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

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-xBi_xI₅ (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_0.39Bi_0.61I₅ 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)SbI₅. Benefiting from these attributes and the intrinsic ferroelectric spontaneous polarization, the X-ray detector based on (HDA)Sb_0.39Bi_0.61I₅ 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.