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Orbital alignment for high performance thermoelectric YbCdSb alloys Xiao Wang, Juan Li, Chen Wang, Binqiang Zhou, Liangtao Zheng, Bo Gao, Yue Chen, and Yanzhong Pei Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b02155 • Publication Date (Web): 10 Jul 2018 Downloaded from http://pubs.acs.org on July 13, 2018
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Chemistry of Materials
1 2 Orbital alignment for high performance thermoelectric YbCd2Sb2 alloys 3 Xiao Wang1, Juan Li1, Chen Wang2, Binqiang Zhou1, Liangtao Zheng1, Bo Gao1, Yue Chen2 and Yanzhong Pei*,1 4 1 Interdisciplinary Materials Research Center, School of Materials Science and Engineering, Tongji Univ., 4800 Caoan Rd., Shanghai 201804, China. 5 2 Department of Mechanical Engineering, The University of Hong Kong, Pokfulam Road, Hong Kong SAR, China 6 *Email:
[email protected] 7 8 Abstract 9 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 10 superior thermoelectric performance, largely stemming from the existence of multiple transporting bands in both conduction types. Being 11 similar to many III-V and elemental semiconductors, the transport of holes in AB2C2 Zintls usually involves multiple valence bands with an 12 extrema at the Brillouin zone center G. However, these valence bands, originating from different orbitals, are unnecessarily aligned in 13 energy due to the crystal field splitting. Formation of solid solutions between constituent compounds having opposite arrangements in 14 energy of band orbitals is believed to be particularly helpful for thermoelectric enhancements, because orbital alignment increases band 15 degeneracy while alloy defects scatter phonons. These effects are simultaneously realized in this work, where the p orbitals of anions in 16 Zn Sb alloys are well-aligned for maximizing the electronic performance, and meanwhile high-concentration Cd/Zn substitutions YbCd 2-x x 2 17 are introduced for minimizing the lattice thermal conductivity. As a result, a significantly enhanced thermoelectric figure of merit, zT~1.3 is 18 achieved, being a record among AB2C2 Zintls in p-type. This work demonstrates not only YbCd2-xZnxSb2 alloys as efficient thermoelectrics 19 but also orbital alignment as an effective strategy for advancing thermoelectrics. 20 21 Introduction compounds involves rich band structures in both conduction 22 types64-67 for charge transport by multiple band valleys. In recent one decade or two, the increase in energy demand 23 In more details for p-type conduction, the p orbits of C anions, and the resulting environmental issues, has largely driven the 24 which are in the form of a doubly-degenerate pxy and a nonadvancement of thermoelectric technology. This is a clean and 25 sustainable energy technology, since it enables a direct conversion degenerate pz at the Brillouin zone center (G) due to the crystal 26 field splitting, determine the valence band structure of AB2C2 between heat and electricity based on Seebeck effect. However, a 27 compounds64. In order to maximize the degeneracy of band valleys large-scale application of thermoelectricity is still a worldwide 28 (Nv) for enhancing electronic performance, alignment of these challenge due to its relatively low conversion efficiency, which is 29 orbitals in energy seems to be quite straightforward and effective55, determined by the thermoelectric materials’ dimensionless figure 30 64 of merit, zT=S2T/r(kE+kL) , where S, T, r, kE, and kL are the . Once an energy offset between orbitals (DE=E(G(pxy))-E(G(pz))) 31 Seebeck coefficient, absolute temperature, resistivity, electronic is defined, this leads a design of solid solutions to have a DE as 32 and lattice components of thermal conductivity, respectively. close to zero as possible45, 64. In addition, such an alloying process 33 would lead to a reduction in lattice thermal conductivity, due to In order to enhance zT, optimizing the electronic transport 34 phonon scattering by the introduced mass and strain fluctuations properties and reducing the lattice thermal conductivity are 35 68-71 particularly effective and have been widely used. However, . The between host and guest atoms in solid solutions 36 because of the strong coupling effect among S, r and kE, most combination of orbital alignment and alloy defect phonon 37 scattering is expected to improve thermoelectric AB2C2 Zintls thermoelectric research activities for zT-enhancements focused on 38 reducing the lattice thermal conductivity, which is the only one significantly. 