A Halogen-Substitution Strategy for Tuning Structure, Phase Transition, and Ferroelectric/Nonlinear Optical Properties in Fe-Based ABX4-Type Organic–Inorganic Hybrid Molecular Ferroelectrics

This study designs and synthesizes a series of ABX4-type organic-inorganic hybrid molecular ferroelectrics [Et2Me(CH2X)N][FeBr4] (X = Cl, Br, I; compounds 1-3). Single-crystal X-ray diffraction (SCXRD), differential scanning calorimetry (DSC), dielectric measurements, and second-harmonic generation (SHG) measurements are adopted to explore their crystal structures, phase transitions, and optoelectronic performances. Structural analyses confirm that compounds 1 and 2 crystallize in noncentrosymmetric hexagonal P63mc, while iodine-triggered lattice symmetry breaking drives compound 3 into orthorhombic Pca21. All three samples possess zero-dimensional frameworks with interleaved organic cations and tetrahedral [FeBr4]- anions. Upon halogen substitution from Cl to I, extended C-X bonds and larger cation quasi-spherical radii weaken cation-anion electrostatic forces and halogen bonds, leading to gradually reduced phase transition temperatures of 405 K (1), 395 K (2) and 301/339 K (3, cooling/heating). Regulated by weak intermolecular forces and cooperative dipole alignment in compact lattices, compound 2 exhibits superior SHG intensity among the three samples. All materials feature reversible order-disorder phase transitions and stable ferroelectric properties. This work achieves tunable structural and functional properties through halogen substitution, which provides experimental and theoretical references for developing high-performance hybrid ferroelectrics and nonlinear optical crystals.