Generalization of piezoelectric quantum anomalous Hall insulator based on monolayer Fe₂I₂: a first-principles study

To easily synthesize a piezoelectric quantum anomalous Hall insulator (PQAHI), the Janus monolayer Fe₂IBr (FeI_0.5Br_0.5) as a representative PQAHI, is generalized to monolayer FeI_1-Br (x = 0.25 and 0.75) with α and β phases. By first-principles calculations, it is proved that monolayer FeI_1-Br (x = 0.25 and 0.75) are dynamically, mechanically and thermally stable. They are excellent room-temperature PQAHIs with high Curie temperatures, sizable gaps and high Chern number (C = 2). Because the considered crystal structures of α and β phases possess M and M mirror symmetries, the topological properties of monolayer FeI_1-Br (x = 0.25 and 0.75) are maintained. Namely, if the constructed structures have M and M mirror symmetries, the mixing ratio of Br and I atoms can be generalized for other proportions. It is also found that different crystal phases have important effects on the out-of-plane piezoelectric response, and the piezoelectric strain coefficient, d₃₂, of the β phase is higher than or comparable with those of other known two-dimensional (2D) materials. To further confirm this idea, the physical and chemical properties of monolayer LiFeSe_0.75S_0.25 with α and β phases, as a generalization of PQAHI LiFeSe_0.5S_0.5, is investigated, as it has a similar electronic structure, magnetic and topological properties as LiFeSe_0.5S_0.5. Our work provides a practical guide to achieve PQAHIs experimentally, and the combination of piezoelectricity, topological and ferromagnetic (FM) orders makes Fe₂I₂-based monolayers a potential platform for multi-functional spintronics and piezoelectric electronics.