Chemical physics of metallic clathrates with low carrier concentration

We discuss physical properties of two metallic clathrate compounds with low carrier concentration: Eu8Ga16Ge30, the first clathrate known to date, for which the 'guest' sites are fully occupied by 'rattling' lanthanide ions, and Ba6Ge25. Eu(8)Ga(16)G(30) exists in two modifications which both contain divalent Eu ions and order ferromagnetically at low temperatures (T-C = 10.5 K and 36 K, respectively). In Ba6Ge25 some of the 'rattling' Ba atoms lock, perhaps randomly, into split positions near 200 K. A BCS-like superconducting phase transition takes place at T-c = 0.14 K. T-c increases almost 20-fold, as the structural distortion is suppressed under hydrostatic pressure. (C) 2002 Elsevier Science B.V. All rights reserved