Extrinsic Self‐Trapped‐Exciton Emission in Cs₅Cu₃Cl₆I₂ for Efficient X‐Ray Scintillation

Efficient and stable scintillators play a crucial role in X‐ray detection applications. To enhance the luminescence efficiency under X‐ray excitation, the incorporation of multiple emission centers into scintillators is widely explored. Here, it is found that the cesium copper halide Cs 5 Cu 3 Cl 6 I 2 exhibits dual emission centers, enabling high‐performance scintillators with an X‐ray light yield of 49000 photon MeV −1 and a low detection limit of 4 nGy s −1 . The emissions of Cs 5 Cu 3 Cl 6 I 2 are from intrinsic self‐trapped exciton (STE) and Frenkel defect‐assisted STE. High‐energy X‐rays can induce an increased fraction of Frenkel defect‐assisted STEs, which can serve as an effective scintillation channel. Furthermore, large‐area flexible scintillators with a high resolution of 18 lp mm −1 are developed, making them suitable for X‐ray imaging applications. These findings offer promising insights for developing more efficient scintillators.