Exploring Ultralow Lattice Thermal Conductivity in Mg 3 AsX 3 (X = Br, Cl): Implications for Thermoelectric Applications from Density Functional Theory

The structural flexibility and unique bonding of antiperovskites Mg 3 AsX 3 (X = Cl, Br) make them promising for thermoelectric (TE) applications. First‐principles density functional theory using the generalized gradient approximation‐Perdew–Burke–Ernzerhof functional was employed to investigate their structural, electronic, and TE properties. Thermal, dynamical, and mechanical stability were confirmed through ab initio molecular dynamics simulations at 500 K, phonon dispersion, and elastic constant analyses. PBE + SOC calculations show that Mg 3 AsBr 3 and Mg 3 AsCl 3 are indirect bandgap semiconductors with gaps of 1.23 eV and 2.07 eV, respectively. Sharp peaks near the Γ point, arising from orbital degeneracy at the band edges, increase carrier effective mass and enhance the Seebeck coefficient, reaching 727 μV/K for Mg 3 AsCl 3 . Covalent bonding, supported by electron localization function analysis and negative Cauchy pressure, influences lattice dynamics and suppresses thermal conductivity. High electron mobility leads to electrical conductivities of S/m (Mg 3 AsBr 3 ) and S/m (Mg 3 AsCl 3 ). Notably, Mg 3 AsBr 3 exhibits a promising TE figure of merit of ∼0.846 for p ‐type carriers, approaching benchmark values.