Traversing the metal-insulator transition in a zintl phase: Rational enhancement of thermoelectric efficiency in Yb_(14)Mn_(1-x)Al_xSb_(11)

For high temperature thermoelectric applications, Yb_(14)MnSb_(11) has a maximum thermoelectric figure of merit (zT) of ~1.0 at 1273 K. Such a high zT is found despite a carrier concentration that is higher than typical thermoelectric materials. Here, we reduce the carrier concentration with the discovery of a continuous transition between metallic Yb_(14)MnSb_(11) and semiconducting Yb_(14)AlSb_(11). Yb_(14)Mn_(1-x)Al_xSb_(11) forms a solid solution where the free carrier concentration gradually changes as expected from the Zintl valence formalism. Throughout this transition the electronic properties are found to obey a rigid band model with a band gap of 0.5 eV and an effective mass of 3 m_e. As the carrier concentration decreases, an increase in the Seebeck coefficient is observed at the expense of an increased electrical resistivity. At the optimum carrier concentration, a maximum zT of 1.3 at 1223K is obtained, which is more than twice that of the state-of-the-art Si_(0.8)Ge_(0.2) flown by NASA.