Sr4Cu25.37(18)Sb12 and Eu4Cu26.06(13)Sb12: Copper-Rich

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Sr4Cu25.37(18)Sb12 and Eu4Cu26.06(13)Sb12: CopperRich Antimonide Intermetallics with Cage Structure Min Zhu, Wenjie Tan, Zhen Wu, Xutang Tao, Baibiao Huang, and Sheng-Qing Xia Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.7b01645 • Publication Date (Web): 27 Jan 2018 Downloaded from http://pubs.acs.org on January 29, 2018

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Crystal Growth & Design

Sr4Cu25.37(18)Sb12 and Eu4Cu26.06(13)Sb12: Copper-Rich Antimonide Intermetallics with Cage Structure Min Zhu, WenJie Tan, Zhen Wu, Xu-Tang Tao, Baibiao Huang and Sheng-Qing Xia* State Key Laboratory of Crystal Materials, Institute of Crystal Materials, Shandong University, Jinan, Shandong 250100, People’s Republic of China Dedicated to the 80th birthday for Prof. Academician Xin-Tao Wu

AUTHOR EMAIL ADDRESS: [email protected] CORRESPONDING AUTHORS FOOTNOTE: Prof. Sheng-Qing Xia, State Key Laboratory of Crystal Materials, Institute of Crystal Materials, Shandong University, Jinan, ShanDong 250100; Phone: (531) 883-62519, Fax: (531) 883-62519.

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Abstract

Two new copper-rich antimonide polar intermetallics, Sr4Cu25.37(18)Sb12 and Eu4Cu26.06(13)Sb12, were synthesized through the high-temperature flux reactions and their structures were accurately determined through the single-crystal X-ray diffraction technique. Sr4Cu25.37(18)Sb12 and Eu4Cu26.06(13)Sb12 crystallize in their own structure type with an orthorhombic space group Cmcm (no. 63) (Cell parameters: a = 4.3200(7)/4.2874(7) Å, b = 14.528(2)/ 14.611(2) Å, c = 12.2594(19)/12.1946(19) Å, respectively). The anionic frameworks of these compounds can be described as composed of 16-vertex Cu10Sb6 polyhedra, which form edge-sharing cages filled with the cations. A simple electron counting implies that the onstoichiometric composition as Sr4Cu28Sb12 or Eu4Cu28Sb12 should be electron-precise, however, metallic conducting properties were suggested by the measured electrical transport properties as well as the extensive Cu vacancies in the structure. Magnetic susceptibility measurements on Eu4Cu26.06(13)Sb12 indicated divalent Eu2+ ions coupled by the weak ferromagnetic interactions. Theoretical calculations were performed on the basis of the hypothetical structure of Sr4Cu28Sb12 as well, which provides more understanding on the correlations between the structure and properties of these phases.

Keywords: copper-rich antimonides, polar intermetallics, magnetism, theoretical calculations

