Alloying n‐Butylamine into CsPbBr3 To Give a Two‐Dimensional Bilayered Perovskite Ferroelectric Material

Cesium-lead halide perovskites (e.g. CsPbBr3 ) have gained attention because of their rich physical properties, but their bulk ferroelectricity remains unexplored. Herein, by alloying flexible organic cations into the cubic CsPbBr3 , we design the first cesium-based two-dimensional (2D) perovskite ferroelectric material with both inorganic alkali metal and organic cations, (C4 H9 NH3 )2 CsPb2 Br7 (1). Strikingly, 1 shows a high Curie temperature (Tc =412 K) above that of BaTiO3 (ca. 393 K) and notable spontaneous polarization (ca. 4.2 μC cm-2 ), triggered by not only the ordering of organic cations but also atomic displacement of inorganic Cs+ ions. To our knowledge, such a 2D bilayered Cs+ -based metal-halide perovskite ferroelectric material with inorganic and organic cations is unprecedented. 1 also shows photoelectric semiconducting behavior with large "on/off" ratios of photoconductivity (>103 ).