Orbital alignment for high performance thermoelectric YbCd2Sb2

Jul 10, 2018 - These effects are simultaneously realized in this work, where the p orbitals of anions in YbCd2-xZnxSb2 alloys are well-aligned for max...
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Orbital Alignment for High Performance Thermoelectric YbCd2Sb2 Alloys Xiao Wang,† Juan Li,† Chen Wang,‡ Binqiang Zhou,† Liangtao Zheng,† Bo Gao,† Yue Chen,‡ and Yanzhong Pei*,† †

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Interdisciplinary Materials Research Center, School of Materials Science and Engineering, Tongji University, 4800 Caoan Road, Shanghai 201804, China ‡ Department of Mechanical Engineering, The University of Hong Kong, Pokfulam Road, Hong Kong SAR, China S Supporting Information *

ABSTRACT: As a typical class of Zintl thermoelectrics, AB2C2 (A = Eu, Yb, Ba, Ca, Mg; B = Zn, Cd, Mg, and C = Sb, Bi) compounds have shown a superior thermoelectric performance, largely stemming from the existence of multiple transporting bands in both conduction types. Being similar to many III−V and elemental semiconductors, the transport of holes in AB2C2 Zintls usually involves multiple valence bands with extrema at the Brillouin zone center Γ. However, these valence bands, originating from different orbitals, are unnecessarily aligned in energy due to the crystal field splitting. Formation of solid solutions between constituent compounds having opposite arrangements in energy of band orbitals is believed to be particularly helpful for thermoelectric enhancements, because orbital alignment increases band degeneracy while alloy defects scatter phonons. These effects are simultaneously realized in this work, where the p orbitals of anions in YbCd2−xZnxSb2 alloys are well-aligned for maximizing the electronic performance, and meanwhile high-concentration Cd/Zn substitutions are introduced for minimizing the lattice thermal conductivity. As a result, a significantly enhanced thermoelectric figure of merit, zT ∼ 1.3, is achieved, being a record among AB2C2 Zintls in p-type. This work demonstrates not only YbCd2−xZnxSb2 alloys as efficient thermoelectrics but also orbital alignment as an effective strategy for advancing thermoelectrics.

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INTRODUCTION In the last decade or two, the increase in energy demand and the resulting environmental issues have largely driven the advancement of thermoelectric technology. This is a clean and sustainable energy technology, since it enables a direct conversion between heat and electricity based on the Seebeck effect. However, a large-scale application of thermoelectricity is still a worldwide challenge due to its relatively low conversion efficiency, which is determined by the thermoelectric materials’ dimensionless figure of merit, zT = S2T/ρ(κE + κL), where S, T, ρ, κE, and κL are the Seebeck coefficient, absolute temperature, resistivity, and electronic and lattice components of thermal conductivity, respectively. In order to enhance zT, optimizing the electronic transport properties and reducing the lattice thermal conductivity are particularly effective and have been widely used. However, because of the strong coupling effect among S, ρ, and κE, most thermoelectric research activities for zT enhancements focused on reducing the lattice thermal conductivity, which is the only independent parameter determining zT. Successful thermal strategies are typified by nanostructuring,1,2 lattice anharmonicity,3,4 liquid phonons,5 dislocations,6−8 and point defects including vacancies,9−11 substitutions, 12,13 and interstitials,14−16 as well as low sound velocity17 and complex crystal structures.17−20 Recently, decoupling the electronic transport properties by the concept of band engineering has been proven © 2018 American Chemical Society

to be effective for zT improvements. These electronic approaches include band convergence,21,22 band nestification,23 and low band effective mass,24 which have been successfully demonstrated in various thermoelectric materials, such as PbTe,25−30 SnTe,31−34 GeTe,35−38 Mg2Si,39,40 halfHeusler,41−43 and Te.23 Zintl compounds, which usually have a complex crystal structure and are rich in materials chemistry, enabling an intrinsic low lattice thermal conductivity and a broad range of manipulation on transport properties, have attracted increasing attention as potential thermoelectric materials.44−51 As a typical family of thermoelectric Zintls crystallizing in the CaAl2Si2 structure (space group P3̅m1),52 AB2C2 (A = Eu, Yb, Ba, Ca; B = Zn, Cd, Mg, and C = As, Sb, Bi)44,45,48,53−63 have been intensively investigated for the past decade. It is recently revealed that this class of Zintl compounds involves rich band structures in both conduction types64−67 for charge transport by multiple band valleys. In more detail for p-type conduction, the p orbitals of C anions, which are in the form of a doubly degenerate pxy and a nondegenerate pz at the Brillouin zone center (Γ) due to the crystal field splitting, determine the valence band structure of Received: May 22, 2018 Revised: July 10, 2018 Published: July 10, 2018 5339

