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

KBi 6.33 S 10 and K 2 Bi 8 S 13 were synthesized by the direct combination of K 2 S/Bi 2 S 3 at high temperature (>700 °C). The reaction of K 2 S/3.3Bi 2 S 3 at 800 °C revealed the presence of a new ternary sulfide KBi 6.33 S 10 ( 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 3, Z = 4, and d c = 5.828 g/cm 3 . Data with F o 2 > 3σ( F o 2 ), 862; no. of variables 108, 2θ max 50°. The final R / R w = 4.3/4.7%. The structure consists of blocks of Bi 2 Te 3 - and CdI 2 -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 K 2 Bi 8 S 13 ( II, 94% yield). This compound crystallizes in the monoclinic space group P 2 1 / 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) Å 3, Z = 2, and d c = 5.900 g/cm 3 . Data with F o 2 > 3σ( F o 2 ), 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 Bi 2 Te 3 - and CdI 2 -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 10 2 S/cm at 300 K. Compound II possesses an electrical conductivity of 10 2 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 6.33 S 10 is found to be substantially lower than that of Bi 2 Te 3 .