Off-Center Guest Vibrations and Their Effect on Lattice Thermal Conductivity in n- and p-Type β-Ba8Ga16Sn30

We report on thermoelectric and Raman scattering studies of single crystalline samples of a type I clathrate Ba8Ga16Sn30. The n- and p-type samples have a thermopower of −300 μV/K and +270 μV/K, and electrical resistivity of 20 mΩ cm and 40 mΩ cm at room temperature, respectively. Regardless of the charge carrier type, the lattice thermal conductivity κL shows typical glass-like behavior. The low-energy vibration of Ba(2) in the tetrakaidecahedron is manifested in the Raman scattering spectrum as a peak at 15 cm−1. This energy agrees with the characteristic vibrational temperature of 20 K derived from the analysis of the specific heat data with a soft-potential model. The fact that both the vibrational energy and the magnitude of κL for Ba8Ga16Sn30 are lowest among the type I clathrates is evidence that off-center vibrations are responsible for the strong reduction of κL. Analysis of κL(T) revealed that the positional disorder associated with the off-center vibrations acts as the Rayleigh scattering center, and the coupling between guest vibrational modes and acoustic phonons is stronger than that in Sr8Ga16Ge30 and Eu8Ga16Ge30.