Structure, Magnetism, and Thermoelectric Properties of Magnesium

Jan 10, 2017 - New Mg-containing antimonide Zintl phases, Sr14MgSb11 and Eu14MgSb11, were synthesized from high-temperature solid-state reactions in ...
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Structure, Magnetism, and Thermoelectric Properties of MagnesiumContaining Antimonide Zintl Phases Sr14MgSb11 and Eu14MgSb11 Wen-jie Tan,† Yin-tu Liu,‡ Min Zhu,† Tie-jun Zhu,‡ Xin-bing Zhao,‡ Xu-tang Tao,† and Sheng-qing Xia*,† †

State Key Laboratory of Crystal Materials, Institute of Crystal Materials, Shandong University, Jinan, ShanDong 250100, China State Key Laboratory of Silicon Materials, School of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, China

‡

S Supporting Information *

ABSTRACT: New Mg-containing antimonide Zintl phases, Sr14MgSb11 and Eu14MgSb11, were synthesized from high-temperature solid-state reactions in Ta tubes at 1323 K. Their structures can be viewed as derived from the Ca14AlSb11 structure type, which adopt the tetragonal space group I41/acd (No. 142, Z = 8) with the cell parameters of a = 17.5691(14)/17.3442(11) Å and c = 23.399(4)/22.981(3) Å for the Srand Eu-containing compounds, respectively. The corresponding thermoelectric properties were probed, which demonstrated high potential of these compounds as new thermoelectrics for their very low thermal conductivity and moderate Seebeck coefficient. Magnetism studies and theoretical calculations were conducted as well to better understand the structure-and-property correlation of these materials.

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Yb14ZnSb11,25 and the defect variants, Sr13□NbAs11 and Eu13□NbAs11, exhibit a vacancy on the cation site owing to the substitution of Nb.19 Within these compounds, novel properties such as colossal magnetoresistance, mixed valence, and thermoelectrics have been well demonstrated and studied. For example, ferromagnetism was discovered for Ca14MnBi11, Sr14MnBi 11, and Yb 14MnSb11 , but for Ba 14MnBi 11 and Eu14InBi11 antiferromagnetism was found instead.6,20,26−32 With the pnictogen elements changed from As to Bi, A14MnPn11 become more electrical conducting.17 Negative magnetoresistance was observed for Eu 14 MnSb 14 and Eu14MnBi11, due to the complex magnetic coupling of the Eu2+ sublattice.33,34 Besides the magnetism, the unit cell parameters as well as the electrical transport properties seem closely related to the ionic radius and electronegativity of the atoms at the triel site, which has been proved by the Ca14AlSb11-type compounds.35−39 Perhaps one of the most interesting aspects related to these 14−1−11 phases is their potential application as highperformance thermoelectrics. Thermoelectric materials can realize the energy conversion between heat and electricity directly, which may play an important role in the energy-saving related issues. Intermetallic materials such as clathrates, HalfHeusler compounds, and complex Zintl phases have been frequently investigated as prospective candidates.40−45 In 2006, Yb14MnSb11 was first reported as a high-temperature thermoelectric material with a high figure of merit zT ∼ 1.0 at 1223 K.46,47 As a consequence, many efforts were taken to improve

INTRODUCTION Compounds with the nominal “14−1−11” formula can be dated back to the 1980s, when compound Ca14AlSb11 was first discovered.1 Since then, this family has grown rapidly and consequently, many triel-containing pnictide analogues (A14TrPn11, A = Ca, Sr, Ba; Tr = Al, Ga, In; Pn = P, As, Sb) have been reported.2−7 On the basis of the Zintl concept, these compounds are charge-balanced and their structure can be understood by using a simple electron counting,8−10 i.e., there are totally 14 divalent cations, one TrPn49− tetrahedron, one linear Pn37− polyatomic anion, and four Pn3− ions in each unit cell. Theoretical analyses on bonding interactions indicate that both TrPn49− tetrahedrons and Pn37− linear units are independent and no evidential connections are found between the anion units.11 For the TrPn4 tetrahedrons, they are slightly distorted and with the cation sizes increased, i.e., from Ca to Ba, the Tr−Pn bonds elongate and the distortions become more significant as well.12 Whereas for the Pn37− polyatomic anion, it features the structure of the I3− anion and the length of two Pn−Pn bonds is identical, but much longer than that of a regular covalent single bond.13 However, for some arsenides, the two As−As bonds in the As37− anion are uneven because of the disorder on the As center, such as in Sr14GaAs11.3 More interestingly, this family can be extended to contain various nontrivalent metals, such as Mn, Zn, Cd, Nb, and Mg.6,12,14−24 For such compounds, the structures may change in order to meet the electronic requirements. For example, interstitial Sb atoms were found in Ca14ZnSb11.20 and Ca14CdSb11.43, which are important in understanding the resultant semiconducting properties of these systems.18 Moreover, mixed valency of Yb2+/Yb3+ was also observed in © XXXX American Chemical Society

