Synergistic Rare‐Earth Alloying and Pseudohalide Coordination Constructing Hybrid Metal Pseudohalide with Robust Crystal Fields for Stable X‐Ray Detection with Record Sensitivity

Metal halides demonstrate superior defect tolerance and carrier transport stemming from their inherent lattice softness, positioning them as promising candidates for X‐ray detection. However, this structural flexibility induces weak interatomic bonding, triggering detrimental issues like ion migration and lattice distortion that ultimately create an inherent trade‐off between detection efficiency and stability. Here, we present a pioneering strategy employing rare‐earth heterobimetallic compounds with pseudohalide‐bridged coordination crystals ((HMT) 2 CsEu(NO 3 ) 6 , HMT = Hexamethylenetetramine) to establish structurally robust lattice, thereby enabling simultaneous achievement of operational stability and detection efficiency. Systematic experiments reveal three synergistic lattice‐stabilizing mechanisms: i) a robust structural framework constructed by strong rare earth‐pseudohalide‐bridged coordination networks, ii) localized 4f‐orbital electron confinement, and iii) Cs‐O bonding interactions. Concurrently, the unique electronic structure characterized by indirect band transitions and low orbital symmetry facilitates the generation of long‐lifetime and low‐concentration carriers. The optimized crystal demonstrates weak electron‐phonon coupling ( S = 0.0159) and exceptional ambient stability. The implemented X‐ray detector achieves a record sensitivity of 3949 µC Gy air −1 cm −2 among hybrid bimetallic halides, a detection limit of 41.2 nGy air s −1 , and a high‐resolution imaging capability at low doses (361 nGy air s −1 ). This work establishes a paradigm for designing stable, high‐performance radiation detection materials through synergistic coordination engineering.