We study the thermal transport properties of three CaF$_{2}$ polymorphs up to a pressure of 30 GPa using first-principle calculations and an interatomic potential based on machine learning. The lattice thermal conductivity $kappa$ is computed by iteratively solving the linearized Boltzmann transport equation (BTE) and by taking into account three-phonon scattering. Overall, $kappa$ increases nearly linearly with pressure, and we show that the recently discovered $delta$-phase with $Pbar{6}2m$ symmetry and the previously known $gamma$-CaF$_{2}$ high-pressure phase have significantly lower lattice thermal conductivities than the ambient-thermodynamic cubic fluorite ($Fmbar{3}m$) structure. We argue that the lower $kappa$ of these two high-pressure phases stems mainly due to a lower contribution of acoustic modes to $kappa$ as a result of their small group velocities. We further show that the phonon mean free paths are very short for the $Pbar{6}2m$ and $Pnma$ structures at high temperatures, and resort to the Cahill-Pohl model to assess the lower limit of thermal conductivity in these domains.
تحميل البحث