Synthesis and Thermoelectric Properties of the New Ternary Bismuth Sulfides KBi_6.33S₁₀ and K₂Bi₈S₁₃

KBi_6.33S₁₀ and K₂Bi₈S₁₃ were synthesized by the direct combination Of K₂S/Bi₂S₃ at high temperature (>700 °C). The reaction of K₂S/3.3Bi₂S₃ at 800 °C revealed the presence of a new ternary sulfide KBi_6.33S₁₀ (I, 92% yield). The shiny, silver polycrystalline material crystallizes in the orthorhombic space group Pnma (No. 62) with a = 24.05(1) Å, b = 4.100-(2) Å, c = 19.44(1) Å, V = 1917(3) Å, Z = 4, and d_c = 5.828 g/cm³. Data with F₀² > 3σ(F_o²), 862; no. of variables 108, 2θ_max 50°. The final R/RW = 4.3/4.7%. The structure consists of blocks of Bi₂Te₃- and CdI₂-type units that are connected to form a three-dimensional network with K⁺ ions located in the channels that run along the b axis. The same reaction but with a different reactant ratio at 750 °C gave the new ternary sulfide K₂Bi₈S₁₃ (II, 94% yield). This compound crystallizes in the monoclinic space group P2₁/m (No. 11) with a = 16.818(2) Å, b = 4.074(5) Å, c = 17.801(3) Å, β = 90.48(1)°, V = 1220(2) ų, Z = 2, and d_c = 5.900 g/cm³. Data with F₀2 > 3σ(F₀²), 1924; no. of variables 131, 2θ_max 50°. The final R/R_W = 7.3/8.2%. The structure of the shiny rodlike crystals is closely related to that of I. As in I, it also consists of Bi2Te3- and CdI₂-type fragments that connect to form K⁺-filled channels. The two potassium atoms and one bismuth atom are disordered over three sites. Electrical conductivity on I show semiconducting behavior with 10² S/cm at 300 K. Compound II possesses an electrical conductivity of 10² S/cm at 300 K. The optical bandgaps of I and II (0.06-0.24 eV) were estimated by infrared diffuse reflectance measurements. Thermal analysis and thermal conductivity data for I and II are reported. The thermal conductivity of KBi_6.33S₁₀ is found to be substantially lower than that of Bi₂Te₃.