  The thermoelectric transport properties of CaMg2Bi2, EuMg2Bi2, and YbMg2Bi2
were characterized between 2 and 650K. 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 CaAl2Si2 structure type, with a mobility of ~740cm$^2$/V/s observed in
EuMg2Bi2 at 50K. Despite decreases in the Seebeck coefficient (\alpha) and
electrical resistivity (\rho) with increasing temperature, the power factor
(\alpha^2/\rho) 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 YbMg2Bi2, with zT approaching 0.4 at 600K for two
samples with carrier densities of approximately 2x10^{18}cm^{-3} and
8x10^{18}cm^{-3} at room temperature. Refinements of neutron powder diffraction
data yield similar behavior for the structures of CaMg2Bi2 and YbMg2Bi2, with
smooth lattice expansion and relative expansion in $c$ being ~35% larger than
relative expansion in $a$ at 973K. First principles calculations reveal an
increasing band gap as Bi is replaced by Sb then As, and subsequent Boltzmann
transport calculations predict an increase in \alpha for a given $n$ associated
with an increased effective mass as the gap opens. The magnitude and
temperature dependence of \alpha 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.
