Stoichiometric starting ingredients (4Bi + K 2 S 5 + K 2 S 3 ), where K 2 S 3 was synthesized in the same way as K 2 S 5 with stoichiometric K and S, produced cubic KBiS 2 . The full refinement results and structures are given in Table and Figure in Supporting Information. UV-vis-NIR measurement: UV-vis-NIR measurements were done for ground powder using Varian Cary 5G, at the Materials Research Laboratory at University of Illinois, and the reflectance vs. wavelength data was converted and linearly fitted with Tauc method. R ∞ is the diffuse reflectance of the sample, and