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N. Melnychenkc-Koblyuk, A. Grytsiv, L. Fornasari, H. Kaldarar, H. Michor, F. Röhrbacher, M. Koza, E. Royanian, E. Bauer, P. Rogl, M. Rotter, H. Schmid, F. Marabelli, A. Devishvili, M. Doerr, G. Giester:
"Ternary clathrates Ba-Zn-Ge: phase equilibria, crystal chemistry and physical properties";
Journal of Physics: Condensed Matter, 19 (2007), 216223; S. 1 - 26.

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

N Melnychenko-Koblyuk 1, A Grytsiv 1, L Fornasari 2,3, H Kaldarar 2, H Michor 2, F Röhrbacher 2, M Koza 4, E Royanian 1,2, E Bauer 2, P Rogl 1,7, M Rotter 1, H Schmid 1, F Marabelli 3, A Devishvili 4, M Doerr 5 and G Giester 6

1 Institute of Physical Chemistry, University of Vienna, A-1090 Wien, Währingerstrasse 42, Austria
2 Institute of Solid State Physics, Vienna University of Technology, A-1040 Wien, Wiedner Hauptstrasse, 8-10, Austria
3 Physics Department 'A Volta', Universita di Pavia, I-27100, Pavia, via Bassi, 6, Italy
4 Institute Laue Langevin, 6 rue Jules Horowitz, 38042 Grenoble, France
5 Institute of Solid State Physics (IFP), TU Dresden, D-01062 Dresden, Germany
6 Institute of Mineralogy and Crystallography, University of Vienna, A-1090 Wien, Althanstrasse 14, Austria
7 Author to whom any correspondence should be addressed
E-mail: peter.franz.rogl@univie.ac.at

Abstract.
The formation, phase relations, crystal chemistry and physical properties were investigated for the solid solution Ba₈Zn_xGe_46-x-y[]_y deriving from binary clathrate Ba₈Ge₄₃[]₃ 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 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, Ba₈Zn_2.1Ge_41.5[]_2.4 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 μ V K⁻¹ at 700 K.
(Some figures in this article are in colour only in the electronic version)

http://dx.doi.org/10.1088/0953-8984/19/21/216223