggered in the In₂Te₅ monolayer by applying tensile strain to soften the transverse optical modes localized at the 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 to produce 2D scale-free FEs of this prototype by demonstrating the stability of M₂S₅ (M = Al, Ga, In, Tl) and M₂Se₅ (M = In, Tl) with intrinsic ferroelectricity. The FE polarizations stem from tailoring the chemical bonds of the chalcogen elements by using tensile strain or ionic radius differences, which gives the definite feature of elemental ferroelectricity but contained within compounds. We believe the M₂X₅ monolayers with the distinctive structural prototype represent unique elemental ferroelectric compounds and will promote development of 2D scale-free ferroelectricity once realized in experiments.