Metal Flux Growth, Structural Relations, and Physical Properties of

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Metal Flux Growth, Structural Relations, and Physical Properties of EuCu2Ge2 and Eu3T2In9 (T = Cu and Ag) Udumula Subbarao, Soumyabrata Roy, Saurav Ch. Sarma, Sumanta Sarkar, Vidyanshu Mishra, Yatish Khulbe, and Sebastian C. Peter* New Chemistry Unit, Jawaharlal Nehru Centre for Advanced Scientific Research, Jakkur, Bangalore 560064, India S Supporting Information *

ABSTRACT: Single crystals (SCs) of the compounds Eu3Ag2In9 and EuCu2Ge2 were synthesized through the reactions run in liquid indium. Eu3Ag2In9 crystallizes in the La3Al11 structure type [orthorhombic space group (SG) Immm] with the lattice parameters: a = 4.8370(1) Å, b = 10.6078(3) Å, and c = 13.9195(4) Å. EuCu2Ge2 crystallizes in the tetragonal ThCr2Si2 structure type (SG I4/mmm) with the lattice parameters: a = b = 4.2218(1) Å, and c = 10.3394(5) Å. The crystal structure of Eu3Ag2In9 is comprised of edge-shared hexagonal rings consisting of indium. The one-dimensional chains of In6 rings are shared through the edges, which are further interconnected with other sixmembered rings forming a three-dimensional (3D) stable crystal structure along the bc plane. The crystal structure of EuCu2Ge2 can be explained as the complex [CuGe](2+δ)− polyanionic network embedded with Eu ions. These polyanionic networks present in the crystal structure of EuCu2Ge2 are shared through the edges of the 011 plane containing Cu and Ge atoms, resulting in a 3D network. The structural relationship between Eu3T2In9 and EuCu2Ge2 has been discussed in detail, and we conclude that Eu3T2In9 is the metal deficient variant of EuCu2Ge2. The magnetic susceptibilities of Eu3T2In9 (T = Cu and Ag) and EuCu2Ge2 were measured between 2 and 300 K. In all cases, magnetic susceptibility data followed Curie−Weiss law above 150 K. Magnetic moment values obtained from the measurements indicate the probable mixed/intermediate valent behavior of the europium atoms, which was further confirmed by X-ray absorption studies and bond distances around the Eu atoms. Electrical resistivity measurements suggest that Eu3T2In9 and EuCu2Ge2 are metallic in nature.

1. INTRODUCTION The structural features of the intermetallic compounds depend on the synthetic methods adopted, which eventually affect the physical properties of these materials as the two are strongly interrelated. In this work, the single crystals of europium compounds were grown using indium as both active and inactive metal flux. The indium flux technique has been used as a handy tool for synthesizing novel intermetallic phases.1−5 In, as a solvent, has been used to synthesize several new polyindides having diverse structures and compositions.1,2 While the metal flux method can be utilized to form SCs of known phases to study various properties, it can also be used to explore completely new phases.6−16 EuInGe is an interesting example in this respect, reported by Mao et al.,17 in the orthorhombic system and the Pnma space group (SG) when synthesized via high-frequency induction heating. However, we found that the same composition crystallized in the monoclinic P21/c SG when the metal flux method was used.18 Similarly, YbCuGa3 was previously reported to crystallize in a tetragonal I4/mmm structure when it was obtained by an arc melting method;19 however, later, when Ga was used an as active flux, the material crystallized in the monoclinic system (SG: C2/ m).20 © XXXX American Chemical Society

Eu-containing rare earth intermetallic compounds are particularly interesting as they exhibit two degenerate electronic configurations such as Eu3+ (4f6, nonmagnetic) and magnetic Eu2+ (4f7, magnetic). Eu based compounds are famous for interesting physical properties like multiple valency, Kondo behavior, heavy Fermion behavior, unusual magnetism,21−26 and low-temperature superconductivity.27,28 These properties generally originate from the strong hybridization between the delocalized s, p, and d conduction electrons and the localized 4f electrons.29,30 Our research also aimed to develop various novel Eu-based intermetallic compounds,14,18,31−34 which in fact resulted in several interesting properties. According to Pearson’s Crystal Data (PCD)35 and the Inorganic Crystal Structure Database (ICSD),36 so far EuAg4In8 is the only ternary compound reported with Eu, Ag, and In metals. Sysa et al. reported EuAg4In8 in a hexagonal system,37 and we discovered its tetragonal polymorph very recently.38 During our systematic studies of the Eu−Ag−In system, we have produced one more new phase, Eu3Ag2In9, which is in fact the first indide in the RE3T2X9 series. Although more than 80 Received: July 5, 2016

A

DOI: 10.1021/acs.inorgchem.6b01598 Inorg. Chem. XXXX, XXX, XXX−XXX

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2.3. Elemental Analysis. Semiquantitative microanalyses were performed on the SCs grown via the flux techniques using a Leica 220i scaning electron microscope (SEM) equipped with a Bruker 129 eV energy dispersive X-ray analyzer (EDS). The SEM images of normal single crystals of EuCu2Ge2 and Eu3T2In9 are shown in Figure 1.

