Chemical synthesis of type I clathrates

We report synthesis of type I clathrates in a metathesis reaction between alkali metal (Na, K, Rb, and Cs) silicides (or germanides) and ammonium halides. In this reaction, clathrates such as Na$_{8}$Si$_{46}$ are formed. The byproducts are alkali halides, ammonia, and hydrogen. The purity of the clathrate depends strongly on pressure and temperature, with the best product formed at t = 300 $^{o}$C and pressures 20 -- 30 atm. In the same conditions, lithium silicides form only amorphous/nanocrystalline silicon (or germanium). The most interesting aspect of this approach is synthesis of mixed, silicon/germanium clathrates.