Silicon clathrates and carbon analogs: high pressure synthesis, structure, and superconductivity

Compounds with cage-like structures of elemental silicon and carbon are comparatively reviewed. Barium containing silicon clathrate compounds isomorphous with type I gas hydrates were prepared using high pressure and high temperature (HPHT) conditions, and found to become superconductors. The application of HPHT conditions to Zintl binary silicides have produced a number of silicon-rich cage-like structures including new clathrate structures; most of them are superconductors. Carbon analogs of silicon clathrates can be prepared by 3D polymerization of C-60 under HPHT conditions, which are new allotropes of carbon with expanded framework structures. The crystal chemistry and characteristic properties of some related compounds are also reviewed.