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 T_c and FE polarization below T_c 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.