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: La₃Pd₂₀X₆ (X = Ge, Si), X₈Ga₁₆Ge₃₀ (X = Ba, Sr, Eu), and KOs₂O₆. 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 X₈Ga₁₆Ge₃₀, La₃Pd₂₀ Ge₆, and KOs₂O₆, where the rattling motion of the guest ion has been reported. However, such anomalous decrease has not been observed in La₃Pd₂₀Si₆ (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