Conduction band engineering of half-Heusler thermoelectrics using orbital chemistry

Guo, Shuping; Anand, Shashwat; Brod, Madison K.; Zhang, Yongsheng; Snyder, G. Jeffrey

doi:10.1039/d1ta09377k

Cited by 36 publications

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“…c) The energy of the L VBM decreases relative to that of the Γ VBM when the difference in group number (Y-X) increases, [40] and d) the energy of the X 3 CBM decreases relative to the energy of the X 2 CBM as the group number difference increases. [42] The Fermi surfaces in (a) and the plot in (c) are adapted with permission. [40] Copyright 2020, Maxwell T. Dylla et al Exclusive Licensee Science and Technology Review Publishing House.…”

Section: Avoided Crossings In the Hh Electronic Structurementioning

confidence: 99%

“…That is, decreasing the group number difference between X and Y increases the energy of the X 3 CBM relative to the X 2 CBM (Figure 2b–d). [ 42 ] Thus, tuning the VBs and CBs is chemically quite simple: adjust the group number of the X and Y elements, with secondary effects from the electronegativity.…”

Section: Introductionmentioning

confidence: 99%

“…b)The X 3 (red) and X 2 (blue) CBMs can also be converged by tuning the group number (or electronegativity) difference to approximately the same value required for VBM convergence. c) The energy of the L VBM decreases relative to that of the Γ VBM when the difference in group number (Y-X) increases,[40] and d) the energy of the X 3 CBM decreases relative to the energy of the X 2 CBM as the group number difference increases [42]. The Fermi surfaces in (a) and the plot in (c) are adapted with permission [40].…”

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confidence: 99%

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The Importance of Avoided Crossings in Understanding High Valley Degeneracy in Half‐Heusler Thermoelectric Semiconductors

Brod

Anand

Snyder

2022

Adv Elect Materials

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Half‐Heusler (hH) compounds are promising candidates for inexpensive, low‐toxicity thermoelectric materials. It is well known that engineering electronic bands with high valley degeneracy is an effective approach for enhancing the performance of thermoelectric materials, and there are several routes for achieving high valley degeneracy in hH systems. For instance, there are multiple locations in the first Brillouin zone where the valence band maximum can be found (at the Γ‐, L‐, or W‐point), and there are two competing low‐lying conduction bands at the X‐point, where the conduction band minimum is located. By converging the multiple valence band and conduction band extrema, the valley degeneracy, and hence, performance of these materials can be improved. Here, group theoretical and tight‐binding approaches, in addition to first‐principles density functional theory calculations, are used to study the chemical origins of various band extrema in both the n‐type and p‐type compounds, with particular focus on ZrNiSn and NbFeSb. Specifically, the importance of avoided crossings is explained. The results of this work can be used to better understand and develop design strategies for engineering better performing hH thermoelectrics.

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Section: Avoided Crossings In the Hh Electronic Structurementioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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confidence: 99%

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The Importance of Avoided Crossings in Understanding High Valley Degeneracy in Half‐Heusler Thermoelectric Semiconductors

Brod

Anand

Snyder

2022

Adv Elect Materials

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“…Furthermore, it does not explain semiconducting properties in emerging examples of defective ternary compounds such as Nb 0.8 CoSb and nontransition metals based Heusler types at other electron counts (e.g., VEC = 8 for LiAlSi, and VEC = 27 for DyNiSb). Since semiconducting properties, atomic contributions to electronic structure, , and compositional stability are all essential to understand the thermoelectric properties of defective Heuslers, it is important to understand them using a single framework which explains the TiFe 1.5 Sb example.…”

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confidence: 99%

“…Currently, the semiconducting properties of TiFe 1.5 Sb are rationalized using the Slater−Pauling rule, 8,14 Since semiconducting properties, atomic contributions to electronic structure, 20,21 and compositional stability are all essential to understand the thermoelectric properties of defective Heuslers, it is important to understand them using a single framework which explains the TiFe 1.5 Sb example. The Zintl interpretation of half-Heusler semiconductors 22 allows for understanding Heusler semiconductors as ionic compounds despite their intermetallic chemistry.…”

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confidence: 99%

Structural Understanding of the Slater–Pauling Electron Count in Defective Heusler Thermoelectric TiFe_1.5Sb as a Valence Balanced Semiconductor

Anand

Snyder

2022

ACS Appl. Electron. Mater.

