In-situ observation of formation of Si clathrates

Introduction Type-I Si clathrates, whose general chemical formula is A8Si46, have a unique structure. In the compounds, Si atoms form Si20 dodecahedra and Si24 tetrakaidehedra, and these polyhedra are linked by shared faces. The A atoms are located in the center of the polyhedra. A search on the superconducting materials has been conducted in Si clathrates since discovery of superconducting fullerides because these polyhedra are isostructural with the fullerene C20 and C24. Yamanaka and his coworkers synthesized the superconducting Si clathrate (Na,Ba)8Si46 by removing Na atoms from Na2BaSi4 at high temperatures under vacuum [1]. They, furthermore, succeeded in a synthesis of Ba8Si46 from a mixture of Si and orthorhombic BaSi2 at pressures ranging from 3 to 5GPa and temperatures ranging from 1073 to 1673K [2,3]. On the other hand, BaSi2 has been reported to undergo two structural phase transitions, the orthorhombic-totrigonal, and the trigonal-to-cubic transitions, in this pressure-temperature region [4]. It is, therefore, interesting to see which kind of phases (the orothorhombic, the trigonal or the cubic) of BaSi2 is responsible for the formation of Ba8Si46 from the mixture of BaSi2 and Si at high pressures and high temperatures. In this research, we have tried to observe the formation of Ba8Si46 from the mixture of BaSi2 and Si in-situ at high pressures and high temperatures.