Quartic Anharmonicity of Rattlers and its Effects on Exceptional Thermal Transport in Intermetallic Clathrates: A First-Principles Investigation

The rattling atoms in host-guest structures, such as clathrates and skutterudites, cause many intriguing thermal properties including low-frequency vibrational modes showing strong temperature (T) dependence and very low lattice thermal conductivity κ_L. While small κ_L values of host-guest systems can mainly be attributed to the strong Umklapp scattering induced by rattlers [1], the atomistic origin of anomalous T-dependence of κ_L still remains unclear. In this study, we have investigated the role of the strong quartic anharmonicity of rattlers in lattice dynamics and thermal transport of type-I clathrate Ba₈Ga₁₆Ge₃₀ based on ab initio self-consistent phonon calculations [2]. 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 at high temperature [3]. The origin of the glasslike κ_L is also discussed.
[1] T. Tadano, Y. Gohda, and S. Tsuneyuki, Phys. Rev. Lett. 114, 095501 (2015).;[2] T. Tadano and S. Tsuneyuki, Phys. Rev. B 92, 054301 (2015).; [3] T. Tadano and S. Tsuneyuki, arXiv:1710.0031.