Raman study of Si clathrates Sr8AlxGa16-xSi30 under High Pressure

High-pressure Raman scattering experiments were carried out for Si clathrates (Sr₈Al_xGa_16-xSi₃₀) with structures of type I and type VIII. The type I clathrate (Sr₈Al₇Ga₉Si₃₀) indicated spectral changes at 13 GPa and 19 GPa. The spectral shape was characteristic of amorphous above 19 GPa. For type VIII (Sr₈Al₈Ga₈Si₃₀), the spectral shape changed to amorphous like one at 20 GPa. After releasing the pressure after the amorphizations, the measured Raman spectra were the same as that of type I, for both the cases of type I and VIII, which suggests that type VIII structure irreversibly transforms to type I via an amorphous phase.