Aliovalent–Isovalent In/Bi Codoping Strategy for Enhanced Thermoelectric Performance in SrCuSb Zintl Phases

SrCuSb-based Zintl compounds are promising midtemperature thermoelectric materials but suffer from high lattice thermal conductivity and suboptimal charge transport. In this work, a codoping approach is implemented by substituting aliovalent In and isovalent Bi for Sb in SrCuSb, thereby simultaneously tuning carrier transport and enhancing phonon scattering. Polycrystalline SrCuSb1–x–yInxBiy (0 ≤ x ≤ 0.125, y = 0, 0.05, 0.07), synthesized by melting, ball milling, and spark plasma sintering, shows that In adjusts the hole concentration and preserves high mobility. At the same time, Bi introduces mass/strain-field disorder that softens the lattice and thereby suppresses lattice thermal conductivity. Codoping also suppresses the formation of secondary phases, improving phase purity and carrier mobility. The optimized SrCuSb0.825In0.125Bi0.05 composition exhibits a high power factor of ∼1.9 mW/m·K2 and a low lattice thermal conductivity of ∼0.52 W/m·K at 773 K, yielding a peak zT of ∼0.65. The average zT over 300–773 K increases from 0.17 for pristine SrCuSb to 0.32, corresponding to an ∼87% enhancement. These results demonstrate that aliovalent–isovalent codoping is an effective and widely applicable route to simultaneously engineer carrier transport and phonon scattering, advancing midtemperature thermoelectric materials based on planar Zintl phases.