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

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We report the new layered chalcogenides A x Cd x Bi 4– x Q 6 (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 1.43 Cd 1.43 Bi 2.57 S 6 ( x = 1.43); a = 3.9968(8) Å, b = 6.9243(14) Å, c = 23.700(5) Å for Rb 1.54 Cd 1.54 Bi 2.46 S 6 ( x = 1.54); a = 3.9986(8) Å, b = 6.9200(14) Å, c = 23.184(5) Å for K 1.83 Cd 1.83 Bi 2.17 S 6 ( x = 1.83) and a = 4.1363(8) Å, b = 7.1476(14) Å, c = 25.047(5) Å for Cs 1.13 Cd 1.13 Bi 2.87 Se 6 ( x = 1.13). These structures are intercalated derivatives of the Bi 2 Se 3 structure by way of replacing some Bi 3+ atoms with divalent Cd 2+ atoms forming negatively charged Bi 2 Se 3 -type quintuple [Cd x Bi 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.