39 This work focuses on p-type YbCd2-xZnxSb2 (x≤0.9) solid independent parameter determining zT. Successful thermal 40 solutions, where a precise control of x enables a certain alloy strategies are typified by nanostructuring1, 2, lattice anharmonicity3, 414 composition showing the minimal energy offset between the p , liquid phonons5, dislocations6-8, point defects including 42 vacancies9-11, substitutions12, 13 and interstitials14-16, as well as low band orbitals. In addition, this is a solid solution system in the 43 sound velocity17 and complex crystal structures17-20. Recently, entire composition range, ensuring an alignment of G(pxy) and G(pz) 44 decoupling the electronic transport properties by the concept of orbitals to be achieved (DE~0) for maximizing the valence band 45 band engineering has been proven to be effective for zTdegeneracy. Furthermore, Zn/Cd substitutions with both large mass 46 and strain fluctuations for phonon scattering, enable an effective improvements. These electronic approaches include band 47 convergence21, 22, band nestification23 and low band effective reduction in lattice thermal conductivity. Eventually, a 48 mass24, which have been successfully demonstrated in various significantly enhanced thermoelectric figure of merit (zT ~ 1.3 at 49 thermoelectric materials, such as PbTe25-30, SnTe31-34, GeTe35-38, 700 K) is achieved, being the highest among p-type AB2C2 Zintl 50 Mg2Si39, 40, half-Heusler41-43 and Te23. thermoelectrics reported so far. 51 Zintl compounds, which usually have a complex crystal 52 structure and are rich in materials chemistry, enabling an intrinsic Result and discussion 53 Details on synthesis, characterization, measurements and low lattice thermal conductivity and a broad range of manipulation 54 band structure calculations can be found in the supplementary. The on transport properties, have attracted increasing attentions as 55 room temperature powder X-ray diffraction (XRD) patterns for potential thermoelectric materials 44-51. As a typical family of 56 thermoelectric Zintls crystallizing in the CaAl2Si2 structure (space YbCd2-xZnxSb2 (x£0.9) are shown in Fig. 1a. All the main _ 57 group P 3 m1)52, AB2C2 (A=Eu, Yb, Ba, Ca; B=Zn, Cd, Mg and diffraction peaks can be well indexed to_ a trigonal CaAl2Si2-type 58 C=As, Sb, Bi) 44, 45, 48, 53-63 have been intensively investigated for crystal structure with a space group of P3m152, indicating the high 59 the past decade. It is recently revealed that this class of Zintl purity of the samples made in this work. As compared to pristine 60 YbCd2Sb2, YbCd2-xZnxSb2 alloys show diffraction peaks shifting to 1
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Chemistry of Materials 1 2 3 higher diffraction angles with increasing x, indicating a lattice 4 shrinkage. As shown in Fig. 1b, both lattice parameters a and c 5 linearly decrease with the increasing Zn content, which nicely 6 follows the Vegard’s Law (dashed lines without any fitting 7 parameters). Similarly, the room temperature Hall carrier 8 concentration linearly increases with the increasing x. Zn/Cd 9 substitutions increase the hole concentration, which can be 10understood by the resultant decrease in formation energy of Yb 11vacancies72, since the vacancy formation energy is found to be 12very low in YbZn Sb 72 These results consistently indicate a 2 2 . 13successful Cd substitution by Zn in the matrix phase. In addition, 14tracing amount of Cd Sb /ZnSb impurities are also observed in 13 10 15Fig. 1a, which are further confirmed by Scanning Electron 16Microscope (SEM) and Energy Dispersive Spectrometer (EDS) 17analyses (Fig. S1). Such a precipitation has been frequently 18observed in AB Sb compounds44, 53, 72, 73. Importantly, the matrix 2 2 19phase is found to be very homogeneous as illustrated in 20YbCd Zn Sb (Fig. 1c and 1d). 1.5 0.5 2 21 22 a 7.5 b 7.70 YbCd Zn Sb YbCd2-xZnxSb2 7.0 7.65 23 300 K 300 K ZnSb, Cd Sb 6.5 7.60 c 24 6.0 7.55 x=0.9 25 x=0.8 5.5 7.50 x=0.7 26 x=0.6 5.0 a 4.65 4.5 27 x=0.5 x=0.4 4.60 x=0.3 4.0 x=0.2 28 x=0.1 4.55 3.5 29 x=0 4.50 3.0 30 2.5 4.45 0.0 0.2 0.4 0.6 0.8 20 40 60 80 x 31 2q (deg.) 32 c Yb Cd x=0.5 d 33 34 35 36 37 Zn Sb 38 39 40 41 42 10µm 43 44 Fig. 1. XRD patterns (a) and Zn concentration dependent lattice 45 parameters and room temperature carrier concentration (b) for YbCd246 Zn Sb (x£0.9). SEM image for YbCd1.5Zn0.5Sb2 (c) and the 47x x 2 corresponding EDS composition mapping (d). 48 49 Since both YbCd2Sb2 and YbZn2Sb2 show a very small energy 50 offset between G(pxy) and G(pz) orbitals64, a fine manipulation in 51 orbital alignment can be expected in YbCd2-xZnxSb2 solid solutions. 52 This is qualitatively supported by our band structure calculations 53 as shown in Fig. 2a. Note that the absence of temperature effects 54 on calculated band structures and instrumental uncertainties 55 challenge a quantitative comparison between calculations and 56 experimental results, since the band offset change involved is 57 extremely small (
