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

Abstract

Achieving ultralow lattice thermal conductivity is crucial for high-performance thermoelectric materials and challengeable. Here, we investigate the YbBi₂ 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⁻¹K⁻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⁻¹K⁻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 YbBi₂ monolayer exhibits a peak power factor of 1.47 mWm⁻¹K⁻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 YbBi₂ monolayer as a promising candidate for thermoelectric applications, providing new insights into phonon scattering mechanisms in low-dimensional materials.