Ambipolar Thermoelectric Performance Enabled by Low Lattice Thermal Conductivity and High-Mobility Carriers in Ag 3 SX ( X = Cl , Br, I) Antiperovskites

High thermoelectric efficiency has been reported in several antiperovskites, but strongly unbalanced n- and p-type thermoelectric performances still limit practical device integration. Here, we show that Ag3SX (X = Cl, Br, I) antiperovskites exhibit simultaneously favorable p- and n-type transport, and that halogen substitution further improves this ambipolar behavior through coupled lattice and electronic mechanisms. On the lattice side, heavier halogens soften the Ag-X framework and strengthen anharmonic phonon scattering, which markedly lowers the lattice thermal conductivity and helps drive the materials toward a glasslike transport regime. On the electronic side, halogen substitution modifies the relative contributions and hybridization of Ag-4d and chalcogen/halogen p states near the valence-band edge; together with the stronger spin-orbit coupling in the iodide, this leads to enhanced valence-band degeneracy while retaining sufficient band dispersion. As a result, Ag3SI exhibits ultralow lattice thermal conductivity (0.29 W/m K) and high carrier mobility (10-200 cm2/V s), achieving a maximum power factor of 2.5 mW/m K2. Under optimal doping, ZT at 800 K reaches approximate to 2.42 for p-type and approximate to 1.87 for n-type carriers. These results identify Ag3SX compounds as promising ambipolar thermoelectric materials and highlight how lattice anharmonicity and band-edge reconstruction can be combined to optimize heat and charge transport.