Impact of Lu-Substitution in Yb 14- x Lu x ZnSb 11 : Thermoelectric Properties and Oxidation Studies.

YbZnSb is one of the newest additions to the high-performance YbMSb (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 YbREMnSb single crystals suggested that substituting late RE elements may form more stable passivation oxide coatings. YbLuZnSb ( = 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 ∼ 0.3; samples with > 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 . This contrasts with the impact of Lu substitution in YbMnSb, where the addition of Lu for Yb results in increases in both resistivity and the Seebeck coefficient. Oxidation of the = 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 YbLuZnSb remained beneath an oxide coating, suggesting that passivation may be achievable in oxygen environments.