Ternary clathrates Ba-Zn-Ge: phase equilibria, crystal chemistry and physical properties

The formation, phase relations, crystal chemistry and physical properties were investigated for the solid solution Ba8ZnxGe46-x-yBoxy deriving from binary clathrate Ba8Ge43 Box3 with a solubility limit of 8 Zn atoms per formula unit at 800 deg. C (Box is a vacancy). Single-crystal x-ray data throughout the homogeneity region confirm the clathrate type I structure with cubic primitive space group type Pm3-bar3n. Temperature-dependent x-ray spectra as well as heat capacity define a low-lying, almost localized, phonon branch, whereas neutron spectroscopy indicates a phonon mode with significant correlations. The transport properties are strongly determined by the Ge/Zn ratio in the framework of the structure. Increasing Zn content drives the system towards a metal-to-insulator transition; for example, Ba8Zn2.1Ge41.5Box2.4 shows metallic behaviour at low temperatures, whilst at high temperatures semiconducting features become obvious. A model based on a gap of the electronic density of states slightly above the Fermi energy was able to explain the temperature dependences of the transport properties. The thermal conductivity exhibits a pronounced low-temperature maximum, dominated by the lattice contribution, while at higher temperatures the electronic part gains weight. Zn-rich compositions reveal attractive Seebeck coefficients approaching -180 μV K-1 at 700 K