Evolution of Thermoelectric and Oxidation Properties in Lu-Substituted Yb₁₄MnSb₁₁

Yb₁₄MnSb₁₁ is one of the state-of-the-art high-temperature p-type thermoelectric materials with reported zTs of 1.2–1.3 at 1273 K. Site substitution of Yb₁₄MnSb₁₁ provides a means to control carrier concentration and impacts the oxidation kinetics. Substitution of Lu³⁺ for Yb²⁺ in Yb₁₄MnSb₁₁ single crystals has been shown to provide faster oxidation kinetics compared with the pristine phase, and it has been speculated that this may lead to surface passivation. Polycrystalline samples of Yb_14–xLu_xMnSb₁₁ were synthesized from the elements and through a route utilizing YbH₂, MnSb, and Yb₄Sb₃ as reactive precursors. The solubility of Lu was found to be less than x = 0.5, and at that composition and above LuSb impurities are observed in the powder X-ray diffraction. Because Lu³⁺ is substituting for Yb²⁺ in a p-type system, there is an increase in both the electrical resistivity and Seebeck coefficient with Lu substitution. As Yb₁₄MnSb₁₁ has been predicted to be optimally doped ∼1.5 × 10²¹ h⁺ cm^–3, this reduction in carrier concentration in the solid solution of Yb_14–xLu_xMnSb₁₁ decreases the peak zT. Oxidation of Yb_13.7Lu_0.3MnSb₁₁ as a function of temperature was studied. At low temperatures, the Lu-substituted sample may have improved oxidation resistance through forming a passivating oxide film on the surface.