Decisive role of interstitial defects in half-Heusler semiconductors: An ab initio study

Abstract

Half-Heusler semiconductors satisfying 18-electron rule typically display promising characteristics for thermoelectric applications. A persistent inconsistency between the type of charge carriers in some of these alloys as obtained from experiment and theory however casts serious doubt on the computational prediction of new and efficient half-Heusler alloys. To gain insights into the origin of this disparity, we have investigated the effect of intrinsic point defects on the electronic structure of four frequently studied half-Heusler alloys of the form XY Z with Y being Ni or Co. Using state-of-the-art ab initio calculations, our study reveals that interstitial Ni and Co are energetically most stable point defects in these alloys. Remarkably, interstitial defect modifies the location of the Fermi level inside the band gap as well as the value of the band gap, thereby bringing in close agreement with the corresponding experimental result. This work thus highlights the decisive role played by interstitial defects in thermoelectric half-Heusler alloys, which may open a new avenue for deliberately utilizing these defects as a strategy for tailoring electronic structure and hence the corresponding thermoelectric properties.

Electronic structure and low-temperature thermoelectric transport of TiCoSb single crystals

“…These defects alter the electronic transport properties, while the grain boundaries of polycrystalline samples have been shown to limit the scattering length of charge carriers 9,50-52 . These results agreed with a recent theoretical study, which attributed the n-type character of the polycrystalline samples to an increased E F and to the Cointerstitial in-gap states, while indicating the p-type behavior of the defect-free materials 53 .…”

“…9,50–52 These results agree with a recent theoretical study, which attributed the n-type character of the polycrystalline samples to an increased E F and to the Co-interstitial in-gap states, while indicating the p-type behavior of the defect-free materials. 53…”

“…9,[50][51][52] These results agree with a recent theoretical study, which attributed the n-type character of the polycrystalline samples to an increased E F and to the Co-interstitial in-gap states, while indicating the p-type behavior of the defect-free materials. 53 The carrier mobility evolution with temperature (Fig. 4c) displayed acoustic phonon scattering at temperatures above 100 K for batches 1 and 2.…”

Chemical Interpretation of Charged Point Defects in Semiconductors: A Case Study of Mg2Si

“…In particular, the framework reviewed here for Mg 2 Si can directly be applied to isostructural compounds such as fluorite CeO 2 , [122] and modified to compounds with crystal structures having the same atomic positions such as half‐Heusler systems [123,124] . For example, controlling interstitial defects in half‐Heusler systems may affect the thermoelectric performance of the material, [125,126] as interstitials can induce mid‐gap states in the electronic structure [127–129] . More generally, a molecular orbital theory‐based understanding of the electronic structures of defects can be built for any material using the local symmetry of the defect site, like the tetrahedral symmetry of the Mg site and the cubic coordination of the interstitial and Si sites in Mg 2 Si.…”

Experimental and computational study of the role of defects and secondary phases on the thermoelectric properties of TiNi1+xSn (0 ≤ x ≤ 0.12) half Heusler compounds

“…electrical resistivity and Seebeck coefficient) [1,2]. Since then, this family of compounds has been extensively studied both from the experimental [3][4][5][6][7][8][9][10] and computational [11][12][13][14][15][16][17][18][19] point of view, leading to the development of high performing thermoelectric materials (ZT max = 1.5 at 820 K in Nanotechnology Nanotechnology 34 (2023) 315703 (9pp) https://doi.org/10.1088/1361-6528/acd120 Ti 0.5 Zr 0.25 Hf 0.25 NiSn [20]). Among HH compounds, the TiNiSn is considered a model system for understanding the relationship among structural, microstructural and thermoelectric properties.…”