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要旨:
Measurements of the optical conductivity on Ba6Ge25 reveal a shift of optical spectral weights towards higher energies, as temperature is lowered below the structural phase transition. This behavior may be understood from the structural modifications revealed from the new structure refinements of x-ray diffraction data from high quality single crystals. Apart from Ba atoms, some Ge cage atoms are also shifted into distant split sites below the phase transition. In this way one covalent bond is broken between the corresponding Ge atoms and they become threefold bonded. Electronic band structure calculations for the low symmetry ordered model show that the bond breaking causes a shifting of three bands from the conduction to the valence region. This leads to a shifting of optical spectral weights towards higher energies, which is in agreement with the experimental data.