ternary compounds have been reported in this system, most of them are Al- and Ga-based compounds. There are no reports of Eu-based compounds in the RE3T2X9 series to the best of our knowledge, and Eu3Ag2In9 can be considered as the first one in this context. Because we have obtained a new compound in the Eu−Ag−In family, we extended our research to the Eu−Cu−Ge series. There is only one compound (EuCu2Ge2) that has been reported so far in this series, which was reported to crystallize in the tetragonal crystal system.39−41 The divalent nature of Eu atoms in EuCu2Ge2 has been proven via 151Eu Mössbauer spectroscopy and magnetic susceptibility measurements,42 but Banik et al. proposed the mixed valent nature through X-ray absorption spectroscopy studies.43 Later, compound Eu3Cu2In9 was synthesized by the high-frequency induction heating (HFIH) method. We have discussed the crystal structures and structural relation between compounds Eu3T2In9 and EuCu2Ge2 in detail. Preliminary magnetic and transport measurements were performed on all three compounds. The valence state of Eu in all three compounds has been discussed in detail using crystal structure and magnetic measurements, which were later confirmed by X-ray absorption near edge spectroscopy (XANES) studies.

Figure 1. SEM images of a typical single crystal of (a) Eu3Ag2In9 and (b) EuCu2Ge2 grown from the indium flux. 2.4. Powder X-ray Diffraction (PXRD). The purity and phase identity of Eu3T2In9 and EuCu2Ge2 were determined by PXRD experiments, which were conducted with a Bruker D8 Discover diffractometer using Cu Kα radiation (λ = 1.5406 Å). The experimental powder patterns of Eu3T2In9 and EuCu2Ge2 were found to be in good agreement (except for the few In peaks in Eu3T2In9) with the simulated pattern obtained from the SC X-ray structure refinement. 2.5. Single-Crystal X-ray Diffraction (SCXRD). SCXRD data were collected on single crystals of Eu3Ag2In9 using a Bruker Smart Apex 2-CCD diffractometer having normal focus, 2.4 kW sealed tube X-ray source with graphite monochromatic Mo Kα radiation (λ = 0.71073 Å). A suitable crystal (0.05 mm × 0.05 mm × 0.1 mm) was mounted on a thin glass (∼0.1 mm) fiber with super glue. SAINT44 was used for integration of diffraction profiles, and SADABS45 was used for the absorption correction. Packing diagrams were generated using Diamond.46 The exact composition was confirmed from the occupancy parameters, which were obtained from the refinement in a separate series of least-squares cycles. All bond lengths were found to be within theoretically acceptable ranges. 2.6. Structural Refinement of Eu3Ag2In9 and EuCu2Ge2. The crystal structures of Eu3Ag2In9 and EuCu2Ge2 were refined using Shelxl-97 (full-matrix least squares on F2).47 The atoms in both compounds were refined anisotropically. The first step of refinement showed the Immm SG with the lattice parameters: a = 4.8370(1) Å, b = 10.6078(3) Å, and c = 13.9195(4) Å. During the isotropic refinement of Eu3Ag2In9, the unacceptable highest difference peak and the deepest hole that is a little bit high (>14 and −9 e Å3) were noticed. The refinement residual value (R1) also was found to be slightly high (12%). Because the scattering powers of Ag and In are similar, we have attempted to mix the Ag and In positions; however, no improvement in refinement was obtained. To tackle these issues in the refinement for Eu3Ag2In9, data were re-collected with a longer exposure time (60 s) and a greater number of frames (90). This revised data collection could solve the problems encountered for the initial data collection (10 s and within 60 frames). On the other hand, the crystal structure of EuCu2Ge2 was refined in the already reported ThCr2Si2 structure type in tetragonal space group I4/mmm.39−41 The details of data collection and structure refinement of Eu3Ag2In9 and EuCu2Ge2 are listed in Table 1. The standard atomic positions and isotropic atomic displacement parameters of this compound are listed in Table 2. The anisotropic displacement parameters and important bond lengths are listed in Tables S1 and 3, respectively. Further information about the structural refinements is available from cif files by quoting the CCDC entries 1436853 (Eu3Ag2In9) and 1436854 (EuCu2Ge2). 2.7. Magnetic Measurements. A Quantum Design MPMSSQUID magnetometer was used for the magnetic measurements on pure powdered polycrystals of Eu3T2In9 and EuCu2Ge2. In the

