Anisotropic Atomic Displacement Induced Thermosalience in Hybrid Zinc Halide Crystals

Thermosalient (TS) crystals can effectively convert thermal energy into mechanical work, holding significant potential for use in advanced actuators and sensors. Although current research mainly focuses on organic crystals and a few metal complexes, discovering TS behavior in other material systems remains challenging. In this work, we present the synthesis and structures of two hybrid organic-inorganic zinc halides, [TAP][ZnCl₄] and [TAP][ZnBr₄] (TAP = 2,4,6-triaminopyrimidinium), which exhibit thermal jumping displacements larger than 20 times their size. Variable temperature and microfocused single crystal X-ray diffraction analysis shows that the TS behavior is likely to arise from the thermally induced breaking of the anisotropic supramolecular structure, where orthogonal atomic displacements could distort the planar hydrogen-bonding network. At a threshold temperature, the structure would partially disassociate, releasing the accumulated internal stress, hence triggering the TS behavior. The TS temperature can be adjusted by about 40 K by replacing Cl^- with Br^- due to the increased atomic mass. Our findings open a new pathway for designing TS crystals in the abundant and structurally diverse hybrid metal halides and other organic-inorganic solids.