  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.
