  The anharmonic phonon properties of type-I filled inorganic clathrates Ba8
Ga16 Ge30 and Sr8 Ga16 Ge30 are obtained from the first-principles calculations
by considering the temperature-dependent sampling of the potential energy
surface and quartic phonon renormalization. Owing to the weak binding of guest
atoms with the host lattice, the obtained guest modes undergo strong
renormalization with temperature and become stiffer by up to 50% at room
temperature in Sr8Ga16Ge30. The calculated phonon frequencies and associated
thermal mean squared displacements are comparable with experiments despite the
on-centering of guest atoms at cage centers in both clathrates. Lattice thermal
conductivities are obtained in the temperature range of 50- 300 K accounting
for three-phonon scattering processes and multi-channel thermal transport. The
contribution of coherent transport channel is significant at room temperature
(13% and 22% in Ba8Ga16Ge30 and Sr8Ga16Ge30) but is insufficient to explain the
experimentally observed glass-like thermal transport in Sr8Ga16Ge30.
