A brief overview on low sodium content silicides:: are they mainly clathrates, fullerenes, intercalation compounds or Zintl phases?

Low sodium content silicides discovered by some of us in the sixties are compared to some analogous fan-Lilies of materials. As endofullerenes constituted by the smallest Si-20, Si-24 and Si-28 clusters, they are polymerized in 3D-structures of face sharing polyhedra, giving rise to clathrate-like structures (similar to those of the gas and liquid hydrates). Several physical properties (conductivity, Na-23 and Si-29 NMR, EPR) show that the electronic transfer from sodium to the silicon host is rather low and appears only gradually with rising Na-content as a consequence of Na 3s and Si 3s-3p orbital mixing which gives rise to an antibonding band. Silicon clathrates differ from the homologous germanium and tin compounds characterized by a large number of defects in the host structure which leads to stronger ionization of the alkali atoms and to the formation of some Ge- anions within Zintl-Klenim-type phases. (C) 2002 editions scientifiques et medicales Elsevier SAS. All rights reserved.