2. EXPERIMENTAL SECTION 2.1. Reagents. The reagents purchased from Alfa Aesar (≥99.99%) were used as obtained, without any further purification: Eu (metal pieces cut from chunks), Cu (powder), Ag (powder), Ge (powder), and In (pieces). 2.2. Synthesis. 2.2.1. Metal Flux Synthesis. Well-shaped single crystals of Eu3Ag2In9 and EuCu2Ge2 were grown by combining europium metal (0.2 g), silver (0.28 g), and indium (2 g) for Eu3Ag2In9 and europium metal (0.3 g), copper (0.37 g), germanium (0.14 g), and indium (1.5 g) for EuCu2Ge2 in alumina crucibles. For the synthesis of EuCu2Ge2, the crucibles were placed in a fused silica tube (13 mm dia), under an argon atmosphere using a glovebox. The silica tube was sealed under a vacuum of 10−3 Torr. The tube containing the reaction mixture was then heated to 1000 °C within 5 h, kept at the same temperature for 1 h for homogenization, cooled to 800 °C in 30 min, and held there for 48 h. Finally, the sample was cooled slowly to 30 °C over 48 h. A different temperature profile has been used for the synthesis of Eu3Ag2In9. The reactants were heated to 1000 °C over 10 h and maintained there for 5 h for homogenization. Next, they were cooled to 900 °C in 2 h and kept at this temperature for 96 h. The sample was finally cooled to 30 °C slowly over 96 h. The samples were heated at 400 °C and centrifuged through coarse frit for isolation. Excess flux was removed by sonicating the isolated products in glacial acetic acid. The final product was washed with distilled water and dried with acetone. Small (0.5 mm length) single crystals of Eu3Ag2In9 and EuCu2Ge2 were selected for structural characterization. 2.2.2. High-Frequency Induction Heating Method. Europium, copper/silver, and indium were mixed in 3:2:9 and 1:2:2 atomic ratios for the synthesis of Eu3T2In9 and EuCu2Ge2, respectively, and sealed in tantalum ampules under an argon atmosphere in an arc melting instrument. The ampules were then placed in the chamber of an induction furnace (Easy Heat induction heating system, model 7590). Tantalum ampules were heated rapidly to 181 A (approximately 1200−1300 K) and held there for 1 h for Eu3T2In9 and 30 min for EuCu2Ge2. Finally, both reaction mixtures were cooled to room temperature by turning off the power supply. No reactions with the ampule were detected after the samples had been removed from the tantalum tube. The polycrystalline samples were light gray in color. There was negligible weight loss (2σ(I)] extinction coefficient largest difference peak and hole (e Å−3) a

Eu3Ag2In9 1705 a = 4.8370(1) b = 10.6078(3) c = 13.9195(4) 0.71073 tetragonal I4/mmm

orthorhombic Immm 714.21(3) 2 7.9278 29.792 1447.8 0.1 × 0.05 × 0.05 2.4−35.8 −7 ≤ h ≤ 7, −17 ≤ k ≤ 12, −22 ≤ l ≤ 22 6194 972 (Rint = 0.0321)

184.28(1) 2 7.645 43.975 370 0.25 × 0.1 × 0.05 3.942−29.398 −5 ≤ h ≤ 5, −5 ≤ k ≤ 5, −14 ≤ l ≤ 14 1057 100 (Rint = 0.0453) 100% full-matrix least squares on F2 100/0/9 972/0/28 1.394 1.177 Robs = 0.0179, wRobs = 0.0518 Robs = 0.038, wRobs = 0.106 0.052(4) 0.000195(7) 1.235 and −1.221 6.886 and −3.899

R = ∑||Fo| − |Fc||/∑|Fo|. wR = {∑[w(|Fo|2 − |Fc|2)2]/∑[w(|Fo|4)]}1/2. calcw = 1/[σ2(Fo2) + (0.0359P)2 + 6.1794P], where P = (Fo2 + 2Fc2)/3.

Table 2. Atomic Coordinates (×104) and Equivalent Isotropic Displacement Parameters (Å2 × 103) of Eu3Ag2In9 and EuCu2Ge2 at 296(2) K with Estimated Standard Deviations in Parentheses EuCu2Ge2 (I4/mmm)

Eu3Ag2In9 (Immm)

a

label

Wyckoff site

x

y

z

occupancy

Ueqa

Eu1 Cu1 Ge1 Eu1 Eu2 In1 In2 In3 Ag

2a 4d 4e 2a 4i 2d 8l 8l 4h

0 0 0 0 0 5000 0 0 0

0 5000 0 0 0 0 3653(8) 2838(8) 1751(16)

0 2500 3784(1) 0 2994(5) 5000 3497(6) 1428(6) 5000

1 1 1 1 1 1 1 1 1

9(1) 10(1) 13(1) 14(2) 15(1) 21(3) 16(1) 16(1) 25(2)

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

temperature range of 2−300 K, temperature-dependent data were collected for the field-cooled mode (FC) in an applied field (H) of 1000 Oe. For Eu3T2In9 and EuCu2Ge2 at 2 and 300 K, magnetization data were also collected with a field sweeping from −60000 to 60000 Oe. 2.8. Electrical Resistivity. In a 1 T field, the resistivity measurements were performed on the Eu3T2In9 and EuCu2Ge2 pellets with a conventional AC four-probe setup. Strong conducting silver epoxy paste was used to create the contacts through four very thin copper wires. In the temperature range of 3−300 K, the data were collected using a commercial Quantum Design Physical Property Measurement System (QD-PPMS). Reproducible results were obtained for several batches. 2.9. X-ray Absorption Near Edge Spectroscopy (XANES). Room-temperature XANES experiments were performed on the Eu3T2In9 and EuCu2Ge2 samples at PETRA III, P06 beamline of DESY, and at the Sector 20 bending magnet beamline (PNC/XSD, 20-BM) of the Advanced Photon Source at the Argonne National Laboratory. At ambient pressure, measurements at the Eu LIII edge were performed in transmission mode. To monitor the incident and transmitted X-ray intensities, gas ionization chambers were used. A Si (111) double-crystal monochromator was used to obtain mono-

