HIGH THERMOPOWER AND LOW THERMAL-CONDUCTIVITY IN SEMICONDUCTING TERNARY K-BI-SE COMPOUNDS - SYNTHESIS AND PROPERTIES OF BETA-K2BI8SE13 AND K2.5BI8.5SE14 AND THEIR SB ANALOGS

beta-K2Bi8Se13 (I), K2Sb8Se13 (II), K2.5Bi8.5Se14 (III), and K2.5Sb8.5 Se14 (IV) were synthesized by a molten flux method. The black needles of compound I were formed at 600 degrees C and crystallized in the mon oclinic P2(1)/m space group (No. 11) with a = 17.492(3) Angstrom, b = 4.205(1) Angstrom c = 18.461(4) Angstrom, beta = 90.49(2)degrees. The final R/R-w = 6.7/5.7%. Compound II is isostructural to I. Both I and II are isostructural with K2Bi8S13 which is composed of NaCl-, Bi2Te3- , and CdI2-type units connecting to form K+-filled channels. The thin black needles of III and TV obtained at 530 degrees C crystallize in t he same space group P2(1)/m with a = 17.534(4) Angstrom, b = 4.206(1) Angstrom, c = 21.387(5) Angstrom, beta = 109.65(2)degrees and a = 17.2 65(3) Angstrom, b = 4.0801(9)Angstrom, c = 21.280(3) Angstrom, beta = 109.31(1)degrees, respectively. The final R/R-w = 6.3/8.3% and 5.1/3.6 %. Compounds III and TV are isostructural and potassium and bismuth/an timony atoms are disordered over two crystallographic sites. The struc ture type is very closely related to that of I. Electrical conductivit y and thermopower measurements show semiconductor behavior with simila r to 250 S/cm and similar to 200 mu V/K for a single crystal of I and similar to 150 S/cm and similar to 100 mu V/K for a polycrystalline in got of III at room temperature. The effect of vaccum annealing on thes e compounds is explored. The optical bandgaps of all compounds were de termined to be 0.59, 0.78, 0.56, and 0.82 eV, respectively. The therma l conductivities of melt-grown polycrystalline ingots of I and III are reported.