Homologous Series of 2D Chalcogenides Cs–Ag–Bi–Q (Q = S, Se) with Ion-Exchange Properties

Four new layered chalcogenides Cs1.2Ag0.6Bi3.4S6, Cs1.2Ag0.6Bi3.4Se6, Cs0.6Ag0.8Bi2.2S4, and Cs2Ag2.5Bi8.5Se15 are described. Cs1.2Ag0.6Bi3.4S6 and Cs1.2Ag0.6Bi3.4Se6 are isostructural and have a hexagonal P63/mmc space group; their structures consist of [Ag/Bi]2Q3 (Q = S, Se) quintuple layers intercalated with disordered Cs cations. Cs0.6Ag0.8Bi2.2S4 also adopts a structure with the hexagonal P63/mmc space group and its structure has an [Ag/Bi]3S4 layer intercalated with a Cs layer. Cs1.2Ag0.6Bi3.4S6 and Cs0.6Ag0.8Bi2.2S4 can be ascribed to a new homologous family Ax[MmS1+m] (m = 1, 2, 3···). Cs2Ag2.5Bi7.5Se15 is orthorhombic with Pnnm space group, and it is a new member of the A2[M5+nSe9+n] homology with n = 6. The Cs ions in Cs1.2Ag0.6Bi3.4S6 and Cs0.6Ag0.8Bi2.2S4 can be exchanged with other cations, such as Ag+, Cd2+, Co2+, Pb2+, and Zn2+ forming new phases with tunable band gaps between 0.66 and 1.20 eV. Cs1.2Ag0.6Bi3.4S6 and Cs0.6Ag0.8Bi2.2S4 possess extremely low thermal conductivity (–1·K–1).