chromatic X-rays. The calibration of the Si (111) double crystal was done by defining the inflection point (first-derivative maxima) of Cu foil as 8980.5 eV. A Kirkpatrick−Baez (K−B) mirror optic was employed to focus the beam. To suppress higher-order harmonics, a rhodium-coated X-ray mirror was used. The transmitted signals were recorded by using a CCD detector. The samples were prepared by mixing an appropriate amount of finely ground powder with boron nitride and cold pressing them into a pellet.

3. RESULTS AND DISCUSSION 3.1. Reaction Chemistry. The SCs of the new compound Eu3Ag2In9 were obtained from the reaction that was initially designed to optimize the synthesis of the compounds within the Eu−Ag−Ge family. However, indium acted as the active solvent, which resulted in the formation of Eu3Ag2In9. Figure 1a shows a selected rod-shaped gray single crystal of Eu3Ag2In9 grown from the indium metal flux reaction. After refining the absolute crystal structure of Eu3Ag2In9, we have attempted the synthesis of Eu3Cu2In9 using similar synthesis strategies. However, indium in this case acted as the nonactive solvent, C

DOI: 10.1021/acs.inorgchem.6b01598 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 2. Crystal structure of Eu3Ag2In9. (a) As viewed along the c-axis. The unit cell is outlined with blue solid lines. (b) Six-membered rings made up of indium along the a−b plane and corner shared by four-membered rings with four indium atoms shown with red dotted lines. (c) Fivemembered rings made up of indium atoms shared by a six-membered ring along the b−c plane.

Figure 3. Group−subgroup scheme for the evolution of Eu3Ag2In9 from EuGa4 through the structure of EuCu2Ge2. The indices for the translationengleiche (t) transition from tetragonal EuCu2Ge2 to orthorhombic Eu3Ag2In9 and the unit cell transformations are given. The evolution of the atomic parameters is shown at the right.

synthesize Eu3Cu2In9 by a metal flux method, later we have synthesized the same by a high-frequency induction method. Selected single crystals of Eu3Ag2In9 and EuCu2Ge2 were used to collect the XRD data. Compounds Eu3Ag2In9 and EuCu2Ge2 were later synthesized by high-frequency induction heating in

which resulted in the formation of the single crystals of EuCu2Ge2. A SEM image of a SC of EuCu2Ge2 is shown in Figure 1b. Both Eu3Ag2In9 and EuCu2Ge2 are stable for several months under ambient conditions, and no decomposition was observed during this period. Although we have failed to D

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CaBa2Ge2 (tetragonal P4/mmm with lattice parameters of a = b = 4.309 Å and c = 10.792 Å). The crystal structure of RE3T2X9 is closely related to the compounds within the RETX3 and RE(TX)4 families.74 Amerioun et al. explained the structural relation between RE3T2X9 (La3Al11 type) and RETX3 (BaAl4 type).74 Here, we explain this relationship among BaAl4, ThCr2Si2, and La3Al11 structure types in detail as shown in Figure 4. The interchange

