Sn-based type-VIII single-crystal clathrates with a large figure of merit

Single-crystal samples of type-VIII Ba8Ga16−xCuxSn30 (x = 0, 0.03, 0.06, 0.15) clathrates were prepared using the Sn-flux method. At room temperature the carrier density, n, is 3.5–5 × 1019 cm−3 for all the samples, the carrier mobility, μH, increases to more than twice that of Ba8Ga16Sn30 for all the Cu doping samples, and consequently the electrical conductivity is enhanced distinctly from 1.90 × 104 S/m to 4.40 × 104 S/m, with the Cu composition increasing from x = 0 to x = 0.15. The Seebeck coefficient, α, decreases slightly with the increases in Cu composition. The κ values are about 0.72 W/mK at 300 K and are almost invariant with temperature up to 500 K for the samples with x = 0 and x = 0.03. The lattice thermal conductivity, κL, decreases from 0.59 W/mK for x = 0 to 0.50 W/mK for x = 0.03 at 300 K. The figure of merit for x = 0.03 reaches 1.35 at 540 K. (condensed matter: electronic structure, electrical, magnetic, and optical properties)