Formation of clathrates

The present paper focuses on a systematic study of clathrate formation, clathrate structures and bonding in clathrate materials. The analysis shows the difficulties in preparation and design of clathrate compounds at a given electron/atom concentration. Validity and shortcomings of the Zintl concept for clathrates will be outlined. Nevertheless the information obtained, although not complete for many systems, may provide useful in defining compositional regions of interest for further search for novel clathrate materials with interesting thermoelectric properties.