High thermopower and low thermal conductivity in semiconducting ternary K-Bi-Se compounds.: Synthesis and properties of β-K2Bi8Se13 and K2.5Bi8.5Se14 and their Sb analogues

beta-K2Bi8Se13 (I), K2Sb8Se13 (II), K2.5Bi8.5Se14 (III), and K2.5Sb8.5Se14 (IV) were synthesized by a molten flux method. The black needles of compound I were formed at 600 degrees C and crystallized in the monoclinic 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 the 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 = 17.265(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/antimony atoms are disordered over two crystallographic sites. The structure type is very closely related to that of I. Electrical conductivity and thermopower measurements show semiconductor behavior with similar 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 ingot of III at room temperature. The effect of vaccum annealing on these compounds is explored. The optical bandgaps of all compounds were determined to be 0.59, 0.78, 0.56, and 0.82 eV, respectively. The thermal conductivities of melt-grown polycrystalline ingots of I and III are reported.