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X. Yan, A. Grytsiv, G. Giester, E. Bauer, P. Rogl, S. Paschen:
"Single-Crystal Investigations on Quaternary Clathrates Ba₈Cu₅Si_xGe_41−x (x = 6, 18, 41)";
Journal of Electronic Materials, 40 (2011), 5; S. 589 - 596.

Type I clathrates have been considered as promising thermoelectric materials
due to their special structural characteristics: the ``rattling´´ guest atoms in the
larger of the two cages of the clathrate I structure are frequently held
responsible for the low lattice thermal conductivity. By single-crystal x-ray
diffraction, we investigated the quaternary clathrates Ba8Cu5SixGe41 x (x = 6,
18, 41). Rietveld refinements confirmed that the clathrates in this system
crystallize with cubic primitive symmetry, in the type I clathrate structure,
and that no phase transitions occur in the temperature range investigated
(100 K to 300 K). We derive the concentration dependencies of the Debye
temperature, the Einstein temperatures, the static disorder parameters, and
the size of the two cages and argue that these dependencies underpin the
previously assumed different bonding character of the Ba guest atoms in
the larger and smaller cages. The linear thermal expansion coefficients for the
samples are derived.

Clathrate, single-crystal x-ray diffraction, cage size, thermal expansion coefficient

http://dx.doi.org/10.1007/s11664-010-1464-1