High Temperature Thermoelectric Properties of Yb<sub>14</sub>MnSb<sub>11</sub> Prepared from Reaction of MnSb with the Elements

Compounds of the Yb<sub>14</sub>MnSb<sub>11</sub> structure type are the highest efficiency bulk p-type materials for high temperature thermoelectric applications, with reported figures of merit (ZTs) as high as ∼1.3 at 1275 K. Further optimization of ZT for this structure type is possible with the development of a simple synthetic route. However, this has been difficult to achieve because of the small amount of Mn required compared with Yb and Sb. A simple synthetic route for Yb<sub>14</sub>MnSb<sub>11</sub> has been developed utilizing a combination of ball milling and annealing to produce phase-pure material followed by spark plasma sintering for consolidation. The materials have been characterized by powder X-ray diffraction before and after spark plasma sintering. The stoichiometric reaction of Yb, Sb, and MnSb provides phase-pure powder by X-ray diffraction. Upon cycling to temperatures greater than 1272 K, Yb<sub>14</sub>MnSb<sub>11</sub> shows the presence of Yb<sub>11</sub>Sb<sub>10</sub>. Additional samples with 5% and 10% excess Mn were also investigated. Adding 5–10% excess Mn does not change the low temperature properties and improves the high temperature ZT, resulting in a ZT of 1.1–1.2 at 1000 K for Yb<sub>14</sub>Mn<sub>1.05</sub>Sb<sub>11</sub>, 30–40% improvement over that of the Sn flux reaction. The increase in ZT is attributed to optimization of the carrier concentration. These results provide a reliable method of bulk synthesis of this Zintl phase and open the way for discovery of new compounds with potential for even higher ZT.