Quartic Anharmonicity of Rattlers and Its Effect on Lattice Thermal Conductivity of Clathrates from First Principles.

We investigate the role of the quartic anharmonicity in the lattice dynamics and thermal transport of the type-I clathrate Ba_{8}Ga_{16}Ge_{30} based on ab initio self-consistent phonon calculations. We show that the strong quartic anharmonicity of rattling guest atoms causes the hardening of vibrational frequencies of low-lying optical modes and thereby affects calculated lattice thermal conductivities κ_{L} significantly, resulting in an improved agreement with experimental results including the deviation from κ_{L}∝T^{-1} at high temperature. Moreover, our static simulations with various different cell volumes shows a transition from crystal-like to glasslike κ_{L} around 20 K. Our analyses suggest that the resonance dip of κ_{L} observed in clathrates with large guest free spaces is attributed mainly to the strong three-phonon scattering of acoustic modes along with the presence of higher-frequency dispersive optical modes.