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Zusammenfassung:
Electronic band structures for Ba6Ge25 and Ba4Na2Ge25 clathrates are calculated using linear muffin-tin orbital method within the local-density approximation. It is found that barium states strongly contribute to the density of states at the Fermi level and thus can influence the transport properties of the compounds. A sharp peak of the density of states is found just at the Fermi level. It is also shown that the shifting of barium atoms toward experimentally deduced split positions in Ba6Ge25 produces a splitting of this peak which may be interpreted as a band Jahn-Teller effect. If the locking of the barium atoms at the observed structural phase transition is assumed, this reduction of the density of states at the Fermi level can account for the experimentally observed decrease of the magnetic susceptibility and electrical resistivity at the phase transition, and the values of density of states are in agreement with low-temperature specific-heat measurements and variation of superconducting transition temperature with pressure.
