Article pubs.acs.org/IC
A14MgBi11 (A = Ca, Sr, Eu): Magnesium Bismuth Based Zintl Phases as Potential Thermoelectric Materials Wenjie Tan,† Zhen Wu,† Min Zhu,† Jiajun Shen,‡ Tiejun Zhu,‡ Xinbing Zhao,‡ Baibiao Huang,† Xu-tang Tao,† and Sheng-qing Xia*,† †
State Key Laboratory of Crystal Materials, Institute of Crystal Materials, Shandong University, Jinan, ShanDong 250100, People’s Republic of China ‡ State Key Laboratory of Silicon Materials, School of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, People’s Republic of China S Supporting Information *
ABSTRACT: A series of new magnesium bismuth Zintl phases, A14MgBi11 (A = Ca, Sr, Eu), have been synthesized, and their thermoelectric properties were systematically evaluated. These novel phases belong to the well-known Yb14MnSb11 family, whose structure adopts the tetragonal space group I41/acd (No. 142) with cell parameters of a = 17.0470(17)/17.854(2)/17.6660(7) Å and c = 22.665(5)/23.580(6)/23.2446(18) Å for Ca14MgBi11, Sr14MgBi11, and Eu14MgBi11, respectively. Without intentional optimization, these materials exhibit high potential as new thermoelectric candidates. Especially for Sr14MgBi11, a high zT value of 0.72 has been approached at 1073 K. The discovery of these new Zintl series is very interesting, which implies the high possibility of extending the 141-11 thermoelectric system to the bismuth analogues in the development of highly efficient thermoelectric materials. Density functional theory (DFT) calculations were incorporated as well to help better understand the properties of these important compounds.
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include, but are not limited to, Ca9Zn4+xSb9,19,20 CaMg2Bi2,21 Ca5Al2Sb6,22 and Yb14MnSb11.23 Especially, Yb14MnSb11 is probably one of the most promising candidates, owing to its high energy-conversion efficiency (zT ≈ 1.0 at 1223 K) and good compatibilities in comparison to the Si1−xGex alloys.24 In addition, the carrier optimization in Yb14MnSb11 is also very flexible, which can be realized by selective doping on the Mn or Yb sites. For example, Yb14Mn1−xZnxSb11 achieved an increase in zT by more than 10%. 2 5 The solid solutions Yb14Mn1−xAlxSb11 can have carrier concentrations tuned without significant diminishing of the band gap and effective mass, which resulted in a figure of merit improved by more than 30%.26 Substitution of the trivalent rare-earth elements on the Yb sites also corresponded to improved zT values by about 45%.27 On the basis of above progress, exploration of new thermoelectric analogues related to the 14-1-11 system is a very valuable work, which may result in more highly efficient thermoelectric materials. Initialized by the high-zT magnesium antimonide analogue Yb14MgSb11 (zT ≈ 1.03 at 1075 K),28 more analogues such as Sr14MgSb11 and Eu14MgSb11 have also been synthesized, which exhibit high potential as new thermoelectric materials.29 Nevertheless, in comparison with the antimonides bismuth compounds with the 14-1-11 structure have been seldom investigated for their thermoelectric
INTRODUCTION With the combustion of fossil fuels, the resultant energy crises and environmental pollution have become more and more deteriorated nowadays, which makes the development of clean and sustainable energy urgent. Thermoelectric materials, which may convert waste heat into reusable electricity, are considered good solutions to improve the efficiency of energy usage.1 The performance of a thermoelectric material is evaluated on the basis of the dimensionless thermoelectric figure of merit, zT = α2T/(ρκ), where α is the Seebeck coefficient, ρ is electrical resistivity, κ is thermal conductivity, and T is absolute temperature.2,3 Since these parameters are correlated with each other through the electronic structure, optimization of the thermoelectric properties in a material is thus a very challenging work. With the various strategies utilized, many state of the art thermoelectric materials have been developed during the past decades.4−7 Among these systems, chalcogenide-based semiconductors,8,9 filled skutterudites,10,11 half-Heusler alloys,12,13 and Zintl phases14,15 have proved very promising and have been the subject of intense focus in the past decade. Meanwhile, the exploration of new thermoelectric materials with high potential is also a perpetual topic. For power generation, Zintl phases have exhibited excellent performance at high temperature, benefiting from their unique crystal and electronic structures.16 Bearing complex crystal structures, weak chemical bonds, and heavy component atoms, Zintl phases usually have low thermal conductivity, which significantly contributes to a high figure of merit.17,18 Such compounds © 2017 American Chemical Society
Received: June 18, 2017 Published: August 22, 2017 10576
DOI: 10.1021/acs.inorgchem.7b01548 Inorg. Chem. 2017, 56, 10576−10583
Article
Inorganic Chemistry
bonding interactions are thus summarized in Tables 1−3 or are provided in the Supporting Information. Further information in the
applications in spite of their well-known magnetoresistance effects.30,31 Up to now only one such report has appeared for Yb14MgBi11 very recently, which has a zT value of 0.2 at 875 K.32 In this paper, a series of new 14-1-11 bismuth Zintl phases, A14MgBi11 (A = Ca, Sr, Eu), were synthesized and their thermoelectric performances were systematically evaluated. These materials exhibit high potential as new thermoelectric materials. Without any intentional optimization, a maximum zT value of 0.72 has already been achieved for Sr14MgBi11 at 1073 K. The discovery of these new compounds strongly suggests that high-performance thermoelectric materials are very promising for the bismuth-based 14-1-11 Zintl phases. Density functional theory (DFT) calculations were incorporated as well to help better understand the properties of these important compounds.
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Table 1. Selected Crystal Data and Structure Refinement Parameters for A14MgBi11 (A = Ca, Sr, Eu) Ca14MgBi11 formula wt T (K) radiation wavelength space group, Z a (Å) c (Å) V (Å3) ρcalcd (g/cm3) abs coeff GOF on F2 R1 (I > 2σI)a wR2 (I > 2σI)a R1 (all data)a wR2 (all data)a
EXPERIMENTAL DETAILS
Synthesis. The reactants were commercially purchased and used as received: Ca blocks (Afla, 99%), Sr ingots (Afla, 99%), Eu ingots (Afla, 99.9%), Yb blocks (Afla, 99.9%), Mg slices (Alfa, 99.8%), and Bi particles (Alfa, 99.99%). All manipulations were conducted in an argon-filled glovebox with an oxygen level of 1 in Inexpensive Zintl Phase Ca9Zn4+xSb9 by Phase Boundary Mapping. Adv. Funct. Mater. 2017, 27, 1606361. (21) Shuai, J.; Liu, Z.; Kim, H. S.; Wang, Y.; Mao, J.; He, R.; Sui, J.; Ren, Z. Thermoelectric properties of Bi-based Zintl compounds Ca1−xYbxMg2Bi2. J. Mater. Chem. A 2016, 4, 4312−4320. (22) Toberer, E. S.; Zevalkink, A.; Crisosto, N.; Snyder, G. J. The Zintl Compound Ca5Al2Sb6 for Low-Cost Thermoelectric Power Generation. Adv. Funct. Mater. 2010, 20, 4375−4380. (23) Toberer, E. S.; Brown, S. R.; Ikeda, T.; Kauzlarich, S. M.; Snyder, G. J. High thermoelectric efficiency in lanthanum doped Yb14MnSb11. Appl. Phys. Lett. 2008, 93, 062110. 10583
DOI: 10.1021/acs.inorgchem.7b01548 Inorg. Chem. 2017, 56, 10576−10583
