# Phonon transport in $\mathrm{Na_2He}$ at high pressure from a first-principles study

## TL;DR

This study uses first-principles calculations to analyze phonon transport in Na₂He at high pressure, revealing its high thermal conductivity and factors influencing heat transfer, with implications for planetary science.

## Contribution

It provides the first detailed phonon transport analysis of Na₂He at high pressure using ab initio methods and Boltzmann transport equations.

## Key findings

- Room-temperature lattice thermal conductivity is 149.19 W/m·K.
- Low-frequency optical modes contribute 16% to thermal conductivity.
- Reducing characteristic length below 100 nm decreases conductivity by up to 36%.

## Abstract

Phonon transport of recently-fabricated $\mathrm{Na_2He}$ at high pressure is investigated from a combination of first-principles calculations and the linearized phonon Boltzmann equation within the single-mode relaxation time approximation (RTA). The calculated room-temperature lattice thermal conductivity is 149.19 $\mathrm{W m^{-1} K^{-1}}$, which is very close to one of Si. It is found that low-frequency optical modes comprise 16\% of the lattice thermal conductivity, while high-frequency optical modes have negligible contribution. The high lattice thermal conductivity is due to large group velocities, small Gr$\mathrm{\ddot{u}}$neisen parameters, and long phonon lifetimes. The size effects on lattice thermal conductivity are considered by cumulative thermal conductivity with respect to phonon mean free path(MFP). To significantly reduce the lattice thermal conductivity, the characteristic length smaller than 100 nm is required, and can reach a decrease of 36\%. These results may be useful to understand thermal transport processes that occur inside giant planets.