Ferroelectricity and Phase Transition in Sn₄X₄ (X = O, S, Se) Monolayers

The emergence of two-dimensional (2D) ferroelectric (FE) materials has provided new opportunities for fundamental research and the application of future nanodevices. However, as dimensionality reduced, the Curie temperature Tc and FE polarization below Tc tend to typically decrease due to the limitations of depolarization fields in 2D materials. Here, based on first-principles calculations and orbital analysis, we report a series of novel 2D material candidates, Sn4X4 (X = O, S, and Se), which are found to exhibit strong FE polarization, a low transition barrier, and a high Curie temperature. Interestingly, we found a new route to enhance ferroelectricity by orbital differences between cations and anions, as well as in-plane strain. The discovery of this comprehensive mechanism enriches our understanding of the causes of ferroelectricity and, to some extent, provides guidance for experimental synthesis and utilization.