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 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 Pca 2 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 P 2 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.