Regensburg 2004 – wissenschaftliches Programm

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CPP: Chemische Physik und Polymerphysik

CPP 10: Light Induced Phenomena

CPP 10.7: Vortrag

Montag, 8. März 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. Steglich11Max-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)

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