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Introduction Polar intermetallics are very interesting for their diverse crystal structures and closely related physical properties.1-3 Among them, compounds formed with the 3d-4f metals are especially attractive, which have been extensively investigated for the potential applications such as magnetism,4-8 catalysts,9-11 thermoelectrics,12-15 and superconductivity.16-19 For example, Eu5+xMg18-xSi13 (x = 2.2) bears an unusual magnetoresistance effect,4 and the magnetism of Yb14MnSb11 can be flexibly tuned by the chemical substitution on the Yb sites.5 As a well-known p-type thermoelectric material, Yb14MnSb11 has exhibited decent performance in the high-temperature power generation12 and many other antimonide analogues, i.e., α-MgAgSb,13 Ca9Zn4+xSb9,14 and Ca1–xRExAg1–ySb (RE = La, Ce, Pr, Nd, Sm; 0 < x < 1, 0 < y < 1)15, were also considered as potential candidates. Besides, superconductivity can even be observed for some polar intermetallics such as SrAuSi3,16 La13Ga8Sb21,17 SrAl4-xSix (0≤x≤2),18 and BaGe3.19 For the A−Cu−Pn system (A = alkaline-earth or rare-earth metals; Pn = pnicogen elements), there have been abundant structure motifs built of various Cu-Pn polyhedra. Among these phases, compounds with the ZrBeSi- and ThCr2Si2-structure type, nominally with the “1-1-1” and “1-2-2” compositions, often feature a layered anionic structure, like BaCuPn (Pn = P, As),20 ACu2-xPn2 (A = Ca, Sr, Ba, Yb; Pn = P, As, Sb).21 With the Cu contents increased, three dimensional frameworks are usually resulted, such as in A2Cu6P5 (A = Sr, Eu) and EuCu4P3. 22 For even higher Cu concentrations, the structures may be much more complex, which lead to various cage structures, which were represented by Eu1.5Ba6.5Cu16P30,23 Ba8Cu16P30,24 A7Cu44As23 (A=Sr,Eu),25 Ba2Cu16.33As1026 and BaCu7.31Sb5. 27 Apparently, such phases fulfill the “PGEC (phonon glass-electron crystal)” criteria,28 and as a result they may be good candidates for the thermoelectric applications, which are beneficial from their low thermal conductivity and high electrical conductivity. Especially, some compounds with the extensive Cu vacancies such as BaCu5.95P2,29 CaCu3.887As2,30 Ba2Cu16.33As10,26 BaCu7.31Sb527 and BaCu1.96Sb2,31 may also provide flexible sites to tune the electrical transport properties. The important role of the Cu defects on the 3



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thermoelectric properties has been well proven by many high-zT materials such as Cu2-xSe,32 Cu3xSbSe,

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BaCu5.9STe6 and BaCu5.9SeTe6.34

Although there have been a number of P- or As-containing copper-rich compounds reported with cage structures, such antimonide analogues are mostly condensed with alkali metals rather than alkaline-earth elements.35 Even by now, only a few simple ternary examples were found for the Sr(Eu)–Cu–Sb ternary system, i.e., SrCuSb, EuCuSb, SrCu2Sb2 and EuCu2Sb2.36-39 In this work, we extend the pnictogenbased Cu-rich compounds to the antimonides, for which good electrical transport properties are obtained owing to the high electrical conductivity and Seebeck coefficient. The discovery of two Cu-rich title compounds, Sr4Cu25.37(18)Sb12 and Eu4Cu26.06(13)Sb12, may suggest interesting applications such as thermoelectrics for these novel phases, in combination of their complex cage structures for which low thermal conductivity is expected. Magnetic properties, electrical transport properties and the theoretical calculations were provided to better understand the electronic structure and related physical properties of these new phases. Experimental Synthesis All synthetic processes were carried out in an argon-filled glovebox with O2 level controlled below 0.1 ppm or under vacuum. All metals used in the reactions were purchased from Alfa: Sr (metal ingot, 99.5%), Eu (metal pieces, 99.9%), Cu (grain, 99.999%), Sb (grain, 99.99%), Pb (grain, 99.99%). Single crystals of the title compounds were obtained by the Pb-flux reactions with an reactant ratio A:Cu:Sb:Pb of 0.9:2:2:15 based on the following procedure: the starting materials were first loaded in an alumina crucible, which were subsequently flame-sealed in a silica tube under vacuum. The reaction was heated to 900 °C at a rate of 200 °C/hr and kept at this temperature for 20 hrs; after above homogeneity process, the products were slowly cooled down to 500 °C at a rate of 5 °C/hr and the excess molten Pb-flux was removed by centrifuge. The title compounds obtained in such a way are black needle-shaped crystals. After the structures and compositions of the title compounds were accurately determined with the aid of the single-crystal X-ray diffraction, the stoichiometric reactions 4


