Long- and short-range structures of Ti1−xHfxNi1.0/1.1Sn half-Heusler compounds and their electric transport properties

Abstract

A commercially feasible, nontoxic material that would convert heat into electricity in an efficient and cheap way has not been yet identified. Half-Heusler compounds are one the most interesting candidates but to become competitive their thermoelectric properties still need to be improved. Atomic substitution and excess Ni in the crystal structure of ternary MNiSn half-Heuslers are among the most successful approaches used to boost their performance. Here, we report on the effect of both methods applied simultaneously to the series of polycrystalline Ti_1−xHf_xNi_1.0/1.1Sn, x = 0.00, 0.10, 0.15, 0.20, samples. High-resolution synchrotron powder X-ray diffraction studies combined with transmission electron microscopy demonstrate the crystallization of single or multiple Hf-substituted half-Heusler phase(s). The analysis of their long-range atomic structures shows that most of them contain interstitial Ni atoms disorderly distributed at the nominally vacant 4d sites. However, the short-range atomic correlations suggest that, for some compositions, excess Ni creates an orderly arranged additional atomic plane at the vacant fcc sublattice, which can be seen as a defective half- and/or full-Heusler phase. The results also show that the Ni-rich samples crystallize with the micrometer-sized full-Heusler phases, while all Hf-incorporating compositions present dispersion of HfO₂ nanoprecipitates in the grains of the half-Heusler matrix. This research is complemented by thermoelectric transport measurements of the studied compositions in the range of 300–750 K. The results suggest that neither Seebeck coefficient nor electrical resistivity shows an obvious correlation with the observed microstructures. These finding are discussed with respect to previously proposed transport mechanisms.