Received: November 16, 2016

A

DOI: 10.1021/acs.inorgchem.6b02724 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

diffraction (Supporting Information). Sr14MgSb11 is a little sensitive to oxygen and but Eu14MgSb11 is quite stable in air. SPS Experiment. The title compounds were ground to fine powder in a glovebox and then moved to a graphite mold to obtain densified materials by spark plasma sintering (SPS 1050: Sumitomo Coal Mining Co, Ltd.). Samples were made into pellets with a diameter of 12.6 mm at 800 °C for 5 min under 50 MPa and the density is around 95%. No significant changes on the diffraction patterns were observed for the samples after SPS, which indicates that no decomposition or phase transformation took place during the material preparation process. Thermoelectric Properties. Thermal conductivity was measured by using a NETZSCH LFA457 instrument in the argon atmosphere. A standard sample of pyroceram 9606 (Ø12.7 × 1.98 mm) was used as the reference for the heat capacity measurements. The thermal diffusivity D was measured directly and thermal conductivity was then calculated by the equation κ = D·d·Cp, in which d is the experimental density (determined by the measured mass and geometric volume of the sample) and Cp is the heat capacity of the sample. After thermal conductivity measurements, the samples were cut into bars to measure the Seebeck coefficient and electrical resistivity simultaneously through a Linseis LSR-3 instrument. Single Crystal X-ray Diffraction and Structure Determination. Single crystals of A14MgSb11 (A = Sr, Eu) were selected and sealed in a capillary glass tube under the protection of argon. The Xray diffraction data were then collected on a Bruker CCD-based diffractometer at 296 K by utilizing a Mo Kα radiation (0.71703 Å). Data reduction and integration, together with global unit cell refinements, were done by the INTEGRATE program incorporated in the APEX2 software.56 Semiempirical absorption corrections were applied using the SCALE program for area detector.57 The structures were solved by direct methods and refined by full-matrix least-squares methods on F2 using SHELX.58 In the last refinement cycles, the atomic positions were standardized using the program STRUCTURE TIDY.59,60 All structures were refined to convergence with anisotropic displacement parameters.61 Similar to Sr14ZnSb11, the split of Sb atoms was also observed in compound Sr14MgSb11.18 For such positions, if refining with fully occupied Sb, an obvious residual peak was found about 0.7 Å away, which has an approximate intense of 11e−l/Å3 in the Fourier difference map. Thus, this disorder was modeled as two split sites, i.e., Sb3A and Sb3B, which were statistically occupied. The refinements then converged quickly with the occupancy of 0.883(3) and 0.117(3) for Sb3A and Sb3B, respectively. In subsequent refinements, all atoms except Sb3B were refined with anisotropic atomic displacement parameters. Crystallographic data and structural refinements are summarized in Table 1. Atomic positions and selected bond lengths are listed in Table 2 and Table 3, respectively. Further information in the form of CIF has been deposited with Fachinformationszentrum Karlsruhe, 76344 Eggenstein-Leopoldshafen, Germany (fax: 49-7247-808-666; e-mail: crysdata@fiz-karlsruhe.de); depository CSD-432173, 432174 for Sr14MgSb11 and Eu14MgSb11, respectively. Elemental Analysis. Energy dispersive X-ray spectroscopy (EDS) was taken on the hand-selected single crystals of Sr14MgSb11 and Eu14MgSb11 with a Hitachi FESEM-4800 field emission microscopy equipped with a Horiba EX-450 EDS. The measured compositions are Sr13.81MgSb11.40 and Eu14.01MgSb11.08, which match well the results obtained from the single-crystal X-ray diffraction data. Magnetic Susceptibility Measurements. Magnetic susceptibility measurements were carried out by using a Quantum Design MPMS superconducting quantum interference device (SQUID) magnetometer. For Eu14MgSb11, both zero-field-cooling (ZFC) and fieldcooling (FC) data were obtained over a temperature range from 5 to 300 K with an applied field of 1000 Oe. The field dependence of magnetization was also measured between −6 and 6 T at 5 K. Computational Details. Electronic band structure and the crystal orbital Hamilton population (COHP) were calculated on Sr14MgSb11 by means of the self-consistent linear-muffin-tin-orbital (LMTO) method in the local density and atomic sphere (ASA) approximations,

