Ferroelectricity induced by lone pair electron effect in halide perovskite monolayers

Two-dimensional (2D) ferroelectricity has attracted considerable interest since its demonstration in van der Waals monolayers. Research on 2D ferroelectricity has so far focused on van der Waals materials, and whether some of the intrinsic ferroelectric mechanisms found in perovskite bulks can be retained to the monolayer limit remains an issue to be solved. Here, we demonstrate that intrinsic ferroelectricity caused by the lone pair electron effect can exist in the inorganic halide perovskite monolayers. This ferroelectricity occurs in halide perovskite monolayers with smaller anions, involving ${\mathrm{Cs}}_{2}{\mathrm{GeF}}_{4}, {\mathrm{Cs}}_{2}{\mathrm{SnF}}_{4}$, and ${\mathrm{Cs}}_{2}{\mathrm{SnCl}}_{4}$. The first two are direct band-gap semiconductors while the latter is an indirect band-gap semiconductor. The octahedral tilt mode also appears in the ground-state structures of these 2D ferroelectrics. The ferroelectric monolayers containing ${\mathrm{F}}^{\ensuremath{-}}$ ions have relatively greater ferroelectric polarization, but also have a higher-energy barrier for ferroelectric switching. This work helps to design 2D ferroelectricity beyond van der Waals materials.