High-pressure Raman study of the Ba-doped silicon clathrate Ba 24Si100 up to 27 GPa

High-pressure Raman scattering of type-III silicon clathrate Ba 24Si100 has been measured up to 27 GPa at room temperature. Low-frequency vibrational modes associated with Ba atoms inside three kinds of cages were found around 45-90 cm-1. The Si framework Raman bands were observed around 115-415 cm-1, which are altogether shifted toward lower frequencies as compared to those of type-I Ba 8Si46. High-pressure phase transition occurs at 6.5 GPa, which seems to be due to the structural distortion induced by the increasing guest(Ba)-host(Si) couplings. Ba24Si100 becomes irreversibly amorphous above 23 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