the efficiency of Yb14MnSb11 by various doping strategies as in Yb14Mn1−xAlxSb11.38,39,48−52 For example, carrier concentration can be adjusted accurately by modulating the doping ratio, which is a critical factor for Seebeck coefficient and electrical resistivity. With Yb2+ replaced by RE3+ (RE = La, Pr, Sm), more electrons are introduced and carrier concentration can be regulated. For example, with the Pr3+ content x increased from 0 to 0.55 in Yb14‑xPrxMnSb11, the hole concentration changes from 1.0 × 1021 to 5.2 × 1020 and the zT value can be increased by 30−45% if the material is optimized with appropriate amount of rare-earth cations.53,54 Other attempts such as replacement with homovalent cations were also frequently utilized, which can lead to improved zT contributed by reduced lattice thermal conductivity, such as in Yb14‑xCaxMnSb11.55 Interestingly, with the magnetic Mn2+ substituted by nonmagnetic Zn2+, spin disorder scattering and resistivity are reduced but Seebeck coefficient of Yb14Mn1−xZnxSb11 remains unchanged, which also results in optimized thermoelectric properties.48 Very recently, the first two Mg-containing compounds belonging to this family, Ca14MgSb11 and Yb14MgSb11, were just discovered and they also exhibited high conversion efficiency with zT ∼ 1.02 at 1075 K for Yb14MgSb11.23 In this work, we extended this system and discovered two more magnesium-containing analogues, Sr14MgSb11 and Eu14MgSb11. With their crystal structures accurately determined by singlecrystal X-ray diffraction, the structure complexity related to such phases was well illustrated and interestingly, the structures of Sr 14 MgSb 11 and Eu 14 MgSb 11 are slightly different. Sr14MgSb11 crystallizes in the Sr14ZnSb11 structure type, whereas Eu14MgSb11 is exactly isotypic to Ca14AlSb11. Such structural discrepancy also leads to significantly different thermoelectric properties of these two materials, which were compared and discussed in detail in this paper. Magnetic properties of Eu14MgSb11 were studied and density functional theoretical calculations were performed to better under the electronic structure of these new materials.

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EXPERIMENTAL SECTION

Synthesis. All syntheses were performed in an argon-filled glovebox with the oxygen level below 0.1 ppm or under vacuum. The elements were commercially purchased and used as received: Sr (Afla, 99%), Eu (Afla, 99.9%), Mg (Alfa, 99.8%), Sb (Alfa, 99.999%). Single crystals were obtained with the ratio of Sr(Eu):Mg:Sb = 14:2.5:11 in Ta tubes and the following procedure was applied: The Ta tubes were sealed in an evacuated fused silica jacket and the container was then moved to a programmable furnace. The reactants were first heated to 1050 °C and homogenized at this temperature for 3 days, and then slowly cooled to the room temperature at a rate of 5 °C/h. The products consisted of a mixture of irregular binary phases and the needle crystals of title compounds. Metal flux methods using Sn or Pb were also tried to reproduce the title compounds but unsuccessful. Shining needle-shaped crystals of title compounds were carefully selected from the products for EDS and magnetic measurements. To prepare materials for thermoelectric-property studies, the products were reproduced by a homemade induction melting furnace incorporated in the glovebox. Reactants with the nearly stoichiometric ratio of Sr(Eu):Mg:Sb = 14:1.1:11 were sealed in the Ta tubes (Note that a little high content of Mg is necessary in order to obtain pure targeted products). The Ta tubes were first heated to 1400 °C in 30 s by the induction melting furnace and then kept at this temperature for 3 min, followed by a quick cooling process with the current terminated immediately. The purity of materials was verified by power X-ray B

DOI: 10.1021/acs.inorgchem.6b02724 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Table 1. Selected Crystal Data and Structure Refinement Parameters for Sr14MgSb11 and Eu14MgSb11 formula −1

