Dynamical properties of guest ions in the type-I clathrate compounds X8 Ga16 Ge30 (X=Eu,Sr,Ba) investigated by Raman scattering

Raman active phonons in the type-I clathrate compounds of X8 Ga16 Ge30 (X=Eu,Sr,Ba) have been fully assigned by the combination of polarized Raman scattering and first principles calculations. The dynamical motions of guest ions in the 6d -site cage have been clearly identified. The energy of the modes, related to a rattling motion, decreases with decreasing temperature. The energy-decrease at low temperatures is a common property for thermal rattling in X8 Ga16 Ge30. The anomalous decreases are originated from the quartic anharmonic potential, and its contribution has been experimentally determined. A mixed state between thermal rattling and quantum tunneling has been observed below 10 K. A soft mode due to the guest ion motion has been observed, but the transition due to this soft mode is somewhat different from the ordinary structural transition because of the shielding by carriers. In addition, it is found that the elastic dispersion of Sr8 Ga16 Ge30 is caused by the interference with the rattling motion of the guest ion in the cage, and that the rattling at the off-center location plays an important role to suppress a lattice thermal conductivity and to achieve the phonon-glass state in X8 Ga16 Ge30.