Exploring Aqueous Solution‐Processed Pseudohalide Rare‐Earth Double Perovskite Ferroelectrics toward X‐Ray Detection with High Sensitivity

Three-dimensional (3D) pseudohalide rare-earth double perovskites (PREDPs) have garnered significant attention for their versatile physical properties, including ferroelectricity, ferroelasticity, large piezoelectric responses, and circularly polarized luminescence. However, their potential for X-ray detection remains unexplored, and the low Curie temperature (T_C) limits the performance window for PREDP ferroelectrics. Here, by applying the chemical regulation strategies involving halogen substitution on the organic cation and Rb/Cs substitution to the PREDP [(R)-M3HQ]₂RbEu(NO₃)₆ [(R)-M3HQ=(R)-N-methyl-3-hydroxylquinuclidinium] with a low T_C of 285 K, a novel 3D PREDP ferroelectric [(R)-CM3HQ]₂CsEu(NO₃)₆ [(R)-CM3HQ=(R)-N-chloromethyl-3-hydroxylquinuclidinium] are successfully synthesized, for which the T_C reaches 344 K. More importantly, such a strategy endowed [(R)-CM3HQ]₂CsEu(NO₃)₆ with notable X-ray detection capabilities. Centimeter-sized [(R)-CM3HQ]₂CsEu(NO₃)₆ single crystals fabricated from aqueous solutions demonstrated a sensitivity of 1307 μC Gy_air ^-1 cm^-2 and a low detectable dose rate of 152 nGy_air s^-1, the highest sensitivity reported for hybrid double perovskite ferroelectric detectors. This work positions PREDPs as promising candidates for the next generation of eco-friendly optoelectronic materials and also offers substantial insights into the interaction between structure, composition, and functionality in ferroelectric materials.