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Chinese Science Bulletin, Volume 64, Issue 16: 1679-1690(2019) https://doi.org/10.1360/N972018-01178

Sr/Eu cation doping and thermoelectric properties of the compound Ca9Zn4.5δSb9 (0<δ<0.5)

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  • ReceivedNov 29, 2018
  • AcceptedJan 4, 2019
  • PublishedFeb 22, 2019

Abstract

Previous studies suggested that Zintl phases related to the Ca9Zn4.5−δSb9 (0<δ<0.5) structure may be very promising thermoelectric candidates because of the complex crystal structure and extensively existing transition metal defects. Studies on compounds Yb9Mn4.2Sb9, Eu9Cd4Sb9 and Ca9Zn4.5−xCuxSb9 indicated that they could achieve an optimized zT around 0.7. Especially for Ca9Zn4.5−δSb9, a very recent report suggested that a high zT of 1.1 at 875 K could be obtained in combination of various secondary phases introduced by the variation of Zn contents, which provided a chemical potential to change the composition. However, to change the composition of Ca9Zn4.5−δSb9 is actually very difficult to implement owing to the narrow homogeneity range. In this work, with Ca substituted by big cations such as Eu2+ and Sr2+, the unit cell of Ca9Zn4.5−δSb9 was expanded, and similar significant enhancement on the thermoelectric performance was resulted.

All syntheses were handled inside a glovebox, then the reactants were loaded in the Nb tube and arc-welded, which were enclosed in a fused silica tube. The reactions were firstly heated to 1173 K in 6 h and then maintained at this temperature for 24 h. After the homogeneity process, the furnace was slowly cooled down to 873 K at a rate of 6 K/h, followed by another dwelling for 6 hours and finally cooled down to 573 K at a rate of 10 K/h. At last, the furnace was shut down and the reactions were opened in the glovebox. The compound were fully characterized by single-crystal X-ray diffraction (SXRD) and powder X-ray diffraction(PXRD), Energy dispersive spectrometer (EDS) and Hall effect measurement, etc. Single-crystal X-ray diffraction (SXRD) proves that with the increase of Sr component, the crystal parameters are slightly increasing. Hall effect measurement show that the substitution of Ca by Eu or Sr can both lead to an obvious decreasing on the carrier (holes) concentration, and the resultant mobility for Sr/Eu-doped materials is very similar if compared by the same doping level. It is also noted that neither the carrier concentration nor the mobility seems to vary linearly with the Eu/Sr-doping contents, which means a saturated state will be eventually approached. This phenomenon is consistent with the speculation above that the changes on the properties should predominantly originate from the change of interstitial Zn content, which the size effect dominated. While the decreasing carrier concentrations as well as the increasing mobility and lattice thermal conductivity all point to a structure modification, the crystal structures of these doped materials were systematically verified through the single crystal X-ray diffraction.

In conclusion, although a direct measurement on such a small composition variation is very difficult (the X-ray diffraction results prove it), the significantly reduced carrier concentration and increased mobility as well as lattice thermal conductivity can still provide some useful hints on understanding the property optimization of these materials. For material Ca8.2Eu0.8Zn4.5−δSb9, high figure of merit with ZT~0.81 has been achieved at 873 K.


Funded by

国家自然科学基金(51771105)


Supplement

补充材料

图S1 Ca9Zn4.45(2)Sb9 EDS数据

图S2 Ca8.79(2)Sr0.21(2)Zn4.44(1)Sb9 EDS数据

图S3 Ca8.57(2)Sr0.43(2)Zn4.45(1)Sb9 EDS数据

图S4 Ca8.42(2)Sr0.58(2)Zn4.47(2)Sb9 EDS数据

表S1 Ca9Zn4.45(2)Sb9原子坐标、Wyckoff位置、占据率及温度因子等参数

表S2 Ca8.79(2)Sr0.21(2)Zn4.44(1)Sb9原子坐标、Wyckoff位置、占据率及温度因子等参数

表S3 Ca8.57(2)Sr0.43(2)Zn4.45(1)Sb9原子坐标、Wyckoff位置、占据率及温度因子等参数

表S4 Ca8.42(2)Sr0.58(2)Zn4.47(2)Sb9原子坐标、Wyckoff位置、占据率及温度因子等参数

表S5 Ca9Zn4.45(2)Sb9和Ca8.42(2)Sr0.58(2)Zn4.47(2)Sb9的部分键长

本文以上补充材料见网络版csb.scichina.com. 补充材料为作者提供的原始数据, 作者对其学术质量和内容负责.


