An environmentally friendly synthesis route: Low-temperature preparation of vacancy-ordered double perovskites Cs2SnX6 (X = Cl, Br, I) via ionic liquid

• Developed a green, low-temperature, ambient-pressure synthesis for lead-free vacancy-ordered double perovskites Cs2SnX6 (X=Cl, Br, I) using ionic liquids and ammonium halides, replacing toxic acids and organic solvents. • The resulting perovskites exhibit high crystallinity, well-defined morphology, and improved thermal stability due to hydrogen bonding interactions and defect passivation from the halogen-rich environment. • The method is versatile, enabling the synthesis of doped crystals such as Bi-doped Cs2SnCl6 with a photoluminescence quantum efficiency of 12.73%, expanding potential optoelectronic applications. • This work provides a scalable and environmentally friendly pathway for producing high-quality lead-free perovskites, addressing toxicity and stability challenges for commercial deployment.