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TT: Tiefe Temperaturen
TT 21: Metall Isolatorüberg
änge
TT 21.5: Vortrag
Donnerstag, 29. März 2001, 10:30–10:45, J
The new clathrate Ba6Ge25 — •S. Paschen, W. Carrillo-Cabrera, V.H. Tran, M. Baenitz, R. Michalak, Yu. Grin, and F. Steglich — MPI für Chemische Physik fester Stoffe, Nöthnitzer Str. 40, D - 01187 Dresden
The new clathrate compound Ba6Ge25 crystallizes in the cubic space group P4132 and belongs to the structure family of chiral clathrates cP124. The Ge atoms form a non-space-filling network of pentagonal dodecahedra (‘cages’) and the Ba atoms are located both within and between the cages. A first-order phase transition takes place at T1"3 K, as evidenced by a symmetric peak in the specific heat. Our x-ray data show that, at room temperature, the inter-cage Ba atoms have elongated displacement ellipsoids, giving evidence for the ‘rattling’ of these Ba atoms. Below T1, the lengths of the displacement ellipsoids increase sharply indicating that the Ba atoms ‘lock-in’ at new equilibrium positions. A rearrangement of the Ge network follows. The structural phase transition strongly affects both the magnetic and the electronic-transport properties. In particular, the electrical resistivity increases and the magnetic susceptibility decreases sharply upon cooling below T1. The Hall constant is, however, virtually unaffected by the phase transition. Thus, the increase of the electrical resistivity must be due to a decrease of the Hall mobility below T1. A plausible explanation for this mobility reduction is that the charge carriers tend to be localized by the structural reorganization of the system, forming some sort of polaronic species below T1. The drop of the magnetic susceptibility may, in this context, be taken as an indication for the formation of (spin-singlet) bipolarons.