FW (g·mol ) T (K) radiation wavelength space group, Z a (Å) c (Å) V (Å3) ρcalc (g·cm−3) abs coeff (mm−1) GOF on F2 R1 (I > 2σI)a wR2 (I > 2σI)a R1 (all data)a wR2 (all data)a

Sr14MgSb11

Eu14MgSb11

2590.24 296

3491.00

Table 3. Selected Important Interatomic Distances (Å) and Bond Angles (deg) in A14MgSb11 (A = Sr, Eu)

Mo−Kα 0.71073 Å I41/acd (No. 142), 8 17.5691(14) 17.3442(11) 23.399(4) 22.981(3) 7222.6(14) 6913.3(11) 4.764 6.708 28.55 33.40 1.061 1.008 0.0354 0.0357 0.0646 0.0654 0.0615 0.0615 0.0721 0.0706

R1 = ∑||F 0 | − |F c ||/∑|F 0 |; wR2 = [∑[w(F 0 2 − F c 2 ) 2 ]/ ∑[w(F02)2]]1/2, and w = 1/[σ2F02 + (A·P)2 + B·P], P = (F02 + 2Fc2)/3; A and B are weight coefficients. a

using the tight-binding (TB) program TB-LMTO-ASA.62−65 The basis sets of Sr 4d, 5s, 5p, Mg 3s, 3p, and Sb 4d, 5s, 5p were included, using a down-folding technique. The band structure was sampled for 12 × 12 × 12 k points in the irreducible Brillouin zone.

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RESULTS AND DISCUSSION Crystal Structures. Both Sr14MgSb11 and Eu14MgSb11 crystallize in the tetragonal space group of I41/acd (No. 142). However, for Sr14MgSb11 a disorder appears on the Sb3 atom, which results in a split of about 88% Sb3A and 12% Sb3B around this site. Similar disorder has also been observed for Sr14ZnSb11,18 yet not discovered in A14MgSb11 (A = Ca, Yb),23 which means such a phenomenon is probably only related to the cation types. As reported by many references,3,4,12,14,17 the

atom pair

Sr14MgSb11

Eu14MgSb11

A1−Sb1 A1−Sb2 A1−Sb2 A1−Sb3 A1−Sb3 A1−Sb4 A2−Mg1 A2−Sb1 A2−Sb2 A2−Sb2 A2−Sb3 A2−Sb3 A2−Sb4 A3−Sb1 × 2 A3−Sb2 × 2 A3−Sb3 × 2 A4−Sb1 A4−Sb2 A4−Sb2 A4−Sb3 A4−Sb3 A4−Sb3 Mg1−Sb2 × 4 Sb1−Sb4 Sb2−Mg1−Sb2 Sb2−Mg1−Sb2 Sb3A−Sb3Ba Sb3B−Sb3B

3.3775(9) 3.3641(10) 3.3677(10) 3.4014(12) 3.4189(11) 3.3944(8) 3.7609(9) 3.4368(10) 3.2894(10) 3.9019(10) 3.3424(9) 3.4770(14) 3.5728(9) 3.5644(5) 3.45001(10) 3.3088(2) 3.3892(10) 3.5424(11) 3.3148(10) 3.4341(12) 3.4267(13) 3.868 (7) 2.8752(5) 3.342(3) 106.732(11) 115.10(2) 0.708(8) 3.013(18)

3.3432(9) 3.3100(11) 3.3323(12) 3.3165(12) 3.3584(12) 3.3394(7) 3.6776(8) 3.4175(10) 3.2650(12) 3.8750(12) 3.3206(12) 3.4183(12) 3.5733(8) 3.5098(6) 3.3798(11) 3.2584(12) 3.3391(12) 3.2924(11) 3.5353(12) 3.3465(12) 3.3568(12) 3.8941(13) 2.8515(9) 3.2907(12) 106.074(17) 116.50(4)

a

Sb3 in Sr14MgSb11 experiences a small positional disorder, modeled as a majority site (Sb3A, 88%) and a minority site (Sb3B, 12%).

