Thermoelectric properties of Sr3GaSb3 – a chain-forming Zintl compound

Abstract

Inspired by the promising thermoelectric properties in the Zintl compounds Ca₃AlSb₃ and Ca₅Al₂Sb₆, we investigate here the closely related compound Sr₃GaSb₃. Although the crystal structure of Sr₃GaSb₃ contains infinite chains of corner-linked tetrahedra, in common with Ca₃AlSb₃ and Ca₅Al₂Sb₆, it has twice as many atoms per unit cell (N = 56). This contributes to the exceptionally low lattice thermal conductivity (κ_L = 0.45 W m⁻¹ K⁻¹ at 1000 K) observed in Sr₃GaSb₃ samples synthesized for this study by ball milling followed by hot pressing. High temperature transport measurements reveal that Sr₃GaSb₃ is a nondegenerate semiconductor (consistent with Zintl charge-counting conventions) with relatively high p-type electronic mobility (∼30 cm² V⁻¹ s⁻¹ at 300 K). Density functional calculations yield a band gap of ∼0.75 eV and predict a light valence band edge (∼0.5 m_e), in qualitative agreement with experiment. To rationally optimize the electronic transport properties of Sr₃GaSb₃ in accordance with a single band model, doping with Zn²⁺ on the Ga³⁺ site was used to increase the p-type carrier concentration. In optimally hole-doped Sr₃Ga_1−xZn_xSb₃ (x = 0.0 to 0.1), we demonstrate a maximum figure of merit of greater than 0.9 at 1000 K.