Synthesis, Structure, and Magnetic Properties of A2Cu5(TeO3)(SO4)3

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Synthesis, Structure, and Magnetic Properties of A2Cu5(TeO3)(SO4)3(OH)4 (A = Na, K): The First Compounds with a 1D Kagomé Strip Lattice Yingying Tang,†,‡ Wenbin Guo,† Hongping Xiang,† Suyun Zhang,† Ming Yang,† Meiyan Cui,†,‡ Nannan Wang,† and Zhangzhen He*,† †

State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, People’s Republic of China ‡ University of the Chinese Academy of Sciences, Beijing 100039, People’s Republic of China S Supporting Information *

ABSTRACT: Two new tellurite−sulfates A 2 Cu 5 (TeO 3 )(SO4)3(OH)4 (A = Na, K) have been synthesized by a conventional hydrothermal method. Both compounds feature 1D kagomé strip structure built by distorted CuO6 octahedra, which can be regarded as the dimensional reduction of kagomé lattice. Magnetic measurements confirmed that the titled compounds possess antiferromagnetic ordering at low temperature, while a field-induced magnetic transition can be observed at critical field. To the best of our knowledge, this is the first time to obtain distorted kagomé strip compounds.

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INTRODUCTION Transition-metal-based oxides with a two-dimensional (2D) kagomé lattice have attracted much attention in the fields of material science and condensed matter physics due to their exotic magnetic phenomena. Since kagomé lattice features a unique planar array of corner-sharing triangles constructed by magnetic ions (Figure 1a), strong geometrical frustration

kagomé structure, which cannot show a long-range magnetic ordering at low temperature due to a quite strong geometrical frustration. On the other hand, most of kagomé compounds are found to exhibit a distorted structure, which usually show a long-range ordering at low temperature and do not indicate a remarkable geometrical frustration. In some cases, such distorted kagomé compounds are also found to exhibit unusual magnetic phenomena.6−11 A typical example is Ni3V2O8 with a staircase kagomé structure, which is confirmed to show exotic magnetic behaviors such as multiple magnetic orderings at low temperature,12,13 field-induced successive magnetic transitions,14,15 and magnetically driven ferroelectric polarization.16,17 Generally, kagomé compounds give an interesting issue for the correlation of structural features and magnetic properties. To further identify the correlation of the structural features and magnetic properties of kagomé compounds, some theorists have tried to use a simplified 1D kagomé strip model (Figure 1) for understanding the magnetic nature of the 2D kagomé lattice.18−21 In this model, the kagomé strip can be considered as a dimensional reduction of kagomé lattice, which further retains the local symmetry of the kagomé lattice. As a result, the magnetic ground states with a spin-gapped or a gapless lowlying spin excitation can be suggested based on the value of J⊥/ J∥ for the S = 1/2 system (J⊥ and J∥ can be equal or unequal). Such dimension-reduced method should make a complicated model simple. Unfortunately, the reliability of the simplified kagomé strip model has not been confirmed experimentally,

Figure 1. Correlation of (a) 2D kagome lattice and (b) 1D kagome strip with two interaction exchanges of J∥ and J⊥ in an ideal model.

usually happens when competing antiferromagnetic interactions cannot be satisfied simultaneously. A perfect kagomé lattice is thus considered as one of the most promising candidates for the study of magnetic frustration. The early interest in kagomé compounds may be traced back to the search for the jarosite KFe3(SO4)2(OH)6 and its analogues.1 Current interest in kagomé compounds is however focused on the spin-1/2 system that may be expected for experimental realization of the quantum ground state responsible for the resonating valence bond model.2 Herbertsmithite ZnCu3(OH)6Cl2,3 α-vesignieite BaCu3V2O8(OH)2,4 and [NH4]2[C7H14N][V7O6F18]5 are all representative compounds with a S = 1/2 nearly regular © XXXX American Chemical Society

Received: August 20, 2015

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DOI: 10.1021/acs.inorgchem.5b01889 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

2 (19.66 mg) were placed in a gel capsule sample holder which was suspended in a plastic drinking straw. Magnetic susceptibility was measured at 0.1 T from 300 to 2 K (temperature scan of 5 K/min). Low-temperature magnetization was measured at different magnetic fields from 2 to 10 K (temperature scan of 5 K/min). Isothermal magnetization was measured at 2 K in applied field from 0 to 8 T (field scan of 0.1 T/step). Diamagnetic corrections were estimated by Pascal constant, and background correction was measured on the sample holder. Thermal Analysis. Thermogravimetric analysis (TGA) of 1 and 2 was performed in the NETZSCH STA 449C instruments in a nitrogen atmosphere at a heating rate of 10 °C/min. The samples were placed in Al2O3 crucibles and heated from room temperature to 1200 °C.

