Three-dimensional perchlorate-based alkali metal hybrid perovskite molecular ferroelastic crystals

Hybrid perovskites possessing structural diversity and solution processability have been extensively studied in numerous application scenarios and have attracted significant interest in the design of high-performance molecular ferroelectric and ferroelastic materials. However, reports on the construction of three-dimensional (3D) perchlorate-based alkali metal hybrid perovskite molecular ferroelastics are scarce. Herein, dual-site substitution was implemented on the 3D non-perovskite network (MDABCO)K(ClO₄)₃ (MDABCO = N-methyl-N'-diazabicyclo[2.2.2]octonium) to achieve a series of 3D perchlorate-based alkali metal perovskite ferroelastics (FMDABCO)M(ClO₄)₃ (FMDABCO = N-fluoromethyl-N'-diazabicyclo[2.2.2]octonium, M = K, Rb, and Cs). The H/F substitution on the organic motif of (MDABCO)K(ClO₄)₃ facilitates a significant structural transformation leading to a perovskite stacking of (FMDABCO)K(ClO₄)₃ accompanied by high-temperature structural phase transition and ferroelasticity. Through further substitutions on the alkali metals according to the fitted tolerance factor, (FMDABCO)Rb(ClO₄)₃ and (FMDABCO)Cs(ClO₄)₃ can not only maintain the 3D perovskite framework but also exhibit ferroelastic phase transitions at a higher temperature. Besides, (FMDABCO)Cs(ClO₄)₃ shows dual types of ferroelastic domain evolution with the Aizu notations of mmmF2/m and m3̄mFmmm. This work offers great inspiration for the design of ferroelastic materials through rational chemical strategies.