  Copper-based sulfide is an attractive material for Earth-abundant
thermoelectrics. In this study, we demonstrate the effect of Sn-substitution on
the electrical and thermal transport properties of fematinite Cu3SbS4 from 300
to 573 K. The carrier concentration is controlled in the range from 4 \times
10^18 to 8 \times 10^20 cm^-3 by Sn-substitution. The density-of-states
effective mass is found to be ~3.0 me, assuming the single parabolic band
model. The direct-type optical band gap is ~0.9 eV, which is consistent with
the density functional theory calculation. The dimensionless figure of merit
reaches 0.1 for Sn-doped samples at 573 K.