Table 2. Refined Atomic Coordinates and Isotropic Displacement Parameters for Sr14MgSb11 and Eu14MgSb11 Atom

Wyckoff

x

Sr1 Sr2 Sr3 Sr4 Mg1 Sb1 Sb2 Sb3Ab Sb3B Sb4

32g 32g 16e 32g 8a 16f 32g 32g 32g 8b

0.04179(4) 0.02216(5) 0.35504(6) 0.34614(5) 0 0.13452(3) 0.36196(3) 0.12920(6) 0.15480(4) 0

Eu1 Eu2 Eu3 Eu4 Mg1 Sb1 Sb2 Sb3 Sb4

32g 32g 16e 32g 8a 16f 32g 32g 8b

0.04315(4) 0.02207(4) 0.35589(5) 0.34251(4) 0 0.13416(5) 0.36026(5) 0.13130(6) 0

y Sr14MgSb11 0.07231(5) 0.37384(5) 0 0.07200(5) 1/4 0.38452(3) 0.25411(3) 0.02526(4) 0.03570(3) 1/4 Eu14MgSb11 0.07301(4) 0.37557(4) 0 0.07060(4) 1/4 0.38416(5) 0.25426(5) 0.02580(5) 1/4

z

Ueqa (Å2)

occupancy

0.17254(3) 0.00504(3) 1/4 0.09410(3) 3/8 1/8 0.05907(2) 0.04686(4) 0.06890(3) 1/8

0.0199(1) 0.0243(2) 0.0174(2) 0.0262(2) 0.0172(1) 0.0195(9) 0.0167(4) 0.0216(2) 0.0216(2) 0.0281(3)

1.0 1.0 1.0 1.0 1.0 1.0 1.0 0.883(3) 0.117(3) 1.0

0.17203(3) 0.00287(3) 1/4 0.09336(3) 3/8 1/8 0.05971(4) 0.04689(4) 1/8

0.0174(8) 0.0225(9) 0.0163(2) 0.0225(2) 0.018(2) 0.0157(3) 0.0169(2) 0.0204(2) 0.0239(5)

1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0

a

Ueq is defined as one-third of the trace of the orthogonalized Uij tensor. bSb3 in Sr14MgSb11 experiences a small positional disorder, modeled as a majority site (Sb3A, 88%) and a minority site (Sb3B, 12%). C

DOI: 10.1021/acs.inorgchem.6b02724 Inorg. Chem. XXXX, XXX, XXX−XXX

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in energy. Thus, the actual site occupancies of the disordered Sb3 atom may vary with different systems or significantly deviate from the theoretical value. By a further comparison of the structure between Sr14MgSb11 and Eu14MgSb11, a slight difference on their crystal structure can be observed. Although both Sr and Eu cations adopt the octahedral coordination geometry, the Sr cations seem to feature more distorted octahedrons. The Sr−Sb bonds obviously fall into a wide range from 3.064(6) Å to 3.9019(10) Å, whereas the corresponding Eu−Sb bonds vary from 3.2584(12) Å to 3.894(13) Å. As discussed above, for the divalent alkaline-earth containing compounds, a disorder on the Sb3 site in Sr14MgSb11 is often seen, as in Sr14ZnSb11 and Ca14CdAs11,18,24 which seems to compensate the electron deficiency of this system. However, for Eu14MgSb11, it turns out to be a perfect ordered structure instead and is certainly worthy of more detailed examination on its electronic structure. The MgSb4 tetrahedrons are both very distorted in Sr14MgSb11 and Eu14MgSb11. The bond angles of Sb−Mg−Sb range from 106.732(11)° to 115.10(2)° for Sr14MgSb11, and 106.074(17)° to 116.50(4)° for Eu14MgSb11. Although Sr and Eu cations are similar in the charges and sizes, the bond angles of MgSb4 tetrahedron in Eu14MgSb11 seems more deviating from the ideal tetrahedral angle (109.47°). This phenomenon was also found for Ca14MgSb11 and Yb14MgSb11.23 The Mg−Sb distances in Sr14MgSb11 and Eu14MgSb11 are 2.8752(5) Å and 2.8515(9) Å, respectively, comparable to those reported magnesium−antimony-based analogues such as SrMg2Sb2 (2.857 Å to 2.909 Å).66 The Sb37− linear chain is characteristic in the 14−1−11 compounds, which can be described as a hypervalent, three-center, four-electron bonded structure rather than a dsp3 hybridized group.11 Each Sb37− linear chain consists of one Sb4 center bonded by two Sb1 atoms. For antimonide analogues, the Sb4 atom is generally ordered, however, partial occupancy has also been reported for arsenides at this site such as Sr14GaAs11.3 The bonding distances between Sb1 and Sb4 are 3.342 Å in Sr14MgSb11 and 3.2907 Å in Eu14MgSb11, respectively, which are in accordance with those of other 14− 1−11 antimonides, such as 3.220(1) Å in Ca14MgSb11 and 3.258(2) Å in Eu14MnSb11.6,23 Magnetic Properties. The zero-field-cooled (ZFC) and field-cooled (FC) susceptibilities of Eu14MgSb11 are shown in the range of 5−300 K in Figure 2. Over the range from 50 to 300 K, the FC and ZFC data are almost identical. The 1/χ curves can be fit with the Curie−Weiss law χ(T) = C/(T − θ), where C is the Curie constant and θ is the Weiss constant. The Curie constant is expressed as C = NAμeff2/3kB, where NA is Avogadro’s number, μeff is an effective magnetic moment, and kB is Boltzmann’s constant. The resultant values of C and θ are 8.26 emu·K/Eu-mol and −9.49 K. The μeff calculated from experimental data is 8.13 μB, which is close to the theoretical value of 7.937 μB, expected for the Eu2+ ion with total angular momentum J = 7/2 and the Landé g factor = 2. There is also a magnetic transition peak at 7 K in the temperature-dependent susceptibility data, and the negative θ value indicates that the coupling between the Eu2+ ions is antiferromagnetic. This behavior is common in the Eu-containing compounds of this family, such as Eu14InBi11.6 A small separation between FC and ZFC data at low temperature was also observed in Figure 2. Such a behavior may be attributed to the trace ferromagnetic impurities in the sample. Although there is no direct evidence of such impurities based on the powder diffraction pattern, this suspicion is

