Synergistic band and defect engineering in ZrNiSn alloys via Ni-vacancy regulation for realizing high thermoelectric performance

Of the n-type half-Heusler material systems, Zr _{1- x}Hf _xNiSn _{1- y}Sb _y alloys have excellent thermoelectric properties. Typically, the Hf atoms replace Zr site atoms, thus reducing the lattice thermal conductivity. Nonetheless, the excess use of Hf increases the cost of half-Heusler thermoelectric materials, limiting their commercial use. In this study, we used band and defect engineering concurrently to enhance the thermoelectric properties of ZrNiSn-based alloys through the modulation of Ni-structural vacancies. The induction of Ni-structural vacancies not only enhances phonon scattering, decreasing both bipolar and lattice thermal conductivities at high temperatures, but also promotes the formation of multiple electronic valleys close to the Fermi level, enhancing the electrical transport properties. Concurrently, doping ZrNi _{1- x}Sn samples with Nb increases the stability of the crystal structure and further refines the thermoelectric parameters. Notably, a thermoelectric figure of merit ( ZT) of 1.16 was achieved at 1073 K for Zr _0.96Nb _0.04Ni _0.9Sn, as high as that of Zr _{1- x}Hf _xNiSn _{1- y}Sb _y samples but without the addition of Hf. This is a novel example of integrating multiple electronic valleys into a ZrNiSn system by introducing Ni-structural vacancies. Crucially, this approach to band structure engineering could be applied to other thermoelectric materials.