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Journal Article

Resolving the true band gap of ZrNiSn half-Heusler thermoelectric materials


Felser,  Claudia
Claudia Felser, Inorganic Chemistry, Max Planck Institute for Chemical Physics of Solids, Max Planck Society;

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Schmitt, J., Gibbs, Z. M., Snyder, G. J., & Felser, C. (2015). Resolving the true band gap of ZrNiSn half-Heusler thermoelectric materials. Materials Horizons, 2(1), 68-75. doi:10.1039/c4mh00142g.

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N-type XNiSn (X = Ti, Zr, Hf) half-Heusler (HH) compounds possess excellent thermoelectric properties, which are believed to be attributed to their relatively high mobility. However, p-type XNiSn HH compounds have poor figures of merit, zT, compared to XCoSb compounds. This can be traced to the suppression of the magnitude of the thermopower at high temperatures. E-g = 2eS(max)T(max) relates the band gap to the thermopower peak. However, from this formula, one would conclude that the band gap of p-type XNiSn solid solutions is only one-third that of n-type XNiSn, which effectively prevents p-type XNiSn HHs from being useful thermoelectric materials. The study of p-type HH Zr1-xScxNiSn solid solutions show that the large mobility difference between electrons and holes in XNiSn results in a significant correction to the Goldsmid-Sharp formula. This finding explains the difference in the thermopower band gap between n-type and p-type HH. The high electron-to-hole weighted mobility ratio leads to an effective suppression of the bipolar effect in the thermoelectric transport properties which is essential for high zT values in n-type XNiSn (X = Ti, Zr, Hf) HH compounds.