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

We report the new layered chalcogenides ACdBiQ (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 CsCdBiS (x = 1.43); a = 3.9968(8) Å, b = 6.9243(14) Å, c = 23.700(5) Å for RbCdBiS (x = 1.54); a = 3.9986(8) Å, b = 6.9200(14) Å, c = 23.184(5) Å for KCdBiS (x = 1.83) and a = 4.1363(8) Å, b = 7.1476(14) Å, c = 25.047(5) Å for CsCdBiSe (x = 1.13). These structures are intercalated derivatives of the BiSe structure by way of replacing some Bi atoms with divalent Cd atoms forming negatively charged BiSe-type quintuple [CdBiSe] 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 CsCdBiSe and CsCdBiS to be direct band gap semiconductors. Polycrystalline CsCdBiS samples show n-type conduction and an extremely low thermal conductivity of 0.33 W·m·K at 773 K. The cesium ions between the layers of CsCdBiS are mobile and can be topotactically exchanged with Pb, Zn, Co and Cd in aqueous solution. The intercalation of metal cations presents a direct "soft chemical" route to create new materials.