The Two-Dimensional A_xCd_xBi_4-xQ₆ (A = K, Rb, Cs; Q = S, Se): Direct Bandgap Semiconductors and Ion-Exchange Materials

We report the new layered chalcogenides A_xCd_xBi_4-xQ₆ (A = Cs, Rb, K; Q = S and A = Cs; Q = Se). All compounds are isostructural crystallizing in the orthorhombic space group Cmcm, with a = 4.0216(8) Å, b = 6.9537(14) Å, c = 24.203(5) Å for Cs_1.43Cd_1.43Bi_2.57S₆ (x = 1.43); a = 3.9968(8) Å, b = 6.9243(14) Å, c = 23.700(5) Å for Rb_1.54Cd_1.54Bi_2.46S₆ (x = 1.54); a = 3.9986(8) Å, b = 6.9200(14) Å, c = 23.184(5) Å for K_1.83Cd_1.83Bi_2.17S₆ (x = 1.83) and a = 4.1363(8) Å, b = 7.1476(14) Å, c = 25.047(5) Å for Cs_1.13Cd_1.13Bi_2.87Se₆ (x = 1.13). These structures are intercalated derivatives of the Bi₂Se₃ structure by way of replacing some Bi³⁺ atoms with divalent Cd²⁺ atoms forming negatively charged Bi₂Se₃-type quintuple [Cd_xBi_2-xSe₃]^x- layers. The bandgaps of these compounds are between 1.00 eV for Q = Se and 1.37 eV for Q = S. Electronic band structure calculations at the density functional theory (DFT) level indicate Cs_1.13Cd_1.13Bi_2.87Se₆ and Cs_1.43Cd_1.43Bi_2.57S₆ to be direct band gap semiconductors. Polycrystalline Cs_1.43Cd_1.43Bi_2.57S₆ samples show n-type conduction and an extremely low thermal conductivity of 0.33 W·m^-1·K^-1 at 773 K. The cesium ions between the layers of Cs_1.43Cd_1.43Bi_2.57S₆ are mobile and can be topotactically exchanged with Pb²⁺, Zn²⁺, Co²⁺ and Cd²⁺ in aqueous solution. The intercalation of metal cations presents a direct "soft chemical" route to create new materials.