Estimates of crystalline LiF thermal conductivity at high temperature and pressure by a Green-Kubo method

TL;DR

This study estimates the high-temperature, high-pressure thermal conductivity of LiF using molecular simulations and compares results with ab initio data, providing insights into phase stability and anisotropic effects under extreme conditions.

Contribution

It introduces a Green-Kubo based simulation approach for LiF's thermal conductivity at extreme conditions, validated against ab initio data, and explores phase stability and anisotropy effects.

Findings

Thermal conductivity of LiF increases with temperature and pressure.

Phase stability favors B1 structure at high conditions.

Uniaxial loading induces anisotropy in thermal conductivity.

Abstract

Given the unique optical properties of LiF, it is often used as an observation window in high-temperature and pressure experiments; and, hence, estimates of its transmission properties are necessary to interpret observations. Since direct measurements of the thermal conductivity of LiF at the appropriate conditions are difficult, we resort to molecular simulation methods. Using the Belonoshko et al. (2000) empirical potential validated against ab initio phonon density of states, we estimate the thermal conductivity of LiF at high temperatures (1000-4000K) and pressures (100-400 GPa) with the Green-Kubo method. We also compare these estimates to those derived directly from ab initio data. To ascertain the correct phase of LiF at these extreme conditions we calculate the (relative) phase stability of the B1 and B2 structures using a quasiharmonic ab initio model of the free energy. We also estimate the thermal conductivity of LiF in an uniaxial loading state that emulates initial stages of compression in high-stress ramp loading experiments and show the degree of anisotropy induced in the conductivity due to deformation.