Unprecedented Mixed Low‐Dimensional Metal Halide Double Perovskite for Stable and Efficient X‐Ray Detection

Double perovskites have shown great promise for direct X‐ray detection due to their nontoxic nature and excellent phase stability. However, mixed low‐dimensional double perovskites are still a virgin land with direct X‐ray detection. Here, a millimeter‐scale mixed low‐dimensional double perovskite (4‐AP)5AgBi3Br20 (1, 4‐AP = 4‐amidinopyridine) is successfully synthesized with a coexistence of 1D–0D structural motifs. Noteworthily, the 0D motif is isolated {BiBr₆} octahedra and the 1D unit is the infinite chain ({AgBi₂Br₁₄}∞). This is the first reported mixed low‐dimensional (1D–0D) structure in Ag‐Bi double perovskite. The unique 1D‐0D architecture incorporates N─H···Br hydrogen bonding and π–π interactions, which enhances material stability, restricts ion migration, and raises the ionic mobility activation energy (Ea) to 0.87 eV in the parallel direction (∥). Furthermore, 1 single crystal detector exhibits high sensitivity (1567.0 µC Gy−1 cm−2, ∥), and an ultra‐low detection limit (38.1 nGy s−1, ∥) which is more than 100 times lower than the conventional medical diagnostic value of 5.5 µGy s−1. In addition, 1 detector further achieves ultralow dark current drift (3.42 × 10−9 nA cm−1 s−1 V−1, at 100 V, ∥). This study knocks on the door of novel mixed low‐dimensional double perovskite with stable and efficient X‐ray detection performances.