Mastering extreme size constraints in the clathrate‐I borosilicide Cs 8 B 8 Si 38

Abstract Cs 8 B 8 Si 38 is obtained by high‐pressure high‐temperature synthesis at p =8 GPa and T =1273 K. The new compound (space group Pm n ; a =10.0312(3) Å) is the third example for a clathrate‐I borosilicide after the potassium and rubidium varieties, respectively. The phase slowly deteriorates in air and moisture but is thermally stable up to 1050 K at ambient pressure. [Cs + ] 8 [B – ] 8 [Si 0 ] 38 is electronically balanced, diamagnetic, and shows semiconducting behavior.