Recent Developments on the Clathrates M8Si46 and MxSi136 (M = Na, K, Rb, Cs, Ba; 0 ≤ x ≤ 24) as Inclusion Compounds of Alkali Metals in Connected Fullerene-like Cages of Silicon

In the structure of the clathrate-type silicides M(8)Si(46) and M(x)Si(136) (M = Na, K, Rb, Cs, Ba; 0 <= x <= 24), the silicon atoms form 3D covalent sublattices which correspond to the association of two kinds of fullerene-like cages : Si(20) and Si(24), and Si(20) and Si(28) respectively. The M atoms are trapped at the centre of all or part of the two different cages and are distributed according to a A15 and a C15 type sublattice, respectively. Due to their clathrate-type structure, these silicides exhibit original and interesting physical properties. The two structures are described and a short review is given on the previous and recent developments on these cage-like compounds.