High-pressure Raman study of the Ba-doped silicon clathrateBa24Si100up to27GPa

High-pressure Raman scattering of type-III silicon clathrate ${\mathrm{Ba}}_{24}{\mathrm{Si}}_{100}$ has been measured up to $27\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$ at room temperature. Low-frequency vibrational modes associated with Ba atoms inside three kinds of cages were found around $45--90\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$. The Si framework Raman bands were observed around $115--415\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$, which are altogether shifted toward lower frequencies as compared to those of type-I ${\mathrm{Ba}}_{8}{\mathrm{Si}}_{46}$. High-pressure phase transition occurs at $6.5\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$, which seems to be due to the structural distortion induced by the increasing guest(Ba)-host(Si) couplings. ${\mathrm{Ba}}_{24}{\mathrm{Si}}_{100}$ becomes irreversibly amorphous above $23\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$. This pressure is lower than those of type-I Si clathrates, which suggests that type-III structure is less stable than type-I under high pressures.