SYNTHESIS AND THERMOELECTRIC PROPERTIES OF THE NEW TERNARY BISMUTH SULFIDES KBI6.33S10 AND K2BI8S13

KBi6.33S10 and K2Bi8S13 were synthesized by the direct combination of K2S/Bi2S3 at high temperature (> 700 degrees C). The reaction of K2S/3 .3Bi(2)S(3) at 800 degrees C revealed the presence of a new ternary su lfide KBi6.33S10 (I, 92% yield). The shiny, silver polycrystalline mat erial crystallizes in the orthorhombic space group Pnma (No. 62) with a = 24.05(1) Angstrom, b = 4.100(2) Angstrom, c = 19.44(1) Angstrom, V = 1917(3) Angstrom(3), Z = 4, and d(c) = 5.828 g/cm(3). Data with F-o (2) > 3 sigma(F-o(2)), 862; no. of variables 108, 2 theta(max)50 degre es. The final R/R(w) = 4.3/4.7%. The structure consists of blocks of B i2Te3- and CdI2-type units that are connected to form a three-dimensio nal network with Kf ions located in the channels that run along the b axis. The same reaction but with a different reactant ratio at 750 deg rees C gave the new ternary sulfide K2Bi8S13 (II, 94% yield). This com pound crystallizes in the monoclinic space group P2(1)/m (No. 11) with a = 16.818(2) Angstrom, b = 4.074(5) degrees, c = 17.801(3) Angstrom, beta = 90.48(1)degrees V = 1220(2) Angstrom(3), Z = 2, and d(c) = 5.9 00 g/cm(3). Data with F-0(2) > 3 sigma(F-0(2)), 1924; no. of variables 131, 2 theta(max) 50 degrees. The final R/R(w) = 7.3/8.2%. The struct ure 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 Kt-filled channels. The two potassium atoms and one bismuth a tom are disordered over three sites. Electrical conductivity on I show semiconducting behavior with 10(2) S/cm at 300 K. Compound II possess es an electrical conductivity of 10(2) S/cm at 300 K. The optical band gaps of I and II (0.06-0.24 eV) were estimated by infrared diffuse ref lectance measurements. Thermal analysis and thermal conductivity data for I and II are reported. The thermal conductivity of KBi6.33S10 is f ound to be substantially lower than that of Bi2Te3.