Thermal conductivity of<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mi>Ca</mml:mi><mml:msub><mml:mi mathvariant="normal">F</mml:mi><mml:mn>2</mml:mn></mml:msub></mml:mrow></mml:math>at high pressure
Somayeh Faraji, S. Mehdi Vaez Allaei, Maximilian Amsler
Abstract
We study the thermal transport properties of three ${\mathrm{CaF}}_{2}$ polymorphs up to a pressure of 30 GPa using first-principles calculations and an interatomic potential based on machine learning. The lattice thermal conductivity $\ensuremath{\kappa}$ is computed by iteratively solving the linearized Boltzmann transport equation and by taking into account three-phonon scattering. Overall, $\ensuremath{\kappa}$ increases nearly linearly with pressure, and we show that the recently discovered $\ensuremath{\delta}$-phase with $P\overline{6}2m$ symmetry and the previously known $\ensuremath{\gamma}\ensuremath{-}{\mathrm{CaF}}_{2}$ high-pressure phase have significantly lower lattice thermal conductivities than the ambient-thermodynamic cubic fluorite ($Fm\overline{3}m$) structure. We argue that the lower $\ensuremath{\kappa}$ of these two high-pressure phases stems mainly due to a lower contribution of acoustic modes to $\ensuremath{\kappa}$ as a result of their small group velocities. We further show that the phonon mean free paths are very short for the $P\overline{6}2m$ and $Pnma$ structures at high temperatures, and we use the Cahill-Pohl model to assess the lower limit of thermal conductivity in these domains.