Large contributions from optical phonons to thermal transport in hexagonal carbon-boron-nitrogen monolayers

As representative semiconducting hexagonal carbon-boron-nitride lattices, ${\mathrm{C}}_{6}\mathrm{BN}$ and ${\mathrm{C}}_{2}\mathrm{BN}$ are experimentally realized two-dimensional (2D) plane materials and have recently become the focus of research. Herein, combining first-principles calculations with the Boltzmann transport equation, we performed a comprehensive study on the phonon interaction and thermal conductivity in ${\mathrm{C}}_{6}\mathrm{BN}$ and ${\mathrm{C}}_{2}\mathrm{BN}$ monolayers. It is found that the thermal conductivities of ${\mathrm{C}}_{6}\mathrm{BN}$ and ${\mathrm{C}}_{2}\mathrm{BN}$ monolayers at room temperature are reduced by 79% and 73%, respectively, due to four-phonon scattering, compared with the results including three-phonon scattering only. We can attribute this phenomenon to giant four-phonon scattering exclusive for the heat-carrying out-of-plane acoustic (ZA) phonons, because the reflection symmetry allows four-ZA processes much higher than three-ZA processes, and the quasiparallel behavior between the ZA and low-lying out-of-plane optical (ZO) branches contribute to a broad phase space for four-phonon scattering as well. Moreover, ${\mathrm{C}}_{6}\mathrm{BN}$ monolayer exhibits unusual behavior that optical phonons contribute about \ensuremath{\sim}60% to the overall thermal conductivity under the four-phonon picture, which differs from the traditional case that acoustic phonons dominate thermal conductivity. Unexpectedly, two low-lying ZO modes have as high as 38% contributions to the thermal transport at 300 K under the four-phonon picture, causing 60% contribution of optical phonon modes, apparently larger than that of the three-phonon case (15%) and many other 2D materials, also indicating the four-phonon scattering has a more significant effect on acoustic phonons than on optical phonons. This finding not only highlights insight into the nature of phonon transport, but also provides a promising strategy for manipulation of heat transport based on optical phonon modes.