Band-structure calculations forBa6Ge25and

Abstract

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 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.

ClathrateBa6Ge25:Thermodynamic, magnetic, and transport properties

“…If states are shifted from the Fermi level to below the Fermi level when cooling Ba 6 Ge 25 below T S1,S2 , e would be reduced while n remains unchanged, just as observed in our experiments. Band structure calculations 33 indicate that such a scenario may indeed apply to Ba 6 Ge 25 .…”

Ferromagnetism in Fe-doped Ba6Ge25 chiral clathrate

“…For comparison, in (Ga 1−x Mn x )As, Mn carries about 5 µ B , and T c scales linearly with small values of x, reaching T c = 110 K for x = 0.05. 7 The present system is quite similar in T c vs. x. Band-structure calculations 22 indicate a gap below the Fermi level in Ba 6 Ge 25 which may quite possibly be the E F location in the Zintl composition Ba 6 Ge 24 . The Zintl concept 23 involves valence counting to give a closed-shell bonding configuration.…”

Positional disorder of Ba in the thermoelectric germanium clathrate Ba6Ge25

“…From a structural point of view the observed shifting of Ba atoms toward one of the split positions may be viewed as a lowering of the local symmetry of Ba(2) and Ba(3) sites from S.G. P4 1 32 to S.G. P2 1 3 as discussed in Ref. [7]. In line with previous structural studies [4,6] the present one did not find any tendency of Ba(1) atoms to move off the centers of Ge dodecahedra upon going through the phase transition.…”

“…The formation of such polaronic quasiparticles is evoked to explain the observed strong reduction of the carrier mobility at low temperatures [6]. Also, it has been predicted that a shifting of Ba(2) and Ba(3) atoms toward split positions in the framework channels would reduce the density of states at the Fermi level which may account for the observed change in the transport properties at low temperatures [7]. Obviously understanding the properties of Ba 6 Ge 25 requires a detailed knowledge of the changes its atomic structure undergoes with temperature.…”