Fluorination-Induced Room-Temperature Ferroelectricity in a Quasi-spherical Bismuth(III) Bromide Hybrid

Molecular fluorination, as a precise halogen-engineering strategy, effectively enhances material properties by modulating rotational energy barriers and dipole moments. We report a room-temperature ferroelectric quasi-spherical amine bismuth(III) halide compound [1,4-3.2.2-H₂dabcn-CH₂F][BiBr₅] (1 F), which is obtained by fluorination of [1,4-3.2.2-H₂dabcn][BiBr₅] (1) (1,4-3.2.2-H₂dabcn = 1.4-diazabicyclo [3.2.2] nonane). In contrast, the nonfluorinated analogue 1 is nonferroelectric. Compound 1 F crystallizes in the orthorhombic system with the noncentrosymmetric space group Pca2₁, exhibiting ferroelectricity. Variable-temperature single-crystal X-ray diffraction (SC-XRD), differential scanning calorimetry (DSC), second-harmonic generation (SHG), and other characterizations confirm its phase transition from the ferroelectric phase to the paraelectric phase. Polarization-electric field (P-E) hysteresis loop measurements on its thin film show a saturation polarization (Ps) of 2.23 μC/cm². Piezoelectric force microscopy (PFM) analysis reveals domain structures, hysteresis loops, and butterfly curves, confirming the ferroelectric and piezoelectric properties of this compound. This study demonstrates fluorination as a potent molecular design strategy for exploring and constructing new ferroelectric materials.