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Semiconducting properties in Heusler phases are of great interest for thermoelectric applications. Historically, transition metal based Heuslers semiconductors were associated with total valence electron counts (VECs) of 18 or 24 and were not expected to form at the compositions other than XYZ and XY 2 Z. The semiconducting defective Heusler phase TiFe1.5Sba low-cost example for emerging low thermal conductivity (κ) XY 1.5 Z compoundsbreaks both these stereotypes, stabilizing with an unusual VEC = 21. Although TiFe1.5Sb is identified as a nonmagnetic semiconductor using the Slater–Pauling rule, this rule offers little structural understanding of its semiconducting properties. Using first-principles based electronic structure analysis, we establish that Feunlike in traditional semiconducting Heusler stoichiometriesacts both as a d6 Fe2+ cation and a covalently bonded d10 Fe2– species in TiFe1.5Sb. In structures where these two types of Fe-atoms are indistinguishable by symmetry the electronic properties are metallic, indicating that a Slater–Pauling electron count alone does not guarantee semiconducting properties and thereby good thermoelectric efficiency. This insight into the semiconducting properties will assist in engineering thermoelectric performance of similar emerging low-κ compounds such as MRu1.5+x Sb and MCo1.5Sn (M = Ti, Zr, and Hf).

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Selective Scatterings of Phonons and Electrons in Defective Half‐Heusler Nb₁₋_δCoSb for the Figure of Merit zT > 1

Gao

Xia

Nan

et al. 2023

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The recently developed defective 19‐electron half‐Heusler (HH) compounds, represented by Nb1−δCoSb, possess massive intrinsic vacancies at the cation site and thus intrinsically low lattice thermal conductivity that is desirable for thermoelectric (TE) applications. Yet the TE performance of defective HHs with a maximum figure of merit (zT) <1.0 is still inferior to that of the conventional 18‐electron ones. Here, a peak zT exceeding unity is obtained at 1123 K for both Nb0.7Ta0.13CoSb and Nb0.6Ta0.23CoSb, a benchmark value for defective 19‐electron HHs. The improved zT results from the achievement of selective scatterings of phonons and electrons in defective Nb0.83CoSb, using lanthanide contraction as a design factor to select alloying elements that can strongly impede the phonon propagation but weakly disturb the periodic potential. Despite the massive vacancies induced strong point defect scattering of phonons in Nb0.83CoSb, Ta alloying is still found effective in suppressing lattice thermal conductivity while maintaining the carrier mobility almost unchanged. In comparison, V alloying significantly deteriorates the carrier transport and thus the TE performance. These results enlarge the category of high‐performance HH TE materials beyond the conventional 18‐electron ones and highlight the effectiveness of selective scatterings of phonons and electrons in developing TE materials even with massive vacancies.

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Conduction band engineering of half-Heusler thermoelectrics using orbital chemistry

Abstract: Semiconducting half-Heusler (HH, XYZ) phases are promising thermoelectric materials owing to their versatile electronic properties. Because the valence band of half-Heusler phases benefits from the valence band extrema at several...

Cited by 36 publications

References 47 publications

The Importance of Avoided Crossings in Understanding High Valley Degeneracy in Half‐Heusler Thermoelectric Semiconductors

The Importance of Avoided Crossings in Understanding High Valley Degeneracy in Half‐Heusler Thermoelectric Semiconductors

Structural Understanding of the Slater–Pauling Electron Count in Defective Heusler Thermoelectric TiFe_1.5Sb as a Valence Balanced Semiconductor

Selective Scatterings of Phonons and Electrons in Defective Half‐Heusler Nb₁₋_δCoSb for the Figure of Merit zT > 1

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Conduction band engineering of half-Heusler thermoelectrics using orbital chemistry

Abstract: Semiconducting half-Heusler (HH, XYZ) phases are promising thermoelectric materials owing to their versatile electronic properties. Because the valence band of half-Heusler phases benefits from the valence band extrema at several...

Cited by 36 publications

References 47 publications

The Importance of Avoided Crossings in Understanding High Valley Degeneracy in Half‐Heusler Thermoelectric Semiconductors

The Importance of Avoided Crossings in Understanding High Valley Degeneracy in Half‐Heusler Thermoelectric Semiconductors

Structural Understanding of the Slater–Pauling Electron Count in Defective Heusler Thermoelectric TiFe1.5Sb as a Valence Balanced Semiconductor

Selective Scatterings of Phonons and Electrons in Defective Half‐Heusler Nb1−δCoSb for the Figure of Merit zT > 1

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Structural Understanding of the Slater–Pauling Electron Count in Defective Heusler Thermoelectric TiFe_1.5Sb as a Valence Balanced Semiconductor

Selective Scatterings of Phonons and Electrons in Defective Half‐Heusler Nb₁₋_δCoSb for the Figure of Merit zT > 1