Structural relationships between intermetallic clathrates, porous tectosilicates and clathrate hydrates

Abstract

Intermetallic clathrate-type compounds, such as |Eu8|[Ga 16Ge30], are Zintl phases in which a formal charge transfer from the more electropositive guest atoms (Eu) to the more electronegative host atoms (Ga or Ge) allows the latter to form skeletons with strong directed covalent bonds (polyanions [Ga16Ge30] 16-). On the other hand, in clathrasils, porous silica polymorphs, such as melanophlogite |(CH4, N2, CO2,...) 8| [Si 46[4] O 92[2] ] the Si atoms form skeletons which are related to those of Zintl phases, if oxygen atoms are neglected. As in other silicates, in clathrasils the oxygen atoms are located near to hypothetical Si(Al)-Si(Al) bonds, thus producing the tetrahedral coordination around the Si(Al) atoms. The similarities between the structures of intermetallic clathrates and clathrasils can be understood in the light of both the extended Zintl-Klemm concept and the Pearson's generalised octet rule. It is recalled that these principles were successfully applied to describe the structures of other ternary and quaternary aluminates and silicates. In this article, we report a comprehensive and comparative study of both clathrate-like and zeolite-like porous tectosilicate structures to show that their skeletons obey the same general principles. Clathrate hydrates, which also adopt similar skeletons to clathrasils, are also discussed in detail. © 2010 Springer-Verlag Berlin Heidelberg.