  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.
