An organic–inorganic hybrid ferroelastic with a near-room-temperature phase transition†

Organic–inorganic hybrid metal halides (OIMHs) with ferroelastic phase transition properties have recently attracted great attention due to their widespread application prospects in the fields of energy storage, sensors, switches, etc. However, most of the hybrid ferroelastics exhibit phase transition points (T_c) far beyond room temperature, which may limit their applications in mechanical switches and energy storage for daily working requirements. Herein, we synthesized a new zinc halide OIMH ferroelastic (E,E)-[BPHD]ZnBr₄ (BPHD = N1,N6-bis(piperidine-1-yl) hexa-2,4-diene diamide), which experiences a 2/mF type paraelastic–ferroelastic phase transition at a near-room-temperature T_c of 285 K. It crystallizes in the monoclinic P2₁/c space group at 298 K, undergoing a change to the triclinic P space group in the low-temperature phase, with the number of molecules in asymmetric units doubling. Differential scanning calorimetry, variable-temperature electrical resistivity (ρ) and Raman spectrum tests confirm the occurrence of the phase transition, accompanied by a change in ρ of nearly two orders of magnitude, and reversible evolution of ferroelastic domains has been observed. In addition, (E,E)-[BPHD]ZnBr₄ also exhibits remarkable compliance and softness within the category of hybrid crystals. This work will inspire further exploration of new hybrid ferroelastic materials with near-room-temperature phase transitions.