2 + 2 = 3: Making Ternary Phases through a Binary Approach

Abstract

Synthetic organic chemists have a large toolbox of named reactions to form structural motifs through a retrosynthetic approach when targeting a complex molecule. On the other hand, a comparatively complex inorganic compound may be made through simple mechanochemical reactions of the elements followed by annealing. For complex phases that involve more than two elements, the simple mechanochemical process can be complex with many competing phases, which can negatively impact desired properties. This point has been made recently with a revelation of improved properties of thermoelectric materials upon the removal of impurities. Compounds of the Yb14AlSb11 structure type represent complex Zintl phases with exceptional high-temperature thermoelectric properties but are difficult to prepare in high purity. In this work, a quenching study was used to elucidate the pathway taken by reactions from the elements to form the complex ternary phase, Yb14AlSb11. Through that study, two Yb–Sb binary phases, Yb11Sb10 and Yb4Sb3, were identified as intermediates in the reaction. These two Yb–Sb binaries were investigated for use as reactive precursors to form Yb14MnSb11 in reactions with MnSb. Through this pseudoretrosynthetic approach, reactions from Yb4Sb3 allowed for the synthesis of high-purity Yb14MnSb11 and Yb14MgSb11 through balanced, stoichiometric reactions. The apparent Yb2O3 (∼1%) impurity found in these products was systematically reduced with x in the series Yb14‑x MnSb11 (x = 0–0.05), suggesting that the main phase is inherently Yb-deficient and showing the high degree of control obtained through this synthetic approach. The stoichiometric sample of Yb14MgSb11 has a peak zT of 1.3 at 1175 K, and the stoichiometric sample of Yb14MnSb11 has a peak zT of 1.2 at 1275 K. This approach to solid-state synthesis provides reproducible products from balanced stoichiometric reactants to form high-purity complex structure types and can be adapted to other difficult ternary systems.

Unlocking the thermoelectric potential of the Ca 14 AlSb 11 structure type

“…The room temperature carrier mobility of this analog is double that of Yb 14 MnSb 11 (2.2 cm 2 V −1 s −1 ), which is atttibuted to the smaller unit cell volume, leading to better orbital overlap ( 2 , 14 , 21 ). Despite the lower carrier concentrations, Yb 14 MgSb 11 has slightly lower carrier mobilities (4.0 to 4.70 cm 2 V −1 s −1 ) than Yb 14 ZnSb 11 at room temperature likely due to the larger unit cell volume of the Mg analog (6150 Å 3 ) ( 1 , 3 ).…”

“…The density of this disk was measured using the Archimedes method with toluene as the liquid. The density of the sample by the Archimedes method using toluene was 8.29(3) g/cm 3 , which is over 98% dense when considering the larger unit cell volume of this sample of Yb 14 ZnSb 11 [V = 6057.65(6) Å 3 ,  calc = 8.38951 (8) (1,2,25). Here, MM is the molar mass of the respective compounds.…”

“…S7 to S9 and lattice parameters in table S1. carrier concentrations to Yb 14 MnSb 11 (8.06 × 10 20 for Mn) with lower values at 600 K (2,3,21). The Mg analog Yb 14 MgSb 11 exhibits lower carrier concentrations (5.3 × 10 20 to 6.5 × 10 20 h + /cm 3 at 300 K) than its Zn counterpart across the entire temperature range (1,3).…”

“…Figure 7A provides the temperature-dependent Seebeck coefficients of Yb 14 ZnSb 11 from both two-probe and off-axis four-probe measurements. The two-probe Seebeck coefficient of Yb 14 ZnSb 11 was measured on a custom instrument at JPL, allowing for direct comparison to previously reported values for Yb 14 MnSb 11 and Yb 14 MgSb 11 across the entire operating temperature range (1)(2)(3)23). The Seebeck coefficient starts at 32.27 V K −1 at 311 K and increases linearly until it levels off to a peak of 211.25 V K −1 at 1181 K. After that point, the Seebeck coefficients begin to decrease attributed to the onset of bipolar conductance (22).…”

“…Yb 14 MSb 11 (M = Mn and Mg) are among the most efficient hightemperature p-type thermoelectric materials exhibiting a unitless thermoelectric figure of merit, zT, of 1.33 at 1275 K and 1.28 at 1175 K, respectively (1)(2)(3). The high zT depends on electronic and thermal transport, and a rationale for their high performance has recently emerged (4)(5)(6).…”