Molten Salt Synthesis and Properties of Three New Solid-State Ternary Bismuth Chalcogenides, β-CsBiS₂, γ-CsBiS₂, and K₂Bi₈Se₁₃

β-CsBiS₂, γ-CsBiS₂, and K₂Bi₈Se₁₃ were synthesized by the alkali metal polychalcogenide flux technique. An investigation of the Bi/Cs₂S_x system at 290 °C revealed the presence of a new ternary sulfide β-CsBiS₂ (I) (83% yield). The red substance crystallizes in the monoclinic P2₁/c space group (No. 14) with a = 7.794(5) Å, b = 9.610(6) Å, c = 7.329(4) Å, β = 102.16(5)°. The final R/R_w = 7.4/10%. The structure consists of BiS₃ trigonal pyramids connected at two vertices to form (BiS₂)_n ⁿ⁻ chains along the a axis with Cs⁺ ions in between the chains. β-CsBiS₂ is an insulator. γ-CsBiS₂ was obtained as above but at 350 °C. It crystallizes in the rhombohedral system with a = 4.166(4) Å and c = 48.77(5) Å. It has a superstructure related to that of RbBiS₂. Similar investigations in the Bi/K₂Se_x system at 330 °C gave the new ternary selenide K₂Bi₈Se₁₃ (II) (88% yield). This compound crystallizes in the triclinic P1̄ space group (No. 2) with a = 13.768(2) A, b = 12.093(3) Å, c = 4.1656(6) Å, α = 89.98(1)°, β = 98.64(1)°, γ = 87.96(1)°. The final R/R_w = 8.2/8.9%. The structure of the black crystals consists of layers of BiSe₆ octahedra with BiSe₅ square pyramidal units connecting the layers to form a three-dimensional network with K⁺ ions located in the channels. The [Bi₈Se₁₃]²⁻ framework can be thought of as a hybrid of three different layered structure types interconnected to form a 3-D network. Structural features from the Bi₂Te₃, Sb₂S₃, and CdI₂ lattices are represented in this framework. Electrical conductivity and thermopower measurements on γ-CsBiS₂ and K₂Bi₈Se₁₃ show n-type semiconducting behavior. For both materials the room temperature conductivity is ∼ 10⁻² S/cm. The reaction of Bi with Cs₂S_x and K₂Se_x fluxes was investigated with differential scanning calorimetry (DSC). The optical band gaps of β-CsBiS₂, γ-CsBiS₂, and K₂Bi₈Se₁₃ were determined by UV/vis/near-IR diffuse reflectance measurements to be 1.43, 1.11, and 0.76 eV, respectively.