Ab initio investigations of the structural, electronic, and thermoelectric properties of Fe₂NbAl-based alloys

Ab initio calculations are performed to investigate the electronic structure of Fe₂NbAl full-Heusler alloy as well as the non-stoichiometric Fe_1.75X_0.25NbAl and Fe₂Y_0.25Nb_0.75Al (X, Y = Cr, Mn, Co) alloys. The thermoelectric properties of these alloys are computed using Boltzmann transport formalism. Fe₂NbAl alloy is found to exhibit a semiconductor structure with an indirect band gap of 0.3 eV along the Γ–X high symmetry line. However, Fe_1.75X_0.25NbAl (X = Cr, Mn, Co) and Fe₂Y_0.25Nb_0.75Al (Y = Cr, Co) are found to be metallic, whereas Fe₂Mn_0.25Nb_0.75Al is semiconductor with a band gap of 0.2 eV at Γ-point. The p-type of Fe₂NbAl alloy shows a maximum Seebeck coefficient of 220 μV/K at 800 K, whereas the n-type shows a peak of 280 μV/K at 150 K. The maximum power factor reaches 12 and 51 × 10⁻³ W/mK² at hole concentrations of ~1.60 × 10²¹ and 1.45 × 10⁻³ cm⁻³ for the p-type material and about 18 and 42 × 10⁻³ W/mK² at electron concentrations of ~1.65 × 10²¹ and 1.6 × 10⁻³ cm⁻³ for the n-type at 300 and 800 K, respectively. Alloying Fe₂NbAl with Cr, Mn, and Co at Fe and Nb sites leads to an appreciable increase in the PF values of the non-stoichiometric alloys, which indicates higher efficiencies and potential thermoelectric applications.