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, <i>etc</i>. However, most of the hybrid ferroelastics exhibit phase transition points (<i>T</i>_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 (<i>E</i>,<i>E</i>)-[BPHD]ZnBr₄ (BPHD = <i>N</i>1,<i>N</i>6-bis(piperidine-1-yl) hexa-2,4-diene diamide), which experiences a 2/<i>mF</i>1̄ type paraelastic-ferroelastic phase transition at a near-room-temperature <i>T</i>_c of 285 K. It crystallizes in the monoclinic <i>P</i>2₁/<i>c</i> space group at 298 K, undergoing a change to the triclinic <i>P</i>1̄ space group in the low-temperature phase, with the number of molecules in asymmetric units doubling. Differential scanning calorimetry, variable-temperature electrical resistivity (<i>ρ</i>) and Raman spectrum tests confirm the occurrence of the phase transition, accompanied by a change in <i>ρ</i> of nearly two orders of magnitude, and reversible evolution of ferroelastic domains has been observed. In addition, (<i>E</i>,<i>E</i>)-[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.