Thermoelectric transport properties of CaMg 2Bi 2, EuMg 2Bi 2, and YbMg 2Bi 2

Abstract

The thermoelectric transport properties of CaMg 2Bi 2, EuMg 2Bi 2, and YbMg 2Bi 2 were characterized between 2 and 650 K. As synthesized, the polycrystalline samples are found to have lower p-type carrier concentrations than single-crystalline samples of the same empirical formula. These low carrier concentration samples possess the highest mobilities yet reported for materials with the CaAl 2Si 2 structure type, with a mobility of ∼740 cm2/V/s observed in EuMg 2Bi 2 at 50 K. Despite decreases in the Seebeck coefficient (α) and electrical resistivity (ρ) with increasing temperature, the power factor (α2ρ) increases for all temperatures examined. This behavior suggests a strong asymmetry in the conduction of electrons and holes. The highest figure of merit (zT) is observed in YbMg 2Bi 2, with zT approaching 0.4 at 600 K for two samples with carrier densities of approximately 2×1018cm -3 and 8×1018 cm -3 at room temperature. Refinements of neutron powder diffraction data yield similar behavior for the structures of CaMg 2Bi 2 and YbMg 2Bi 2, with smooth lattice expansion and relative expansion in c being ∼35% larger than relative expansion in a at 973 K. First-principles calculations reveal an increasing band gap as Bi is replaced by Sb and then As, and subsequent Boltzmann transport calculations predict an increase in α for a given n associated with an increased effective mass as the gap opens. The magnitude and temperature dependence of α suggests higher zT is likely to be achieved at larger carrier concentrations, roughly an order of magnitude higher than those in the current polycrystalline samples, which is also expected from the detailed calculations. © 2012 American Physical Society.