Kinetic Trapping of Pressure‐Induced Symmetry Breaking in Hybrid Metal Halides via Fluorine‐Mediated Molecular Locking

Permanently capturing pressure‐induced metastable phases at ambient conditions is a fundamental challenge that, if solved, would unlock a new realm of materials with exotic functionalities. A central obstacle is the reversibility of pressure‐driven transitions, particularly symmetry‐breaking ones that yield valuable properties like nonlinear optical activity. Here, we report a kinetic trapping strategy that permanently preserves high‐pressure noncentrosymmetric phases in hybrid tin bromides via a fluorine‐mediated “molecular lock.” Using a two‐dimensional perovskite (3‐CF 3 py) 2 SnBr 4 (3‐CF 3 py + = 3‐(trifluoromethyl)pyridinium) as a model system, we demonstrate that hydrostatic compression induces a symmetry‐breaking transition at ∼4.0 GPa with a strong second‐harmonic generation (SHG) response, which peaks at ∼8 GPa with a staggering ∼400‐fold enhancement relative to the signal at ∼4.0 GPa. Remarkably, the noncentrosymmetric phase and its SHG activity remain after full pressure release. Single‐crystal x‐ray diffraction reveals that pressure‐enhanced directional F···F interactions cooperatively reorganize the organic–inorganic framework, suppressing elastic recovery and locking in the metastable structure. Importantly, this design principle is generalizable, as demonstrated by trapping of the high‐pressure noncentrosymmetric phase in the one‐dimensional analogue, (3‐CF 3 py)SnBr 3 . Our work establishes a powerful design strategy to induce and stabilize high‐pressure phases, making their otherwise inaccessible functionalities available for ambient‐condition applications.