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Synthesis and Thermoelectric Properties of the New Ternary Bismuth Sulfides KBi6.33S10 and K2Bi8S13

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posted on 11.07.1996, 00:00 by Mercouri G. Kanatzidis, Timothy J. McCarthy, Troy A. Tanzer, Li-Heng Chen, Lykourgos Iordanidis, Tim Hogan, Carl R. Kannewurf, Ctirad Uher, Baoxing Chen
KBi6.33S10 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 KBi6.33S10 (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) Å3, Z = 4, and dc = 5.828 g/cm3. Data with Fo2 > 3σ(Fo2), 862; no. of variables 108, 2θmax 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) Å3, Z = 2, and dc = 5.900 g/cm3. Data with Fo2 > 3σ(Fo2), 1924; no. of variables 131, 2θmax 50°. The final R/Rw = 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 KBi6.33S10 is found to be substantially lower than that of Bi2Te3.