Mixed‐Ligand‐Induced Crystal Symmetry Breaking and Lattice Distortion in Hybrid Cu(I) Halides for Near‐Unity Quantum Yield Scintillators

Abstract Cu(I)‐based metal halides have emerged as promising scintillators due to their efficient self‐trapped exciton (STE) emission. However, the radiative efficiency of STE emission is mainly determined by structural distortion, making it challenging to precisely control the distortion for optimal luminescence. Here, inspired by symmetry‐breaking principles, we developed a universal asymmetric structure transformation strategy through mixed‐ligand engineering to modulate structural distortion and enhance intramolecular charge transfer, thereby boosting radiative STE emission. Mechanistic studies demonstrate that this mixed‐ligand approach effectively tunes bond lengths and angles, intensifying structural distortion while simultaneously promoting charge transfer for improved luminescence. By optimizing structural distortion, the (C 8 H 20 N) 1 (C 12 H 28 N) 1 Cu 4 Br 6 crystal achieved a 138% enhancement in emission efficiency with a near‐unity photoluminescence quantum yield (99% PLQY). Consequently, the ​radioluminescence intensity increased by 187%, reaching 2.67 times light output that of (Lu, Y) 2 SiO 5 : Ce (LYSO). Owing to this remarkable improvement in radioluminescence, large‐area (C 8 H 20 N) 1 (C 12 H 28 N) 1 Cu 4 Br 6 single‐crystal films with low light scattering exhibited outstanding X‐ray imaging performance, achieving a spatial resolution exceeding 29 lp/mm, 2.64 times higher than that of (C 8 H 20 N) 1 (C 12 H 28 N) 1 Cu 4 Br 6 @PMMA films (11 lp/mm). This work establishes mixed‐ligand engineering as an effective approach for structural asymmetry design and demonstrates the material's potential for advanced radiation detection and imaging.