Thermal conductivity and guest vibration in cage-size controlled Ba ₈in₁₆Ge_30-xM_x (M=Si, Sn)

We report thermal and transport measurements on single crystalline samples of Ba8In16Ge30-xMx (M=Si, Sn, x ≤, 2.2), which crystallize in the type-I clathrate structure. On going from Mx = Si 1.6 to Mx = Sn2.2, the lattice parameter increases by 1% from 11.20 to 11.30 Å. All the samples show semi-metallic behavior with n-type majority carries. The analysis of the specific heat indicates that the local vibration energy of the Ba guests in the tetrakaidecahedron decreases from 39 to 35 K with the increase of the lattice parameter. Concomitantly, the lattice thermal conductivity is largely suppressed and that for Mx = Sn2.2 shows a glasslike temperature dependence. The phonon scattering is greatly increased as a consequence of the strengthened coupling between guest vibration and the cage's acoustic phonon.