Site occupancy preference, electrical transport property and thermoelectric performance of Ba8Cu6−xGe40+x single crystals grown by using different metal fluxes

Different fluxes, Ga, Bi, In, Sb, and Sn, have been used to grow clathrate Ba8Cu5.3Ge40.7 (BCG) single crystals using a flux method, and BCG single crystals have been successfully synthesized by Bi-, In- and Sn-flux. The grown crystals were characterized by single crystal and power X-ray diffraction as well as energy dispersive X-ray spectroscopy measurements. The solubility of flux inside the crystals was determined and the actual chemical composition was decided to be: Ba8Cu5.1Ge40.5 (BCG-Bi), Ba8Cu4.6In1.4Ge40.0 (BCG-In) and Ba8Cu5.1Ge40.2Sn0.7 (BCG-Sn) for the compounds grown from Bi-, In- and Sn-flux, respectively. A high electron mobility (μ), 44.8 cm2 V−1 s−1 at 300 K, is observed for BCG-In, which is almost 5 times higher than the reported data at a similar carrier concentration. Therefore, enhanced thermoelectric power factors and ZT values are obtained. The relationship between the structure and electrical/thermoelectric properties is discussed. The high μ is most probably due to the relatively ordered cage framework structure in BCG-In, because the 6c site is fully occupied by Cu and In, and the other two sites are uniformly occupied by Ge.