Off-center rattling and cage vibration of the carrier-tuned type-I clathrateBa8Ga16Ge30studied by Raman …

To reveal dynamical property of a guest ion in type-I clathrate compound, n- and p-type clathrates Ba8Ga16Ge30 have been investigated by Raman scattering. It is found that the guest ion in a 6d-site cage 6d-cage rotationally moves for both n- and p-type since the additional guest mode EgA has been observed regardless of its carrier. The potential-energy difference between 100 and 110 directions in the 6d-cage is proportional to the off-center distance of the guest-ion position from the cage center and this off-center distance for p-type is much larger than that for n-type Ba8Ga16Ge30. In addition, the Raman intensity of the cage vibration at a 6c site for p-type is weaker than that for n-type. Thus, the amplitude of the vibration at the 6c site becomes small for p-type, and this small amplitude induces a large movable space for the guest ion, i.e., this vibrational amplitude of the 6c-site atom works as the barrier for the off-center position. For both systems, the guest ion in the 6d-cage shows an anharmonic vibration, judging from the anomalous energy decrease in the guest ion with decreasing temperature. The energy difference between T2g and T1u T. Mori et al., Phys. Rev. B 79, 212301 2009 of the guest mode clearly supports the theoretical prediction of an interacting dipoles picture that explains the glasslike properties of the off-centered clathrate. It is concluded that the off-center rattling plays an important role to suppress a lattice thermal conductivity.