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

TL;DR

This study uses first-principles calculations to reveal how quartic anharmonicity in rattling guest atoms influences lattice thermal conductivity in clathrates, improving agreement with experiments and explaining glasslike behavior at low temperatures.

Contribution

It demonstrates the significant impact of quartic anharmonicity on vibrational frequencies and thermal transport in clathrates, providing new insights into their thermal conductivity mechanisms.

Findings

Quartic anharmonicity causes hardening of low-lying optical modes.

Lattice thermal conductivity deviates from T^{-1} at high temperatures.

Transition from crystal-like to glasslike thermal conductivity around 20 K.

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.