Ideal Charge Compensation via Monoprotonated Ligand: Highly Efficient Copper Halide Scintillators Toward High‐Resolution X‐Ray Tomography

Hybrid copper(I) halides feature excellent luminescence and scintillation performance, yet their structure‐property relationships and A‐site cation effects are poorly understood. Herein, we synthesized four copper(I) halide single crystals using two piperidine cations (protonated 4‐aminopiperidine vs. 4‐methylpiperidine) and different halogens, attempting to decipher the influence of A‐site cation charge number, halogen species on the crystal structure and luminescence properties. The diprotonated 4‐aminopiperidine forms 0D (C 5 H 14 N 2 ) 3 Cu 2 X 8 (X = I, Br) with extra iodide counterions and 1D (C 5 H 14 N 2 )Cu 2 Cl 4 , showing dual self‐trapped exciton (STE) emission. In contrast, monoprotonated 4‐methylpiperidine yields counterion‐free (C 6 H 14 N) 4 Cu 2 I 6 with single STE emission. Benefiting from ideal charge compensation and potentially less defects, counterion‐free (C 6 H 14 N) 4 Cu 2 I 6 achieves a radiative decay rate 15.7 times the non‐radiative rate, a PLQY of 94% and an impressive x‐ray light yield of 60 047 photons/MeV. Its flexible scintillation screen reaches 18 lp/mm resolution and realizes high‐quality 3D x‐ray tomography. This work highlights the synergistic A‐site cation and halogen engineering of copper(I) halides for advanced x‐ray imaging scintillators.