Stronger polar ferromagnetism and narrower optical band gaps in two-dimensional hybrid copper perovskites induced by bromine-substitution

Two-dimensional (2D) organic-inorganic hybrid perovskites are attracting great interest due to their structural tunability and rich functional characteristics. Here, we report a series of 2D layered hybrid copper(ii) halide perovskites (n-BrbaH)₂CuX₄ (n = 2, 3 or 4, X = Cl or Br) by a ‘dual-action’ halogen-substitution strategy, i.e. a heavier atom, Br, is substituted in the organic moieties at different positions and also within the inorganic octahedral moieties. Br-substitution in the organic moieties at meta- and para- positions results in stronger polar ferromagnets (3/4-BrbaH)₂CuCl₄ with narrower optical band gaps, compared to (2-BrbaH)₂CuCl₄. When the inorganic octahedral ligand Cl⁻ is further substituted by Br⁻, two polar structures (3/4-BrbaH)₂CuBr₄ are obtained, which have similar structural features to (3/4-BrbaH)₂CuCl₄. However, the two structures (3/4-BrbaH)₂CuBr₄ possess enhanced magnetic and optical properties, with the highest Curie temperatures and the narrowest optical band gaps, due to the substitution of higher-energy Br 4p for Cl 3p orbitals at the inorganic octahedral ligand. Our study demonstrates the rich and interesting magnetic and optical properties of these layered copper halide perovskites and paves the way for the design of multifunctional high-performance magnetic materials.