Effect of Aliovalent Doping on the Thermoelectric Performance of Double Half-Heusler Alloys

The conversion of waste heat into valuable green energy by employing thermoelectric devices has received significant attention worldwide due to the rapid depletion of fossil fuels and the availability of enormous waste heat resources. In this context, the surge for new thermoelectric (TE) materials development with high TE performance is the need of the hour. Half-Heusler (HH) materials are considered the best candidate TE materials in the mid-temperature range (673-873 K) apart from the silicides. Furthermore, double half-Heusler (DHH) materials embracing two aliovalent HH alloys are considered to be potential candidates for TE devices due to their inherent low lattice thermal conductivity. A few DHH alloys have been synthesized in the current study, in particular Ti2FeNiSb2, MgTiNi2Sb2, and Nb2FeNiSn2. Furthermore, the thermoelectric transport properties have been measured and compared with conventional HH compounds, such as TiCoSb, ScNiSb, and NbCoSn. The Ti2FeNiSb2 exhibited the higher Seebeck coefficient of - 120 mu V/K among the three compounds at 813 K. As a result, with an increased power factor and reduced thermal conductivity, the Ti2FeNiSb2 DHH has exhibited a figure-of-merit (ZT) of similar to 0.1 at 813 K. This enhancement was mainly due to the aliovalent substitution (1:1) of Fe and Ni at the Co-site in the DHH compound, significantly reducing the lattice thermal conductivity and maintaining the band gap. Furthermore, the synthesized compounds possess a net valence value equal to zero with a valence electron count equal to 18, and these compounds have excellent thermal stability. These results are discussed in detail, delineating the underlying physics to support the experimentally realized results. Suitable aliovalent dopants can further improve the thermoelectric performance of DHH with the optimization of process parameters. Finally, the enhancement of ZT for DHH materials has been suitably corroborated with the appropriate structural and microstructural characterizations.