A Lead‐Free Ferroelectric 2D Dion–Jacobson Tin Iodide Perovskite

Abstract 2D hybrid organic–inorganic halide perovskites emerge as a new class of 2D semiconductors with the potential to combine excellent optoelectronic properties with symmetry‐enabled properties such as ferroelectricity. Although many lead‐based ferroelectric 2D halide perovskites are reported, there is yet to be a conclusive report of ferroelectricity in tin‐based 2D perovskites. Here, the structures and properties of a new series of 2D Dion–Jacobson (DJ) Sn perovskites: (4AMP)SnI₄, (4AMP)(MA)Sn₂I₇, and (4AMP)(FA)Sn₂I₇ (4AMP = 4‐(aminomethyl)piperidinium, MA = methylammonium, and FA = formamidinium), are reported. Structural characterization reveals that (4AMP)SnI₄ is polar with in‐plane spontaneous polarization whereas (4AMP)(MA)Sn₂I₇ and (4AMP)(FA)Sn₂I₇ are centrosymmetric. Further, (4AMP)SnI₄ displays second harmonic generation (SHG) and polarization‐electric field hysteresis measurements confirm it is ferroelectric with a spontaneous polarization of 10.0 µC cm⁻² at room temperature. (4AMP)SnI₄ transitions into a centrosymmetric structure above 367 K. As the first direct experimental observation of the spontaneous ferroelectric polarization of a Sn‐based 2D hybrid perovskite, this work opens up environmentally friendly 2D tin halide perovskites for ferroelectricity and other physical property studies.