Achieving n–p Transformation and Thermoelectric Performance Enhancement of ZrNiSn Half-Heusler Alloys via Sc-Y Co-Doping

The development of high-performance thermoelectric (TE) devices with superior thermal stability relies critically on the availability of both p- and n-type materials with well-matched compositions. Although ZrNiSn is recognized as a well-established n-type Half-Heusler alloy with excellent TE performance, its p-type counterpart exhibits a fairly low TE figure of merit (zT). In this work, we successfully converted n-type ZrNiSn to its p-type TE material by partially substituting Zr with Sc and Y. The resulting defects, including interfaces, dislocations, and nanoprecipitates, can effectively scatter low- and mid-frequency phonons, while heavy-element doping modulates the phonon group velocity and scattering rates. Consequently, the Sc0.25Y0.25Zr0.5NiSn achieves an ultra-low lattice thermal conductivity of 1.44 W m-1 K-1 at 923 K. Ultimately, the p-type Sc0.25Y0.25Zr0.5NiSn alloy attains a peak zT of 0.14 at 873 K, demonstrating strong competitiveness among Hf-free p-type (Ti, Zr)NiSn alloy systems. Moreover, the synergistic effect of multiple strengthening mechanisms increases the Vickers hardness of the Sc0.25Y0.25Zr0.5NiSn alloy to 1145 HV, which is 57.6% higher than the pristine ZrNiSn. This work not only provides an effective approach for developing p-type ZrNiSn alloys but also establishes a feasible modulation strategy for realizing n–p conversion in other TE materials.