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The clathrate I BaGe□ [space group $$ Ia\bar{3}d $$, no. 230, a = 21.307(1) Å] has been synthesized as a single phase and characterized by x-ray powder diffraction and metallographic analysis. Electrical and thermal transport measurements have been performed in the temperature range of 5 K to 673 K. BaGe□ displays the electrical resistivity of a poor metal at low temperatures, with semiconducting-like behavior appearing above 300 K. [ABSTRACT FROM AUTHOR]

The single phase clathrate-I Ba8Ge433(space group Ia3d(no. 230), a= 21.307(1) Å) was synthesized by quenching the melt between cold steel plates. Specimens for physical property measurements were characterized by microstructure analysis and X-ray diffraction on polycrystalline samples as well as single crystals. Transport properties including thermopower, electrical resistivity, thermal conductivity and specific heat were investigated in a temperature range of 2–673 K. The electrical resistivity exhibits a metal-like temperature dependence below 300 K turning into a semiconductor-like behaviour above 300 K. The analysis of the specific heat at low temperature indicates a finite density of states at the Fermi level, thus corroborating the metallic character below 300 K. The temperature dependence of the specific heat was modelled assuming Einstein-like localized vibrations of Ba atoms inside the cages of the Ge framework. A conventional crystal-like behaviour of the thermal conductivity with a low lattice contribution (κl(300 K) = 2.7 W m−1K−1) has been evidenced. [ABSTRACT FROM AUTHOR]

Single crystals of the type-I clathrate Ba8Ni3.5Ge42.10.4(space group Pm3n, no. 223, a= 10.798(2) Å, l= 30 mm, ∅ = 8 mm) were grown from the melt using the Bridgman technique. Their composition, determined by microprobe analysis, reveals a distinctly lower Ni content than previously reported for the lower limit (x= 5.4) of the homogeneity range of the clathrate-I phase Ba8NixGe46−x. From single crystal X-ray diffraction data we introduce a crystal structure model that takes point defects (vacancies) in the Ge network into account. It reveals that both Ni and accumulate at a single site (6c) and that, as a consequence, the Ge network distorts considerably. Ba8Ni3.5Ge42.10.4shows metal-like behaviour (dρ/dT> 0) albeit with high resistivity at room temperature (ρ(300 K) 1 mΩ cm). Together with the low charge carrier concentration of 2.3 e−/unit cell at 300 K this is typical of a degenerate semiconductor. The lattice thermal conductivity is distinctly smaller than that of Ba8Ge433, where the vacancies partially order, and smaller than those of Ba–Ni–Ge type-I clathrates without vacancies, suggesting that disordered vacancies efficiently scatter heat-transporting phonons. We provide evidence that the maximum value of the thermoelectric figure of merit reached in Ba8Ni3.5Ge42.10.4, ZT(680 K) ≅ 0.21, can be further improved by adjusting the charge carrier concentration. [ABSTRACT FROM AUTHOR]