Pressure‐Induced Bandgap Narrowing and Irreversible Color Shift in Cadmium Halides Through Constrained Structural Amorphization

Abstract Cadmium halides have recently emerged as promising alternatives to conventional optoelectronic materials because of their outstanding optical properties. Nevertheless, challenges such as limited bandgap tunability and stability under ambient conditions continue to hinder their practical applications. Herein, pressure engineering is employed to achieve advanced optical properties in a series of distinctive cadmium halides, [BPy] 2 CdX 4 (BPy + = Butylpyridinium (C 9 H 14 N + ), X = I, Br, Cl). Remarkably, pressure‐induced emission enhancement of 8‐fold, 28‐fold, and 41‐fold are observed for [BPy] 2 CdI 4 , [BPy] 2 CdBr 4 , and [BPy] 2 CdCl 4 , respectively. In addition, pressured‐treated samples exhibited notable bandgap narrowing of 0.67, 0.50, and 0.98 eV from [BPy] 2 CdI 4 to [BPy] 2 CdCl 4 , accompanied by irreversible color shifts compared to the initial states. Structural analysis reveals that pressure‐induced inhomogeneous distortion of the [CdX 4 ] 2− tetrahedra leads to deeper self‐trapped states, enhancing emission efficiency. Meanwhile, upon decompression, the loss of long‐range order and the strengthening of hydrogen bonds in the formed amorphous samples, along with the local structural reorganization, which is conducive to achieving efficient exciton capture at the deformed lattice sites. Collectively, these findings highlight the power of pressure engineering in tailoring the optical properties of cadmium halides and broaden the prospects of amorphous‐state design in the development of flexible optoelectronics.