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were repeated with a loading ratio of A:Cu:Sb = 4:28:12 and the products were reproduced. Typical SEM images of needle-shaped single crystals of Sr4Cu25.37(18)Sb12 and Eu4Cu26.06(13)Sb12 are shown in Figure 1. Single-crystal X-ray diffraction. Single crystals suitable for the structure determination were handselected under the microscope and cut into desired sizes, which were then mounted on a glass fibre by using epoxy resin AB glue adhesive. The data collections were carried out at room temperature through a Bruker SMART APEX-II CCD area detector on a D8 goniometer, using the graphite-monochromated Mo-Kα radiation (λ = 0.71073 Å) with a exposure time of 10 s and θmax = 28.8°. Since all title compounds are air stable (Proven by the powder diffraction patterns on samples exposed in air for 2 days), the data can be recorded without any inert gas protection at room temperature. Data collection, reduction and integration, together with global unit cell refinements were performed by the INTEGRATE program of the APEX2 software.40 Structure refinements were done by the SHELX (version 6.12) program package,41 and all atoms were treated with anisotropic atomic displacement parameters. The centrosymmetric space group Cmcm was suggested on the basis of the Laue symmetry and systematic absences. Important information on the data collection and structure refinement of Sr4Cu25.37(18)Sb12 and Eu4Cu26.06(13)Sb12 were summarized in Table 1. Standardized atomic coordinates and isotropic atomic displacement parameters of Sr4Cu25.37(18)Sb12 and Eu4Cu26.06(13)Sb12 were collected in Table 2. Important bonding distances were tabulated in Tables 3. Further information in the form of CIF has been deposited with Fachinformationszentrum Karlsruhe, 76344 Eggenstein-Leopoldshafen, Germany, (fax: (49) 7247808-666; e-mail: [email protected]) – depository CSD-number 433802, 433803 for Sr4Cu25.37(18)Sb12 and Eu4Cu26.06(13)Sb12, respectively. Powder X-ray diffraction Powder X-ray diffraction patterns were taken at room temperature by a Bruker AXS X-ray powder diffractometer using Cu-Kα radiation to identify the purity of Eu4Cu26.06(13)Sb12, which were proceeded to magnetic susceptibility measurements. Data acquisition was

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fulfilled by the Bruker software with a step size of 0.04° of 2θ (Figures S1 in Supporting Information). Elemental Analysis Crystals of both Sr4Cu25.37(18)Sb12 and Eu4Cu26.06(13)Sb12 were picked for Energy Dispersive X-ray Spectroscopy (EDS) measurements, which operated by a Hitachi FESEM-4800 field emission microscopy equipped with a Horiba EX-450 EDS. The measured compositions are in good agreement with the results obtained from the single-crystal X-ray diffraction data (Supporting Information). Magnetic Measurements Field-cooled direct-current (DC) magnetization measurements were performed with a MPMS Quantum Design SQUID magnetometer. In order to prepare samples with high purity, single crystals of Eu4Cu26.06(13)Sb12 were carefully selected under the microscope and then soaked in a mixed solution with ~10% H2O2 and ~20% CH3COOH for a few hours to remove the excess Pb flux. The applied magnetic field was 500 Oe, and the temperature interval was 5−300 K. The collected data were converted to molar magnetic susceptibility [χ (T) = M/H]. Electrical Resistivity The electrical conductivity was measured on the needle single crystals of Sr4Cu25.37(18)Sb12 and Eu4Cu26.06(13)Sb12 on a Quantum Design Physical Property Measurement System (PPMS) equipped with an ac transport controller (model 7100) under 100 µA current, over a temperature range from 5 to 300 K. The measurements were done by applying the four-probe method with four thin copper wires glued to crystals by the silver epoxy electrical conducting paste. Electronic Structure Calculations A hypothetical model of Sr4Cu28Sb12 was constructed for calculations. Electronic band structure and the total density of states (DOS) were calculated by Wien2k42 with the full potential linearized augmented plane wave method (FP-LAPW).43,44 In this method, the unit cell is divided into nonoverlapping muffin-tin (MT) spheres and an interstitial region. The wave functions in the interstitial regions are expanded in plane waves up to RMT×Kmax = 7, in which RMT is the smallest radius of all MT spheres and Kmax is the plane wave cutoff. The valence wave functions inside the MT spheres are expanded up to lmax = 10, whereas the charge density was Fourier expanded up to Gmax = 12 (au)-1. The MT radii were chosen to be 2.5, 2.3 and 2.25 Bohr for Sr, Cu and Sb atoms,

