Structural chemistry, constitution and properties of clathrates
P. Rogl1, Y. Mudryk1, C. Paul2, S. Berger2, E. Bauer2, G. Hilscher2, C. Godart3, H. Noël4
1Institut für Physikalische Chemie, Universität Wien, Währingerstrasse 42, 1090 Wien, Austria
2Institut für Festkörperphysik, TU Wien, Wiedner Hauptstrasse 8-10, A-1040 Wien, Austria
3C.N.R.S. – F94320 Thiais, France and L.U.R.E., Université Paris Sud, 91405 Orsay, France
4Lab. de Chimie du Solide et Inorgan. Moléculaire, Univ. de Rennes I, F-35042 Rennes, France
Intermetallides, labelled "clathrates", have recently attracted widespread attention revealing a high potential for thermoelectric applications. Geometric crystallographic relations were employed to construct a general classification scheme for intermetallide crystal structures with four-coordinated networks, which are isomorphous or isopointal with hydrate clathrates. Based on earlier attempts to classify the structures of hydrate clathrates by Jeffrey , we define nine different groups of hydrate clathrates. For five of these classes intermetallide compounds have been hitherto observed. Formation of intermetallide clathrates, information of the corresponding phase diagrams and physical properties will be reviewed.
Novel europium substituted clathrates, Eu2-x(Sr,Ba)6-xMySi46-y (M = Al, Ga), consistent with the standardized clathrate I - Ba8Al16Ge30 type structure (space group Pmn), have been prepared in the Vienna laboratory. Europium atoms in Ba compounds preferentially occupy the 2a position and thus form a new quaternary version of the Ba8Al16Ge30 structure type. All clathrates studied are metals with low electrical conductivity partially exhibiting long-range magnetic order below 38 K of presumably ferromagnetic type. Magnetic susceptibilities indicate a Eu2+ ground state, in fine agreement with LIII absorption edge spectra.
 G. Jeffrey, in: Inclusion Compounds, Eds. J. Atwood, et al., (Acad. Press, 1984)