Eco‐Friendly Preparation of High‐Quality Lead‐Free Halide Ferroelectric Solid‐Solution Toward High‐Performance X‐ray Detection

Abstract Bi/Sb‐based halide ferroelectrics have become promising lead‐free alternatives due to their excellent stability and environmental friendliness. However, their wide bandgaps and limited charge‐carrier mobility‐lifetime ( µτ ) product impede their optoelectronic performance. Here, we report a new eco‐friendly method for growing lead‐free halide ferroelectric solid‐solution (HDA)Sb 1‐x Bi x I 5 (x = 0–1) (HDA = hexane‐1,6‐diammonium), by isothermal evaporation from a biomass‐derived solvent, γ ‐valerolactone (GVL). The solid‐solution strategy, good material stability,‐ and high‐quality centimeter‐sized single‐crystals not only endow the (HDA)Sb 0.39 Bi 0.61 I 5 with the narrowest bandgap of 1.64 eV among Bi/Sb‐based halide ferroelectrics, but also enhance its µτ product by approximately two orders of magnitude compared with the parent (HDA)SbI 5 . Benefiting from these attributes and the intrinsic ferroelectric spontaneous polarization, the X‐ray detector based on (HDA)Sb 0.39 Bi 0.61 I 5 exhibits excellent self‐powered detection performance with a high sensitivity of 1,040 µC Gy air −1 cm −2 and a low detection limit of 0.25 nGy air s −1 . More excitingly, its detection sensitivity can reach 17,560 µC Gy air −1 cm −2 , which is the highest among reported halide ferroelectrics, and superior to that of most halide detectors. This work opens new avenues for the rational engineering of Bi/Sb‐based halide ferroelectrics for advanced optoelectronic applications.