Impact of Lu-Substitution in Yb_14-LuZnSb₁₁: Thermoelectric Properties and Oxidation Studies

Yb₁₄ZnSb₁₁ is one of the newest additions to the high-performance Yb₁₄MSb₁₁ (M = Mn, Mg, and Zn) family of p-type high-temperature thermoelectric materials and shows promise for forming passivating oxide coatings. Work on the oxidation of rare earth (RE)-substituted Yb_14-REMnSb₁₁ single crystals suggested that substituting late RE elements may form more stable passivation oxide coatings. Yb_14-LuZnSb₁₁ (x = 0.1, 0.2, 0.3, 0.4, 0.5, and 0.7) samples were synthesized, and Lu-substitution's effects on thermoelectric and oxidation properties are investigated. The solubility of Lu within the system was found to be quite low with x_max ∼ 0.3; samples with x > 0.3 contained impurities of LuSb. Goldsmid-Sharp band gap estimations show that introducing Lu reduces the apparent band gap. Because of this, the Lu-substituted samples show a reduction in the maximum Seebeck coefficient, decreasing the high-temperature zT. This contrasts with the impact of Lu³⁺ substitution in Yb₁₄MnSb₁₁, where the addition of Lu³⁺ for Yb²⁺ results in increases in both resistivity and the Seebeck coefficient. Oxidation of the x = 0.3 solid solution was studied by thermogravimetric- differential scanning calorimetry , powder X-ray diffraction, scanning electron microscopy-energy-dispersive spectroscopy, and optical images. The samples show no mass gain before 785 K, and ensuing oxidation reactions are proposed. At the highest temperatures, significant amounts of Yb_14-LuZnSb₁₁ remained beneath an oxide coating, suggesting that passivation may be achievable in oxygen environments.