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  • Figure 1

    (Color online) Side-by-side structure comparison of Ca9Zn4.5−δSb9 with δ=0.5 (a) and δ=0 (b), viewed along the c axis. The Ca cations are removed for clarity, and the Zn and Sb atoms locate on the tetrahedral centers and vertexes, respectively. The interstitial Zn atoms are linked by three neighboring tetrahedrons

  • Figure 2

    (Color online) Powder X-ray diffraction patterns of Ca9−xEuxZn4.5Sb9 (a) and Ca9−xSrxZn4.5Sb9 (b) (x = 0, 0.2, 0.4, 0.6, 0.8). The simulated pattern is presented for comparison and the diffraction peaks between 24° and 25° is zoomed in for better illustration

  • Figure 3

    (Color online) Carrier concentration and mobility of Ca9−xEuxZn4.5Sb9 (a, c) and Ca9−xSrxZn4.5Sb9 (b, d) (x=0, 0.2, 0.4, 0.6, 0.8)

  • Figure 4

    (Color online) Resistivity and Seebeck coefficient of Ca9−xEuxZn4.5Sb9 (a, c) and Ca9−xSrxZn4.5Sb9 (b, d) (x = 0, 0.2, 0.4, 0.6, 0.8)

  • Figure 5

    (Color online) Seebeck Pisarenko plot of Ca9−xSrxZn4.5Sb9 (a) and Ca9−xEuxZn4.5Sb9 (b). x = 0, 0.2, 0.4, 0.6, 0.8

  • Figure 6

    (Color online) Electronic and lattice components of thermal conductivity for Ca9−xEuxZn4.5Sb9 ((a), (c)) and Ca9−xSrxZn4.5Sb9 ((b), (d)) (x= 0, 0.2, 0.4, 0.6, 0.8)

  • Figure 7

    (Color online) Thermal conductivity and calculated figure of merit for materials Ca9−xEuxZn4.5Sb9 (a), (c) and Ca9−xSrxZn4.5Sb9 (b), (d) (x=0, 0.2, 0.4, 0.6, 0.8)

  • Table 1   Crystal data and structure refinement for series of CaSrZnSb samples

    分子式

    Ca8.42(2)Sr0.58(2)Zn4.47(2)Sb9

    Ca8.57(2)Sr0.43(2)Zn4.45(1)Sb9

    Ca8.79(2)Sr0.21(2)Zn4.44(1)Sb9

    Ca9Zn4.45(2)Sb9

    分子量

    1776.01

    1767.9

    1756.7

    1747.69

    温度(K)

    273(2)

    273(2)

    273(2)

    273(2)

    辐射波长

    Mo Kα, 0.71073 Å

    Mo Kα, 0.71073 Å

    Mo Kα, 0.71073 Å

    Mo Kα, 0.71073 Å

    空间群

    Pbam(NO.55)

    Pbam(NO.55)

    Pbam(NO.55)

    Pbam(NO.55)

    Z

    2

    2

    2

    2

    a(Å)

    12.5563(8)

    12.538(4)

    12.5185(8)

    12.5098(10)

    b(Å)

    21.9624(13)

    21.963(7)

    21.9244(13)

    21.9023(18)

    c(Å)

    4.5620(3)

    4.5625(14)

    4.5571(3)

    4.5561(4)

    V3)

    1258.05(14)

    1256.4(7)

    1250.74(14)

    1248.34(18)

    理论密度(g/cm3)

    4.688

    4.673

    4.665

    4.635

    吸收系数(cm–1)

    1.6571

    1.6294

    1.5944

    1.5508

    R1a)

    R1=0.0378

    R1=0.0316

    R1=0.0341

    R1=0.0350

    wR2b)

    wR2=0.0864

    wR2=0.0628

    wR2=0.0615

    wR2=0.0732

    Rint

    R1=0.0414

    R1=0.0377

    R1=0.0430

    R1=0.0424

    wR2b)

    wR2=0.0886

    wR2=0.0649

    wR2=0.0639

    wR2=0.0761

    R1||Fo|−|Fc||/Σ|Fo|; b) wR2=Σ[w(Fo2Fc2)2]/Σ[w(Fo2)2]1/2. 从上至下依次表示: R1, 对可观察衍射点的R1值; wR2, 对可观察衍射点的wR2值; Rint, 对全部衍射点的R1值; wR2, 对全部衍射点的wR2

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