Four-phonon scattering-induced ultralow thermal conductivity in Ba3X2 (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 (κL). The κL 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, κL 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 ZT 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.