Heat‐ and Pressure‐driven Room‐temperature Polymorphic Transition Accompanied with Switchable SHG Signal in a New Chiral Hexagonal Perovskite

Abstract Endowing room‐temperature polymorphs with both long‐term stability and easy interconvertibility is a big challenge due to the complexity of intermolecular interactions. Herein, we present a chiral hexagonal perovskite ( R ‐3‐hydroxy‐1‐methylpiperidinium)[CdCl 3 ] having two room‐temperature crystalline forms featuring obviously distinct second‐harmonic‐generation (SHG) signals with a high switching contrast of ~18 times. The two room‐temperature forms could be long‐term stable yet easily interconvertible through an irreversible thermal‐induced phase transition and a pressure‐driven backward transition, by switching hydrogen bonds via collective reorientation of ordered homochiral cations. Based on the essential role of homochiral organic cations in inducing switchable hydrogen bond linkages, this present instance provides good evidence that relatively irregular organic cations could induce more obvious inorganic chain deformations, thus endowing polymorphs with significantly different SHG signals at room temperature.