because transition-metal-based compounds still have not been found to exhibit a 1D kagomé strip lattice so far. Recently, we successfully synthesized two new tellurite− sulfates A2Cu5(TeO3)(SO4)3(OH)4 [A = Na (1) and K (2)] by a conventional hydrothermal reaction. The topological structure of Cu2+ ions is found to exhibit a 1D distorted kagomé strip lattice. To the best of our knowledge, to date this is the first time to obtain kagomé strip compounds. Here we report on the detailed structural features and magnetic properties of two compounds.

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EXPERIMENTAL SECTION

Preparation. Single crystals of 1 and 2 were synthesized by a conventional hydrothermal method. A mixture of 1.5 mmol of CuSO4· 5H2O (3 N, 0.3745 g), 0.5 mmol of TeO2 (3 N, 0.0798 g), 2 mmol of NaOH (3 N, 0.08 g) or KOH (3 N, 0.1122 g), and 1 mL of deionized water was sealed in an autoclave equipped with a Teflon liner (28 mL). The autoclaves were put into a furnace which was heated at 190 °C for 5 days under autogenous pressure and then cooled to room temperature at a rate of ∼3 °C/h for 2 days. Some green bulk crystals were obtained and further dried at 60 °C for 2 h. The powdered samples were prepared by crushing single crystals, which were used for various physical measurements. The purity of powered samples was confirmed by powder X-ray diffraction (Figure S1). X-ray Crystallographic Studies. Data collection was performed on a Rigaku Mercury CCD diffractometer equipped with graphitemonochromated Mo Kα radiation (λ = 0.71073 Å) at 293 K. The data sets were corrected for Lorentz and polarization factors as well as for absorption by the multiscan method.22 The structure was solved by direct methods and refined by full-matrix least-squares fitting on F2 by SHELX-97.23 All non-hydrogen atoms were refined with anisotropic thermal parameters. If this constraint is not applied, its displacement parameters will be abnormally small. The final refined structural parameters were checked by the PLATON program.24 Crystallographic data and structural refinements are summarized in Table 1. The final refined atomic positions and structural parameters are seen in the Supporting Information (Tables S1−S6). Magnetic Measurements. Magnetic measurements were performed using a commercial Quantum Design Physical Property Measurement System (PPMS). Powdered samples of 1 (8.28 mg) and

RESULTS AND DISCUSSION Structural Analysis. 1 and 2 crystallize in the monoclinic system with a space group of P21/m. Since they are isostructural, 1 is selected as a representative for a description of structural feature. There are three crystallographic Cu, one Te, two S, and two Na sites in an asymmetrical unit (Figure S2, Supporting Information). All of the Cu atoms form distorted CuO6 octahedra with bond lengths ranging from 1.85 to 2.52 Å. Te atoms are coordinated by three oxygen atoms, forming TeO3 trigonal pyramids with bond lengths around 1.90 Å, while S atoms form SO4 tetrahedra with bond lengths ranging from 1.46 to 1.49 Å. As shown in Figure 2a, the complicated 3D

Table 1. Crystal Data and Structure Refinements for A2Cu5(TeO3)(SO4)3(OH)4 (A = Na, K) formula fw T (K) λ (Å) space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3) Z Dcalcd (g cm−3) μ (cm−1) GOF on F2 R1, wR2 [I > 2σ(I)]a R1, wR2 (all data)

Na2Cu5(TeO3) (SO4)3(OH)4 895.52 room temp 0.71073 P21/m 7.294(3) 12.005(4) 9.214(3) 90 111.160(6) 90 752.4(5) 2 3.935 94.38 1.093 0.0204, 0.0545

K2Cu5(TeO3) (SO4)3(OH)4 927.77 room temp 0.71073 P21/m 7.467(6) 12.177(9) 9.397(4) 90 111.352(8) 90 795.9(2) 2 3.854 93.91 1.073 0.0354, 0.0793

