NH4+/K+-substitution-induced C–F–K coordination bonds for designing the highest-temperature hybrid halide double perovskite ferroelastic

Ferroelastic materials with switchable spontaneous strain possess widely potential applications in the field of energy and information conversion. Recently, organic-inorganic hybrid halide double perovskites (OIHHDPs) have become a charming new platform for developing various functional materials, such as ferroelectrics, fluorescence and X‑ray detection. Nevertheless, OIHHDP ferroelastic materials, especially high-temperature ones, are rare. Herein, we initially synthesized an OIHHDP ferroelastic, (2,2-difluoroethanamine)2[(NH4)InCl6] (1), which possesses a ferroelastic phase transition at 407 K. Moreover, thanks to the flexible B-site for OIHHDPs, we replaced the NH4+ ions within [(NH4)InCl6]n2n– formworks with K+ ions, which endows with coordination bonds between 2,2-difluoroethanamine organic cations and [KInCl6]n2n– formworks. Due to the existence of coordination bonds, the phase transition temperature of (2,2-difluoroethanamine)2[KInCl6] (2) can reach 458 K. As far as we know, this value is the highest reported in OIHHDP ferroelastics. This work offers inspiration for the design of high-temperature OIHHDP phase transition materials including ferroelectrics and ferroelastics.