Discovery of multivalley Fermi surface responsible for the high thermoelectric performance in Yb(14)MnSb(11) and Yb(14)MgSb(11).

The Zintl phases, Yb(14) MSb(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) MSb(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) MSb(11) systems.