Ab initio studies on the lattice thermal conductivity of silicon clathrate frameworks II and VIII

TL;DR

This study uses ab initio methods to compare the lattice thermal conductivities of silicon clathrate frameworks II and VIII with diamond silicon, revealing significantly lower conductivities mainly due to phonon spectra and relaxation times.

Contribution

It provides the first ab initio analysis of the thermal conductivity of silicon clathrate frameworks II and VIII, highlighting the dominant role of harmonic phonon spectra and anharmonic effects.

Findings

Clathrate frameworks have lower thermal conductivity than diamond silicon.

Harmonic phonon spectra primarily influence the lower conductivity in structure II.

Anharmonic effects significantly impact thermal conductivity, more than unit cell size.

Abstract

The lattice thermal conductivities of silicon clathrate frameworks II and VIII are investigated by using ab initio lattice dynamics and iterative solution of the linearized Boltzmann transport equation(BTE) for phonons. Within the temperature range 100-350 K, the clathrate structures II and VIII were found to have lower lattice thermal conductivity values than silicon diamond structure (d-Si) by factors of 1/2 and 1/5, respectively. The main reason for the lower lattice thermal conductivity of the clathrate structure II in comparison to d-Si was found to be the harmonic phonon spectra, while in the case of the clathrate structure VIII, the difference is mainly due to the harmonic phonon spectra and partly due to shorter relaxation times of phonons. In the studied clathrate frameworks, the anharmonic effects have larger impact on the lattice thermal conductivity than the size of the unit cell. For the structure II, the predicted lattice thermal conductivity differs approximately by factor of 20 from the previous experimental results obtained for a polycrystalline sample at room temperature.