In situ observation of the formation of Si clathrate Ba8Si46 at high pressures and high temperatures.

X-ray diffraction measurements at high pressures and high temperatures revealed that Si clathrate Ba 8Si 46 is formed by a solid-phase reaction of an 8:30 molar mixture of SrSi 2-phase BaSi 2 and Si after BaSi 2 undergoes the BaSi 2-to-EuGe 2 and the EuGe 2-to-SrSi 2 transitions. The volume reduction during the formation of Ba 8Si 46 is the largest, 7.6%, among the observed transitions. On the other hand, an 8:30 molar mixture of SrSi 2-phase SrSi 2 and Si does not result in the formation of Sr 8Si 46 at high pressures and high temperatures; only SrSi 2 transforms from the SrSi 2 phase into the alpha-ThSi 2 phase, and Si remains in the diamond phase.