Phonons and thermal transport in hafnium disulphide (HfS₂)

Recent studies have shown that hafnium disulphide (HfS₂) exhibits many desirable properties such as high room-temperature mobility, finite bandgap, and high on-off current ratio that are favored by electronic devices. Despite the large number of existing studies of HfS₂, thermal transport properties have not been fully explored and the related question of how temperature affects the thermal properties remain unanswered.
In this work, we report anharmonic phonons in HfS₂ via first-principle calculations. Key calculated properties include the fully anisotropic Gruneissen parameters, specific heats, coefficients of thermal expansion, and anisotropic thermal conductivity. The study of thermal expansion found interesting behaviors that suggest the structure of HfS₂ gives rise to negative Gruneissen parameters where the frequency or energy of a mode can decrease with applied pressure. The associated phonon lifetimes and specific heats also are used to examine mode contributions to the anisotropic thermal conductivity. Our results also confirm that the overall out-of-plane thermal conductivity of HfS₂ is small, on the order of 1 W/mK, suggesting the promise of HfS₂ application to thermoelectric devices.