Persistent Second‐Harmonic Generation Enhancement Across an Unprecedented Pressure Range in a Hybrid Antimony Halide

ABSTRACT Organic‐inorganic hybrid solids combine chemical tunability and lattice flexibility, making them ideal platforms for modulating optical nonlinearities through external stimulus. Conventionally, the stereochemical activity of lone‐pair electrons (LPEs), structural distortion, and second‐harmonic generation (SHG) response are positively correlated. In most halide and related systems that contain stereochemically active LPEs, compression suppresses LPEs activity by increasing cation coordination, and SHG intensities consequently follow a dome‐shaped pressure dependence that limits continuous enhancement below ∼2.5 GPa. Herein, we report a remarkable exception: the zero‐dimensional hybrid halide (TMP) 2 (SbBr 5 )(SbBr 3 ) (TMP = thiomorpholine, (CH 2 ) 4 NH 2 S + ) exhibits a continuous 2.1‐fold SHG enhancement over an unusually wide pressure range of 0–8.88 GPa. The net enhancement persists through an isostructural 0D→1D phase transition at ∼3.5 GPa. Our combined structural, spectroscopic, and DFT analyses show that the formation of new Sb‒S bonds linking SbBr 3 trigonal pyramids to adjacent TMP + cations, together with the assembly of SbBr 5 square pyramids into 1D zigzag chains, increases Sb–Br and Sb–S orbital hybridization and electron delocalization. This enhanced hybridization amplifies the dominant d 33 tensor component and overwhelms the concurrent reduction in Sb 5 s 2 LPEs expression; accordingly, changes in connectivity and hybridization are the primary drivers of the sustained SHG increase.