First-Principles Study of Dual-Ion Substitutions in SnS ₂ : Lattice Softening and Anion Dimerization Leading to Ultralow Thermal Conductivity

Developing thermoelectric materials that can efficiently convert heat into electricity is still a major challenge. Tin-based chalcogenide materials are more attractive in this research because of their layered arrangements, better structural stability, and tunable electronic structure. We investigate the effects of dual-ion substitution on the structural, electronic, and thermoelectric properties of SnS2 using first-principles density functional theory calculations. The optimized substitution causes anion dimer formation and lattice softening, which increases phonon scattering, resulting in an ultralow total thermal conductivity in the Sn0.75Al0.25S0.5Se0.5 system (κe = 7.11 and κL = 0.007 W m−1 K−1 at 800 K). Our calculations show that the system with this composition exhibits excellent structural and thermodynamic stabilities, confirmed through energetics, dynamics, and molecular dynamics analyses. Electronic properties indicate bandgap narrowing and improved carrier transport near the Fermi level, both of which contribute to better thermoelectric performance. This study demonstrates that substitution-induced lattice softening is an effective strategy for tuning phonon electron interactions and provides enhanced performance in thermoelectric applications.