  Here, we investigate the thermoelectric properties of the marcasite-type
compounds MSb$_2$ (M = Ta, Nb) in the temperature range of 310-730 K. These
compounds were synthesized by a solid-state reaction followed by the spark
plasma sintering process. The Rietveld refinement method confirms the
monoclinic phase with space group C2/m for both compounds. The observed values
of Seebeck coefficients exhibit non-monotonic behaviour in the studied
temperature range, with the maximum magnitude of -14.4 and -22.7 $\mu$V
K$^{-1}$ for TaSb$_2$ and NbSb$_2$, respectively at ~444 K. The negative sign
of S in the full temperature window signifies the n-type behaviour of these
compounds. Both electrical and thermal conductivities show an increasing trend
with temperature. The experimentally observed thermoelectric properties are
understood through the first-principles DFT and Boltzmann transport equation. A
pseudogap in the density of states around the Fermi level characterizes the
semimetallic behaviour of these compounds. The multi-band electron and hole
pockets were found to be mainly responsible for the temperature dependence of
transport properties. The experimental power factors are found to be ~0.09 and
~0.42 mW m$^{-1}$ K$^{-2}$ at 310 K for TaSb2 and NbSb2, respectively. From the
DFT-based calculations, the maximum possible power factors for p-type
conduction are predicted as ~1.14 and ~1.74 mW m$^{-1}$ K$^{-2}$, while these
values are found to be ~1.16 and ~1.80 mW m$^{-1}$ K$^{-2}$ for n-type TaSb$_2$
and NbSb$_2$, respectively at 300 K with the corresponding doping
concentrations. The present study suggests that the combined DFT and Boltzmann
transport theory are found to be reasonably good at explaining the experimental
transport properties, and moderate power factors are predicted.
