In Situ Evolution of Secondary Metallic Phases in Off-Stoichiometric ZrNiSn for Enhanced Thermoelectric Performance

The full-Heusler (FH) inclusions in the half-Heusler (HH) matrix is a well-studied approach to reduce the lattice thermal conductivity of ZrNiSn HH alloy. However, excess Ni in ZrNiSn may lead to the in situ formation of FH and/or HH alloys with interstitial Ni defects. The excess Ni develops intermediate electronic states in the band gap of ZrNiSn and also generates defects to scatter phonons, thus providing additional control to tailor electronic and phonon transport properties synergistically. In this work, we present the implication of isoelectronic Ge-doping and excess Ni on the thermoelectric transport of ZrNiSn. The synthesized ZrNi_1.04Sn_1-Ge (x = 0-0.04) samples were prepared by arc-melting and spark plasma sintering, and were extensively probed for microstructural analysis. The in situ evolution of minor secondary phases, i.e., FH, Ni-Sn, and Sn-Zr, primarily observed post sintering resulted in simultaneous optimization of the electrical power factor and lattice thermal conductivity. A ZT of ∼1.06 at ∼873 K was attained, which is among the highest for Hf-free ZrNiSn-based HH alloys. Additionally, ab initio calculations based on density functional theory (DFT) were performed to provide comparative insights into experimentally measured properties and understand underlying physics. Further, mechanical properties were experimentally extracted to determine the usability of synthesized alloys for device fabrication.