We report a study of the thermoelectric properties of the two series of Ru2NbGa Heusler-type alloys by attempting chemical substitutions; (i) making the off-stoichiometry between Ru and Nb in Ru2-xNb1+xGa (x = 0.00, ±0.05, and 0.10) and (ii) doping the sp-elements M = Sn, In, and Ge onto the Ga sites in Ru2NbGa1-xMx (x = 0.05, 0.10, 0.20). All the investigated alloys show predominantly a semiconducting-like feature in the temperature-dependent electrical resistivity and the Seebeck coefficient measurements. Interestingly, the sample with x = −0.05 in the off-stoichiometry Ru2-xNb1+xGa series exhibits a substantial decrease in electrical resistivity as compared to the pristine Ru2NbGa sample and demonstrates metallic behavior at low temperatures. Besides, the Seebeck coefficient for the x = 0.10 alloy displays a positive sign in the temperature range under investigation, indicating that the hole-type carriers dominate the thermoelectric transport. This finding is in contrast to other compositions that show a negative sign in the Seebeck coefficient, suggesting that a significant modification of the electronic structure in the Ru1.90Nb1.10Ga alloy. Temperature-dependent thermal conductivity reveals the behavior of the excitation of phonon modes with temperature and indicates a dominant lattice phonons heat conduction in all alloys. Moreover, the thermal conductivity was found to be little affected by the chemical substitution, which is mainly caused by the crystallographic disorders through the different atomic mass and radius of the host and the substituent. The thermoelectric performances, i.e., the power factor (PF) and figure of merit (ZT), were calculated. We found an enhancement in the PF and ZT values in Ru2NbGa0.90Sn0.10, Ru2NbGa0.90Ge0.10, and Ru2NbGa0.95In0.05 alloys as compared to the parent compound of Ru2NbGa.
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