Thermoelectric properties of marcasite-type compounds MSb₂ (M = Ta, Nb): a combined experimental and computational study

Here, we investigate the thermoelectric properties of the marcasite-type compounds MSb₂(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<i>C</i>2/<i>m</i>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 <i>µ</i>V K^-1for TaSb₂and NbSb₂, respectively at ∼444 K. The negative sign of<i>S</i>in the full temperature window signifies the<i>n</i>-type behaviour of these compounds. Both electrical and thermal conductivities show decreasing trends with increasing 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^-1K^-2at 300 K for TaSb₂and NbSb₂, respectively. We found that there is much room for improvement of power factor by tuning carrier concentration. The DFT-based calculations predict the maximum possible power factors at fairly high 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.