Evolution of the structure and properties of mechanochemically synthesized pyrrolidine incorporated manganese bromide powders

Abstract

Due to the effects of electron correlation and spin–spin coupling, pure transition metal compounds rarely produce luminescence and ferromagnetism. In this work, the Pb-free perovskite materials, C₈H₂₀N₂MnBr₄ and C₄H₁₀NMnBr₃ with high luminescence yields, were obtained via a simple mechanochemical process. In C₈H₂₀N₂MnBr₄ powders, MnBr₄²⁻ coordinated with two pyrrolidine molecules to form an independent mononuclear structure in a crystal, with paramagnetic properties and a strong emission band at 520 nm due to the lowest d–d crystal field radiation transition for individual Mn(II) ions. In C₄H₁₀NMnBr₃ powders, MnX₆⁴⁻ octahedra, coordinated with a much smaller amount of pyrrolidine molecules than that in C₈H₂₀N₂MnBr₄, formed edge-sharing linear chains of Mn-ion octahedra with a much smaller Mn–Mn distance, which produced emission bands at 628 nm due to the ferromagnetic coupling of Mn pairs or clusters. Influenced by the modification in the local crystal structure by incorporated pyrrolidine molecules, the microcrystals in these two powders exhibited different phase transition temperatures and varied lifetimes in their photoluminescence besides their emission colors. By controlling the processing time of mechanochemical reactions and pyrrolidine amount, the pyrrolidine insertion into the lattice of this transition metal halide can be adjusted to be completely realized, which provides a very simple way to change the ligand from a halide ion to an organic molecule, which regulates the Mn–Mn distance in the lattice, modifying the electronic correlation and spin coupling, thereby obtaining new manganese perovskite compounds with both strong luminescence and clear ferromagnetic properties.