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

Abstract 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] 2 RbEu(NO 3 ) 6 [( R )‐M3HQ=( R )‐N‐methyl‐3‐hydroxylquinuclidinium] with a low T C of 285 K, a novel 3D PREDP ferroelectric [( R )‐CM3HQ] 2 CsEu(NO 3 ) 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] 2 CsEu(NO 3 ) 6 with notable X‐ray detection capabilities. Centimeter‐sized [( R )‐CM3HQ] 2 CsEu(NO 3 ) 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.