Polar Ferromagnet Induced by Fluorine Positioning in Isomeric Layered Copper Halide Perovskites

[Image: see text] We present the influence of positional isomerism on the crystal structure of fluorobenzylammonium copper(II) chloride perovskites A(2)CuCl(4) by incorporating ortho-, meta-, and para-fluorine substitution in the benzylamine structure. Two-dimensional (2D) polar ferromagnet (3-FbaH)(2)CuCl(4) (3-FbaH(+) = 3-fluorobenzylammonium) is successfully obtained, which crystallizes in a polar orthorhombic space group Pca2(1) at room temperature. In contrast, both (2-FbaH)(2)CuCl(4) (2-FbaH(+) = 2-fluorobenzylammonium) and (4-FbaH)(2)CuCl(4) (4-FbaH(+) = 4-fluorobenzylammonium) crystallize in centrosymmetric space groups P2(1)/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 behavior in (3-FbaH)(2)CuCl(4). Preliminary magnetic measurements confirm that these three perovskites possess dominant ferromagnetic interactions within the inorganic [CuCl(4)](∞) layers. Therefore, (3-FbaH)(2)CuCl(4) 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.