Effect of four-phonon scattering and strong anharmonicity on the thermal conductivity of the antiperovskite derivatives Ba3BiX3 (X = Br, I)

Cubic antiperovskite derivatives X3BA3 feature perovskite-like structures, but with A-site anions shifted from the corners to the edge-center positions. This structural modification increases the number of atoms per unit cell compared to conventional cubic perovskites, which may result in lower lattice thermal conductivity. In this work, we systematically investigated the phonon thermal transport properties of Ba₃BiX₃ (X = Br, I) using first-principles calculations and Boltzmann transport theory. Our results reveal that the three-phonon lattice thermal conductivity (κ_3ph)of Ba₃BiI₃ (1.81 Wm⁻¹ K⁻¹) is higher than that of Ba₃BiBr₃ (1.48 Wm−1 K−1) at 300 K, which deviates from the expected trend based on halide atomic mass. Inclusion of four-phonon scattering corrects this trend, reducing κ by up to 40.88% for Ba₃BiBr₃ (to 1.07 Wm⁻¹ K⁻¹) and 19.59% for Ba3BiBr3 (to 1.19 Wm−1 K−1). The strong four-phonon scattering in Ba₃BiBr₃, despite the absence of an acoustic-optical gap, is attributed to flattened optical phonon modes and pronounced anharmonicity. These factors contribute to a significantly enlarged four-phonon scattering phase space, leading to scattering strengths comparable to their three-phonon counterparts. Additionally, the wavelike (coherent) contribution is found to play a non-negligible role in the thermal transport of both compounds. These findings provide key insights into the mechanisms underlying low lattice thermal conductivity in newly reported antiperovskite derivatives and establish a promising platform for designing high-performance thermoelectric materials.