Anharmonic phonon renormalization and thermal transport in the type-I Ba$_{\rm 8}$Ga$_{\rm 16}$Sn$_{\rm 30}$ clathrate from first principles

TL;DR

This study uses first-principles calculations to analyze anharmonic phonon effects and thermal transport in a type-I Ba8Ga16Sn30 clathrate, revealing temperature-dependent phonon behavior influencing thermal conductivity.

Contribution

It introduces a self-consistent phonon theory approach to accurately model temperature-dependent phonon properties in clathrates, highlighting the impact of guest atom dynamics on thermal transport.

Findings

Lattice thermal conductivity increases at low frequencies with temperature.

Guest mode hardening affects phonon group velocities and lifetimes.

Thermal conductivity decrease due to nanostructuring is more effective at higher temperatures.

Abstract

Effects of strong phonon anharmonicity of a type-I clathrate Ba$_{\rm 8}$Ga$_{\rm 16}$Sn$_{\rm 30}$ induced by quadruple-well potential of guest atoms were investigated. Phonon transport including coherent interbranch component was analyzed using a first-principles-based self-consistent phonon (SCP) theory that gives temperature-dependent harmonic interatomic force constants and by solving off-diagonal components of group velocity operator. Experimentally observed thermal conductivities have been reasonably reproduced by considering both lattice and electron contributions. Through the analysis with the SCP theory, we found that hardening of guest modes leads to an increase in lattice thermal conductivity at frequencies below those of framework-dominant flat modes (< 40 cm$^{\rm -1}$), which finally results in the slow decay and slight increase in the total lattice thermal conductivity with increasing temperature. Detailed analyses revealed that the increase in lattice thermal conductivity at low frequency is attributed to (a) the increase in group velocities of phonon modes located at frequencies below that of the flat guest modes and (b) abnormal increase in lifetimes of phonon modes located between frequencies of the flat guest and framework modes with increasing temperature. From an engineering point of view, this effect may lead to an intriguing phenomenon, a larger decrease in thermal conductivity due to nanostructuring at higher temperatures.