The formation, phase relations, crystal chemistry and physical properties were investigated for the solid solution deriving from binary clathrate with a solubility limit of 8 Zn atoms per formula unit at 800 °C ( is a vacancy). Single-crystal x-ray data throughout the homogeneity region confirm the clathrate type I structure with cubic primitive space group type . 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, 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.

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Combining experiments and ab initio models we report on SrPt4Ge12 and BaPt4Ge12 as members of a novel class of superconducting skutterudites, where Sr or Ba atoms stabilize a framework entirely formed by Ge atoms. Below T(c)=5.35 and 5.10 K for BaPt4Ge12 and SrPt4Ge12, respectively, electron-phonon coupled superconductivity emerges, ascribed to intrinsic features of the Pt-Ge framework, where Ge-p states dominate the electronic structure at the Fermi energy.

Phase relations at subsolidus temperatures as well as at T = 800°C, crystallographic data, electrical and thermal transport measurements, and heat capacity data are reported for several compositions within the clathrate type-I solid solution: Ba 8 Pd x Ge 46−x−y ᮀ y ͑ᮀ is a vacancy͒. The solid solution derives from binary clathrate Ba 8 Ge 43 ᮀ 3 with a solubility limit of 3.8 Pd atoms per formula unit at T = 800°C. Structural investigations throughout the homogeneity region confirm cubic primitive symmetry consistent with the space group type Pm3n and lattice parameters ranging from a = 1.0657͑2͒ nm for Ba 8 Ge 43 ᮀ 3 to a = 1.077 41͑2͒ nm for Ba 8 Pd 3.8 Ge 42.2 ᮀ 0.0 . The primary field of clathrate crystallization has been elucidated from micrography and differential thermal analyses. Both heat capacity and inelastic neutron diffraction define a low-lying, almost localized, phonon branch. Studies of transport properties evidence electrons as the majority charge carriers for most of the homogeneity region; however, at the Pd-rich limit, holes dominate the electronic transport. The crossover between both regimes provides appropriate conditions for attractively high Seebeck values. The lattice contribution dominates the overall thermal conductivity.

The present paper describes the formation, phase relations at subsolidus temperatures and at 800 • C, crystal chemistry and physical properties of a series of ternary clathrates as part of the solid solution Ba 8 Cd x Ge 43−5x/8 3−3x/8 , derived from binary Ba 8 Ge 43 3 with a solubility limit of 8 Cd per formula unit at 800 • C. Structural investigations in all cases confirm cubic primitive symmetry with a lattice parameter a ≈ 1.1 nm, consistent with the space group type Pm 3n. Both the temperature dependent x-ray spectra and the heat capacity define a low-lying, almost localized, phonon branch.Studies of transport properties show electrons to be the majority charge carriers in the systems. As the Cd content increases, the system is driven towards a metal-to-insulator transition, causing Ba 8 Cd 4.7 Ge 40.3 1.0 , for example, to show metallic behaviour at low temperatures while at high temperatures semiconducting features become obvious. A model based on a gap of the electronic density of states slightly above the Fermi energy perfectly explains such a scenario. Thermal conductivity exhibits a pronounced low temperature maximum, dominated by the lattice contribution, while at higher temperatures the electronic part becomes more important.