Dual‐Anchor Molecular Lock in 3D Lead‐Free Perovskite Quenches Ion Migration for Ultra‐Stable, Nanogray‐Sensitive X‐Ray Detection

ABSTRACT Ion migration severely limits the sensitivity and stability of lead‐free perovskite X‐ray detectors. Here, we propose a “dual‐anchor molecular lock” strategy to quench this migration, realized in a new 3D lead‐free iodide double perovskite, (Thiomorpholine) 2 RbBiI 6 , abbreviated as (Thiomor) 2 RbBiI 6 . In this material, the thiomorpholinium cation is immobilized within the inorganic framework through the cooperative action of an Rb–S contact and a directional N–H···I interaction network. This dual‐anchor design yields an ultrahigh ion‐migration activation energy of 729 meV and an exceptionally low dark‐current drift of 1.36 × 10 − 6 nA cm − 1 s − 1 V − 1 . Consequently, single‐crystal detectors achieve a high sensitivity of 1242 µC Gy a i r − 1 cm − 2 and an ultralow detection limit of 2.22 ± 0.31 nGy a i r s − 1 —approximately 2,500 times below the medical diagnostic threshold—alongside negligible degradation during continuous operation, long‐term ambient storage, and repeated X‐ray ON–OFF cycling. This work demonstrates that molecular‐scale cation immobilization via synergistic dual‐anchor host–guest anchoring is a decisive and generalizable design principle for creating drift‐free, ultra‐stable, and high‐performance lead‐free radiation detectors.