  Electronic band structures for Ba$_{6}$Ge$_{25}$ and
Ba$_{4}$Na$_{2}$Ge$_{25}$ 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
Ba$_{6}$Ge$_{25}$ produces a splitting of this peak which may therefore 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 qualitatively account for the
experimentally observed decrease of the magnetic susceptibility and electrical
resistivity at the phase transition.
