A brief overview on low sodium content silicides : are they mainly clathrates, fullerenes, intercalation compounds or Zintl phases
Résumé
Lowsodiumcontentsilicides discovered by some of us in the sixties are compared to some analogous families of materials. As endofullerenes constituted by the smallest Si20, Si24 and Si28 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, 23Na and 29Si 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-Klemm-type phases.