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respectively. Self-consistency was achieved using 1000 k-points in the irreducible Brillouin zone (IBZ). The program “LMTO 4.7”45 with the linear muffin-tin orbital (LMTO) method46 was used to calculate the crystal Hamilton orbital population (COHP). The basis set included 5s, 5p, 4d for Sr, 4s, 4p, 3d for Cu, and 5s, 5p, 4d for Sb. Results and Discussion Structure Description Sr4Cu25.37(18)Sb12 and Eu4Cu26.06(13)Sb12 crystallize in the same structure type with an orthorhombic space group Cmcm (no. 63), owing to the similar sizes and charges for Sr2+ and Eu2+ cations. The divalent state of the Eu cations was supported by the magnetic susceptibility measurements below. As indicated in Figure 2, the anionic structure of Sr4Cu25.37(18)Sb12 can be described as composed of 16vertex Cu10Sb6 polyhedra, which are linked by the Cu-Sb covalent bonds and filled by the Sr cations. Especially, the Cu-rich antimonide analogues are rather few, though corresponding arsenide compounds have been reported with quite a number of examples such as Eu7Cu44As23 and Sr7Cu44As23

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.

Interestingly, by comparing this Cu10Sb6 cage with the 20-vertex Cu12As8 polyhedra of Eu7Cu44As23,25 the topologic structure of the Cu10Sb6 unit can be readily derived from the Cu12As8 cage by selectively removing two vertex Cu and As atoms, as indicated in Figure 3. For various Cu-centered polyhedra in Sr4Cu25.37(18)Sb12 and Eu4Cu26.06(13)Sb12, there have been different coordination geometry observed. Take Sr4Cu25.37(18)Sb12 for example, Cu1, Cu3 and Cu4 atoms are surrounded by four Sb atoms and thus tetrahedrally coordinated, whereas for Cu2, a trigonal planar coordination sphere is resulted instead. All CuSb4 tetrahedrons are slightly distorted with the Cu-Sb distance ranges of dCu1−Sb = 2.6202(15) ~ 2.8900(19)Å, dCu3−Sb = 2.5470(19) ~ 2.976(2)Å and dCu4−Sb = 2.635(2) ~ 2.744(2)Å, respectively. Similarly, the Cu2 atom does not locate in the center of the trigonal plane either. Note that there also exist short Cu-Cu interactions, as indicated in Figure S2 (Supporting Information), which have suspicious bonding distances ranging from 2.444(2) to 2.916(5) Å. It is known that the Cu-Cu bonds in the Cu metal is about 2.56 Å,47 thus such short Cu-Cu pairs may imply some 7



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weak Cu-Cu interactions, which have been often found for the pnictide intermetallic compounds, i.e., 2.415-2.885 Å in BaCu7.31Sb5,27 and 2.450-3.060 Å in BaCu10P4.48 Certainly, if comparing this interactions with the distance of 2.34 Å for the Cu-Cu single bond,

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possible covalent bonding

interactions may also be reasonable. With above polyhedral building unit connected through a corner- or edge-sharing manner, a complex three dimensional framework is constructed, as indicated in Figure 4. Another important structural feature of these new phases is the extensively existing Cu vacancies in the structure. Although a hypothetic stoichiometric composition [A2+]4[Cu+]28[Sb3-]12 tends to result in the electron-precise state, the actual Cu concentrations are a little lower than the theoretical predications. According to the references, Cu defects seem to be common in the Cu-rich intermetallics, i.e., in ReCuxTe2

