Enhanced thermoelectric properties of Sr₅In₂Sb₆via Zn-doping

Zintl phases exhibit inherently low thermal conductivity and adjustable electronic properties, which are integral to designing high-efficiency thermoelectric materials. Inspired by the promising thermoelectric figure of merit of optimized A₅M₂Sb₆ Zintl phases (A = Ca or Sr, M = Al, Ga, In), Zn-doped Sr₅In_2-xZn_xSb₆ (x = 0, 0.025, 0.05, 0.1) compounds were investigated. Optical absorption measurements combined with band structure calculations indicate two distinct energy transitions for Sr₅In₂Sb₆, one direct (E_g ∼ 0.3 eV) and the other from a lower valence band manifold to the conduction band edge (E_g ∼ 0.55 eV). Sr₅In₂Sb₆ exhibits nondegenerate p-type semiconducting behavior with low carrier concentration (∼4 × 10¹⁸ h⁺ cm^-3 at 300 K). Charge carrier tuning was achieved by Zn²⁺ substitution on the In³⁺ site, increasing carrier concentrations to up to 10²⁰ h⁺ cm^-3. All samples displayed relatively low thermal conductivities (∼0.7 W m^-1 K^-1 at 700 K). The Zn-doped samples exhibited significantly higher zT values compared to the undoped sample, reaching a value of ∼0.4 at 750 K for Sr₅In_1.9Zn_0.1Sb₆.