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

Abstract: We investigate the role of the quartic anharmonicity in lattice dynamics and thermal transport of type-I clathrate Ba${8}$Ga${16}$Ge${30}$ based on \textit{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 $\kappa{L}$ significantly, resulting in an improved agreement with experimental results including the deviation from $\kappa_{L}\propto T^{-1}$ at high temperature. Moreover, our static simulations with various different cell volumes show a transition from crystal-like to \textit{glasslike} $\kappa_{L}$ around 20 K. Our analyses suggest that the resonance dip of $\kappa_{L}$ observed in clathrates with large guest-free-spaces is attributed mainly to the strong Umklapp scattering of acoustic modes along with the presence of higher-frequency dispersive optical modes.