High-pressure Raman scattering of type-III silicon clathrate Ba 24 Si 100 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 8 Si 46 . High-pressure phase transition occurs at 6.5 GPa, which seems to be due to the structural distortion induced by the increasing guest͑Ba͒-host͑Si͒ couplings. Ba 24 Si 100 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.

Raman scattering measurements of an iodine-doped I 8 Si 44 I 2 clathrate have been performed at pressures up to 28 GPa and 296 K. We found two Raman peaks at 75 and 101 cm Ϫ1 associated with the vibrations of guest I atoms inside the host Si cages, and observed some framework vibrations around 120-500 cm Ϫ1. These characteristic Raman bands and their pressure dependence are investigated in consideration of our recent Ba 8 Si 46 studies. The lowest-frequency framework vibration at 133 cm Ϫ1 shows the softening with pressure, which seems to be the common feature of Si clathrates. A strong and broad Raman band centered at 461 cm Ϫ1 is identified to the highest-frequency framework vibration, which is likely intensified and broadened by the considerable framework distortion due to the replacement of framework Si with larger I atom. No obvious pressure-induced phase transition was found up to 28 GPa. The guest-host interactions are investigated by the present vibrational properties and are compared with those of previous neutron studies of I 8 Si 44 I 2 .