structures of the Ca14AlSb11-type compounds can be rationalized simply by using the Zintl concept. In such a description, the formula can be rewritten as a charge-balanced one, which seems composed of various ions, as exemplified by [Ca2+]14[AlSb49−][Sb37−][Sb3−]4. Consequently, the anionic structures of Sr14MgSb11 and Eu14MgSb11 can be viewed as built of such three basic units as well, i.e., MgSb4 tetrahedrons, Sb3 linear chains, and isolated Sb anions, as indicated in Figure 1.

Figure 1. Polyhedral and ball-and-stick representation of the crystal structure for Sr14MgSb11, viewed down the b-axis. The Mg atoms and Sb anions are drawn as pink and green spheres, respectively. The Sr cations are omitted.

However, the electron counting of these compounds is still a little confusing. Compared with Ca14AlSb11, it is very obvious that Sr14MgSb11 is one electron deficient in consideration of the charge difference between Al3+ and Mg2+. Such an electronic structure will lead to the metallic conducting of the materials, which in fact is not supported by the semiconducting properties found for many alkaline-earth antimonide analogues, such as Sr14ZnSb11,18 Ca14MgSb11,23 and Sr14MgSb11 (this work). Thus, if these systems are indeed charge-balanced, the structures then have to be modified in order to meet the electronic requirements, which can be realized through the following two strategies. A simple way to reduce the electron requirements is formation of covalent bonds between anions, typically seen in some strontium containing compounds such as Sr14ZnSb11 and Sr14MgSb11. Take Sr14MgSb11 for example, the disorder of Sb3 atom, i.e., split into Sb3A and Sb3B, will create an additional bond between Sb3B and Sb3B, which has a bonding length of 3.013(18) Å. In such a case, Sr14MgSb11 can then be reformulated as [Sr 2+ ] 14 [MgSb 4 10− ][Sb 3 7− ][Sb24−]x[Sb3−]4−2x (x means the site occupancy of Sb3B and with x = 0.5 the system turns into electron precise). Another way is generation of the Sb defects, which has been noted in Ca14MgSb11. In such a case, a Sb-deficient composition Ca14MgSb11‑x is resulted with the deficient levels x in a range of 0.08−0.16.23 Similarly, a reformulation of [Ca2+]14[MgSb410‑][Sb37−] [Sb3−]4(1‑x) with x = 0.08 will correspond to a charge-balanced system. Please note that above analyses are only for better understanding on the structural modification related to these phases. The real situation may be much more complicated, for example, introducing of the defects or disorders into the crystal may also lead to the lattice distortion, which has to be compensated D

DOI: 10.1021/acs.inorgchem.6b02724 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 3. (a) Resistivity and (b) Seebeck coefficient of Sr14MgSb11 and Eu14MgSb11.

