Abstract
Lattice dynamical properties, especially dynamical motion of a guest ion, of type-I clathrate compounds A 8 Ga 16 Ge 30 ( A = Eu , Sr, Ba) have been studied by Raman scattering. The Raman active phonon modes are fully assigned by polarized Raman scattering on single crystalline A 8 Ga 16 Ge 30 with referring to the results of the first principles calculations. Additional phonon modes due to an off-center rattling of the guest ion, whose energy decreases with decreasing temperature, have been observed. This anomalous temperature dependence is a common property for A 8 Ga 6 Ge 30 . In addition, a soft mode is also observed in the guest modes. This softening suggests a new type transition named an incoherent transition.
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