Mixed A-Site Organic Cation Copper Chloride Perovskite-like: From Thermochromism to Mixed-Valence Cu for Redox Switching

Hybrid ternary copper halides, A₂CuX₄ (where A is a monovalent organic cation and X is a halide), exhibit unique structural, magnetic, and optical properties with potential in several applications, including resistive switching memory devices, batteries, and solar cells. While extensive research has focused on tuning properties through halide composition, exploring mixed A-site cations in A₂CuX₄ remains relatively underdeveloped. Here, we investigate the synthesis and characterization of (MA)₂CuCl₄, (EA)₂CuCl₄ (MA = methylammonium and EA = ethylammonium), and their mixed crystals. The crystals were grown using solvent acidolysis crystallization, forming the organic cations in situ from N-methylformamide and N-ethylformamide in the presence of HCl. X-ray diffraction and Raman spectroscopy confirmed the successful formation of mixed MA/EA crystals. Energy-dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy analyses indicate that crystals are EA/MA- and Cl-rich, thereby reducing surface defects and lowering the metallic copper (Cu⁰) content. Temperature-dependent Raman spectroscopy and optical measurements reveal that mixing the organic cations leads to structural disruptions that can modulate the bandgap and the thermochromic behavior of the crystals. Furthermore, current-voltage measurements demonstrate that mixed cation strategies effectively modulate ion migration and redox paths. This work highlights the unexploited potential of mixed A-site cations in tuning the properties of ternary copper halides.