Manipulating Phase Transition and Ferroelectricity via Enhancing the Stereochemical Activity of Lone-Pair Electrons

The concept of lone-pair electrons (LPEs) has a long history of about one century in chemistry. Previous studies have revealed a vital role of LPEs in determining the physical and chemical properties of many substances, especially post-transition metal compounds. Bismuth oxychalcogenide Bi₂O₂X (X = S, Se, and Te) ultrathin films, particularly Bi₂O₂Se, are attracting intense research interest due to their extremely outstanding semiconducting properties and ferroelectricity. However, the definite role of LPEs in determining the properties such as structural distortions and ferroelectric properties remains unclear in this prominent system. More generally, it remains largely unknown to what extent the stereochemical activity of LPEs can be enhanced to induce and modulate the properties of materials containing LPEs, such as phase transition and ferroelectric properties. Here, considering Bi₂O₂X as a typical example, we demonstrate that ferroelectricity can be induced and modulated by enhancing the activity of LPEs via lattice expansion. We find that the enhanced activity of LPEs plays a crucial role in the structural distortions and tetragonal-to-orthorhombic phase transition in Bi₂O₂X. Lattice expansion favors the expression of LPEs and thus enhances the stereochemical activity, giving rise to ferroelectricity and its tunability in Bi₂O₂Se ultrathin films. We propose a feasible approach to achieve lattice expansion in realistic experimental conditions, allowing induced ferroelectricity in the otherwise nonferroelectric materials. Our results suggest that enhancing the stereochemical activity of LPEs provides a promising approach to seek intriguing ferroelectric materials in post-transition metal compounds.