We utilized density functional theory to systematically investigate the thermoelectric performance of two low-cost, environmentally friendly half-Heusler materials: CoNbSi and CoNbSn. It is interesting that relatively light CoNbSi exhibits a lower lattice thermal conductivity (4.89 W/(m K) @1000 K) than CoNbSn by solving the phonon Boltzmann transport equation, which is mainly due to strong anharmonicity and a large phonon scattering rate. From the calculated electronic structure, we find that band degeneracy near the valence band maximum can reach 12. Such large valence band degeneracy will lead to good electrical transport properties of p-type materials. Owing to the relatively low lattice thermal conductivity and the good electrical transport properties, the highest ZT value can reach 2.1 and 1.6 at 1000 K for p-type and n-type CoNbSi, respectively, which implies that CoNbSi is a promising low-cost half-Heusler thermoelectric material. Our work not only provides a promising candidate for future experimental investigation but also provides a useful guide to seek and design new thermoelectric materials with strong anharmonicity and high band degeneracy.
Half-Heusler compounds are considered promising thermoelectric materials for high-temperature power generation due to their good electrical properties and thermal stability. Some new half-Heuslers with excellent thermoelectric properties are found to be p-type, the discovery of competitive n-type half-Heusler materials has been extremely challenging. Here, we report a new half-Heusler compound FeGeW through first-principles calculation, which exhibits a high ZT of 2.36 at 1000 K. Systematically studied its thermoelectric performance indicates that a large dispersion or small band effective mass of conduction band can efficiently improve the electrical conductivity of n-type FeGeW. From the calculated formation energy of intrinsic point defects, we find that positive charged Fe interstitial are found to be the dominant defect at Fe-rich/Ge-poor condition, which account for the n-type conduction. Moreover the transition levels of Fe interstitial defect is shallow, which means that this donor defect does not damage electrical conductivity and thermoelectric performance. These results not only find a new n-type half-Heusler compound FeGeW, but also are helpful for understanding the roles of point defects in FeGeW, which is expected to encourage more experimental and theoretical investigations to study this kind of n-type half-Heusler thermoelectric material and seek out strategies to optimize thermoelectric performance using intrinsic point defect.
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