Halogen Effects on Thermosalience in Hybrid Cadmium Halide Crystals

Thermosalient (TS) crystals are a class of dynamic crystalline materials with exceptional thermally responsive mechanical effects and efficient thermomechanical energy conversion capability. Hybrid metal halides serve as excellent carriers for TS crystals owing to their highly tunable structures. In this work, a series of chiral hybrid cadmium halide crystals (R/S-3OHPD)CdX3 (3OHPD = 3-hydroxypiperidinium; X = Cl, Br, I) were synthesized, and their phase transitions, crystal structures, TS behaviors, and intrinsic mechanisms were systematically investigated. The halogen substitution effectively regulates the lattice dimensions, rigidity, phase transition type, and TS performance of the crystals. Specifically, (R-3OHPD)CdCl3 undergoes an irreversible solid–solid phase transition accompanied by explosive fragmentation. Within a certain size range, its TS behavior is significantly enhanced with decreasing crystal size; that is, smaller crystals are more prone to explosive cracking due to stress concentration. The Br analogue exhibits a reversible solid–solid phase transition with moderate fragmentation, which is insensitive to crystal size variation within a certain size range. In contrast, the I analogue directly melts at 340 K without a solid–solid phase transition. Mechanistic studies reveal that anisotropic lattice distortion, cation order–disorder transition, hydrogen-bonding network reorganization, and inorganic chain displacement collectively govern the macroscopic thermally responsive mechanical motions. This work clarifies the effects of halogen substitution on the TS properties in hybrid halide crystals and provides a design strategy for controllable dynamic crystalline materials for intelligent responsive and information confidentiality applications.