Abstract
The discovery of a semiconducting ground state XyYZ (y = 0.8 or 0.75) in nominally 19-electron half-Heusler materials warrants a closer look at their apparently metallic properties that often make them good thermoelectric (TE) materials. By systematically investigating the temperature dependence of off-stoichiometry (x) in V0.8+ xCoSb, Nb0.8+ xCoSb, and Ti0.75+ xNiSb it is found that x invariably increases with increasing temperature, leading to an n-type self-doping behavior. In addition, there is also a large phase width (range of x) associated with each phase that is temperature dependent. Thus, unlike in typical 18-electron half-Heuslers (e,g, TiNiSn), the temperature dependence of vacancy and carrier concentration (n) in nominally 19-electron half-Heuslers links its transport properties to synthesis conditions. The temperature dependence of x and n are understood using density functional theory based defect energies (Ed) and phase diagrams. Ed are calculated for 21 systems which can be used in predicting solubility in this family of compounds. Using this simple strategy, suitable composition and temperature synthesis conditions are devised for obtaining an optimized n to engineer TE properties in phase-pure V0.8+ xCoSb, and the previously unexplored Ta0.8+ xCoSb.
Original language | English (US) |
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Article number | 1801409 |
Journal | Advanced Energy Materials |
Volume | 8 |
Issue number | 30 |
DOIs | |
State | Published - Oct 25 2018 |
Funding
Authors would like to thank Prof. Zhifeng Ren for discussions on the subject of 19-electron half-Heuslers. S.A. would like to thank Vinay Ishwar Hegde for fruitful discussion on the subject of thermodynamic phase stability and defect energies. This work was partially supported by “Solid State Solar Thermal Energy Conversion Center (S3TEC),” an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Science under Award No. DE-SC0001299. G.J.S. and S.A. acknowledge support by the “Designing Materials to Revolutionize and Engineer our Future” program of the National Science Foundation, under Award No. 1729487. C.W. (DFT calculations) acknowledges support by the U.S. Department of Energy, Office of Science and Office of Basic Energy Sciences, under Award No. DE-SC0014520. T.Z. acknowledges support by the National Science Fund for Distinguished Young Scholars (51725102) and the National Natural Science Fund (51761135127). The reference list was reordered on October 25, 2018.
Keywords
- 19-electron half-Heuslers
- NbCoSb
- VCoSb
- defects
- vacancy
ASJC Scopus subject areas
- Renewable Energy, Sustainability and the Environment
- General Materials Science