Thermal expansion and transport in van der Waals solids from first-principles calculations

TL;DR

This study uses first-principles calculations to analyze thermal expansion and conductivity in Mo and W-based transition metal dichalcogenides, revealing how structural defects and chemical composition influence heat transport.

Contribution

It provides a comprehensive first-principles analysis of thermal properties in van der Waals solids, including structural and phonon scattering effects, with detailed chemical trend insights.

Findings

Thermal conductivity agrees with experiments when phonon and isotopic scattering are included.

Defects significantly limit phonon mean free paths, affecting conductivity.

Conductivity increases from Te to S in chalcogenides and is higher in WS₂ than MoS₂ due to phonon band gap differences.

Abstract

The lattice thermal expansion and conductivity in bulk Mo and W-based transition metal dichalcogenides are investigated by means of density functional and Boltzmann transport theory calculations. To this end, a recent van der Waals density functional (vdW-DF-CX) is employed, which is shown to yield excellent agreement with reference data for the structural parameters. The calculated in-plane thermal conductivity compares well with experimental room temperature values, when phonon-phonon and isotopic scattering are included. To explain the behavior over the entire available temperature range one must, however, include additional (temperature independent) scattering mechanisms that limit the mean free path. Generally, the primary heat carrying modes have mean free paths of $1\,\mu\text{m}$ or more, which makes these materials very susceptible to structural defects. The conductivity of Mo and W-based TMDs is primarily determined by the chalcogenide species and increases in the order Te-Se-S. While for the tellurides and selenides the transition metal element has a negligible effect, the conductivity of WS$_2$ is notably higher than for MoS$_2$, which can be traced to the much larger phonon band gap of the former. Overall the present study provides a consistent set of thermal conductivities that reveal chemical trends and constitute the basis for future investigations of van der Waals solids.