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

Four new layered chalcogenides Cs1.2Ag0.6Bi3.4S6, Cs1.2Ag0.6Bi3.4Se0.(6), Cs0.6Ag0.8Bi2.2S4, and Cs2Ag2.6Bi8.5Se15 are described. Cs1.2Ag0.6Bi3.4S6 and Cs1.2Ag0.6Bi3.4Se6 are isostructural and have a hexagonal P6(3)/mmc space group; their structures consist of [Ag/Bi](2)Q(3) (Q= S, Se) quintuple layers intercalated with disordered Cs cations. Cs0.6Ag0.8Bi2.2S4 also adopts a structure with the hexagonal P6(3)/mmc space group and its structure has an [Ag/Bi](3)S-4 layer intercalated with a Cs layer. Cs1.2Ag0.6Bi3.4S6 and CS0.6Ag0.8Bi2.2S4 can be ascribed to a new homologous family A(x)[MmS1+m,] (m = 1, 2, 3-.). Cs2Ag2.5Bi7.5Se15 is orthorhombic with Pnnm space group, and it is a new member of the A(2)[M5+n Se9+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 (<0.6 W.m(-1.)K(-1)).