Thermoelectric Properties of Mn-Doped Ca_5Al_2Sb_6

Ca_5Al_2Sb_6 is a relatively inexpensive Zintl compound exhibiting promising thermoelectric efficiency at temperatures suitable for waste heat recovery. Motivated by our previous studies of Ca_5Al_2Sb_6 doped with Na and Zn, this study focuses on doping with Mn^(2+) at the Al^(3+) site. While Mn is a successful p-type dopant in Ca_5Al_2Sb_6, we find that incomplete dopant activation yields lower hole concentrations than obtained with either previously investigated dopant. High-temperature Hall effect and Seebeck coefficient measurements show a transition from nondegenerate to degenerate semiconducting behavior in Ca_5Al_(2−x) Mn_xSb_6 samples (x = 0.05, 0.1, 0.2, 0.3, 0.4) with increasing Mn content. Ultimately, no improvement in zT is achieved via Mn doping, due in part to the limited carrier concentration range achieved.