H/F Substitution Strategy in 0D Organic–Inorganic Hybrid Metal Cyanides Designing Ferroelastic Phase-Transition Materials

The three-dimensional (3D) cyano-based organic-inorganic hybrid double perovskites (CHOIPs) have shown great application potential in the field of multifunctional materials due to their unique ferroelasticity, ferroelectricity, and dielectric-switching properties. However, the inherent spatial limitations of the 3D rigid framework greatly restrict the types and sizes of organic cations that can be introduced, and they limit the further regulation and optimization of material properties. A zero-dimensional (0D) CHOIPs ferroelastic material, (MA)₃[Fe(CN)₆] (MA = CH₃NH₃⁺), was reported to break through this limitation, providing an idea for the development of 0D CHOIPs phase-transition materials, but reports on 0D CHOIPs ferroelastic phase-transition materials are still scarce. In this study, we adopted the H/F substitution strategy and synthesized two 0D organic-inorganic hybrid metal cyanide materials, (EA)₃[Fe(CN)₆] (EA = CH₃CH₂NH₃⁺, ethylamine) and (DFEA)₃[Fe(CN)₆] (DFEA = F₂CHCH₂NH₃⁺). The results show that the phase transition of (DFEA)₃[Fe(CN)₆] originates from the coupling mechanism of the ordered-disordered transformation of organic cations and the coordinated rotation of inorganic [Fe(CN)₆]^3- octahedrons. By introducing fluorine atoms, the phase-transition temperature is significantly higher than that of EA⁺. This work provides ideas for the structural design and material-property optimization of 0D organic-inorganic hybrid metal cyanides.