Yb14MnSb11 is a member of a remarkable structural family of compounds that are classified according to the concept of Zintl. This structure type, of which the prototype is Ca14AlSb11, provides a flexible framework for tuning structure-property relationships and hence the physical and chemical properties of compounds. Compounds within this family show exceptional high temperature thermoelectric performance at temperatures above 300 K and unique magnetic and transport behavior at temperatures below 300 K. This review provides an overview of the structure variants, the magnetic properties, and the thermoelectric properties. Suggestions for directions of future research are provided. One active research area is to systematically explore more complex compositions such as Ca11Sb10, K4Pb9, Na8Si46, Ca14AlSb11 and KBa2InAs3. 3-7 The other direction is to replace the alkaline earth metals with divalent rare earth elements (Sm, Eu and Yb) along with the introduction of transition metals into structures, typically replacing the less electronegative metalloid in the anionic framework. 8-10 Combinations of these two directions led to compounds such as Yb14MnSb11, Pr4MnSb9, Eu10Mn6Sb13, Yb9Zn4+xBi9 and Cs13Nb2In6As10. 11-16 The complexity of compositions can be combined with a small flexibility in electron counting. For example, Yb14MnSb11 and Yb9Zn4+xBi9 do not strictly follow the Zintl-Klemm concept. Yb14MnSb11 has Mn 2+ instead of a group 13 element such as in Ca14AlSb11 and therefore is electron deficient, 17 and Yb9Zn4+xSb9 has interstitial Zn atoms which can be compositionally varied to achieve specific properties. 18 At the same time, the total number of valence electrons within an identical Zintl phase structure type with different elements may also vary slightly but the variance can be quite small and limited for many structure types that can be described by the Zintl concept. Therefore, with the introduction of transition elements, new electronic properties are possible, but complete transfer of electrons and clear counting of valence electrons remains a criterion for describing transition and rare earth metal containing Zintl phase compounds. Binary Zintl phase compounds which have compositions of simple ratios of elements usually adopt the structures of known oxides or halides, in which anions and cations are isolated in the structure with no covalent bonding. 2, 19 Both isolated anions, polyanions or clusters in Zintl phase compounds can provide complex compositions such as those represented by Ca11Sb10 and K4Pb9. 3, 4 Polyanions or clusters are formed to compensate for lack of enough electrons from the electropositive element to satisfy valence to form a simple one atom anion. Sb forms Sb-Sb single bonds in the Ca11Sb10 structure type resulting in Sb2 4and Sb4 2polyanions in the structure. 3 The Zintl electron counting provides the following charge balanced scenario: 11Ca 2+ + 4Sb 3-+ 2Sb2 4-+ Sb4 2-. Two types of clusters exist in K4Pb9 with the same formal oxidation state: a monocapped square antiprism and a tr...

Large crystals of Yb14MgSb11 prepared by Sn flux show the presence of Yb3+ making this compound a Zintl phase.

Yb14ZnSb11 has been of interest for its intermediate valency and possible Kondo designation. It is one of the few transition metal compounds of the Ca14AlSb11 structure type that show metallic behavior. While the solid solution of Yb14Mn1-xZnxSb11 shows an improvement in the high temperature figure of merit of about 10% over Yb14MnSb11, there has been no investigation of optimization of the Zn containing phase. In an effort to expand the possible high temperature p-type thermoelectric materials with this structure type, the rare earth (RE) containing solid solution Yb14-xRExZnSb11 (RE = Y, La) was investigated. The substitution of a small amount of 3+ rare earth (RE) for Yb2+ was employed as a means of optimizing Yb14MnSb11 for use as a thermoelectric material. Yb14ZnSb11 is considered an intermediate valence Kondo system where some percentage of the Yb is formally 3+ and undergoes a reduction to 2+ at ~85 K. The substitution of a 3+ RE element could either replace the Yb3+ or add to the total amount of 3+ RE and provides changes to the electronic states. RE = Y, La were chosen as they represent the two extremes in size as substitutions for Yb: a similar and much larger size RE, respectively, compared with Yb3+. The composition x = 0.5 was chosen as that is the typical amount of RE element that can be substituted into Yb14MnSb11. These two new RE containing compositions show a significant improvement in Seebeck while decreasing thermal conductivity. The addition of RE increases the melting point of Yb14ZnSb11 so that the transport data from 300 K to 1275 K can be collected. The figure of merit is increased five times over that of Yb14ZnSb11 and provides a zT ~0.7 at 1275 K.

