Ultralow lattice thermal conductivity and exceptional thermoelectric performance in the YbBi2 monolayer induced by strong four-phonon scattering

• YbBi 2 monolayer shows low κ l of 0.006 Wm −1 K −1 ( x ) and 0.022 Wm −1 K −1 ( y ) at 300 K. • Four-phonon scattering reduces κ l by 95%. • Combination of the RSSR and the strong anharmonicity enhances four-phonon scattering. • ADP, ODP, POP, and IIS scattering mechanisms are considered. • Evaluate ZT based on intrinsic concentration and bipolar effect. Achieving ultralow lattice thermal conductivity is crucial for high-performance thermoelectric materials and challengeable. Here, we investigate the YbBi 2 monolayer, a mechanically, dynamically, and thermodynamically stable material, and uncover an unprecedented reduction in lattice thermal conductivity (κ l ) driven by four-phonon scattering. Specifically, the κ l values, calculated solely based on three-phonon scattering, are as low as 0.128 (0.377) Wm −1 K −1 in the x ( y ) directions at 300 K. Remarkably, four-phonon scattering further suppresses this by 95%, yielding ultralow values of 0.006 (0.022) Wm −1 K −1 . This extraordinary reduction originates from the interplay of the unique quadratic ZA phonon dispersion, pronounced anharmonicity, and reflection symmetry-imposed selection rules. Additionally, the YbBi 2 monolayer exhibits a peak power factor of 1.47 mWm −1 K −2 and an outstanding thermoelectric figure of merit ( ZT ) of 1.75 at 700 K under intrinsic carrier concentrations with the bipolar effect. These findings position the YbBi 2 monolayer as a promising candidate for thermoelectric applications, providing new insights into phonon scattering mechanisms in low-dimensional materials.