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Contributed by Ching-Wu Chu, May 31, 2016 (sent for review March 29, 2016; reviewed by Peter Rogl and Ronggui Yang)
Significance
The search for high-efficiency thermoelectric materials encompasses many classes of semiconductors. Zintl phases are attractive thermoelectric materials for thermoelectric applications. Here, we report the high thermoelectric performance of the rarely studied bismuth (Bi)-based Zintl phases (Eu0.5Yb0.5)1−xCaxMg2Bi2 with the record figure-of-merit ZT as high as 1.3 at 873 K. This ZT value is, to our knowledge, the highest ever reported in CaAl2Si2-based structures, especially compared with the best antimony (Sb)-based YbZn0.4Cd1.6Sb2 compound. Because Sb-based Zintl compounds have been studied for many decades, this Bi-based Zintl phase with high thermoelectric properties could be a good thermoelectric material candidate in the future.
Abstract
Complex Zintl phases, especially antimony (Sb)-based YbZn0.4Cd1.6Sb2 with figure-of-merit (ZT) of ∼1.2 at 700 K, are good candidates as thermoelectric materials because of their intrinsic “electron–crystal, phonon–glass” nature. Here, we report the rarely studied p-type bismuth (Bi)-based Zintl phases (Ca,Yb,Eu)Mg2Bi2 with a record thermoelectric performance. Phase-pure EuMg2Bi2 is successfully prepared with suppressed bipolar effect to reach ZT ∼ 1. Further partial substitution of Eu by Ca and Yb enhanced ZT to ∼1.3 for Eu0.2Yb0.2Ca0.6Mg2Bi2 at 873 K. Density-functional theory (DFT) simulation indicates the alloying has no effect on the valence band, but does affect the conduction band. Such band engineering results in good p-type thermoelectric properties with high carrier mobility. Using transmission electron microscopy, various types of strains are observed and are believed to be due to atomic mass and size fluctuations. Point defects, strain, dislocations, and nanostructures jointly contribute to phonon scattering, confirmed by the semiclassical theoretical calculations based on a modified Debye–Callaway model of lattice thermal conductivity. This work indicates Bi-based (Ca,Yb,Eu)Mg2Bi2 is better than the Sb-based Zintl phases.
Footnotes
- ↵1To whom correspondence may be addressed. Email: cwchu{at}uh.edu, suijiehe{at}hit.edu.cn, or zren{at}uh.edu.
Author contributions: J. Shuai, J. Sui, and Z.R. designed research; J. Shuai, Y.L., Z.Z., and J.B. performed research; H.G. and C.W. contributed new reagents/analytic tools; J. Shuai, Z.L., C.-W.C., J. Sui, and Z.R. analyzed data; C.-W.C., J. Sui, and Z.R. directed the project; and J. Shuai, J. Sui, and Z.R. wrote the paper.
Reviewers: P.R., Institut für Physikalische Chemie; and R.Y., University of Colorado, Boulder.
The authors declare no conflict of interest.
This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1608794113/-/DCSupplemental.
Zintl phases (Eu0.5Yb0.5)1−xCaxMg2Bi2
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