Transformations of silicon clathrate Si-136 under high hydrogen pressure up to 11 GPa

Phase transformations of silicon clathrate Si-136 under high hydrogen pressure up to 11.5 GPa were studied by in situ Raman spectroscopy and X-ray diffraction at room temperature in a diamond anvil cell. The pressure dependencies of the vibrational mode frequencies of Si-136 agree well with the ab initio calculation results. In addition to the "rigid" one-phonon modes a series of "soft" modes was found in the 300 divided by 400 cm(-1) frequency range, which results from the two-phonon Raman scattering. At the pressure of 10 GPa the silicon clathrate Si-136 transformed into a mixture of the Si-II (beta-Sn-type) and Si-III (BC8-type) phases. During the subsequent decompression down to 9 GPa the Si-II phase transformed to Si-III, which was then recovered to ambient pressure. No interaction between hydrogen and silicon was observed for Si-136 upon compression and for the Si-II and Si-III phases upon decompression.