Synergistic effect of cation substitution and p-type anion doping to improve thermoelectric properties in Zintl phases 

Altogether, seven Zintl compounds in the solid solution Ca_9-xYb_xZn_4.5-yCu_ySb₉ (0 ≤ x ≤ 1.5, 0 ≤ y ≤ 0.15) system were successfully prepared by the molten Pb-flux and hot-pressing method. From the powder X-ray and single crystal X-ray diffraction investigations, all these isotypic phases were observed to have been crystallized in the Ca₉Mn₄Bi₉-type phase (space group Pbam, Z = 2, Pearson code oP45). The overall crystal structure consists of complex anionic  [Zn₄Sb₉] clusters as well as the space-filling cationic elements. Notably, the central Zn1 site in a triangular coordinate exhibited a partial occupation and a relatively large atomic displacement parameter, which was necessary for the charge balance of the title compounds. All five intra-layer and inter-layer cationic sites showed the Ca/Yb mixed-occupancy with Yb presenting a specific site-preference for the A3 site. Density functional theory calculations unveiled a synergistic effect of the Yb-substitution and the p-type Cu-doping increased carrier concentration by reducing bond polarity through the tuning of the electronegativity difference. Thermoelectric property measurements further validated that the given synergistic effect was successful in enhancing the electrical conductivities of the quinary title compounds compared to the parental ternary compound Ca₉Zn_4.5Sb₉. As a result, the title compound Ca₈YbZn_4.4Cu_0.1Sb₉ achieved the largest ZT of 0.81 at 843 K, which should be attributed to improved electrical transport properties while maintaining balanced thermal conductivity.