Regensburg 2004 – scientific programme
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CPP: Chemische Physik und Polymerphysik
CPP 10: Light Induced Phenomena
CPP 10.7: Talk
Monday, March 8, 2004, 17:15–17:30, H 38
Optical investigations of the clathrate compound Ba6Ge25 and new band structure calculations — •V. Voevodin1, J. Sichelschmidt1, S. Kimura2, W. Carrillo-Cabrera1, I. Zerec1, S. Paschen1, Y. Grin1, and F. Steglich1 — 1Max-Planck-Institut für Chemische Physik fester Stoffe, Nöthnitzer Str.40, 01187 Dresden — 2Division of Structural Science, Graduate School of Science and Technology Kobe University, Rokkodai, Nada-ku, Kobe 657-8501, Japan
The compound Ba6Ge25 has a cubic structure which consists of large
Ge cages with Ba atoms
trapped within. At room temperature a large Ba-atom displacement factor
provided evidence for
anharmonical vibrations ("rattling") of Ba atoms. At around TS=200 K
Ba6Ge25 undergoes a
two-step first-order phase transition and Ba atoms are locked into
off-center positions in Ge
cages. [1]
Recently new temperature dependent X-ray diffraction data on
single crystalline
samples revealed that at this phase transition also some of the
cage-building Ge atoms are
displaced from their room temperature sites. Therefore strong band
structure changes near the Fermi
surface are expected at T=TS. We present our optical investigations of
the electronic properties
of Ba6Ge25 in comparison with theoretical band structure
calculations. Near normal
reflectivity measurements for energies 0.01 - 30 eV have been done for
temperatures between 7 K and
300 K . Besides changes of the optical data at T=TS, below T=80 K a
pronounced optical
conductivity peak develops at 0.2 eV.
[1] S. Paschen et al., Phys. Rev. B 65, 134435(2002)