Study for the microscopical and dynamical behavior of the guest ion in clathrate compounds

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Some type-I clathrates show the glass-like thermal conductivity due to the guest-ion vibration(so-called rattling) in spite of the crystal structure. To reveal the microscopic properties of the off-center rattling, I have investigated type-I clathrates by the Raman scattering using the high-quality single crystal. The guest ion rotationally moves with the large off-center distance due to the anharmonic potential of the cage, and I can estimate the off-center distance. Moreover, it is revealed that the large displacement of the guest ion effectively lowers the cage stability, and that the lattice thermal conductivity is suppressed.