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 ACuCl by incorporating , -, and -fluorine substitution in the benzylamine structure. Two-dimensional (2D) polar ferromagnet (3-FbaH)CuCl (3-FbaH = 3-fluorobenzylammonium) is successfully obtained, which crystallizes in a polar orthorhombic space group 2 at room temperature. In contrast, both (2-FbaH)CuCl (2-FbaH = 2-fluorobenzylammonium) and (4-FbaH)CuCl (4-FbaH = 4-fluorobenzylammonium) crystallize in centrosymmetric space groups 2/ and 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)CuCl. Preliminary magnetic measurements confirm that these three perovskites possess dominant ferromagnetic interactions within the inorganic [CuCl] layers. Therefore, (3-FbaH)CuCl 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.