Figure 2. (a) Magnetic susceptibility and (b) reciprocal susceptibility for Eu14MgSb11 measured at 1000 Oe in the range of 5−300 K under the FC and ZFC condition.

622 K, and then decreases with the increasing temperature. As mentioned above, Sr14MgSb11 are semiconducting whereas Eu14MgSb11 is metallic. Thus, the thermal excitation of carriers should play an important role in affecting the electrical transport properties in Sr14MgSb11, which is significantly affected by the temperature. However, for metallic Eu14MgSb11, a relatively high carrier concentration could be expected in the material and a saturated Seebeck value over the measured temperature range was observed instead. The thermal conductivity data of Sr 14 MgSb 11 and Eu14MgSb11 are compared in Figure 4. Both compounds have relatively low lattice thermal conductivity due to the very complex crystal structures, which is typically around 0.5 W·m−1· K−1. These values are comparable to those of 14−1−11 antimonide analogues such as Ca14MgSb11 and Yb14MgSb11.23 However, the electronic thermal conductivities of Sr14MgSb11 and Eu14MgSb11 are significantly different. For Sr14MgSb11, the electronic contribution is obviously minor, and this can be understood by its semiconducting property with relatively high resistivity. For Eu14MgSb11, the metallic property also embodies more significant electronic contribution to the thermal conductivity, which should correspond to higher thermal conductivity compared to Sr14MgSb11. Despite the intrinsically low thermal conductivity of both compounds, the calculated ZT values are not very high (Figure S4) due to the poor electrical transport properties; however, these properties may be improved by utilizing suitable optimization strategies. Electronic Structures. To better understand the electrical transport properties of these materials, electronic band structure calculations were performed on Sr14MgSb11 based on a hypothetical ordered model with the disordered Sb3B atoms removed from the structure. The calculated total density

supported by the temperature dependence of the inverse susceptibility (1/χ), which is actually not very linear. These results may offer a hint to explain for the deviation of the measured effective moment of Eu2+. In addition, the observed coercivity is also very small and does not saturate even at a high field of 6 T (Figure S3), which is another clue of the possible existence of ferromagnetic impurities in the sample. However, limited by the rather low content, the identification of such impurities is very difficult. Thermoelectric Properties. After being densified by SPS, the disk-shaped samples were used to measure thermal conductivity and then cut into bars for resistivity and Seebeck coefficient measurements. The temperature dependence of electrical resistivity and Seebeck coefficient of Sr 14 MgSb 11 and Eu14MgSb11 are presented in Figure 3. The resistivity of Eu14MgSb11 increases gradually from 3.93 to 5.08 mΩ·cm as the temperature increases, indicating metallic electrical conducting property. However, the resistivity of Sr14MgSb11 is much higher at room temperature (408 mΩ·cm) and it decreases rapidly with the increasing temperature (15.62 mΩ· cm at 922 K). Such a behavior is similar to that of intrinsic semiconductors, in which the decreasing resistivity is resulted by the thermal excitation of carriers at high temperature. For Sr14MgSb11 the Seebeck coefficient decreases quickly with the increasing temperature before 423 K, and then remains almost constant until the temperature reaches 723 K. Above this temperature, the Seebeck coefficient turns to decrease again. In comparison with Sr14MgSb11, the Seebeck coefficient behavior of Eu14MgSb11 seems much different, which keeps increasing until it reaches a peak value of about 63.4 μV·K−1 at E

DOI: 10.1021/acs.inorgchem.6b02724 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 4. Thermal conductivity of Sr14MgSb11 and Eu14MgSb11. Electronic thermal conductivity (κe) was extracted by using the Wiedemann−Franz law: κe = LσT, where L is the Lorenz number. The lattice thermal conductivity (κl) was obtained by subtracting electronic thermal conductivity (κe) from total thermal conductivity (κtot).

Figure 5. Total and partial densities of states of Sr14MgSb11. The Fermi level is set at 0 eV.

