Origin of Ultralow Lattice Thermal Conductivity and Enhanced Thermoelectric Performance in X ₂ Se ₂ S (X = Bi, As, and Sb) Through Homologous Cation Substitution

Suppressing lattice thermal conductivity is essential for high‐performance thermoelectric materials. Here, first‐principles calculations combined with the linearized Boltzmann transport equation and deformation potential theory were used to investigate the thermal transport and thermoelectric behavior of X 2 Se 2 S (X = Bi, As, and Sb). A counterintuitive mass‐dependent trend was identified: despite being the lightest compound, As 2 Se 2 S exhibits the lowest lattice thermal conductivity, reaching 0.443 W m −1 K −1 at 300 K. This behavior can be understood within the complete Slack framework, in which lattice anharmonicity dominates over atomic mass in determining heat transport. Phonon dispersion and anharmonic force‐constant analyses show that strong acoustic‐optical coupling and abundant low‐frequency optical modes in As 2 Se 2 S enhance phonon scattering, whereas Bi 2 Se 2 S retains a clear acoustic‐optical gap and correspondingly higher thermal conductivity. Owing to its ductile mechanical response and favorable electronic structure, p‐type As 2 Se 2 S achieves a maximum ZT of 0.503 at 850 K, while Sb 2 Se 2 S shows promise for n‐type transport. These results clarify the role of anharmonicity in X 2 Se 2 S compounds and highlight their potential for low‐cost medium‐temperature thermoelectric applications.