The full-Heusler VFe 2 Al has emerged as an important thermoelectric material in its thin film and bulk phases. VFe 2 Al is attractive for use as a thermoelectric materials because of it contains only low-cost, non-toxic and earth abundant elements. While VFe 2 Al has often been described as a semimetal, here we show the electronic and thermal properties of VFe 2 Al can be explained by considering VFe 2 Al as a valence precise semiconductor like many other thermoelectric materials but with a very small band gap ( E g = 0.03 ± 0.01 eV). Using a two-band model for electrical transport and point-defect scattering model for thermal transport we analyze the thermoelectric properties of bulk full-Heusler VFe 2 Al. We demonstrate that a semiconductor transport model can explain the compilation of data from a variety of n and p-type VFe 2 Al compositions assuming a small band-gap between 0.02 eV and 0.04 eV. In this small E g semiconductor understanding, the model suggests that nominally undoped VFe 2 Al samples appear metallic because of intrinsic defects of the order of ∼10 20 defects per cm −3 . We rationalize the observed trends in weighted mobilities ( μ w ) with dopant atoms from a molecular orbital understanding of the electronic structure. We use a phonon-point-defect scattering model to understand the dopant-concentration (and, therefore, the carrier-concentration) dependence of thermal conductivity. The electrical and thermal models developed allow us to predict the zT versus carrier concentration curve for this material, which maps well to reported experimental investigations.
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Antibonding induced anharmonicity leading to ultralow lattice thermal conductivity and extraordinary thermoelectric performance in CsK 2 X (X = Sb, Bi)
Full Heusler compounds have long been discovered as exceptional n-type thermoelectric materials. However, no p-type compounds could match the high n-type figure of merit ( ZT ). In this work, based on first-principles transport theory, we predict the unprecedentedly high p-type ZT = 2.2 at 300 K and 5.3 at 800 K in full Heusler CsK 2 Bi and CsK 2 Sb, respectively. By incorporating the higher-order phonon scattering, we find that the high ZT value primarily stems from the ultralow lattice thermal conductivity ( κ L ) of less than 0.2 W mK −1 at room temperature, decreased by 40% compared to the calculation only considering three-phonon scattering. Such ultralow κ L is rooted in the enhanced phonon anharmonicity and scattering channels stemming from the coexistence of antibonding-induced anharmonic rattling of Cs atoms and low-lying optical branches. Moreover, the flat and heavy nature of valence band edges leads to a high Seebeck coefficient and moderate power factor at optimal hole concentration, while the dispersive and light conduction band edges yield much larger electrical conductivity and electronic thermal conductivity ( κ e ), and the predominant role of κ e suppresses the n-type ZT . This study offers a deeper insight into the thermal and electronic transport properties in full Heusler compounds with strong phonon anharmonicity and excellent thermoelectric performance.
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- Award ID(s):
- 2030128
- NSF-PAR ID:
- 10427721
- Date Published:
- Journal Name:
- Journal of Materials Chemistry C
- Volume:
- 10
- Issue:
- 42
- ISSN:
- 2050-7526
- Page Range / eLocation ID:
- 15822 to 15832
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1039/D2TC03356A
