Discovery of a 2D Hybrid Silver/Antimony‐Based Iodide Double Perovskite Photoferroelectric with Photostrictive Effect and Efficient X‐Ray Response

2D hybrid halide double perovskites (HHDPs) have been demonstrated to be a promising alternative to conventional lead-based halide perovskites as a new system of photoferroelectrics, due to their unique characteristics of environmental friendliness, favorable stability, and fascinating optoelectronic properties. Herein, for the first time, a 2D iodide double perovskite photoferroelectric is reported based on Ag/Sb ions, (4,4-DFPD)(4)AgSbI8 (4,4-DFPD = 4,4-difluoropiperidinium), which possesses a high Curie temperature of 414 K (above BaTiO3), a large spontaneous polarization of 9.6 mu C cm(-2), ferroelectric photovoltaic effect, and photostrictive effect. Notably, to the best of the authors' knowledge, the discovery of photostriction in HHDP photoferroelectrics is unprecedented. Moreover, (4,4-DFPD)(4)AgSbI8 exhibits an impressive X-ray responsivity, with a sensitivity as high as 704.8 mu C Gy(air)(-1) cm(-2) at 100 V bias and a detection limit as low as 0.36 mu Gy(air) s(-1) at 10 V bias, both of which outperform the current all HHDP photoferroelectrics. This work enriches the photoferroelectric family, and proves that Ag/Sb-based HHDP photoferroelectrics are a promising candidate for the next-generation optoelectronic devices.