  Understanding the covalent clathrate formation is a crucial point for the
design of new superhard materials with intrinsic coupling of superhardness and
metallic conductivity. Silicon clathrates have the archetype structures that
can serve an existant model compounds for superhard clathrate frameworks
"Si-B", "Si-C", "B-C" and "C" with intercalated atoms (e.g. alkali metals or
even halogenes) that can assure the metalic properties. Here we report the in
situ and ex situ studies of high-pressure formation and stability of clathrates
Na8Si46 (structure I) and Na24+xSi136 (structure II). Experiments have been
performed using standard Paris-Edinburgh cells (opposite anvils) up to 6 GPa
and 1500 K. We have established that chemical interactions in Na-Si system and
transition between two structures of clathrates occur at temperatures below
silicon melting. The strong sensitivity of crystallization products to the
sodium concentration have been observed. A tentative diagram of clathrate
transformations has been proposed. At least up to ~6 GPa, Na24+xSi136
(structure II) is stable at lower temperatures as compared to Na8Si46
(structure I).
