Importance of Four-Phonon Interactions in Lattice Thermal Conductivity and Thermoelectrics: A Case Study

The theoretical determination of thermoelectric (TE) properties of a material depends upon the considered order of phonon interactions within the system. While three-phonon interactions have been used widely, four-phonon interactions should be considered in cases where lattice thermal conductivity (κ L ) is overestimated by three-phonon-based calculations, leading to a lower figure of merit (ZT). Here, we have evaluated bulk and bilayer systems of orthorhombic and hexagonal GeS as potential TE materials. The o-GeS and h-GeS bulk systems show higher ZT values of 0.96 than their respective bilayer systems. The effect of four-phonon scattering has been calculated in the o-GeS and h-GeS bilayer systems, revealing that κ L is more dominant in the o-GeS bilayer. This disparity can be attributed to the presence of a larger phonon band gap in the o-GeS bilayer compared to the h-GeS bilayer. The percentage change in κ L upon considering a higher order four-phonon scattering also increases with temperature. The four-phonon interactions lead to lower κ L and higher ZT values of 0.508 for the o-GeS bilayer along the y -axis at 900 K. These findings show the vitality of considering higher order four-phonon interactions in calculating the lattice thermal conductivity and ZT values for such materials.