Defect-free band engineering in Sr-doped ZnS for improved thermoelectric performance

The development of effective and ecologically safe thermoelectric materials for waste heat recovery remains challenging due to the strong correlation between thermal and electrical transport characteristics. Strontium (Sr) doping allows synergistic band and phonon engineering in ZnS, promising simultaneous optimization of carrier transport and thermal suppression. Employing density functional theory, we systematically evaluated the impact of 3% and 6% Sr substitution on the structural, electronic, elastic, vibrational, and thermoelectric behavior of ZnS. Results demonstrated that a slight, direct band gap increases with Sr concentration, from 3.302 eV for pristine ZnS to 3.37 eV for 6% Sr-doped ZnS, attributed to valence band broadening from Sr hybridization. All structures are dynamically and mechanically stable. Sr incorporation decreases elastic anisotropy (Zener factor 2.32 to 1.70), a reduced Debye temperature (from 350 K to 290 K), indicating lattice softening, and favorable phonon scattering for lowering lattice thermal conductivity. Crucially, the Seebeck coefficient increases more than the electrical conductivity per relaxation time decreases with Sr doping, which results in a higher power factor per relaxation time for 3% Sr-doped ZnS and a higher figure of merit (zT ≈ 0.86 at 750 K for τ= 10 fs) for 6% Sr-doped ZnS due to its greater lattice softening. This work establishes that moderate Sr doping effectively engineers the electronic structure and lattice dynamics of ZnS for high-temperature thermoelectric applications.