Anharmonic Stabilization and Thermoelectric Transport in BaBX₃ (B = Zr, Hf; X = S, Se) Perovskites

We investigate the thermoelectric performance of Ba-based chalcogenide perovskites, BaBX₃ (B = Zr, Hf; X = S, Se), in the orthorhombic Pnma phase using a first-principles approach that combines self-consistent phonon theory and Boltzmann transport calculations. Anharmonic lattice dynamics is explicitly included to capture phonon–phonon interactions and the resulting temperature-dependent renormalization. We find that anharmonicity removes dynamic instabilities in the Zr-based compounds and corrects the lattice thermal conductivity (κ_l) that is artificially suppressed by soft phonons in harmonic calculations. Overall, these compounds exhibit ultralow κ_l; for example, BaHfSe₃ reaches average κ_l of 0.43 W m^–1 K^–1 at 900 K. Electronic transport properties, including the Seebeck coefficient, electrical conductivity, and electronic thermal conductivity, are calculated by including the acoustic deformation, polar optical phonon, and ionized impurity scatterings. Among the four compounds, n-type doped BaHfSe₃ achieves the highest TE figure of merit of 1.1 at 900 K due to the low κ_l and a high power factor. Our results establish Ba-based chalcogenide perovskites, especially the Se-containing variants, as promising candidates for high-temperature thermoelectric energy conversion.