Polarity and Ferromagnetism in Two-Dimensional Hybrid Copper Perovskites with Chlorinated Aromatic Spacers

Two-dimensional (2D) organic-inorganic hybrid copper halide perovskites have drawn tremendous attention as promising multifunctional materials. Herein, by incorporating ortho-, meta-, and para-chlorine substitutions in the benzylamine structure, we first report the influence of positional isomerism on the crystal structures of chlorobenzylammonium copper(II) chloride perovskites A₂CuCl₄. 2D polar ferromagnets (3-ClbaH)₂CuCl₄ and (4-ClbaH)₂CuCl₄ (ClbaH⁺ = chlorobenzylammonium) are successfully obtained. They both adopt a polar monoclinic space group Cc at room temperature, displaying significant differences in crystal structures. In contrast, (2-ClbaH)₂CuCl₄ adopts a centrosymmetric space group 2₁/ at room temperature. This associated structural evolution successfully enhances the physical properties of the two polar compounds with high thermal stability, discernible second harmonic generation (SHG) signals, ferromagnetism, and narrow optical band gaps. These findings demonstrate that the introduction of chlorine atoms into the interlayer organic species is a powerful tool to tune crystal symmetries and physical properties, and this inspires further exploration of designing high-performance multifunctional copper-based materials.