Raman scattering study of the guest ion motion in caged crystals

The guest ion motion has been investigated by Raman scattering for the following caged crystals: La
₃
Pd
₂₀
X
₆
(X = Ge, Si), X
₈
Ga
₁₆
Ge
₃₀
(X = Ba, Sr, Eu), and KOs
₂
O
₆
. The observed Raman peaks have been well assigned by the polarization dependence and the first principles calculation. It has been found that the energy of the guest ion anomalously decreases with decreasing temperature for X
₈
Ga
₁₆
Ge
₃₀
, La
₃
Pd
₂₀
Ge
₆
, and KOs
₂
O
₆
, where the rattling motion of the guest ion has been reported. However, such anomalous decrease has not been observed in La
₃
Pd
₂₀
Si
₆
(no rattling). The origin of the energy decrease is understood by the existence of the large quartic anharmonic potential. Furthermore, the existence of carriers is important for the rattling mode