We found that whisker crystals of Mo-doped Nb₄SiTe₄ show high thermoelectric performances at low temperatures, indicated by the largest power factor of ∼70 μW cm⁻¹K^–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 of 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 bandgap 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, indicating the high potential of this system as a practical thermoelectric material for low temperature applications.

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