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the various defect levels are governed by the cation types for which important size and

electronic effects should be taken into account.51 In current cases, although Sr4Cu25.37(18)Sb12 and Eu4Cu26.06(13)Sb12 feature the same structure type, the actual occupancies of the defected Cu sites are somehow different, which also implies the possible electronic effects originating from the cations. Besides, the metallic conducting properties, as discussed below, also support the argument that these compounds might not be charge-balanced. Electronic Structure The calculated total and projected density of states (TDOS and PDOS) are shown in Figure 5. In the vicinity of Fermi level, the conductive bands (CB) are mostly composed of the Sb-p and Cu-d states, while the valence bands (VB) are predominantly contributed by the Sb-p and Sr-d states. The d-orbitals of Cu seem to dominate the CB states below Fermi level from -5.0 to -2.0 eV. However, viewed from the corresponding band structure, the Cu-3d bands are generally very flat, which means the 3d electrons of Cu are basically localized. In such a case, a monovalent state of the Cu ions should be more reasonable, which support the reasonability of the electron counting above. With a pseudo-gap appearing in the DOS curve, these compounds should be poor metallic conducting, and the states around the Fermi level suggest a strong p-d mixing between Sr and Sb, which was also normally observed for

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the alkaline-earth pnictide intermetallics.52,53 The calculated Crystal Orbital Hamilton Population (COHP) of various bonding interactions are provided in Figure 6. The Sr−Sb bonds are almost optimized at the Fermi level, but for the Cu–Sb interactions, they locate in a weak antibonding region. This phenomenon may provide some explanation for the reasonability of Cu vacancies in the structure since obviously a stoichiometric composition of these compounds will lead to the weakening of the Cu-Sb bonds. The calculated -iCOHPs suggests that the Cu−Sb bonds should be covalently bonded, which bears reasonably high -iCOHP values around 1.361 eV/bond. In addition, the Sr–Sb interactions are relatively much weaker with an averaged -iCOHP value around 0.457 eV/bond. As mentioned above, there are also some possible Cu-Cu interactions in these compounds. On the basis of the calculated -iCOHPs, the interactions between the Cu atoms seem to significantly depend on the interatomic distances, i.e., 0.895 eV/bond for Cu-Cu bonds between 2.444 and 2.499 Å, 0.698 eV/bond for 2.607 Å, and 0.351 eV/bond for 2.708~2.916 Å. Thus, the Cu-Cu interactions in these compounds are probably very complicated, which should not be simply treated as the simple two-centered two-electron covalent bonds or multi-centered multi-electron metallic bonds. Magnetic Susceptibility Measurement Figure 7 shows the measured magnetic susceptibility data of Eu4Cu26.06(13)Sb12 from 5 K to 300 K measured in an applied field of 500 Oe. In the low temperature region ( 1 in Inexpensive Zintl Phase Ca9Zn4+ xSb9 by Phase Boundary Mapping. Advanced Functional Materials, Adv. Funct. Mater. 2017, 27, 1606361. 15 Wang, J.; Liu, X. C.; Xia, S. Q.; Tao, X. T. Ca1-xRExAg1-ySb (RE = La, Ce, Pr, Nd, Sm; 0 ≤ x ≤1; 0 ≤ y ≤1): Interesting Structural Transformation and Enhanced High Temperature Thermoelectric Performance. J. Am. Chem. Soc. 2013, 135, 11840–11848. 16 Isobe, M.; Yoshida, H.; Kimoto, K.; Arai, M.; Takayama-Muromachi, E. SrAuSi3: A Noncentrosymmetric Superconductor. Chem. Mater. 2014, 26, 2155-2165. 17 Mills, A. M.; Deakin, L.; Mar, A. Electronic Structures and Properties of RE12Ga4Sb23 (RE = La-Nd, Sm) and Superconducting La13Ga8Sb21. Chem. Mater. 2001, 13, 1778−1788. 18 Zevalkink, A.; Bobnar, M.; Schwarz, U.; Grin, Y. Making and Breaking Bonds in Superconducting SrAl4-xSix (0 ≤ x ≤ 2). Chem. Mater. 2017, 29, 1236-1244. 19 Castillo, R.; Baranov, A. I.; Burkhardt, U.; Cardoso-Gil, R.; Schnelle, W; Bobnar, M.; Schwarz, U. Germanium Dumbbells in a New Superconducting Modification of BaGe3. Inorg. Chem. 2016, 55, 4498-4503. 20 Mewis, A. Darstellung und Struktur der Verbindungen MgCuP, BaCuP (As) und BaAgP (As)/Preparation and Crystal Structure of MgCuP, BaCuP (As), and BaAgP (As). Zeitschrift für 12


