Traversing the Metal‐Insulator Transition in a Zintl Phase: Rational Enhancement of Thermoelectric Efficiency in Yb14Mn1−xAlxSb11

For high temperature thermoelectric applications, Yb₁₄MnSb₁₁ 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 Yb₁₄MnSb₁₁ and semiconducting Yb₁₄AlSb₁₁. Yb₁₄Mn_1‐xAl_xSb₁₁ 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 1223 K is obtained, which is more than twice that of the state‐of‐the‐art Si_0.8Ge_0.2 flown by NASA.