Ultralow Lattice Thermal Conductivity and Enhanced Thermoelectric Performance in AgKTe Monolayer Induced by Four-Phonon Scattering

Abstract

The lattice thermal conductivity (κl) for the AgKTe monolayer is obtained using the second-, third-, and fourth-order interatomic force constants by machine learning methods. Remarkably, the inclusion of four-phonon (4ph) scattering processes leads to a significant reduction—by approximately 50–66%—in the κl value compared to calculations considering only three-phonon (3ph) processes. Consequently, ultralow κl values of 0.12/0.11 W m-1 K-1  are achieved in x/y directions, respectively, at 600 K, placing AgKTe among the 2D materials with the lowest known thermal conductivities. This highlights the pivotal role of 4ph scattering in suppressing phonon transport. Moreover, a highly degenerate valence band substantially boosts the power factor of p-type carriers. Benefiting from both the reduced κl and enhanced electronic transport, the AgKTe monolayer achieves a high thermoelectric figure of merit (ZT) of 3.87 at 600 K under intrinsic carrier concentrations. By revealing the dominance of 4ph scattering in κl, this work not only suggests AgKTe as a viable thermoelectric candidate but also deepens the understanding of thermal behavior in low-dimensional structures.