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By converting waste heat into electricity and improving the efficiency of refrigeration systems, thermoelectric devices could play a significant role in solving today's energy problems. Increasing the thermoelectric efficiency (as measured by the thermoelectric material's figure-of-merit, zT) is critical to the development of this technology. Complex Zintl phases, in particular, make ideal candidates for thermoelectric materials because the necessary "electron-crystal, phonon-glass" properties can be engineered with an understanding of the Zintl chemistry. A recent example is the discovery that Yb 14 MnSb 11 , a transition metal Zintl compound, has twice the zT as the material currently in use at NASA. This perspective outlines a strategy to discover new high zT materials in Zintl phases, and presents results pointing towards the success of this approach.

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. Yb14Mn1‐xAlxSb11 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 me. 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 Si0.8Ge0.2 flown by NASA.

Lanthanum doping of the high-temperature p-type thermoelectric material Yb14MnSb11 enhances the figure of merit (zT) through carrier concentration tuning. This is achieved by substituting La3+ on the Yb2+ site to reduce the free hole concentration as expected from the change in valence. The high-temperature transport properties (Seebeck coefficient, electrical resistivity, Hall mobility, and thermal conductivity) of Yb13.6La0.4MnSb11 are explained by the change in carrier concentration using a simple rigid parabolic band model, similar to that found in Yb14Mn1−xAlxSb11. Together, use of these two dopant sites enables the partial decoupling of electronic and structural properties in Yb14MnSb11-based materials.

Supporting Information.Element maps were taken for the x = 0.6 single crystal sample, and are shown in SFigure 1. These maps show that the distribution of Al and Mn across the crystal is homogeneous. The random light and dark specks on the images are surface imperfections or dust since the crystal was measured as-is, not polished or sanded. SFigure 2 shows the microprobe back scattered electron (BSE) images from the pressed pellets: the light-grey regions are identified as the Yb 14 Mn 1-x Al x Sb 11 phase. There are a few minor small medium-grey regions that were identified as Sn inclusions and a very few dark regions which are voids or Yb inclusions.

Rare-earth transition metal compounds Yb 14 Mn 1-x Zn x Sb 11 , isostructural with Ca 14 AlSb 11 , have been prepared using a metal flux growth technique for thermoelectric property measurements (with x ) 0.0, 0.2, 0.3, 0.4, 0.7, 0.9, and 1.0). Single-crystal X-ray diffraction and electron microprobe analysis data indicate the successful synthesis of a solid-solution for the Yb 14 Mn 1-x Zn x Sb 11 structure type for 0< x < 0.4. Hot-pressed polycrystalline samples showed that the product from the flux reaction was a pure phase from x ) 0 through x ) 0.4 with the presence of a minor secondary phase for compositions x > 0.4. High-temperature (298 K-1275 K) measurements of the Seebeck coefficient, resistivity, and thermal conductivity were performed on hot-pressed, polycrystalline samples. As the concentration of Zn increases in Yb 14 Mn 1-x Zn x Sb 11 , the Seebeck coefficient remains unchanged for 0 e x e 0.7 indicating that the free carrier concentration has remained unchanged. However, as the nonmagnetic Zn 2+ ions replace the magnetic Mn 2+ ions, the spin disorder scattering is reduced, lowering the resistivity. Replacing the magnetic Mn 2+ with non magnetic Zn 2+ provides an independent means to lower resistivity without deleterious effects to the Seebeck values or thermal conduction. Alloying the Mn site with Zn reduces the lattice thermal conductivity at low temperatures but has negligible impact at high temperatures. The reduction of spin disorder scattering leads to an ∼10% improvement over Yb 14 MnSb 11 , revealing a maximum thermoelectric figure of merit (zT) of ∼1.1 at 1275 K for Yb 14 Mn 0.6 Zn 0.4 Sb 11 .

tion. -Single crystals of the title compound are synthesized from mixtures of Yb, Mn, and Sb in a Sn flux (1100°C, 1 h). The samples are characterized by thermal conductivity, Seebeck coefficient, resistivity, and Hall effect measurements. This compound achieves quadrupled efficiency and virtually doubled figure of merit over the current state-of-the-art thermoelectric material, SiGe, thus making it superior for thermoelectric applications in segmented devices. Yb14MnSb11 represents the first complex Zintl phase with substantially higher figure of merit and efficiency than any other competing materials, opening a new class of thermoelectric compounds. -(BROWN, S. R.; KAUZLARICH*, S. M.; GASCOIN, F.; SNYDER, G.

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