  We found that whisker crystals of Mo-doped Nb4SiTe4 show high thermoelectric
performances at low temperatures, indicated by the largest power factor of 70
microW cm-1 K-2 at 230-300 K, much larger than those of Bi2Te3-based practical
materials. This power factor is smaller than the maximum value in the 5d
analogue Ta4SiTe4, but is comparable to that with a similar doping level. First
principles calculation results suggest that the difference in thermoelectric
performances between Nb and Ta compounds is caused by the much smaller band gap
in Nb4SiTe4 than that in Ta4SiTe4, due to the weaker spin-orbit coupling in the
former. We also demonstrated that the solid solution of Nb4SiTe4 and Ta4SiTe4
shows a large power factor, indicating that their combination is promising as a
practical thermoelectric material, as in the case of Bi2Te3 and Sb2Te3. These
results advance our understanding of the mechanism of high thermoelectric
performances in this one-dimensional telluride system, as well as indicating
the high potential of this system as a practical thermoelectric material for
low temperature applications.
