The Cs 2 SnI 6 compound was synthesized by adjusting the procedure detailed by Lee et al. Around 0.5 mmol of Cs 2 CO 3 was dissolved in 3 mL of 57 % hydroiodic acid (aqueous, 1.5 % H 3 PO 2 ) in a beaker and stirred until fully dissolved. The addition of solid Cs 2 CO 3 to hydroiodic acid was carefully done to manage the vigorous reaction between them, leading to heat and gaseous CO 2 production. Meanwhile, the appropriate ratio of SnI 4 precursor was dissolved in 15 mL absolute ethanol and 2 mL of 57 % hydroiodic acid (aqueous, with an additional excess elemental iodine added). This solution was gently heated to approximately T = 60 °C for enhanced solubility. Once both solutions were ready, the Cs 2 CO 3 /HI solution was swiftly mixed with the SnI 4 solution, resulting in immediate black precipitate formation followed by stirring for another half hour. The obtained precipitate underwent centrifugation and washing three times with absolute ethanol before being dried at 60 °C for a day. The powder X-ray diffraction verified phase purity while synthetic yields ranged from 92 -96 %, determined through comparison between collected product mass and theoretically calculated amount based on precursor quantities.