Water-Triggered Domino-Like Phase Transition in a Molecular Ferroelectric

Domino-like phase transitions offer a powerful route to propagate structural changes through local stimuli, yet their coupling with ferroelectricity in molecular materials has remained elusive. Here we show that water enables a domino-like phase transition in the hybrid copper halide Gua₃Cu₂I₅ (Gua⁺ = CN₃H₆ ⁺), providing non-thermal access to a polar molecular ferroelectric. We find that the polar α phase cannot be reached through conventional thermal pathways but instead emerges through a hierarchical transition mechanism. In this mechanism, water first induces a surface solid-liquid-solid conversion via localized dissolution and reprecipitation, which subsequently triggers an interface-propagated domino-like transformation from the non-polar β phase into the ferroelectric α phase in the bulk. This process can be directly visualized through real-time luminescence contrast and is accompanied by robust, switchable ferroelectric polarization at room temperature. By identifying water as an active mediator rather than a passive environment, our results establish a new pathway for accessing polar phases beyond thermal equilibrium and provide a conceptual framework for coupling domino-like phase transitions with ferroelectric functionality in molecular materials.