A Borosilicide with Clathrate VIII Structure.

The high-pressure phase Na 8 B x Si 46- x (3 < x < 5) is the first representative of a borosilicide crystallizing in the rarely occurring clathrate VIII type structure. Crystals with composition Na 8 B 4 Si 42 (space group I 43̅ m ; a = 9.7187(2) Å; Pearson symbol cI 54) 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 ( Pm 3̅ n ; a = 9. 977(2) Å; cP 54). 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.