Switchable coordination bonds in 3D cyano-bridged perovskite ferroelastics: achieving the largest leap of symmetry breaking and enhanced dielectric switching performance

Abstract

Three-dimensional (3D) cyano-bridged perovskites, with a larger framework than their metal–halide counterparts, have shown great promise in the design of molecular ferroelastic, ferroelectric, and even multiferroic materials. However, the relatively weak interaction between the organic cations and the rigid cyano-bridged framework usually makes them exhibit a low phase transition temperature (T_c) and inadequate performance. In this study, we used the parent compounds [C₃H₆NH₂]₂[MFe(CN)₆] (C₃H₆NH₂ = azetidinium, M = K, Rb or Cs) as starting points to design a variety of 3D cyano-bridged perovskite ferroelastic materials [C₃H₅FNH₂]₂[MFe(CN)₆] (C₃H₅FNH₂ = 3-fluoroazetidinium) through the H/F substitution strategy. Notably, in comparison with [C₃H₆NH₂]₂[MFe(CN)₆], the strongest electronegative fluorine atom of [C₃H₅FNH₂]⁺ cations forms a C–F–M coordination bond with alkali metal ions, which greatly enhances the interaction between the cation and the framework, thus increasing T_c. The transition from order to disorder in [C₃H₅FNH₂]⁺ cations, coupled with the generation/breakage of switchable C–F–M coordination bonds, synergistically results in the largest leap of symmetry breaking with an Aizu notation of mmF and is responsible for significantly enhancing the dielectric switching performance. This study offers an effective approach for discovering novel molecular ferroelastics with excellent switchable physical properties.