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Pn= Sb, Bi. J. Am. Chem. Soc. 2007, 129, 10011-10018. 52 Xia, S. Q.; Bobev, S. Cation-Anion Interactions as Structure Directing Factors: Structure and Bonding of Ca2CdSb2 and Yb2CdSb2. J. Am. Chem. Soc. 2007, 129, 4049–4057. 53 Cooley, J.; Kazem, N.; Zaikina, J. V.; Fettinger, J. C.; Kauzlarich, S. M. Effect of Isovalent Substitution on the Structure and Properties of the Zintl Phase Solid Solution Eu7Cd4Sb8−xAsx (2 ≤ x ≤ 5). Inorg. Chem. 2015, 54, 11767–11775. 54 O’Connor, C. J.; Magnetochemistry-Advances in Theory and Experimentation. In Progress in Inorganic Chemistry. 55 Lippard, S. J. Ed. John Wiley & Sons, Inc.: New York, 2007; 29, 203−283. 56 Zhu, M.; Tao, X. T.; Xia, S. Q. Electron-deficient copper pnictides: A2Mg3Cu9Pn7 (A = Sr, Eu; Pn = P, As) and Eu5Mg2.39Cu16.61As12. Inorg. Chem. Front., 2016, 3, 1264–1271. 57 Wang, J.; Yang, M.; Pan, M.-Y.; Xia, S. -Q.; Tao, X. -T.; He, H.; Darone, G.; Bobev, S. Synthesis, Crystal and Electronic Structures, and Properties of the New Pnictide Semiconductors A2CdPn2 (A = Ca, Sr, Ba, Eu; Pn = P, As). Inorg. Chem., 2011, 50, 8020–8027. 58 Wang, J.; Xia, S. Q.; Tao, X. T. A5Sn2As6 (A = Sr, Eu). Synthesis, Crystal and Electronic Structure, and Thermoelectric Properties. Inorg. Chem. 2012, 51, 5771–5778. 59 Liu, X. C.; Xia, S. Q.; Lei, X. W.; Pan, M. Y.; Tao, X. T. Crystal and Electronic Structures and Magnetic Properties of Eu3Tt2As4 (Tt = Si, Ge). Eur. J. Inorg. Chem., 2014, 13, 2248–2253.

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Page 16 of 29

Captions of Figures and Tables Table 1. Selected crystal data and structure refinement parameters for Sr4Cu25.37(18)Sb12 and Eu4Cu26.06(13)Sb12. Table 2. Refined atomic coordinates and isotropic displacement parameters for Sr4Cu25.37(18)Sb12 and Eu4Cu26.06(13)Sb12. Table 3. Selected interatomic distances (Å) in Sr4Cu25.37(18)Sb12 and Eu4Cu26.06(13)Sb12. Figure 1. SEM images of the single crystals of (a) Sr4Cu25.37(18)Sb12 and (b) Eu4Cu26.06(13)Sb12, grown from the Pb-flux reactions. Figure 2. (a) Crystal structure of Sr4Cu25.37(18)Sb12 viewed along the a-axis, showing the complex anionic framework composed of the 16-vertex Cu10Sb6 polyhedra. (b) Channels formed by the edgesharing Cu10Sb6 polyhedra along the c-axis. Figure 3. Structure comparison between A7Cu44As23 (A= Sr, Eu) and A4Cu28-xSb12 (A= Sr, Eu) on the coordination sphere around the cations. The topologic structure of the Cu10Sb6 polyhedra can be obtained by simply removing two Cu and As atoms from the Cu12As8 polyhedra. Figure 4. Polyhedral view of the anion structure of A4Cu28-xSb12 (A= Sr, Eu). The CuSb4 tetrahedrons and the coplanar CuSb3 triangles are emphasized. Figure 5. (a) Total, Partial Density of States (TDOS and PDOS) and (b) electronic band structures for the hypothetical structure configured by Sr4Cu28Sb12. Fermi level is chosen as the energy reference. Figure 6. Density of States (DOS) and Crystal Orbital Hamilton Population (COHP) curves calculated for Sr4Cu28Sb12. For −COHPs, positive values represent the bonding interactions, whereas negative values mean antibonding interactions.

