A Borosilicide with Clathrate VIII Structure

The high-pressure phase Na₈B_xSi_46-x(3 < x < 5) is the first representative of a borosilicide crystallizing in the rarely occurring clathrate VIII type structure. Crystals with composition Na₈B₄Si₄₂(space group I43¯ m; a = 9.7187(2) Å Pearson symbol cI54) were obtained at 5-8 GPa and 1200 K. The clathrate I modification exists for the same composition at lower pressure with a larger cell volume (Pm3¯ n; a = 9. 977(2) Å cP54). Profound structural adaptions allow for a higher density of the clathrate VIII type than clathrate I, opening up the perspective of obtaining clathrate VIII type compounds as high-pressure forms of clathrate I.