pure form confirmed by powder XRD as shown in Figures S1 and S2. Unreacted In could not be avoided in the reaction mixture. A few extra peaks corresponding to In were observed for Eu3T2In9 (marked with asterisks). 3.2. Crystal Chemistry of Eu3Ag2In9 and EuCu2Ge2. The structure of Eu3Ag2In9 along the (110) plane is shown in Figure 2a. Eu3Ag2In9 crystallizes in the body-centered orthorhombic crystal system with the La3Al11 structure type (space group Immm).48 The crystal structure of the compounds with the formula RE3T2X9 can be related to many other structure types. It was reported that preliminary powder XRD of the compounds RE(Ag,Al)4 (RE = Y, Gd, Tb, and Dy)49 and RECuAl3 (RE = Tb, Dy, Ho, Er, Tm, and Yb)50 suggests the CeNi2+xSb2−x structure type,51 which is closely related to the BaAl4 structure type. However, later Stel’makhovych et al. reported the actual formula of these compounds as Y3Ag1.5Al9.5, Dy3Ag2.3Al8.7, Dy3Cu2.6Al8.4, Ho3Ag2.1Al8.9, and Ho3Cu2.4Al8.652 using single-crystal XRD, and they crystallize in the La3Al11 type,48 which is a deficient variant of the BaAl4 structure type. A few other RE3T2X9 compounds reported in the La3Al11 type are Ho3Cu2Al9,52,53 Tb3Cu1.2Al9.8,54 Y3Cu2.5Al8.5,55 Dy3Zn3.4Al7.6,56 Er3Zn4Al7,56 Gd3Zn3.4Al7.6,56 Ho3Zn4.4Al6.6,56 La1.5Nd1.5Al11,57 La2.28Y0.72Al11,58 Tb3Zn3.6Al7.4,59 RE3AgxGa11−x (RE = Y and Gd−Yb), 6 0 Gd 3 Ag 2 . 8 Ga 8 . 2 , 6 1 Lu 3 Pd 1 . 6 G a 9 . 4 , 6 2 and Tb3Ag2.8Ga8.2.63 The crystal structure of Eu3Ag2In9 is composed of sixmembered rings that consist of In atoms (Figure 2b). Eu3Ag2In9 is a building block unit of edge-shared hexagonal indium rings and corner-shared In4 atoms along the [100] direction forming four-membered rings as shown by the red dotted lines in Figure 2b. On the other hand, In6 rings form one-dimensional chains via edge sharing, which are further interconnected to other six-membered rings forming a threedimensional stable crystal structure along the a-axis. Another layer forming five-membered rings with indium atoms shared with six-membered rings that consist of In4Ag2 atoms (Figure 2c). The crystal structure of Eu3Ag2In9 contains six crystallographic positions: two europium atoms occupying 2a and 4i Wyckoff sites (mmm and mm2 point symmetry, respectively), one silver atom occupying the 4h Wyckoff site (m2m point symmetry), and three indium atoms occupying 8l, 8l, and 2d Wyckoff sites (m.., m.., and mmm point symmetry, respectively). The comparison between tetragonal BaAl4 (I4/mmm) and orthorhombic La3Al11 (Immm) structures is reported elsewhere64 and can be explained through group−subgroup relationships in the Barnighausen formalism.65 The crystal structure of EuCu2Ge2 can be explained as originating from EuGa4 with the substitution of Cu and Ge at Ga1 and Ga2, respectively. The subcell originated from EuGa4 transformed into Eu3Ag2In9 via EuCu2Ge2 can be predicted with a lowering symmetry from the tetragonal EuCu2Ge2 (space group I4/ mmm) to the orthorhombic Eu3Ag2In9 via the translationengleiche (t2) transition (Figure 3).65−67 Three types of superstructures of BaAl4 with REM1M2 composition have been reported: (a) ThCr2Si2 type (tetragonal, I4/mmm), (b) CeAl2Ga2 type (tetragonal, I4/mmm), and (c) CaBe2Ge2 type (tetragonal, P4/mmm).68 The crystal structure of EuCu2Ge2 was previously reported only via powder XRD refinement.40,42,43,69−71 The close relation between these structure types is already found in EuZn2Ge2,72,73 which was reported for both types CeAl2Ga2 (tetragonal I4/mmm with lattice parameters of a = b = 4.348 Å and c = 10.589 Å) and

Figure 4. Structural comparison of (a) BaAl4, (b) EuCu2Ge2, (c) La3Al11, and (d) Eu3Ag2In9 compounds.

of b- and c-axes in EuCu2Ge2 (Figure 4b) followed by the tripling of the b-axis resulted in the Eu3Ag2In9 structure, which is crystallizing in the La3Al11 type in SG: Immm (Figure 4d). The crystal structure of Eu3Ag2In9 (Figure 4c) can be considered as the Ag deficient variant of EuCu2Ge2. The silver deficiency is shown with block dotted spheres (Figure 4d). The composition of EuCu2Ge2 can be represented as Eu3(CuGe)12 (=Eu3Cu6Ge6), while that of Eu3Ag2In9 can be represented as Eu3(AgIn)11, which clearly indicates the deficiency at the Ag atom. This deficiency may originated from the difference in the sizes of atoms. 3.3. Physical Properties. 3.3.1. Magnetism. Magnetic susceptibility (χm) and inverse susceptibility (1/χm) measured at an applied magnetic field of 1000 Oe within the temperature range of 2−300 K for Eu3T2In9 and EuCu2Ge2 are shown in Figure 5. The inverse magnetic susceptibility data follow Curie−Weiss law [χ = C/(T − θ)] above 150 K with effective magnetic moments of 2.01, 1.98, and 7.06 μB per Eu atom for Eu3Cu2In9, Eu3Ag2In9, and EuCu2Ge2, respectively, which indicates the presence of mixed/intermediate valent Eu E

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Figure 5. Temperature dependence of magnetic susceptibility (χm) and inverse magnetic susceptibility (1/χm) of (a) Eu3Cu2In9, (b) Eu3Ag2In9, and (c) EuCu2Ge2 measured at 1000 Oe. The magnetic ordering temperatures are marked with red arrows.