DOI: 10.1021/acs.chemmater.8b02155 Chem. Mater. 2018, 30, 5339−5345

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Chemistry of Materials AB2C2 compounds.64 In order to maximize the degeneracy of band valleys (Nv) for enhancing electronic performance, alignment of these orbitals in energy seems to be quite straightforward and effective.55,64 Once an energy offset between orbitals (ΔE = E(Γ(pxy)) − E(Γ(pz))) is defined, this leads a design of solid solutions to have a ΔE as close to zero as possible.45,64 In addition, such an alloying process would lead to a reduction in lattice thermal conductivity, due to phonon scattering by the introduced mass and strain fluctuations between host and guest atoms in solid solutions.68−71 The combination of orbital alignment and alloy defect phonon scattering is expected to improve thermoelectric AB2C2 Zintls significantly. This work focuses on p-type YbCd2−xZnxSb2 (x ≤ 0.9) solid solutions, where a precise control of x enables a certain alloy composition showing the minimal energy offset between the p band orbitals. In addition, this is a solid solution system in the entire composition range, ensuring an alignment of Γ(pxy) and Γ(pz) orbitals to be achieved (ΔE ∼ 0) for maximizing the valence band degeneracy. Furthermore, Zn/Cd substitutions with both large mass and strain fluctuations for phonon scattering enable an effective reduction in lattice thermal conductivity. Eventually, a significantly enhanced thermoelectric figure of merit (zT ∼ 1.3 at 700 K) is achieved, being the highest among p-type AB2C2 Zintl thermoelectrics reported so far.

work. As compared to pristine YbCd2Sb2, YbCd2−xZnxSb2 alloys show diffraction peaks shifting to higher diffraction angles with increasing x, indicating a lattice shrinkage. As shown in Figure 1b, both lattice parameters a and c linearly decrease with the increasing Zn content, which nicely follows the Vegard’s Law (dashed lines without any fitting parameters). Similarly, the room temperature Hall carrier concentration linearly increases with the increasing x. Zn/Cd substitutions increase the hole concentration, which can be understood by the resultant decrease in formation energy of Yb vacancies,72 since the vacancy formation energy is found to be very low in YbZn2Sb2.72 These results consistently indicate a successful Cd substitution by Zn in the matrix phase. In addition, trace amounts of Cd13Sb10/ZnSb impurities are also observed in Figure 1a, which are further confirmed by scanning electron microscope (SEM) and energy dispersive spectrometer (EDS) analyses (Figure S1). Such a precipitation has been frequently observed in AB2Sb2 compounds.44,53,72,73 Importantly, the matrix phase is found to be very homogeneous as illustrated in YbCd1.5Zn0.5Sb2 (Figure 1c,d). Since both YbCd2Sb2 and YbZn2Sb2 show a very small energy offset between Γ(pxy) and Γ(pz) orbitals,64 a fine manipulation in orbital alignment can be expected in YbCd2−xZnxSb2 solid solutions. This is qualitatively supported by our band structure calculations as shown in Figure 2a. Note

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RESULT AND DISCUSSION Details on synthesis, characterization, measurements, and band structure calculations can be found in the Supporting Information. The room temperature powder X-ray diffraction (XRD) patterns for YbCd2−xZnxSb2 (x ≤ 0.9) are shown in Figure 1a. All the main diffraction peaks can be well indexed to a trigonal CaAl2Si2-type crystal structure with a space group of P3̅m1,52 indicating the high purity of the samples made in this

Figure 2. Calculated valence band structures of Yb4ZnzCd8−zSb8 with z = 0, 1, 2, 3, and 4 (a), indicating pz orbital first converges with px and py orbitals at Γ and then diverges from with increasing z. Normalized optical absorption versus photon energy at room temperature for YbCd2−xZnxSb2 (x ≤ 0.9) (b) and its temperature dependence for YbCd1.5Zn0.5Sb2 (c).

that the absence of temperature effects on calculated band structures and instrumental uncertainties challenge a quantitative comparison between calculations and experimental results, since the band offset change involved is extremely small (0.7 are achieved in the measured temperature range, both of which are the highest among AB2C2 Zintl compounds. The 5342

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work demonstrates orbital alignment by alloying as an effective strategy for improving thermoelectrics.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.8b02155.

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Materials and methods; SEM images and EDX analyses for YbCd2Sb2 and Yb0.96Ba0.04Cd1.5Zn0.5Sb2; temperature dependent lattice thermal conductiv ity for YbCd2−xZnxSb2 (x ≤ 0.9); thermoelectric property isotropy; and XRD patterns, composition dependent lattice parameters, electric and thermal transport properties for Yb1−yBayCd1.5Zn0.5Sb2 (y ≤ 0.1) (PDF)

AUTHOR INFORMATION

Corresponding Author

*(Y.P.) E-mail: [email protected]. ORCID

Chen Wang: 0000-0001-5736-4188 Yue Chen: 0000-0001-5811-6936 Yanzhong Pei: 0000-0003-1612-3294 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the National Natural Science Foundation of China (Grant No. 11474219 and 51772215), the National Key Research and Development Program of China (2018YFB0703600), Fundamental Research Funds for Science and Technology Innovation Plan of Shanghai (18JC1414600), the Fok Ying Tung Education Foundation (Grant No. 20170072210001), and “Shu Guang” Project Supported by Shanghai Municipal Education Commission and Shanghai Education Development Foundation. C.W. and Y.C. are grateful for the financial support from RGC under Project Numbers 27202516 and 17200017 and the research computing facilities offered by ITS, HKU.

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DOI: 10.1021/acs.chemmater.8b02155 Chem. Mater. 2018, 30, 5339−5345

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DOI: 10.1021/acs.chemmater.8b02155 Chem. Mater. 2018, 30, 5339−5345