Abstract
Four quaternary Zintl phase thermoelectric (TE) materials belonging to the Ba(1-x)Eu(x)Zn(2)Sb(2) (x = 0.02(1), 0.04(1), 0.08(1), 0.15(1)) system were successfully synthesized using the molten Pb-flux or the conventional high-temperature reaction methods. Their crystal structures were characterized by both powder and single-crystal X-ray diffraction analyses, and all four isotypic title compounds adopted the orthorhombic BaCu(2)S(2)-type (Pnma, Z = 4, Pearson code oP20) structure. The radius ratio criterion, based on the cationic and anionic elements (i.e., r(+)/r(-)), was successfully verified in the title system, as in our previous reports, where r(+)/r(-) > 1 for the BaCu(2)S(2)-type structure. A series of density functional theory calculations were performed using a hypothetical model with the idealized compositions of Ba(0.75)Eu(0.25)Zn(2)Sb(2), and the results were compared with the ternary parental compound BaZn(2)Sb(2) to understand the influence of Eu substituents in the Ba(1-x)Eu(x)Zn(2)Sb(2) system. A similar overall shape of the density of states (DOS) curves and the near-constant DOS values at E(F) before and after the cationic substitution suggest only marginal changes in the carrier concentration. Therefore, carrier mobility has a dominant role in rationalizing the observed variations in the electrical transport properties of the title system. Temperature-dependent TE property measurements proved that an increase in the Seebeck coefficient S and a decrease in electrical conductivity σ were observed as the Eu substituents gradually increased in the Ba(1-x)Eu(x)Zn(2)Sb(2) system, although the overall S and σ values were lower than those in the parental compound BaZn(2)Sb(2). The thermal conductivities of these title compounds were successfully lowered by phonon scattering, but due to the overall smaller electrical transport properties, the observed maximum ZT was 0.49 at 773 K for Ba(0.98(1))Eu(0.02)Zn(2)Sb(2).