The role of interstitial Cu on thermoelectric properties of ZrNiSn half-Heusler compounds

The density functional theory (DFT) calculations and experiments have confirmed that in the ABC-type half-Heusler compounds, the 3d elements occupying the B position are natural over-stoichiometry. These additional atoms are able to synergistically optimize the electrical and thermal transport properties of half-Heusler compounds. In this work, Cu (3d104s1) is intentionally introduced into the ZrNiSn compound to form Cu interstitial defects. The correlations between the phase structure, microstructure, and thermoelectric properties of ZrNiCuxSn (x = 0-0.20) are investigated with X-ray and neutron diffraction, transmission electron microscopy, atom probe tomography, and band structure and phonon spectra calculations. The diffraction results reveal that Ni/Cu atoms partially occupy the 4d position (3/4, 3/4, 3/4) of the half-Heusler crystal structure, forming interstitial defects. The interstitial Cu defects force the conduction band minimum to gradually move close to the valence band maximum and reduce the bandgap, rather than induce in-gap states as typical Ni interstitials. Besides the interstitial defects, a full-Heusler phase is also formed in the half-Heusler matrix with increasing Cu content. Due to the interstitial defects and interface engineering, the thermal conductivity is suppressed. As a result, a higher figure of merit (ZT) value is achieved (∼1.1 at 950 K) in the ZrNiCu0.05Sn sample. This work analyses the possibility of interstitial defects from a thermodynamic point of view and highlights the defect engineering to positively tune the thermoelectric properties in half-Heusler compounds.