Enhanced thermoelectric properties of the Zintl phase BaGa2Sb2via doping with Na or K

Abstract

Na- or K-doped samples of Ba_1−x(Na, K)_xGa₂Sb₂ were prepared by ball-milling followed by hot-pressing. The topological analysis of the electron density of BaGa₂Sb₂ implies a polar covalent nature of the Sb–Ga bonds in which the Sb atoms receive the electrons transferred from Ba rather than the Ga atoms. Successful doping of BaGa₂Sb₂ with Na or K was confirmed with combined microprobe and X-ray diffraction analysis. Alkali metal doping of BaGa₂Sb₂ increased the p-type charge carrier concentration to almost the predicted optimum values (∼10²⁰ h⁺ cm⁻³) needed to achieve high thermoelectric performance. With increasing alkali metal concentration, electronic transport was shifted from non-degenerate semiconducting behaviour observed for BaGa₂Sb₂ to degenerate one for Na- or K-doped compounds. Overall, the thermoelectric figure of merit, zT, values reached up to ∼0.65 at 750 K, considerably higher than the undoped sample (zT ∼ 0.1 at 600 K), and a slight improvement relative to previously reported Zn-doped samples (∼0.6 at 800 K).