Effect of co-substitution on complex thermoelectric compounds: The Zintl phase Ba1-xSrxZn2-yCuySb2 system

Three co-substituted quaternary and quinary Zintl phases belonging to the Ba_1-xSr_xZn_2-yCu_ySb₂ (x = 0, 0.09; 0.24 ≤ y ≤ 0.42) system were prepared by the molten Pb metal-flux method. Co-substitution using the cationic Sr and anionic Cu for Ba and Zn was initially applied to lower the thermal conductivities and to improve the electric conductivities of these title compounds simultaneously. The homogeneities of single-phase products were verified by powder x-ray diffraction analysis, and the BaCu₂S₂-type orthorhombic crystal structures with the Ba/Sr and the Zn/Cu mixed-sites were refined by single crystal x-ray diffraction analysis. Structural selectivity for the observed BaCu₂S₂-type phase was rationalized by the radius ratio of cationic and anionic elements, where r₊/r₋ > 1. DFT calculations using the three structural models revealed that the Sr and Cu substitutions can increase the structural stability and the hole carrier concentration. A series of temperature-dependent electrical transport property measurements for BaZn_1.76Cu_0.24Sb₂ and Ba_0.91Sr_0.09Zn_1.70Cu_0.30Sb₂ successfully proved that the co-substitution using Sr and Cu enhanced electrical conductivities, but reduced the Seebeck coefficients resulting in the slight change in power factor.