Yb 14 MnSb 11 is a magnetic Zintl compound as well as being one of the best high temperature p-type thermoelectric materials. According to the Zintl formalism, which defines intermetallic phases where cations and anions are valence satisfied, this structure type is nominally made up of 14 Yb 2+ , 1 MnSb 9− 4 , 1 Sb 7− 3 , and 4 Sb 3− atoms. When Mn is replaced by Mg or Zn, the Zintl defined motifs become 13 Yb 2+ , 1 Yb 3+ , 1 (Mg, Zn)Sb 10− 4 , 1 Sb 7− 3 , and 4 Sb 3− . The predicted existence of Yb 3+ based on simple electron counting rules of the Zintl formalism calls the Yb valence of these compounds into question. X-ray absorption near-edge structure, magnetic susceptibility, and specific heat measurements on single crystals of the three analogs show signatures of intermediate valence Yb behavior and in particular, reveal the heavy fermion nature of Yb 14 MgSb 11 .Inthese isostructural compounds, Yb can exhibit a variety of electronic configurations from intermediate (M = Zn), mostly 2+ (M = Mn), to 3+ (M = Mg). In all cases, there is a small amount of intermediate valency at the lowest temperatures. The amount of intermediate valency is constant for M = Mn, Mg and temperature dependent for M = Zn. The evolution of the Yb valence correlated to the transport properties of these phases is highlighted. The presence of Yb in this structure type allows for fine tuning of the carrier concentration and thereby the possibility of optimized thermoelectric properties along with unique magnetic phenomena.

The band structure of Yb14MgSb11 is tuned by substituting the more earth-abundant cations, Ca and Ba, on the four crystallographically distinct Yb sites (Yb13–x Ca x BaMgSb11 (x = 1, 2, 3, 4, 5, 6)). Single crystals of composition Yb9.7(2)Ca3.85(5)Ba0.29(4)Mg1.13(3)Sb11.0(1) were grown from Sn flux revealing the cation site preferences. Magnetic measurements on this crystal show paramagnetic behavior consistent with the presence of ∼0.85 Yb3+. High-purity samples (>98%) with compositions close to nominal of Yb13–x Ca x BaMgSb11 (x = 1–6) were prepared by ball milling and spark plasma sintering. The carrier concentration can be rationalized with the presence of Yb3+ for all samples and decreases as a function of x in a systematic fashion at room temperature and increases above ∼600 K for x = 3–6. The temperature dependence of the carrier concentration can be understood considering the electronic structure with a light and heavy band valence band contributing to the properties and suggests the involvement of a localized flat band or impurity state that is active with increasing amounts of Ca. The effect of temperature leads to sustained high Seebeck coefficients with low electrical resistivity arising from the transitioning of the light to heavy band with localization of carriers in the flat band or impurity state for Ca-rich compositions. Speed of sound measurements show that the lattice stiffens with increasing x. Despite the stiffening lattice, the thermal conductivity decreases until x = 3, 4 at which point it increases slightly. The x = 4 sample reaches a peak figure of merit (zT) of 1.32 at 1273 K while being 16% lighter by the molar mass compared to Yb14MnSb11 thereby providing a more power dense material.

The solid solutions of Yb 14−x RE x ZnSb 11 (RE = Sc, Y, La, Lu, and Gd; 0.2 ≤ x ≤ 0.7) were prepared to probe the intermediate valency of Yb in Yb 14 ZnSb 11 . The substitution of Yb with RE 3+ elements should reduce or remove the intermediate valency of the remaining Yb ions. Large crystals are grown from Sn-flux, and the structure and magnetic susceptibility are presented. All compounds crystallize in the Ca 14 AlSb 11 structure type and the RE 3+ ions show Yb site substitution preferences that correlate with size. Two compositions of Yb 14−x Y x ZnSb 11 were investigated [x = 0.38(3), 0.45(3)] by temperature-dependent magnetic susceptibility and the broad feature in magnetic susceptibility measurements at 85 K in pristine Yb 14 ZnSb 11 attributed to valence fluctuation decreases and is absent for x = 0.45(3). In compounds with nonmagnetic RE 3+ substitutions (Sc, Y, La, and Lu), temperaturedependent magnetic susceptibility shows a transition from intermediate valency fluctuation toward temperature-independent (Y, La, and Lu) or Curie−Weiss behavior and possibly low temperature heavy Fermion behavior (Sc). In the example of the magnetic rare earth substitution, RE = Gd, the Curie−Weiss-dependent magnetic moment of Gd 3+ is consistent with x. Hall resistivity of Yb 14−x Y x ZnSb 11 showed that the carrier concentration decreases with x and the signature of the low-T intermediate valence state seen for x = 0 is suppressed for x = 0.38 and gone for x = 0.45.

The title compound with the nominal formula, Sr