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

Abstract

The Zintl phases, Yb14MSb11 (M = Mn, Mg, Al, Zn), are now some of the highest thermoelectric efficiency p-type materials with stability above 873 K. Yb14MnSb11 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 Yb14Mg1−xAlxSb11 (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 (Nv = 8) band is responsible for the groundbreaking performance of Yb14MSb11. This multiband understanding of the properties provides insight into other thermoelectric systems (La3−xTe4, SnTe, Ag9AlSe6, and Eu9CdSb9), and the model predicts that an increase in carrier concentration can lead to zT > 1.5 in Yb14MSb11 systems.

Thermoelectric properties and low thermal conductivity of Zintl compound Yb10MnSb9

“…However, when considering the only the lattice contributions to the total thermal conductivity, the lattice thermal conductivity of Yb 14 MnSb 11 is lower than that of Yb 10 MnSb 9 , although the κ of Yb 14 MnSb 11 appears to be affected by multiple electronic bands making it less clear how to extract a lattice contribution. 26 The total thermal conductivity of Yb 10 MnSb 9 appears to be completely dominated by the lattice thermal conductivity term and thus the values for total thermal conductivity and lattice thermal conductivity are nearly identical. This is because this material has a very high electrical resistivity, and as such very little heat is carried by the electronic charge carriers.…”

“…The Zintl compound Yb 14 MnSb 11 , 23 has been the topic of numerous studies 6,8,9,24–26 due to its high thermoelectric performance at high temperatures making it a desirable candidate for space applications. This compound exists in the complicated Yb–Mn–Sb phase diagram which includes many binary and ternary compounds, with multiple Zintl compounds displaying thermoelectricity.…”

Finding the order in complexity: The electronic structure of 14-1-11 zintl compounds

“…We note that the valence bands in A 14 M Sb 11 compounds are quite similar, with a single dispersive band centered at the Γ point and flatter, non-dispersive bands below the band extremum (see e.g., Yb 14 MnSb 11 25 ). The fact that these bands are dominated by Sb-atoms explains why the p-type thermoelectric properties of all A 14 M Sb 11 compounds are similar for the same hole concentration.…”

“…From detailed experimental and theoretical analysis of the high zT compositions Yb 14 MnSb 11 and Yb 14 MgSb 11 and related compounds it has been shown 15,23,[25][26][27][28] that the valence bands are essentially the same for the Sb compounds. The valence bands contain a simple, light valence band with a second heavier valence band close in energy that has high valley degeneracy which provides additional conduction paths, contributing to a high electrical conductivity, while maintaining a large thermopower 5,6 .…”

“…of 500 eV. The 4f electrons of Yb were not treated explicitly as valence electrons following the methodology of Ref 25…”