Figure 6. Magnetization as a function of applied magnetic field at 2 and 300 K for (a) Eu3Cu2In9 and (c) Eu3Ag2In9 and at 4 and 300 K for (e) EuCu2Ge2 and (b, d, and f) the corresponding compounds at low field.

atoms. The estimated experimental μeff values are approximately 25 and 24% for Eu3Cu2In9 and Eu3Ag2In9, respectively, of the usual value for a free Eu2+ ion moment (7.96 μB/Eu). In the case of EuCu2Ge2, the calculated magnetic moment of 7.06 μB/Eu atom can be assigned as 88% divalent (Eu2+) and 12% trivalent (Eu3+), which corroborates the reported literature.43 Temperature-dependent magnetic susceptibility data of Eu3Cu2In9 show antiferromagnetic transitions at 3.3 K, whereas Eu3Ag2In9 exhibits antiferromagnetic transitions at 14 K. On the other hand, EuCu2Ge2 shows ferromagnetic transitions at 12 K. The mixed/intermediate valent nature of Eu atoms was later verified by XANES and bond length analysis, which are discussed in this section (Table 3). For the ground sample of Eu3T2In9 and EuCu2Ge2, the field dependence of the magnetization M(H) curves were measured in the temperature ranges of 2−300 and 4−300 K, respectively, and are shown in Figure 6a,c,e. There is linear behavior in the magnetic data measured at 300 K up to a higher magnetic moment obtained at 0.02 emu/mol. On the other hand, there is a slight field-dependent response up to ∼10 kOe in the magnetization curve measured at 2 K. It rises slowly up to the

highest obtainable field (60 kOe) for Eu3T2In9 without any further saturation, while EuCu2Ge2 continues to rise slowly up to the highest obtainable field (60 kOe) without any fielddependent response. To confirm the ferromagnetic behavior observed at 70 and 67 K in Eu3T2In9 and 65 K in EuCu2Ge2, the field-dependent magnetization M(H) was performed at 60 and 50 K in Eu3T2In9 and 50 K in EuCu2Ge2, which clearly confirms the weak ferromagnetic behavior (Figure 6b,d,f). The magnetic susceptibility data of all the samples have a ferromagnetic ordering around 69 K, which corresponds to EuO.75 Assuming the possibility of oxidation, all the precautions were taken during the synthesis and kept the sample inside the glovebox having an argon atmosphere but also showed the magnetic ordering at the same temperature for all the samples. The magnetic measurements performed on different batches from the synthesis also could not negate the ordering at 69 K. The experimental powder XRD patterns of Eu3T2In9 (Figure S3) and EuCu2Ge2 (Figure S4) were compared with simulated patterns of EuO, EuO2, and Eu2O3 but did not produce any europium oxide peaks. However, there is the probability of a much smaller amount of europium oxide

Table 3. Selected Bond Lengths (Å) for Eu3Ag2In9 and EuCu2Ge2 at 296(2) K with Estimated Standard Deviations in Parentheses EuCu2Ge2

Eu3Ag2In9

label

distance

label

distance

label

distance

Eu−Ge Eu−Cu Cu−Cu Ge−Ge Cu−Ge

3.2394(4) 3.3373(1) 2.9853(1) 2.515(2) 2.4935(5)

Eu1In2 Eu1In3 Eu2Ag Eu2In1 Eu2In2 Eu2In3 In1Ag In2Ag

3.5020(6) 3.6078(8) 3.3528(11) 3.6931(6) 3.4934(7) 3.4283(6) 3.0497(10) 2.9062(14)

In3Ag In2In2 In2In3 In2In3 In3In1 Cu1Cu2 Cu2Cu2

3.1609(6) 2.8568(17) 2.8919(7) 3.0072(11) 3.0351(8) 2.9781(7) 2.9759(15)

F

DOI: 10.1021/acs.inorgchem.6b01598 Inorg. Chem. XXXX, XXX, XXX−XXX

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2+ or 3+ oxidation states of the Eu atoms or intermediate valent compounds with all the Eu atoms in both compounds having a noninteger valence. The shortest Eu−Eu bond is found in Eu3Ag2In9, 4.1688 Å, which is also close to the value of the mixed valent systems, but more towards the divalent Eu− Eu distances as observed in EuPd3 (4.10 Å),48 suggesting a majority are in the trivalent state. Because there are two unique europium sites (2a and 4i positions) present in the crystal structure of Eu3T2In9, we can expect that 2a europium is divalent and 4i europium is trivalent with an estimate of 33% divalent europium, which is close to the observed value of 25% divalent, from the XANES and magnetic data. To confirm this, the average bond distance around each Eu atom has been calculated and found to be 3.75 Å for Eu1 and 3.60 Å for Eu2, which suggests Eu1 is most likely divalent and Eu2 would be trivalent. It was noted that the value obtained from the magnetic measurements is slightly higher than the value of valence for Eu (2+) estimated from the XANES analysis predicting 25, 24, and 88% of Eu in the divalent state for Eu3Cu2In9, Eu3Ag2In9, and EuCu2Ge2, respectively. The uncertainty of approximately ∼5−10% in the absolute valence is probably due to the systematic errors that originate from the fitting model. 3.3.3. Electrical Resistivity. Eu3T2In9 and EuCu2Ge2 show continuous linear decreases in their electrical resistivity (ρ) with a decrease in temperature (shown in Figure 8) is typical for

at the surface of the compound, which is beyond the detection limit of XRD. 3.3.2. X-ray Absorption Near Edge Spectroscopy (XANES). To determine the actual valence state of Eu in Eu3T2In9 and EuCu2Ge2, Eu LIII edge XANES measurements were performed under ambient conditions to probe the Eu valence state in Eu3T2In9 and EuCu2Ge2. The main absorption peak of the spectrum (Figure 7) at 6973 eV for EuCu2Ge2 and a broad

