Effects of phonon bandgap on phonon-phonon scattering in ultrahigh thermal conductivity θ-phase TaN

Phonon bandgap typically has a significant effect on phonon-phonon scattering process. In this work, the effects of mass modified phonon bandgap in theta-phase TaN are systemically investigated by the means of first-principles calculations with linearized Boltzmann transport equation. Through detailed calculations, we find that phonon bandgap has a significant effect on three-phonon process while exhibits a much weaker effect on four-phonon process. The reason for the ultrahigh thermal conductivity of theta-phase TaN is the long lifetime of phonons including both three-phonon and four-phonon processes, which originates from the weak phonon anharmonicity and large phonon bandgap-induced small phonon-phonon scattering phase space. This work advances the understanding of phonon bandgap effects on phonon transport.