Thermal conductivity of CaF$_{2}$ at high pressure

TL;DR

This study investigates how the thermal conductivity of CaF$_{2}$ varies under high pressure using first-principle calculations, revealing phase-dependent differences and the impact of phonon properties on heat transport.

Contribution

It introduces a comprehensive analysis of high-pressure CaF$_{2}$ phases' thermal conductivity using advanced computational methods, highlighting the role of phonon dynamics.

Findings

Thermal conductivity increases nearly linearly with pressure.

High-pressure phases have significantly lower thermal conductivity than cubic fluorite.

Lower conductivity is mainly due to reduced acoustic phonon contributions.

Abstract

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 $P\bar{6}2m$ symmetry and the previously known $\gamma$-CaF$_{2}$ high-pressure phase have significantly lower lattice thermal conductivities than the ambient-thermodynamic cubic fluorite ($Fm\bar{3}m$) 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 $P\bar{6}2m$ and $Pnma$ structures at high temperatures, and resort to the Cahill-Pohl model to assess the lower limit of thermal conductivity in these domains.