Synthesis and structural characterization of novel clathrate-II compounds of silicon

The search for new and better thermoelectric (TE) materials has recently brought to light a somewhat forgotten class of compounds, the clathrates of group 14. The rich phenomenology of these and related cage-like structures has driven the most recent research on their electronic, magnetic, spectral, and conducting properties [Rowe, ed., 1995]. The present studies try to address the issue whether the clathrates are closed-shell compounds (i.e. semiconductor-like) by nature or whether the metallic behavior in some clathrates is simply a failure of the Zintl concept. The underlying principle in our work is based upon the formal oxidation states - a defect-free clathrate framework is electronically balanced since each group 14 element carries 4 valence electrons, which are shared with the 4 neighbors to form 4 covalent bonds. Each guest atom (alkali, alkaline-earth or rare-earth element) is considered merely as an electron donor that transfers its valence electron(s) to the network. Metallic behavior is expected even if the cages are not fully occupied, provided the framework retains the ideal tetrahedral topology. There are two possible ways to balance the extra electrons: 1) to create a vacancy in the framework, or 2) to substitute network atoms with electron-poorer elements, from groups 13 or 12 for example. Such considerations, however, are not always supported by experiments, suggesting that such a clear-cut approach is perhaps overly simplistic