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Figure 7. Temperature-dependent magnetic susceptibility data measured on Eu4Cu26.06(13)Sb12 polycrystalline samples under an applied field of 500 Oe. Inset shows the inverse susceptibility versus temperature. Figure 8. Electrical resistivity data for Sr4Cu25.37(18)Sb12 and Eu4Cu26.06(13)Sb12 measured over a temperature range from 5 to 300 K.

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Table 1. Selected crystal data and structure refinement parameters for Sr4Cu25.37(18)Sb12 and Eu4Cu26.06(13)Sb12. Formula fw/ g·mol–1

Sr4Cu25.37(18)Sb12

Eu4Cu26.06(13)Sb12

3423.49

3749.47

T/K

296(2)

Radiation, wavelength

Mo-Kα, 0.71073 Å

Space group, No. Z

Cmcm (No. 63) 1

1

a/Å

4.3200(7)

4.2874(7)

b/Å

14.528(2)

14.611(2)

c/Å

12.2594(19)

12.1946(19)

V / Å3

769.4(2)

763.9(2)

ρcalc / g·cm–3

7.389

8.150

µMo Kα / cm–1

3.4268

3.6134

R1 = 0.0374

R1 = 0.0244

wR2 = 0.0828

wR2 = 0.0607

R1 = 0.0466

R1 = 0.0269

wR2 = 0.0878

wR2 = 0.0622

Cell dimensions

Final R indices a [I>2σ(I)]

Final R indices a [all data] a

R1 = ∑||Fo| – |Fc||/∑|Fo|; wR2 = [∑[w(Fo2 – Fc2)2]/∑[w(Fo2)2]]1/2, and w = 1/[σ2Fo2 + (A·P)2 + B·P], P = (Fo2 + 2Fc2)/3; A and B are weight coefficients.

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Table 2. Refined atomic coordinates and isotropic displacement parameters for Sr4Cu25.37(18)Sb12 and Eu4Cu26.06(13)Sb12. Atoms

Wyckoff

occ.

x

y

z

Ueqa (Å2)

Sr4Cu25.37Sb12 Sr1

4c

1

0

0.2487(1)

0.2500

0.019(1)

Cu1

4c

1

0

0.55898(17) 0.2500

0.021(1)

Cu2

8f

0.818(7)

0

0.05151(15) 0.10209(19) 0.028(1)

Cu3

8f

0.936(8)

0

0.4511(1)

0.4030(2)

0.029(1)

Cu4

8f

0.918(8)

0

0.1567(1)

0.5592(1)

0.023(1)

Sb1

8f

1

0

0.3454(1)

0.5688(1)

0.013(1)

Sb2

4c

1

0

0.92681(9)

0.2500

0.019(1)

Eu4Cu26.06Sb12

a

Eu1

4c

1

0

0.24992(4)

0.25

0.0144(2)

Cu1

4c

1

0

0.55883(11) 0.25

0.0179(4)

Cu2

8f

0.925(5)

0

0.05215(9)

0.10264(13) 0.0268(5)

Cu3

8f

0.968(5)

0

0.44995(8)

0.40397(10) 0.0221(4)

Cu4

8f

0.864(6)

0

0.15734(9)

0.55981(11) 0.0170(5)

Sb1

8f

1

0

0.34359(4)

0.57038(4)

0.0108(2)

Sb2

4c

1

0

0.92473(6)

0.25

0.0146(2)

ij

Ueq is defined as one third of the trace of the orthogonalized U tensor.

