Superconductivity in novel Ge-based skutterudites: {Sr,Ba}Pt_4Ge_{12}

Combining experiments and ab initio models we report on $\rm SrPt_4Ge_{12}$ and $\rm BaPt_4Ge_{12}$ as members of a novel class of superconducting skutterudites, where Sr or Ba atoms stabilize a framework entirely formed by Ge-atoms. Below $T_c=5.35$ K, and 5.10 K for $\rm BaPt_4Ge_{12}$ and $\rm SrPt_4Ge_{12}$, 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.Comment: 4 pages, 4 figures, accepted for publication in PR

Structure and physical properties of type-I clathrate solid-solution Ba8PtxGe46-x-y□y (□= vacancy)

Starting from binary Ba8Ge433 a series of clathrates was synthesized forming a solid solution Ba8PtxGe46−x−yy, with a solubility limit of 3.5 Pt atoms/f.u. at 800 °C. Throughout the homogeneity region cubic primitive symmetry consistent with the clathrate type I has been confirmed. Phase relations were derived at subsolidus temperatures as well as at 800 °C. Physical properties, such as charge carrier density and transport quantities, were found to be heavily influenced by the Ge/ Pt substitution in the clathrate framework. From electrical resistivity data the series can be classified to be located close to a semiconducting regime with a gap in the electronic density of states of a few thousand K. The gradual change observed is a function of the combined effect of Ge/ Pt substitution and the reduction of vacancies. Thermal conductivity data are modeled in terms of Callaway and von Baeyer’s theory of heat carrying phonons. The efficiency of scattering vacancies is evidenced from the thermal conductivity study. Thermopower indicates that the majority charge carriers are electrons, with a slight variation of the charge carrier density being in the order of n(10^21 cm−3) as a function of the Pt content. Additionally, S(T) basically defines the figure of merit Z·T, which represents the thermoelectric performance of a certain material. Using the compound with the highest Seebeck coefficient, i.e., x=2.7 reveals ZT about 0.15 at 600 K

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−yy deriving from binary clathrate Ba8Ge433 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 Pm¯3n. Temperature-dependent x-ray spectra as well as heat capacity define a lowlying, 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, Ba8Zn2.1Ge41.52.4 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

Synthesis, Crystal Structure, and Physical Properties of the Type-I Clathrate Ba8−δNixΔySi46−x−yBa_{8-\delta}Ni_{x}\Delta_{y}Si_{46-x-y}

Type-I clathrate phase Ba(8)Ni(x)□(y)Si(46-x-y) (□ = vacancy) was obtained from the elements at 1000 °C with the homogeneity range 2.4 ≤ x ≤ 3.8 and 0 ≤ y ≤ 0.9. In addition, samples with low Ni content (x = 1.4 and 1.6; y = 0) and small Ba deficiency were prepared from the melt by steel-quenching. Compositions were established by microprobe analysis and crystal structure determination. Ba(8-δ)Ni(x)□(y)Si(46-x-y) crystallizes in the space group Pm ̅3n (No. 223) with lattice parameter ranging from a = 10.3088(1) Å for Ba(7.9(1))Ni(1.4(1))Si(44.6(1)) to a = 10.2896(1) Å for Ba(8.00(3))Ni(3.82(4))Si(41.33(6)). Single-crystal X-ray diffraction data together with microprobe analysis indicate an increasing number of framework vacancies toward compositions with higher Ni content. For all compositions investigated, Ni K-edge X-ray absorption spectroscopy measurements showed an electronic state close to that of elemental Ni. All samples exhibit metallic-like behavior with moderate thermopower and low thermal conductivity in the temperature range 300-773 K. Samples with compositions Ba(7.9(1))Ni(1.4(1))Si(44.6(1)) and Ba(7.9(1))Ni(1.6(1))Si(44.4(1)) are superconducting with T(c) values of 6.0 and 5.5 K, respectively