Physical Properties of Single-Crystalline Ba8Ni3.5Ge42.1□0.4

Abstract

Clathrates are candidate materials for thermoelectric applications because of a number of unique properties. The clathrate I phases in the Ba-Ni-Ge ternary system allow controlled variation of the charge carrier concentration by adjusting the Ni content. Depending on the Ni content, the physical properties vary from metal-like to insulator-like and show a transition from p-type to n-type conduction. Here we present first results on the characterization of millimeter-sized single crystals grown by the Bridgman technique. Single crystals with a composition of Ba8Ni3.5Ge42.1□0.4 show metallic behavior (dρ/dT > 0) albeit with high resistivity at room temperature [ρ (300 K) = 1 mΩ cm]. The charge carrier concentration at 300 K, as determined from Hall-effect measurements, is 2.3 e−/unit cell. The dimensionless thermoelectric figure of merit estimated at 680 K is ZT ≈ 0.2.