Thermoelectric and Magnetic Properties of Biphasic ZrFe_0.5Ni_0.5Sb Double Half-Heusler and ZrNiSb Half-Heusler Induced by Co Doping

Improving the efficiency of upcoming thermoelectric (TE) materials and exploring their potential for various niche applications are among the promising strategies for addressing the challenges that impede the commercialization of traditional TE materials. This work reports the results of efforts to improve the performance of the recently optimized ZrFe_0.4Ni_0.6Sb double half-Heusler (DhH) by cobalt (Co) doping. The synthesized ZrFe_0.4-yCo_yNi_0.6Sb samples exhibit phase separation into coherent biphasic DhH and ZrNiSb phases with greatly improved electrical conductivity, which increases from ∼400 S/cm in sample y = 0 to ∼3886 S/cm in sample y = 0.4 at room temperature. In addition to the suppression of bipolar conduction, the thermal conductivity also decreases by ∼18% in sample y = 0.1 compared to sample y = 0, which can be attributed to the enhanced phonon scattering and leads to an increase in peak zT from 0.33 in sample y = 0 to 0.37 in sample y = 0.1 at 973 K. Unfortunately, higher doping concentrations not only deteriorate the Seebeck coefficient but also excessively increase the electronic and lattice thermal conductivities, leading to lower zT values in the samples y > 0.1. By synthesizing and analyzing the (ZrFe_0.4Ni_0.6NiSb)_1-z + (ZrNiSb)_z samples, it is confirmed that the ZrNiSb phases are responsible for the appalling TE performance in the y > 0.1 samples. Further investigation using the recently restructured single parabolic model shows that the ZrFe_0.4Ni_0.6NiSb system was already overdoped before Co doping, which explains the reason for the resulting PF decrease. Finally, the ferromagnetic nature and tunable magnetism of the synthesized samples are revealed by using a vibrating sample magnetometer to study their magnetism, expanding their range of potential niche applications in spintronics.