High-pressure synthesis and structure of a new silicon clathrate Ba24Si100

A new silicon clathrate Ba24Si100 was prepared from a stoichiometric mixture of 12 BaSi2 and 76 Si in a molar ratio using a high pressure of 1.5 GPa at 800°C. The X-ray Rietveld analysis revealed that Ba24Si100 was isotypic with K8Sn25, consisting of Ba containing dodecahedral silicon cages, Ba@Si20, which are linked by sharing the pentagonal faces. A chiral zeolite-like network was formed; the rest of Ba atoms occupied the resulting interstices of Si20 open-cages and Si8 pseudo-cubic spaces. The synthesis using a pressure higher than 3 GPa at 800°C gave a mixture of the type I silicon clathrate Ba8Si46 and Si. It should be noted that the germanium analog Ba24Ge100 was prepared by a simple arc-melting under Ar atmosphere, while the silicon analog required a moderately high pressure for the preparation. Ba24Si100 showed a metallic conductivity