High Thermopower and Low Thermal Conductivity in Semiconducting Ternary K-Bi-Se Compounds. Synthesis and Properties of β-K₂Bi₈Se₁₃ and K_2.5Bi_8.5Se₁₄ and Their Sb Analogues

β-K₂Bi₈Se₁₃ (I), K₂Sb₈Se₁₃ (II), K_2.5Bi_8.5Se₁₄ (III), and K_2.5Sb_8.5Se₁₄ (IV) were synthesized by a molten flux method. The black needles of compound I were formed at 600 °C and crystallized in the monoclinic P2₁/m space group (No. 11) with a -17.492(3) Å, b = 4.205(1) Å, c= 18.461(4) Å, β= 90.49(2)°. 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 IV obtained at 530 °C crystallize in the same space group P2₁/m with a -17.534(4) Å, b = 4.206(1) Å, c = 21.387(5) Å, β= 109.65(2)° and a = 17.265(3) Å, b = 4.0801(9) Å, c = 21.280(3) Å, β = 109.31(1)°, respectively. The final R/R_w = 6.3/8.3% and 5.1/3.6%-. Compounds III and IV 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 ∼250 S/cm and ∼-200 μV/K for a single crystal of I and ∼150 S/cm and ∼-100 μV/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.