Dynamical motion of a guest ion studied by Raman scattering and the lattice thermal conductivity in A8Ga16Si30−xGex (A = Ba, Sr)

Dynamical motions of a guest ion in type-I clathrate compounds have been investigated using Raman scattering. It has been found that the 4th order anharmonic potential is important for the guest ion motion not only for A
₈
Ga
₁₆
Ge
₃₀
(A = Ba, Sr, Eu), but also for A
₈
Ga
₁₆
Si
_30-x
Ge
_x
(A = Ba, Sr). The substitution of Ge expands isotropically the cage at 6d-site for Ba
₈
Ga
₁₆
Si
_30-x
Ge
_x
, however, anisotropically for Sr
₈
Ga
₁₆
Si
_30-x
Ge
_x
. Especially, for Sr
₈
Ga
₁₆
Si
_30-x
Ge
_x
, the off-center rattling develops with increasing Ge concentration and plays an important role to the suppression of the lattice thermal conductivity.