Higher-order anharmonic phonon transport enhanced by flatbands in Pd3S2Cl2 with intrinsic kagome lattice

An efficient computational framework combining machine-learning moment tensor potentials with the phonon Boltzmann transport equation is developed to systematically investigate the anomalous lattice dynamics and thermal transport properties of the geometrically frustrated kagome material Pd3S2Cl2. The results reveal that the geometric constraints inherent to the kagome lattice generate pronounced flat phonon bands over a wide frequency range. These flat-band modes exhibit nearly zero group velocities and therefore make negligible direct contributions to lattice thermal transport. Instead, the low thermal conductivity of Pd3S2Cl2 originates from the synergistic effects of reduced phonon group velocities and enhanced anharmonic scattering. Further analysis demonstrates that the nearly dispersionless phonon bands induce pronounced Van Hove singularities in the phonon density of states, leading to a high density of nearly degenerate phonon states. This characteristic substantially increases the number of phonon combinations satisfying the energy- and momentum-conservation requirements for four-phonon processes, thereby enlarging the available four-phonon scattering phase space. The enhanced four-phonon scattering mainly reduces the lifetimes of heat-carrying dispersive phonons, providing an additional mechanism for suppressing lattice thermal conductivity beyond the intrinsic limitation of phonon group velocities. Consequently, thermal conductivities predicted by considering only three-phonon scattering are significantly overestimated due to the omission of strong four-phonon interactions. Our calculations demonstrate that incorporating four-phonon scattering is essential for accurately describing thermal transport in this frustrated system, with the thermal conductivity reduction reaching 76.6% for the γ phase at room temperature. These findings establish a microscopic connection between geometric frustration, flat-band-induced anharmonic scattering, and ultra-low lattice thermal conductivity, providing new insights into phonon transport in frustrated materials and offering guidance for designing ultralow-thermal-conductivity materials through flat-band engineering.