Advancing very high temperature thermoelectric performance of Yb₄Sb₃ through dual-substitutions: a combined experimental and theoretical study

This article reports and discusses the synthesis and the transport properties of the binary rare-earth antimonide Yb4Sb3 and some of its substituted derivatives. Specifically, co-substitution of La on the Yb site and Bi on the Sb site was attempted to further improve its thermoelectric properties. The solubility limit of the LaxYb4-xSb2.8Bi0.2 solid solution was established to be x = 0.3. Subsequent synthesis of x = 0.1, 0.2, and 0.3 compositions at a larger scale enabled their transport property evaluation and the comparison with Yb4Sb3. The Seebeck coefficient of the substituted compounds was found to be similar to the pristine material from 373 to 1273 K, while an increase in resistivity was observed. Detailed DFT calculations confirmed that the Seebeck coefficient may not be significantly improved by La substitutions and explained the p-type conducting behavior at high temperatures of the title compounds. The thermal conductivity of La0.2Yb3.8Sb2.8Bi0.2 was found to be reduced by about 30% compared to that of the binary Yb4Sb3. The figure of merit zT of the parent Yb4Sb3 compound reaches 0.5 at 1273 K. While dual substitutions have not permitted a significant improvement in the figure of merit mostly due to a resistivity increase, this study provides a stepping stone for further optimization.