Mechanochemical Synthesis, Optical and Magnetic Properties of Pb-Free Ruddlesden–Popper-Type Layered Rb₂CuCl₂Br₂ Perovskite

2D layered Ruddlesden–Popper (RP) perovskites gained significant attention owing to their unique optoelectronic properties and ultralow thermal conductivity; however, in comparison to the hybrid Pb-based RP phases, their Pb-free all-inorganic analogues are rarely explored. Herein, we demonstrate the optical and magnetic properties of Pb-free RP-type layered Rb₂CuCl₂Br₂, which was synthesized by liquid-assisted mechanochemistry at ambient temperature and pressure. The dark brown colored material displayed band gap of ∼1.88 eV accompanied by a room-temperature band-edge photoluminescence (PL) at ∼1.97 eV. Moreover, Raman scattering at low-frequency region from the [CuCl₄Br₂]⁴– unit and reasonable thermal and environmental stability were manifested. The Rb₂CuCl₂Br₂ exhibited an excitonic absorption which is characteristic of layered perovskites accompanied by a high exciton binding energy. Large Stokes-shifted PL emission, negligible self-absorption, and fast charge carrier lifetimes were evidenced for Rb₂CuCl₂Br₂. Low-temperature (77 K) measurement revealed red-shifted and intense dual emission in contrast to the room-temperature emission. Finally, the temperature-dependent magnetization measurement indicates a paramagnetic to antiferromagnetic (AFM) transition at ∼16.1 K in Rb₂CuCl₂Br₂, primarily related to the interlayer AFM exchange interaction. Competition between the interlayer AFM interaction and the intralayer ferromagnetic interaction determine the nature of magnetic ground state in Rb₂CuCl₂Br₂.