Thermoelectric properties of chemically substituted Heusler-type Ru2-xNb1+xGa and Ru2NbGa1-xMx (M = In, Ge, and Sn) alloys
Pallab Bag1*, W. Z. Liu1, Yung-Kang Kuo1, C. N. Kuo2, C. S. Lue2
1Department of Physics, National Dong Hwa University, Hualien-97401, Taiwan
2Department of Physics, National Cheng Kung University, Tainan-70101, Taiwan
* Presenter:Pallab Bag,
Investigation of thermoelectric (TE) properties in full Heusler alloys (FHAs) with cubic L21-type crystal structure is an active research area for the last few years due to its multifunctional applications [1]. The TE properties in FHAs strongly depend on the structural order and atomic arrangements in the crystals and the number of the valence electrons per formula unit of the constituents [1-5]. Lately, the nonmagnetic Ru-based FHAs such as Ru2NbGa, Ru2NbAl, Ru2VAl, and Ru2TaAl have considerable interest in thermoelectricity, as they exhibited narrow bandgap semiconducting or semimetallic behavior [2-5]. A substantial increase of ZT values by chemical substitutions in Ru2TaAl and Ru2VAl alloys were studied [3,5]. Therefore, a similar study on Ru2NbGa alloy is highly anticipated for the improvement of ZT value. Here, we present the TE properties of the two series of Ru2NbGa 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) [6]. All these alloys show a semiconducting-like feature. Interestingly, the x = -0.05 sample in Ru2-xNb1+xGa series exhibits a substantial decrease in electrical resistivity 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, indicating that the hole-type carriers dominate the TE transport. Thermal conductivity (κ) reveals the behavior of the excitation of phonon modes with temperature and indicates a dominant lattice phonons heat conduction in all alloys. Moreover, the value of κ was found to be little affected by the chemical substitutions due to the mass-difference phonon scattering. As a result, the TE performances, i.e., the power factor and figure of merit values in Ru2NbGa0.90Sn0.10, Ru2NbGa0.90Ge0.10, and Ru2NbGa0.95In0.05 alloys, show a substantial enhancement as compared to the parent compound.

[1] T. Graf, C. Felser, S.S. Parkin, Prog. Solid State Chem. 39 (2011) 1.
[2] C. N. Kuo, H. W. Lee, C. M. Wei, Y. Lin, Y. K. Kuo, and C. S. Lue, Phys. Rev. B 94 (2016) 205116.
[3] C. W. Tseng, C. N. Kuo, H. W. Lee, K. Chen, R. Huang, C. M. Wei, Y. K. Kuo, C.S. Lue, Phys. Rev. B 96 (2017), 125106.
[4] S. Mondal, C. Mazumdar, R. Ranganathan, E. Alleno, P.C. Sreeparvathy, V. Kanchana, G. Vaitheeswaran, Phys. Rev. B 98 (2018), 205130.
[5] B. Ramachandran, Y. Lin, Y. K. Kuo, C. N. Kuo, A. Gippius, and C. S. Lue, Intermetallics 92 (2018) 36 .
[6] P. Bag, W. Z. Liu, Y. K Kuo, C. N. Kuo, and C. S. Lue, J. Alloys Compd. 849 (2020) 156617.

Keywords: Full Heusler alloys, Semiconductors, Transport properties, Thermoelectric properties, Point defects