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-CsPbBi₃Se₆ (I), β-CsPbBi₃Se₆ (II), RbPbBi₃Se₆ (III), KPbBi₃Se₆ (IV), CsPbBi₃S₆ (V), and RbPbBi₃S₆ (VI) were synthesized by the polychalcogenide flux method. α-CsPbBi₃Se₆ was obtained at 720 °C and crystallizes in the space group Pnma (no. 62) with α = 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-CsPbBi₃Se₆ were obtained at 400 °C and crystallize in the space group P6₃/mmc (no. 194) with a = 4.213(2) Å, c = 25.22(1) Å, y = 120° at -100 °C. Final R/R_W = 4.2/4.7%. APbBi₃Se₆ (A = Rb, K) and APbBi₃S₆ (A = Cs, Rb) are isostructural with β-CsPbBi₃Se₆ and their hexagonal cell parameters were obtained at room temperature. The structure is composed of negatively charged Bi₂Te₃-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-CsPbBi₃Se₆ and RbPbBi₃Se₆, 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 Pb²⁺ 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.