Thermoelectric transport properties of Sc₂X₂Te₂ (X = Cl, Br, I) predicted with four-phonon scattering

In this work, we systematically investigate the thermoelectric transport properties of monolayer Sc₂X₂Te₂ (X = Cl, Br, I) by combining first-principles calculations with machine-learned moment tensor potentials (MTP), a two-channel phonon transport model, and Boltzmann transport theory. Our results reveal that the strong anharmonicity induced by weak chemical bonds is the root cause of enhanced higher-order phonon scattering, which significantly shortens the phonon lifetime. Although heavier halogen substitution can effectively suppress the lattice thermal conductivity by strengthening the anharmonicity associated with the weak bonds, we find that the overall thermoelectric performance decreases progressively as the halogen varies from Cl to Br to I. Consequently, Sc₂Cl₂Te₂ achieves the maximum figure of merit (ZT) of 3.76 at 800 K along the x-direction under n-type doping. This seemingly counterintuitive behavior arises from a drastic reduction in carrier mobility, whose detrimental impact on the power factor outweighs the beneficial effect of reduced lattice thermal conductivity. Our work thus elucidates the physical mechanism of synergy and competition between the "thermal-transport-dominated ZT" and the "electrical-transport-dominated ZT", providing a new physical picture for the co-optimization of thermal and electrical transport in two-dimensional thermoelectric materials.