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

Abstract

Four new layered chalcogenides Cs_1.2Ag_0.6Bi_3.4S₆, Cs_1.2Ag_0.6Bi_3.4Se₆, Cs_0.6Ag_0.8Bi_2.2S₄, and Cs₂Ag_2.5Bi_8.5Se₁₅ are described. Cs_1.2Ag_0.6Bi_3.4S₆ and Cs_1.2Ag_0.6Bi_3.4Se₆ 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. Cs_0.6Ag_0.8Bi_2.2S₄ 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. Cs_1.2Ag_0.6Bi_3.4S₆ and Cs_0.6Ag_0.8Bi_2.2S₄ can be ascribed to a new homologous family A_x[M_mS_1+m] (m = 1, 2, 3···). Cs₂Ag_2.5Bi_7.5Se₁₅ is orthorhombic with Pnnm space group, and it is a new member of the A₂[M_5+nSe_9+n] homology with n = 6. The Cs ions in Cs_1.2Ag_0.6Bi_3.4S₆ and Cs_0.6Ag_0.8Bi_2.2S₄ 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. Cs_1.2Ag_0.6Bi_3.4S₆ and Cs_0.6Ag_0.8Bi_2.2S₄ possess extremely low thermal conductivity (<0.6 W·m^-1·K^-1).