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The Zintl phases, Yb 14 M Sb 11 ( M = Mn, Mg, Al, Zn), are now some of the highest thermoelectric efficiency p-type materials with stability above 873 K. Yb 14 MnSb 11 gained prominence as the first p-type thermoelectric material to double the efficiency of SiGe alloy, the heritage material in radioisotope thermoelectric generators used to power NASA’s deep space exploration. This study investigates the solid solution of Yb 14 Mg 1− x Al x Sb 11 (0 ≤ x ≤ 1), which enables a full mapping of the metal-to-semiconductor transition. Using a combined theoretical and experimental approach, we show that a second, high valley degeneracy ( N v = 8) band is responsible for the groundbreaking performance of Yb 14 M Sb 11 . This multiband understanding of the properties provides insight into other thermoelectric systems (La 3− x Te 4 , SnTe, Ag 9 AlSe 6 , and Eu 9 CdSb 9 ), and the model predicts that an increase in carrier concentration can lead to zT > 1.5 in Yb 14 M Sb 11 systems. 

Abstract Engineering semiconductor devices requires an understanding of charge carrier mobility. Typically, mobilities are estimated using Hall effect and electrical resistivity meausrements, which are are routinely performed at room temperature and below, in materials with mobilities greater than 1 cm²V^‐1s^‐1. With the availability of combined Seebeck coefficient and electrical resistivity measurement systems, it is now easy to measure the weighted mobility (electron mobility weighted by the density of electronic states). A simple method to calculate the weighted mobility from Seebeck coefficient and electrical resistivity measurements is introduced, which gives good results at room temperature and above, and for mobilities as low as 10⁻³cm²V^‐1s^‐1,Here, μ_wis the weighted mobility, ρ is the electrical resistivity measured in mΩ cm,Tis the absolute temperature in K,Sis the Seebeck coefficient, andk_B/e = 86.3 µV K^–1. Weighted mobility analysis can elucidate the electronic structure and scattering mechanisms in materials and is particularly helpful in understanding and optimizing thermoelectric systems.