Fermi level and its 3p-orbitals locate at relatively low energies from −3.4 to 0.3 eV, which mainly contributes to the valence band. Interestingly, an intermediate band appears in the band gap, which is worthy of special discussion here. This intermediate band is mostly contributed by Sr-4d and Sb-5p orbitals, and the mixing from the Mg atom is negligible. Combined with the COHP analysis (Figure 6), this band corresponds to a weak bonding region of the Sr−Sb pairs but a strong antibonding

of states (TDOS) and the projected density of states (PDOS) for various constituent atoms are presented in Figure 5. At first glance, it is clear that Sr14MgSb11 exhibits a band gap of about 1.18 eV, which is also supported by the temperature-dependent resistivity data. However, the position of the Fermi level is suspicious, which obviously crosses the top of the valence bands and thus suggests metallic conducting behavior for this compound. The reason for such a contradiction is because the model chosen for this theoretical modulation does not really represent the “real structure”. As already discussed above, a disorder on Sb3 site will actually balance the charge of this system since it will introduce additional Sb−Sb bonds to reduce the total electron numbers required by a Zintl system. At this aspect, it is not difficult to understand why disordered Sr14MgSb11 is semiconducting, but ordered Eu14MgSb11 is metallic, although these two compounds nominally have the same valence electron numbers according to the magnetic susceptibility measurements. In Figure 5, the states of Sb-5s orbitals are shown in the lowest energy levels, from −8.3 to −7.6 eV, while the Sb-5p orbitals basically predominate the bands right below the Fermi level. Although the Sr atoms are usually treated as cations in Zintl compounds, the contribution from Sr-4d orbitals has significant contribution to the bands in the vicinity of Fermi level. This implies that for such Zintl compounds, the role of cations can not only be considered as electron donors and space fillers, and possible bonding interactions with anions should also be counted.29 For the Mg atom, it does show very little mixing with the bands close to

Figure 6. Calculated COHP curves for (a) average Sr−Sb and Mg−Sb bonds in Sr14MgSb11, (b) Sb−Sb bonding interaction in Sb37− linear chains. F

DOI: 10.1021/acs.inorgchem.6b02724 Inorg. Chem. XXXX, XXX, XXX−XXX

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ACKNOWLEDGMENTS We acknowledge the financial support from the National Natural Science Foundation of China (S.-Q.X.: 51271098; X.T.T.: 51321091; T.-J.Z.: 11574267 and 51571177).

interaction between the Sb1 and Sb4 atoms in the trimeric Sb37− linear unit. These results indicate that for an ordered Sr14MgSb11 structure, the Sr−Sb bonds are obviously electron deficient with the Fermi level crossing the bonding regions. However, for Mg−Sb and Sb−Sb interactions (originating from the isolated MgSb4 tetrahedron and trimeric Sb37− linear unit), the bonds seem well optimized. In such a circumstance, a disorder on the isolated Sb3 atom will be an efficient way to reduce the electrons required by the system. In such a case, extra Sb−Sb bonds are introduced whereas the original covalent anion structure is kept intact, as proved by the crystallographic data. Because such an intermediate band is close to the Fermi level and as well constructs the bottom of the conduction bands, it will probably play an important role in affecting the physical properties of these materials, which certainly calls for more in-depth investigation. However, such a issue obviously requires more substantial and systematic theoretical and experimental work and as well goes beyond the topic of this paper.

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CONCLUSIONS In summary, two new Mg-containing Zintl compounds, Sr14MgSb11 and Eu14MgSb11, were discovered and their crystal structures were determined by single crystal X-ray diffraction. These compounds belong to the well-known 14−1−11 family, for which promising thermoelectric properties can be expected. Sr14MgSb11 is semiconducting, whereas Eu14MgSb11 is poor metallic, and such a discrepancy in the electrical transport properties can be well understood in combination of their crystal and electronic band structures. Their low thermal conductivity and moderate Seebeck coefficient imply that these materials might bear high potential as new thermoelectrics if the electrical transport properties can be tuned. ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b02724. Elemental analysis on single crystals of Sr14MgSb11 and Eu14MgSb11 by EDS; powder X-ray diffraction patterns of Sr14MgSb11 and Eu14MgSb11; field-dependent magnetic susceptibility of Eu14MgSb11; figure of merits calculated for Sr14MgSb11 and Eu14MgSb11; calculation on the Lorenz number (PDF) Crystallographic data in CIF format for Sr14MgSb11 (CIF) Crystallographic data in CIF format for Eu14MgSb11 (CIF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: (531) 883-62519. Fax: (531) 883-62519. ORCID

Sheng-qing Xia: 0000-0002-6199-2491 Notes

The authors declare no competing financial interest. G

DOI: 10.1021/acs.inorgchem.6b02724 Inorg. Chem. XXXX, XXX, XXX−XXX

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