Polar ferromagnet induced by fluorine positioning in isomeric layered copper halide perovskites

We present the influence of positional isomerism on the crystal structure of fluorobenzylammonium copper (II) chloride perovskites A₂CuCl₄, by incorporating ortho, meta and para- fluorine substitution in the benzylamine structure. Two-dimensional (2D) polar ferromagnet (3-FbaH)₂CuCl₄ (3-FbaH⁺ = 3-fluorobenzylammonium) is successfully obtained, which crystallizes in a polar orthorhombic space group Pca2₁ at room temperature. In contrast, both (2-FbaH)₂CuCl₄ (2-FbaH⁺ = 2-fluorobenzylammonium) and (4-FbaH)₂CuCl₄ (4-FbaH⁺ = 4-fluorobenzylammonium) crystallize in centrosymmetric space groups P2₁/c and Pnma at room temperature, respectively, displaying significant differences in crystal structures. These differences indicate that the position of the fluorine atom is a driver for the polar behaviour in (3-FbaH)₂CuCl₄. Preliminary magnetic measurements confirm that these three perovskites possess dominant ferromagnetic interactions within the inorganic [CuCl₄]_∞ layers. Therefore, (3-FbaH)₂CuCl₄ is a polar ferromagnet, with potential as a type I multiferroic. This work is expected to promote further development of high performance 2D copper (II) halide perovskite multiferroic materials.