Alkali Metal Substitution for Modulating Three-Dimensional Halide Double Perovskite Ferroelectric

Organic-inorganic hybrid halide double perovskites have garnered significant attention because of their broad utility in data storage, transducers, and signal processing. However, halide double perovskites incorporating alkali metal components with ferroelectricity are still relatively scarce. Herein, we utilize the alkali metal substitution strategy to fabricate a new homochiral K/Bi-based halide double perovskite ferroelectric, (S-3-hydroxypyrrolidinium)₂KBiBr₆ (1). Compound 1 forms a three-dimensional (3D) inorganic framework and features a lon topology characterized by a Schläfli symbol of 6⁶. The sp³ O atoms from the organic cations coordinate with the alkali metal K atoms, forming the twist dodecahedron and creating a large distortion in the inorganic framework. Structural analysis reveals 1 undergoes two phase transitions at 357 and 420 K. Symmetry-breaking with the 2F1 structural species leads to the emergence of ferroelasticity and ferroelectricity. 1 possesses clear ferroelectricity with a remnant polarization (P_r) value of 1.86 μC cm^-2 and a low coercive field (E_c) value of 1.4 kV cm^-1. Interestingly, the P_r value remains nearly unchanged, while the E_c value decreases by 17.6% after the alkali metal substitution. This work enlarges the family of hybrid bimetal halides containing alkali metals and provides new insights for exploring stable multifunctional ferroelectric materials.