The Two-Dimensional AxCdxBi4–xQ6 (A = K, Rb, Cs; Q = S, Se): Direct Bandgap Semiconductors and Ion-Exchange Materials

We report the new layered chalcogenides AxCdxBi4–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) A, b = 6.9537(14) A, c = 24.203(5) A for Cs1.43Cd1.43Bi2.57S6 (x = 1.43); a = 3.9968(8) A, b = 6.9243(14) A, c = 23.700(5) A for Rb1.54Cd1.54Bi2.46S6 (x = 1.54); a = 3.9986(8) A, b = 6.9200(14) A, c = 23.184(5) A for K1.83Cd1.83Bi2.17S6 (x = 1.83) and a = 4.1363(8) A, b = 7.1476(14) A, c = 25.047(5) A for Cs1.13Cd1.13Bi2.87Se6 (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 [CdxBi2–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 Cs1.13Cd1.13Bi2.87Se6 and Cs1.43Cd1.43Bi2.57S6 to be direct band gap semiconductors. Polycrystall...