Ultralow lattice thermal conductivity and enhanced thermoelectric performance in the AgKTe monolayer induced by four-phonon scattering

The lattice thermal conductivity (<i>κ</i>_l) 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 <i>κ</i>_l value compared to calculations considering only three-phonon (3ph) processes. Consequently, an ultralow <i>κ</i>_l value of 0.03 W m^-1 K^-1 is achieved along the <i>x</i> and <i>y</i> 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 <i>κ</i>_l and enhanced electronic transport, the AgKTe monolayer achieves a high thermoelectric figure of merit (<i>ZT</i>) of 3.87 at 600 K and intrinsic carrier concentrations. By revealing the dominance of 4ph scattering in <i>κ</i>_l, this work not only suggests AgKTe as a viable thermoelectric candidate but also deepens the understanding of thermal behavior in low-dimensional structures.