Off-center guest vibrations and their effect on lattice thermal conductivity in n- and p-type β-Ba ₈Ga ₁₆Sn ₃₀

We report on thermoelectric and Raman scattering studies of single crystalline samples of a type I clathrate Ba ₈Ga ₁₆Sn ₃₀. 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 κ _L 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 κ _L for Ba ₈Ga ₁₆Sn ₃₀ are lowest among the type I clathrates is evidence that off-center vibrations are responsible for the strong reduction of κ _L. Analysis of κ _L(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 Sr ₈Ga ₁₆Ge ₃₀ and Eu ₈Ga ₁₆Ge ₃₀.