Structure and thermoelectric properties of the n-type clathrate Ba8Cu5.1Ge40.2Sn0.7

Abstract

We study type I clathrate Ba₈Cu_5.1Ge_40.2Sn_0.7 single crystals (space group Pmn, no. 223, a = 10.7151(3)) grown using a Sn flux method. Microprobe analysis and single-crystal X-ray diffraction reveal a small amount of Sn embedded in the Cu/Ge framework, which increases disorder at the guest Ba sites. Ba₈Cu_5.1Ge_40.2Sn_0.7 is diamagnetic with a susceptibility of ∼2.8 × 10⁻⁷ emu g⁻¹ and shows metal-like behavior (dρ/dT > 0) with a low charge carrier concentration of 0.5 e⁻ per unit cell at 300 K. The single crystals show a relatively high carrier mobility (μ (300 K) = 11.9 cm² V⁻¹) and very low lattice thermal conductivity (∼0.6 W m⁻¹ K⁻¹). The thermoelectric figure of merit ZT of Ba₈Cu_5.1Ge_40.2Sn_0.7 single crystals reaches a maximum value of 0.6 at 773 K, which can be further improved by adjusting the chemical composition.