The Two-Dimensional A_xCd_xBi_(4_x)Q_6 (A = K, Rb, Cs; Q = S, Se): Direct Bandgap Semiconductors and Ion-Exchange Materials

摘要

We report the new layered chalcogenides A_xCd_xBi_(4-x)Q_6 (A = Cs, Rb, K; Q = S and A = Cs; Q = Se). All compounds are isostructural crystallizing in the ortho-rhombic space group Cmcm, with a = 4.0216(8) A, b = 6.9537(14) A, c = 24.203(5) A for Cs_(1.43) Cd_(1.43)Bi_(2.57)S_6 (x = 1.43); a = 3.9968(8) A, b = 6.9243(14) A, c = 23.700(5) A for Rb_(1.54)Cd_(1.54)Bi_(2.46)S_6 (x = 1.54); a = 3.9986(8) A, b = 6.9200(14) A, c = 23.184(5) A for K_(1.83)Cd_(1.83)Bi_(2.17)S_6 (x = 1.83) and a = 4.1363(8) A, b = 7.1476(14) A, c = 25.047(5) A for Cs_(1.13)Cd_(1.13)Bi_(2.87)Se_6 (x = 1.13). These structures are intercalated derivatives of the Bi_2Se_3 structure by way of replacing some Bi~(3+) atoms with divalent Cd~(2+) atoms forming negatively charged Bi_2Se_3-type quintuple [Cd_xBi_(2-x)Se_3]~(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_(1.13)Cd_(1.13)Bi_(2.87)Se_6 and Cs_(1.43)Cd_(1.43)Bi_(2.57)S_6 to be direct band gap semiconductors. Polycrystalline Cs_(1.43)Cd_(1.43)Bi_(2.57)S_6 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_(1.43)Cd_(1.43)Bi_(2.57)S_6 are mobile and can be topotactically exchanged with Pb~(2+), Zn~(2+), Co~(2+) and Cd~(2+) in aqueous solution. The intercalation of metal cations presents a direct "soft chemical" route to create new materials.