Figure 7. LIII absorption edge spectra of Eu in (a) Eu3Cu2In9, (b) Eu3Ag2In9, and (c) EuCu2Ge2.

hump for Eu3T2In9 were observed in the Eu LIII X-ray absorption spectrum, which arise from the 2p3/2 to 5d transition and are features of the 4f7 (Eu2+) configuration.76 Another main absorption peak at around 6982 eV for Eu3T2In9 and a broad hump for EuCu2Ge2 were observed, which correspond to 4f7 (Eu3+).77 Lorentzian fitting was used for integrating over the respective areas, which yield 22, 20, and 80% of Eu in the divalent state for Eu3Cu2In9, Eu3Ag2In9, and EuCu2Ge2, respectively (see Figure 7a−c). Because the crystal structure of EuCu2Ge2 was reported and experimentally proven to be in the ThCr2Si2 type structure having only a Eu site, an intermediate valent picture of Eu can be proposed. This observation was further confirmed by the bond distance analysis of the single-crystal XRD data. The Eu−Eu bond distance observed along the ab plane in EuCu2Ge2 is 4.2218(2) Å, which is close to the distances observed in EuPd2Si2 (4.1800 Å) with mixed valent Eu.78 A slightly larger value close to the distances reported for purely divalent Eu intermetallics, such as Eu2AuGe3 (4.2875 Å)37 and Eu2AgGe3 (4.3384 Å), suggests the major contribution of divalent Eu atoms in EuCu2Ge2. The mixed or intermediate valence behavior in EuCu2Ge2 observed during X-ray absorption spectroscopy studies was reported previously.43 On the other hand, the compounds Eu3T2In9 can be assigned as either heterogeneous mixed valent compounds with exactly

Figure 8. Temperature dependence of the electrical resistivity (ρ) of (a) Eu3Cu2In9, (b) Eu3Ag2In9, and (c) EuCu2Ge2 with zero applied magnetic field. Insets show the corresponding fittings with the power law ρ = ρ0 + ATn in the low-temperature range.

metallic systems.79,80 The low-temperature data within the range of 3−12 K were fitted for Eu3T2In9 using the power law equation ρ = ρ0 + ATn, where ρ0 is the residual resistivity and A and n are the fitting parameters.81 The fitting yields a residual resistivity of 0.06 mΩ cm and an n value of 0.99 for Eu3Cu2In9 (Figure 8a) and a residual resistivity of 0.09 mΩ cm and an n value of 0.87 for Eu3Ag2In9 (Figure 8b). The value of ρ varies with T2 at low temperatures, which indicates non-Fermi liquid G

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Inorganic Chemistry behavior of these compounds at low temperatures.82 To confirm this, the resistivity data are plotted as ρ − ρ0 versus T2 as the inset of panels a and b of Figure 8 for Eu3T2In9. The nonlinearity curve in the data indicates low-temperature nonFermi liquid behavior in these compounds. The slight drop in EuCu2Ge2 is due to the magnetic transition that is clearly shown in the previously reported EuCu2Ge2 compound.71 Surprisingly, no magnetic ordering was observed in the resistivity data of the Eu3T2In9 compounds, as it cannot be ascribed to the Kondo effect resulting from the Eu f electrons.83,84 The presence of a dip at 11 K in the resistivity plot of EuCu2Ge2 is consistent with the previously reported EuCu2Ge2,71 and its supporting magnetic data. Similar behavior was also noticed in other reported GdPd2Si,83 Nd2AgGe385 and GdNi2Si284 compounds, where spin fluctuations at the transition metal lattices are proposed as the possible reason for increase in resistivity at low temperature. Further investigations are required to understand the actual reason for this kind of phenomenon.

JNCASR for research fellowships, respectively in Integrated PhD, PGDMS, and POCE programmes. S.C.P. thanks DST for a Ramanujan fellowship (Grant SR/S2/RJN-24/2010). We acknowledge Somnath Ghara and Bhavya for performing various measurements. Parts of this research were carried out at the light source PETRA III at DESY, a member of the Helmholtz Association (HGF). We would like to thank Dr. Gerald Falkenberg for assistance in using beamline P06. Authors thank the DST (SR/NM/Z-07/2015) for the financial support and JNCASR for managing the project for the measurements at APS beamline PNC/XSD, 20-BM at ANL, Chicago, USA, and grateful to Dr. Mahalingam Balasubramanian for the assistance.

