In-Cage Interactions in the Clathrate Superconductor Sr₈ Si₄₆

The clathrate I superconductor Sr₈ Si₄₆ is obtained under high-pressure high-temperature conditions, at 5 GPa and temperatures in the range of 1273 to 1373 K. At ambient pressure, the compound decomposes upon heating at T=796(5) K into Si and SrSi₂ . The crystal structure of the clathrate is isotypic to that of Na₈ Si₄₆ . Chemical bonding analysis reveals conventional covalent bonding within the silicon network as well as additional multi-atomic interactions between Sr and Si within the framework cages. Physical measurements indicate a bulk BCS type II superconducting state below T_c =3.8(3) K.