Constant‐Pressure Atmosphere Engineering Induced δ‐α Phase Transition in FAPbI ₃ Single Crystal for X‐Ray Detection

ABSTRACT The lead halide perovskite single crystals (SCs) have already emerged as a series of star materials for solar cells, lasers and detectors due to the complete crystal structure, fewer defects and grain boundaries. Previous studies have demonstrated that MASCN gas can convert δ‐FAPbI 3 films into α‐FAPbI 3 below the phase transition temperature. However, applying this strategy to the exploration of FAPbI 3 SCs remains a challenge. Herein, we designed a constant‐pressure gas diffusion setup to treat δ‐FAPbI 3 SCs with MASCN vapor, successfully obtaining photoactive α‐FAPbI 3 SCs with enhanced optoelectronic properties. X‐ray detectors based on the processed SCs exhibited significantly improved performance, achieving a high sensitivity of 4.46 × 10 4 µC Gy air −1 cm −2 , a ratio of sensitivity to dark current density as high as 1.06 × 10 12 µs Gy air −1 , and a detection limit as low as 150 nGy air s −1 . Furthermore, due to the strong polarizing nature of sulfur, some SCN − ions formed stable bonds with Pb 2+ in the crystal lattice, thereby enhancing the environmental stability of FAPbI 3 . The processed SCs maintained their phase purity without degradation after one month of storage in ambient conditions. This work offers a new strategy for repurposing δ‐FAPbI 3 SCs and advances the development of perovskite SC X‐ray detectors.