Synthesis: The flux for KBiS 2 crystal growth is K 2 S 5 , which was synthesized by stoichiometric reaction of potassium (0.356 g, 99%) and sulfur (0.729 g, 99.9%). The potassium chunk was loaded into a one-side-open small quartz tube, which was loaded into a larger quartz tube with sulfur at the bottom to avoid direct contact between sulfur and potassium.