Phonon transport properties of two-dimensional electride Ca₂N - A first-principles study

We investigate phonon transport in dicalcium nitride (Ca₂N), an electride with two-dimensional confined electron layers, using first-principles density functional theory and the phonon Boltzmann transport equation. The in-plane ( κ100) and out-of-plane ( κ001) lattice thermal conductivities at 300 K are found to be 11.72 W m⁻¹ K⁻¹ and 2.50 W m⁻¹ K⁻¹, respectively. Spectral analysis of lattice thermal conductivity shows that ∼85% of κ100 and κ001 is accumulated by phonons with frequencies less than 5.5 THz and 2.5 THz, respectively. Modal decomposition of lattice thermal conductivity further reveals that the optical phonons contribute to ∼68% and ∼55% of overall κ100 and κ001, respectively. Phonon dispersion suggests that the large optical phonon contribution is a result of low frequency optical phonons with high group velocities and the lack of phonon bandgap between the acoustic and optical phonon branches. We find that the optical phonons with frequencies below ∼5.5 THz have similar three-phonon phase space and scattering rates as acoustic phonons. Comparison of the contributions from emission and absorption processes reveals that the three-phonon phase space and scattering rates of phonons - optical or acoustic - with frequencies below 5.5 THz are largely dominated by absorption processes. We conclude that the large contribution to lattice thermal conductivity by optical phonons is due to the presence of multiple low frequency optical phonon modes with high group velocities and similar phase space and scattering rates as the acoustic phonons. This study provides the frequency and temperature dependent lattice thermal conductivity and insights into phonon transport in Ca₂N, both of which have important implications for the development of Ca₂N based devices.