Phonon transport in Na2He at high pressure from a first-principles study

Abstract

Phonon transport of recently fabricated Na2He 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. The calculated room-temperature lattice thermal conductivity is 149.19 W m−1 K−1, which is very close to that 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 Grüneisen parameters, and long phonon lifetimes. The size effects on lattice thermal conductivity are considered by cumulative thermal conductivity with respect to the phonon mean free path. 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.