A Borosilicide with Clathrate VIII Structure

[Image: see text] 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 I43̅m; 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 (Pm3̅n; 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.