Manipulation of Ni Interstitials for Realizing Large Power Factor in TiNiSn‐Based Materials

Abstract

Defect engineering has been identified as an effective strategy for improving thermoelectric performance by tailoring electron and phonon transport. TiNiSn is unique due to its naturally formed Ni interstitials, where the interstitial atoms enable strong phonon scattering that results in reduced lattice thermal conductivity, although an adverse effect on mobility is inevitable. Rather than pursuing the conventional strategy of strengthening the interstitial scattering to improve the performance of TiNiSn‐based materials, an attempt is made to minimize the atomic disorder in order to enhance the mobility, which in turn favors a higher power factor. The altered bandgap, and electrical and thermal properties demonstrate that the interstitials can be effectively controlled by intentionally reducing the amount of Ni. Benefiting from the manipulation of the interstitials, significantly enhanced mobility is achieved in the Ni‐deficient composition, resulting in peak power factor of ≈50 µW cm−1 K−2, which is comparable to the best n‐type half‐Heusler compounds. Additionally, the well‐designed composition employing Ni interstitial manipulation and heavy‐element doping exhibits peak ZT of ≈0.73, higher than that of all other reported TiNiSn‐based materials. The unique role of interstitials in either electron or phonon transport is emphasized, and further encouragement for engineering thermoelectric properties by manipulating intrinsic disorder, especially in materials with complex structures, is provided.

Suppression of thermal conductivity without impeding electron mobility in n-type XNiSn half-Heusler thermoelectrics

“…60 For our samples, the alloy scattering most likely originates from the X-site, as there is no evidence for the presence of substantial amounts of interstitial Ni from the NPD analysis (Table S1 †). Recent literature has conrmed the suppression of the electron mobility by interstitial Ni, 46,62,63 and our results on interstitial Cu are therefore highly relevant as they afford the possibility of increased phonon scattering without detriment to the electrical transport. This very different impact is supported by our DFT calculations that show the presence of in-gap states for interstitial Ni, which provide a possible route for carrier scattering, while these are absent in the Cu case.…”

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

“…It has been speculated that slight change in alloy composition in the experimental samples due to point defects may explain this deviation from the usual trend observed in semiconductors [57,64,65]. While external doping is the commonly employed point defect engineering strategy for tailoring carrier concentration and mobility [44][45][46][88][89][90][91][92][93][94], intrinsic point defects can also modify electronic structure, carrier concentration, and hence transport properties of half-Heusler alloys [95][96][97][98][99][100][101][102]. The high-resolution angle-resolved photoemission spectroscopy (ARPES) measurements can be used to gain insights into the defect-induced changes in the electronic structure [99,103].…”

Anomalous Defect Dependence of Thermal Conductivity in Epitaxial WO3 Thin Films

“…One common strategy for the manipulation of phonon transport has been to introduce lattice imperfections in materials which alters the mean free path of phonons, particularly for applications in thermoelectric energy conversion . Based on the well‐established Abeles theory of interactions between phonons and defects in alloys or semiconductors, point defects, including substitutionals, interstitials, and vacancies, are considered to cause additional scattering and therefore lower κ L due to the mass differences and local strain that they introduce. However, in addition to localized mass and strain effects, point defects can also yield significant structural changes, particularly in complex oxides .…”