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Table 3. Selected interatomic distances (Å) in Sr4Cu25.37(18)Sb12 and Eu4Cu26.06(13)Sb12. Distances (Å) Atom pairs A1 –

Cu1 –

Cu2 –

Cu3 –

Cu4 –

Sr4Cu25.37Sb12

Eu4Cu26.06Sb12

Sb1 × 4

3.386(1)

3.356(1)

Sb2 × 2

3.371(8)

3.334(1)

Sb1 × 2

2.620(2)

2.614(1)

Sb2 × 2

2.890(2)

2.904(1)

Sb1× 2

2.660(1)

2.659(1)

Sb2

2.563(2)

2.588(2)

Sb1

2.547(2)

2.556(1)

Sb1

2.976(2)

3.033(1)

Sb2× 2

2.882(1)

2.873(1)

Sb1

2.744(2)

2.724(2)

Sb1 × 2

2.670(1)

2.668(1)

Sb2

2.635(2)

2.611(1)

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Figure 1. SEM images of the single crystals of (a) Sr4Cu25.37(18)Sb12 and (b) Eu4Cu26.06(13)Sb12, grown from the Pb-flux reactions.

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Figure 2. (a) Crystal structure of Sr4Cu25.37(18)Sb12 viewed along the a-axis, showing the complex anionic framework composed of the 16-vertex Cu10Sb6 polyhedra. (b) Channels formed by the edgesharing Cu10Sb6 polyhedra along the c-axis.

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Figure 3. Structure comparison between A7Cu44As23 (A= Sr, Eu) and A4Cu28-xSb12 (A= Sr, Eu) on the coordination sphere around the cations. The topologic structure of the Cu10Sb6 polyhedra can be obtained by simply removing two Cu and As atoms from the Cu12As8 polyhedra.

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Figure 4. Polyhedral view of the anion structure of A4Cu28-xSb12 (A= Sr, Eu). The CuSb4 tetrahedrons and the coplanar CuSb3 triangles are emphasized.

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Figure 5. (a) Total, Partial Density of States (TDOS and PDOS) and (b) electronic band structures for the hypothetical structure configured by Sr4Cu28Sb12. Fermi level is chosen as the energy reference.

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Figure 6. Density of States (DOS) and Crystal Orbital Hamilton Population (COHP) curves calculated for Sr4Cu28Sb12. For −COHPs, positive values represent the bonding interactions, whereas negative values mean antibonding interactions.

26


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Figure 7. Temperature-dependent magnetic susceptibility data measured on Eu4Cu26.06(13)Sb12 polycrystalline samples under an applied field of 500 Oe. Inset shows the inverse susceptibility versus temperature.

27



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Figure 8. Electrical resistivity data for Sr4Cu25.37(18)Sb12 and Eu4Cu26.06(13)Sb12 measured over a temperature range from 5 to 300 K.

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For Table of Contents Use Only

Sr4Cu25.37(18)Sb12 and Eu4Cu26.06(13)Sb12: Copper-Rich Antimonide Intermetallics with Cage Structure Min Zhu, WenJie Tan, Zhen Wu, Xu-Tang Tao, Baibiao Huang and Sheng-Qing Xia*

Synopsis:

Two new copper-rich ternary antimonides, Sr4Cu25.37(18)Sb12 and Eu4Cu26.06(13)Sb12, were synthesized by the Pb-flux reactions. Their anionic framework feature cage structures composed of the unique 16vertex Cu10Sb6 polyhedra. Both Sr4Cu25.37(18)Sb12 and Eu4Cu26.06(13)Sb12 are metallic conducting, supported by the electrical resistivity measurements. With the magnetic susceptibility data measured on Eu4Cu26.06(13)Sb12 as well as the theoretical calculations, the electron deficiency of these phases are verified, which may indicate interesting thermoelectric properties in consideration of the complex cage structures and the extensively existing Cu vacancies.

29


Sr4Cu25.37(18)Sb12 and Eu4Cu26.06(13)Sb12: Copper-Rich

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