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

Abstract

We present the influence of positional isomerism on the crystal structure of fluorobenzylammonium copper(II) chloride perovskites A₂CuCl₄ by incorporating <i>ortho-</i>, <i>meta</i>-, and <i>para</i>-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 <i>Pca</i>2₁ 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 <i>P</i>2₁/<i>c</i> and <i>Pnma</i> 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)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.