First Quaternary A−Pb−Bi−Q (A = K, Rb, Cs; Q = S, Se) Compounds: Synthesis, Structure, and Properties of α- and β-CsPbBi3Se6, APbBi3Se6, (A = K, Rb), and APbBi3S6 (A = Rb, Cs)

α-CsPbBi 3 Se 6 ( I ), β-CsPbBi 3 Se 6 ( II ), RbPbBi 3 Se 6 ( III ), KPbBi 3 Se 6 ( IV ), CsPbBi 3 S 6 ( V ), and RbPbBi 3 S 6 ( VI ) were synthesized by the polychalcogenide flux method. α-CsPbBi 3 Se 6 was obtained at 720 °C and crystallizes in the space group Pnma (no. 62) with a = 23.564(6) Å, b = 4.210(2) Å, c = 13.798(3) Å at room temperature. Final R / R w = 3.0/3.6%. In this compound, parallel NaCl-type Pb/Bi/Se columns with rectangularly shaped cross-sections are interconnected by edge sharing to build a 3-D tunnel framework with Cs atoms located inside the tunnels. The hexagonal plates of β-CsPbBi 3 Se 6 were obtained at 400 °C and crystallize in the space group P 6 3 / mmc (no. 194) with a = 4.213(2) Å, c = 25.22(1) Å, γ = 120° at −100 °C. Final R / R w = 4.2/4.7%. APbBi 3 Se 6 (A = Rb, K) and APbBi 3 S 6 (A = Cs, Rb) are isostructural with β-CsPbBi 3 Se 6 and their hexagonal cell parameters were obtained at room temperature. The structure is composed of negatively charged Bi 2 Te 3 -type bilayers separated by alkali metals, which are distributed over two different crystallographic sites. The alkali metal ions are loosely packed in the interlayer space making them mobile. Topotactic ion-exchange reactions of two compounds, β-CsPbBi 3 Se 6 and RbPbBi 3 Se 6, were examined with LiI and NaI in the solid state and in aqueous solution. Prolonged water contact of the hexagonal compounds leads to decomposition and leaching of alkali metal and Pb 2+ ions. Electrical conductivity and thermopower measurements for single crystals of I, II, and III show n-type semiconductor behavior with 0.6, 0.3, and 0.3 S/cm and −730, −550, and −560 μV/K at room temperature, respectively. The optical band gaps of all compounds range from 0.27 to 0.89 eV. Thermal properties of the compounds are reported.