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(1) Peter, S. C.; Subbarao, U.; Sarkar, S.; Vaitheeswaran, G.; Svane, A.; Kanatzidis, M. G. Crystal structure of Yb2CuGe6 and Yb3Cu4Ge4 and the valency of ytterbium. J. Alloys Compd. 2014, 589, 405−411. (2) Chondroudi, M.; Balasubramanian, M.; Welp, U.; Kwok, W. K.; Kanatzidis, M. G. Mixed Valency in Yb7Co4InGe12: A Novel Intermetallic Compound Stabilized in Liquid Indium. Chem. Mater. 2007, 19, 4769−4775. (3) Peter, S. C.; Subbarao, U.; Rayaprol, S.; Martin, J. B.; Balasubramanian, M.; Malliakas, C. D.; Kanatzidis, M. G. Flux Growth of Yb6.6Ir6Sn16 Having Mixed-Valent Ytterbium. Inorg. Chem. 2014, 53, 6615−6623. (4) Subbarao, U.; Sarkar, S.; Gudelli, V. K.; Kanchana, V.; Vaitheeswaran, G.; Peter, S. C. Yb5Ga2Sb6: A Mixed Valent and Narrow-Band Gap Material in the RE5M2X6 Family. Inorg. Chem. 2013, 52, 13631−13638. (5) Subbarao, U.; Peter, S. C. Single Crystal X-ray Diffraction studies on Magnetic Yb5Co4Ge10. Adv. Mater. Phys. Chem. 2013, 3, 54−59. (6) Sarkar, S.; Peter, S. C. Single crystal growth of europium and ytterbium based intermetallic compounds using metal flux technique. J. Chem. Sci. 2012, 124, 1385−1390. (7) Peter, S. C.; Sarkar, S.; Kanatzidis, M. G. Metallic Yb2AuGe3: An Ordered Superstructure in the AlB2-Type Family with Mixed-Valent Yb and a High-Temperature Phase Transition. Inorg. Chem. 2012, 51, 10793−10799. (8) Peter, S. C.; Rayaprol, S.; Francisco, M. C.; Kanatzidis, M. G. Crystal Structure and Properties of Yb5Ni4Ge10. Eur. J. Inorg. Chem. 2011, 2011, 3963−3968. (9) Peter, S. C.; Malliakas, C. D.; Kanatzidis, M. G. Structure and Unusual Magnetic Properties of YbMn0.17Si1.88. Inorg. Chem. 2013, 52, 4909−4915. (10) Peter, S. C.; Kanatzidis, M. G. ThSi2 Type Ytterbium Disilicide and its Analogues YbTxSi2‑x (T = Cr, Fe, Co). Z. Anorg. Allg. Chem. 2012, 638, 287−293. (11) Chondroudi, M.; Peter, S. C.; Malliakas, C. D.; Balasubramanian, M.; Li, Q. A.; Kanatzidis, M. G. Yb3AuGe2In3: An Ordered Variant of the YbAuIn Structure Exhibiting Mixed-Valent Yb Behavior. Inorg. Chem. 2011, 50, 1184−1193. (12) Peter, S. C.; Kanatzidis, M. G. The New Binary Intermetallic YbGe2.83. J. Solid State Chem. 2010, 183, 2077−2081. (13) Peter, S. C.; Chondroudi, M.; Malliakas, C. D.; Balasubramanian, M.; Kanatzidis, M. G. Anomalous Thermal Expansion in the Square-Net Compounds RE4TGe8 (RE = Yb, Gd; T = Cr-Ni, Ag). J. Am. Chem. Soc. 2011, 133, 13840−13843. (14) Peter, S. C.; Malliakas, C. D.; Chondroudi, M.; Schellenberg, I.; Rayaprol, S.; Hoffmann, R. D.; Pöttgen, R.; Kanatzidis, M. G. Indium Flux-Growth of Eu2 AuGe 3 : A New Germanide with an AlB 2 Superstructure. Inorg. Chem. 2010, 49, 9574−9580. (15) Peter, S. C.; Salvador, J.; Martin, J. B.; Kanatzidis, M. G. New Intermetallics YbAu2In4 and Yb2Au3In5. Inorg. Chem. 2010, 49, 10468−10474.

4. CONCLUDING REMARKS The single crystals of Eu3Ag2In9 and EuCu2Ge2 were grown through the reactions conducted in indium as the metal flux technique. Crystallographic investigations of Eu3Ag2In9 and EuCu2Ge2 were performed using single-crystal XRD. The compounds Eu3Cu2In9 and Eu3Ag2In9 are being reported for the first time, and in fact, they are the first Eu- and In-based members of the large RE3T2X9 family. The structural relationship between Eu3T2In9 and EuCu2Ge2 and parent EuGa4 has been discussed in detail, and we can conclude that Eu3T2In9 is the metal deficient variant of EuCu2Ge2. The mixed/ intermediate valent behavior for the europium atoms in the compounds Eu3T2In9 and EuCu2Ge2 has been examined via magnetic measurements and was substantiated by XANES measurements and bond length analysis, which pointed to the probable potential of these materials because of their interesting electronic and magnetic properties. Currently, we are exploring similar metal flux strategies to grow many other In-, Si-, and Ge-based compounds in the RE3T2X9 series.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b01598. Crystallographic information files (CIF) Crystallographic information files (CIF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*Phone: 080-22082998. Fax: 080-22082627. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We express gratitude to Prof. C. N. R. Rao for his continuous encouragement. We are grateful to the Department of Science and Technology (DST), Sheikh Saqr Laboratory (SSL), and the Jawaharlal Nehru Centre for Advanced Scientific Research (JNCASR) for financial support. U.S. and S.S. thank the CSIR. S.R. thanks the UGC, and S.Ch.S., V.M., and Y.K. thank H

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