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By investigations of phase relations in the alloy system Ba-Pt-Si at 900°C we observe the formation of the compound BaPtSi 3 , which crystallizes in the noncentrosymmetric BaNiSn 3 structure type. Its space group is I4mm with the tetragonal lattice parameters a = 0.44094͑2͒nm and c = 1.0013͑2͒nm for the arc-melted compound annealed at 900°C. The characterization of the physical properties of BaPtSi 3 reveals a superconducting transition at 2.25 K with an upper critical field at T =0 K of Ϸ0.05 T. For analyzing the electronic structure, density-functional theory calculations are performed yielding very good agreement between theory and experiment for the structural properties. From relativistic electronic-structure calculations, Fermi surface nesting features are found for two characteristic double sets of bands. The spin-orbit splitting of the relativistic electronic bands is in general rather small at Fermi energy and, therefore, superconductivity adheres to an almost undisturbed BCS state.

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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