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Using the 18-Electron Rule To Understand the Nominal 19-Electron Half-Heusler NbCoSb with Nb Vacancies
journal contribution
posted on 2017-01-17, 00:00 authored by Wolfgang G. Zeier, Shashwat Anand, Lihong Huang, Ran He, Hao Zhang, Zhifeng Ren, Chris Wolverton, G. Jeffrey SnyderThe 18-electron rule is a widely
used criterion in the search for
new half-Heusler thermoelectric materials. However, several 19-electron
compounds such as NbCoSb have been found to be stable and exhibit
thermoelectric properties rivaling state-of-the art materials. Using
synchrotron X-ray diffraction and density functional theory calculations,
we show that samples with nominal (19-electron) composition NbCoSb
actually contain a half-Heusler phase with composition Nb0.84CoSb. The large amount of stable Nb vacancies reduces the overall
electron count, which brings the stoichiometry of the compound close
to an 18-electron count, and stabilizes the material. Excess electrons
beyond 18 electrons provide heavy doping needed to make these good
thermoelectric materials. This work demonstrates that considering
possible defect chemistry and allowing small variation of electron
counting leads to extra degrees of freedom for tailoring thermoelectric
properties and exploring new compounds. Here we discuss the 18-electron
rule as a guide to find defect-free half-Heusler semiconductors. Other
electron counts such as 19-electron NbCoSb can also be expected to
be stable as n-type metals, perhaps with cation vacancy defects to
reduce the electron count.
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state-of-the art materialscomposition Nb 0.84 CoSbcation vacancy defectsOther electron countssynchrotron X-ray diffractionNominal 19- Electron Half-Heusler NbCoSb18- electron count19- electron compoundsdefect-free half-Heusler semiconductorselectron count18- Electron Rule19- electron NbCoSb18- electron rule
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