Four-phonon scattering-induced ultralow thermal conductivity in Ba ₃ X ₂ (X = P, As) monolayers: promising high-performance thermoelectric candidates at room temperature

Two-dimensional thermoelectric materials hold great promise for efficient energy harvesting and self-powered electronic devices. In this study, we systematically investigate the thermoelectric properties of Ba₃X₂ (X = P, As) monolayers at 300 K by combining first-principles calculations with a machine-learning-assisted four-phonon scattering framework. The results reveal that Ba₃X₂ monolayers overcome the conventional limitation imposed by narrow band gaps and exhibit outstanding thermoelectric performance at room temperature. Specifically, both materials demonstrate excellent electron mobilities (∼10⁴ cm² V^-1 s^-1) and favorable power factors, reaching as high as 0.089 and 0.027 W m^-1 K^-2 for Ba₃P₂ and Ba₃As₂, respectively. Due to the pronounced anharmonicity arising from anti-bonding states and weak interatomic forces, the inclusion of four-phonon scattering results in a marked decrease in the lattice thermal conductivity (<i>κ</i>_L). The <i>κ</i>_L is remarkably suppressed to ultralow levels, with Ba₃P₂ and Ba₃As₂ exhibiting values of 2.27 W m^-1 K^-1 (2.56 W m^-1 K^-1) and 0.95 W m^-1 K^-1 (0.98 W m^-1 K^-1), respectively. These values represent reductions of 25.8% (28.8%) and 44.7% (34.4%) relative to predictions, considering only three-phonon scattering processes. Moreover, <i>κ</i>_L exhibits a nonmonotonic response to biaxial strain, with Ba₃P₂ displaying a nearly linear decrease within the 0.5% to 3% strain range, indicating exceptional tunability of thermal transport. In summary, the room-temperature <i>ZT</i> values reach ∼2.62 for Ba₃P₂ and ∼4.0 for Ba₃As₂ outperforming many two-dimensional thermoelectric materials. These findings underscore the pivotal role of four-phonon scattering in tailoring low-dimensional thermal transport and highlight Ba₃X₂ monolayers as highly promising candidates for high-performance thermoelectric applications at room temperature.