Intrinsic Lattice‐Driven Mn ²⁺ ‐Mn ²⁺ Dimers in CdGa ₂ S ₄ : Lifting Spin Forbiddance for High‐Efficiency Flexible X‐Ray Scintillation

ABSTRACT Red‐emitting Mn 2+ ‐activated scintillators are highly desirable for X‐ray imaging due to their well matched with silicon‐based detectors and reduced light scattering in biological tissues. However, achieving efficient red emission from tetrahedral Mn 2+ remains a significant challenge. Herein, we demonstrate that CdGa 2 S 4 , with its unique all‐cation‐tetrahedral chain structure, intrinsically facilitates the formation of Mn 2+ ‐Mn 2+ dimers even at relatively low Mn 2+ concentrations. This dimer configuration relaxes the spin‐ and parity‐forbidden nature of isolated Mn 2+ transitions, generating an intense red emission at 615 nm, in contrast to the weak green luminescence of isolated Mn 2+ ions. The resulting CdGa 2 S 4 : Mn 2+ scintillator exhibits a high light yield of 22 816 photons/MeV, an ultralow detection limit of 490 nGy/s, and a short radioluminescence lifetime of 0.314 ms. Furthermore, a flexible scintillator film fabricated with this material achieves high‐resolution X‐ray imaging at 12.2 lp/mm, surpasses that of Mn 2+ based hybrid materials such as (C 8 H 20 N) 2 MnBr 4 (5 lp/mm) and most reported metal halide scintillators. This study not only provides a new host design paradigm for activating dimer emission at low doping concentrations but also establishes a promising pathway for developing next‐generation flexible, high‐sensitivity X‐ray scintillators for non‐destructive testing and medical radiography.