Dynamical properties of guest ions in the type-I clathrate compoundsX8Ga16Ge30(X=Eu,Sr,Ba)investigated by Raman scattering

Abstract

Raman active phonons in the type-I clathrate compounds of ${X}_{8}{\mathrm{Ga}}_{16}{\mathrm{Ge}}_{30}$ $(X=\mathrm{Eu},\mathrm{Sr},\mathrm{Ba})$ have been fully assigned by the combination of polarized Raman scattering and first principles calculations. The dynamical motions of guest ions in the $6d$-site cage have been clearly identified. The energy of the modes, related to a rattling motion, decreases with decreasing temperature. The energy-decrease at low temperatures is a common property for thermal rattling in ${X}_{8}{\mathrm{Ga}}_{16}{\mathrm{Ge}}_{30}$. The anomalous decreases are originated from the quartic anharmonic potential, and its contribution has been experimentally determined. A mixed state between thermal rattling and quantum tunneling has been observed below $10\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. A soft mode due to the guest ion motion has been observed, but the transition due to this soft mode is somewhat different from the ordinary structural transition because of the shielding by carriers. In addition, it is found that the elastic dispersion of ${\mathrm{Sr}}_{8}{\mathrm{Ga}}_{16}{\mathrm{Ge}}_{30}$ is caused by the interference with the rattling motion of the guest ion in the cage, and that the rattling at the off-center location plays an important role to suppress a lattice thermal conductivity and to achieve the phonon-glass state in ${X}_{8}{\mathrm{Ga}}_{16}{\mathrm{Ge}}_{30}$.