Tuning the Thermoelectric Properties of Ca₉Zn_4+xSb₉ by Controlled Doping on the Interstitial Structure

The complex Zintl phase Ca 9 Zn 4.5 Sb 9 is a promising thermoelectric material due to its low thermal conductivity. Initial studies revealed that undoped Ca 9 Zn 4.5 Sb 9 has a maximum zT value of 0.37 at 823 K. Hall effect measurements on Ca 9 Zn 4.5– x Al 2 x /3 Sb 9 ( x = 0, 0.5, 1), which nominally feature identical carrier concentration, suggested that control of the interstitial chemistry is critical in affecting the mobility of this system. Removal of the interstitial atoms results in rapid decrease in the carrier mobility, which has negative effects on the overall thermoelectric performance. Further material optimization was successfully carried out via Cu-doping in Ca 9 Zn 4.5 Sb 9, with the aid of theoretical predictions. This resulted in high concentration of interstitial atoms as in Ca 9 Zn 4.5– x Cu x Sb 9 ( x = 0.05, 0.1, 0.15, 0.2). For the optimized composition Ca 9 Zn 4.35 Cu 0.15 Sb 9, a maximum of the figure of merit ( zT = 0.72) was obtained at 873 K, which is increased by more than twice compared to the undoped material Ca 9 Zn 4.5 Sb 9 .