The Two-Dimensional A_<i>x</i>Cd_<i>x</i>Bi_4–<i>x</i>Q₆ (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) Å, b = 6.9537(14) Å, c = 24.203(5) Å
for Cs1.43Cd1.43Bi2.57S6 (x = 1.43); a = 3.9968(8) Å, b = 6.9243(14) Å, c = 23.700(5) Å
for Rb1.54Cd1.54Bi2.46S6 (x = 1.54); a = 3.9986(8) Å, b = 6.9200(14) Å, c = 23.184(5) Å
for K1.83Cd1.83Bi2.17S6 (x = 1.83) and a = 4.1363(8) Å, b = 7.1476(14) Å, c = 25.047(5) Å
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. Polycrystalline
Cs1.43Cd1.43Bi2.57S6 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 Cs1.43Cd1.43Bi2.57S6 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.