Optical conductivity spectra of rattling phonons and charge carriers in the type-VIII clathrate Ba8Ga16Sn30.

We have investigated optical conductivity spectra of n- and p-type Ba8Ga16Sn30 (α-BGS) with type-VIII clathrate structure, at temperatures from 296 K down to 6 K with a terahertz time-domain spectrometer (0.2-2.5 THz). The continuous spectra contributed from charge carriers are dispersive in this frequency range and also temperature and carrier-type dependent. The Drude-Smith model taking multiple scatterings of charge carriers into account well reproduces those data. The relaxation rate of the rc-type carriers decreases more sharply than that in the p-type material, suggesting a stronger electron-phonon interaction in the ra-type material than in the p-type. On the other hand, the localized infrared-active modes observed at 1.3 and 1.7 THz, identified as the rattling phonons of the Ba2+ ion quasi-on-center vibrations become soft and broad significantly with decreasing temperature as well as observed in type-I BGS and BGG (Ba8Ga16Ge30 ) clathrates. The softening in the n-type material is smaller by about 30% than in the p-type one, whereas the linewidth broadening is almost independent of the carrier type. The difference in the softening is discussed with a scenario where the interaction of rattling phonons with carriers can modify the anharmonic potential of the guest ions. The anomalous broadening at low temperatures is also discussed with the use of the impurity-scattering model presented for a rattling-phonon system strongly hybridized with acoustic cage phonons [ABSTRACT FROM AUTHOR]