Lattice dynamics of thermoelectric palladium sulfide

Highly efficient thermoelectric materials always have low thermal conductivities. Their phonon spectrum information is essential for understanding the procedure of thermal transport on thermoelectrics. Recently, palladium sulfide was found to be a potential thermoelectric material. However, the high thermal conductivity limits its thermoelectric performance and technological applications. Here, the phonon dispersion and phonon density of state in PdS are presented by using the first-principles theory. The phonon modes are assigned and compared with experiments. The evolution of optical modes with pressure is studied by using Raman spectroscopy. The low-energy and high-energy phonon bands are related to the vibrations of the heavy atom and the light atom, respectively. By combining Raman scattering and X-ray diffraction measurements, we obtain the mode-Grüneisen parameters for the detected phonon modes. The small mode-Grüneisen parameters indicate a weak anharmonicity in this material. This offers an explanation for its high thermal conductivity. The evolution of linewidths with pressure accounts for the decrease of the thermal conductivity upon compression.