0.0221, 0.0555

0.0432, 0.0852

Figure 2. (a) Three-dimensional framework of Na2Cu5(TeO3)(SO4)3(OH)4 showing a layered structure with (b) linkage of CuO6 octahedra, TeO3 trigonal pyramids, and SO4 tetrahedra, in which (c) a distorted spin−lattice built by magnetic Cu2+ ions is seen.

framework with a layered structure is constructed by {Cu5TeO3SO4OH}∞ units, while the layers are separated by Na + ions. As shown in Figure 2b, two neighboring {Cu5TeO3SO4OH}∞ units connect to each other via two S1O4 tetrahedra along the c axis, which can also be considered that the unit is terminated by S1O42− groups. Moreover, the units form a unique network with six rings running along the b axis, in which CuO6 octahedra connect to each other via edge sharing, while TeO3 trigonal pyramids are located at the centers of the six rings with up−down−up−down manner, and the isolated S2O4 tetrahedra situate at opposite sides of the TeO3 groups. Such network contains linear CuO6 chains with Cu1− Cu1−Cu2−Cu2 manner along the b axis, in which Cu1 atoms connect to each other through edge sharing (O2, O9) and connect to Cu2 atoms through edge sharing (O3, O8), while

R1 = ∑||F0| − |Fc||/∑|F0|, wR2 = {∑w[(F0)2 − (Fc)2]2/ ∑w[(F0)2]2}1/2.

a

B

DOI: 10.1021/acs.inorgchem.5b01889 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 3. (a) Temperature dependence of magnetic susceptibility (χ) and corresponding reciprocal one (χ−1) of A2Cu5(TeO3)(SO4)3(OH)4 (A = Na, K) and (b) the isothermal magnetization as a function of magnetic field at 2 K.

Figure 4. Low-temperature magnetization for (a) Na2Cu5(TeO3)(SO4)3(OH)4 and (b) K2Cu5(TeO3)(SO4)3(OH)4 at different magnetic fields.

Cu distance along the a axis increases obviously upon substituting Na+ for larger K+. Magnetic Properties. Figure 3a shows the temperature dependence of magnetic susceptibility (χ) and corresponding reciprocal one (χ−1) of 1 and 2. The susceptibilities increase with decreasing temperature, while a sharp peak is observed at ∼2.4 K for 1 and ∼5.2 K for 2 (the insets of Figure 3a), indicating the onset of a long-range antiferromagnetic ordering. A typical Curie−Weiss behavior is observed above 50 K, giving the Curie constant C = 2.88(2) emu·K·mol−1 and Weiss temperature θ = −6.1(8) K for 1 and C = 2.91(8) emu·K·mol−1 and θ = −13.9(4) K for 2. The effective magnetic moment of Cu2+ is calculated to be 2.14(7) μB for 1 and 2.16(1) μB for 2, which is a little larger than the theoretical spin value of 1.732 μB for Cu2+ (S = 1/2, g = 2). This indicates a large orbital moment contribution of magnetic ions and magnetic anisotropy in such oxygen-coordinated environment. Similar features are also seen in Cu5V2O10.25 To clarify the nature of the magnetic behaviors of 1 and 2, the isothermal magnetization as a function of magnetic field is shown in Figure 3b. For 1, the magnetization increases rapidly

Cu2 atoms connect to each other through edge sharing (O1, O5). Cu3 atoms are located between linear chains, showing Cu triangles composed of Cu1, Cu2, and Cu3, in which Cu3 atoms connect to Cu1 atoms through edge sharing (O3, O4) and connect to Cu2 atoms through edge sharing (O3, O11). Since Na+, (OH)−, TeO32−, and SO42− groups are nonmagnetic, the spin−lattice in 1 is determined by the topological arrangement of magnetic Cu2+ ions. Removing Na+, (OH)−, TeO32−, and SO42− groups from the structure of 1, magnetic Cu2+ ions are found to form a kagomé strip lattice in Figure 2c, which is similar to that mentioned in Figure 1b. It must be noted that the kagomé strip lattice of 1 is quite distorted, showing five different Cu···Cu distances of 2.84, 2.94, 3.01, 3.07, and 3.08 Å. The nearest Cu···Cu distance between kagomé strips is 4.34 Å, while the shortest distance between the layers is 6.36 Å. A comparison of the Cu···Cu distances in A2Cu5(TeO3)(SO4)3(OH)4 (A = Na, K) is summarized in Table S7. It is noted that the Cu···Cu distances in 2 are larger than those in 1. This may be due to the fact that the distinct distance between kagomé strips is intuitively dictated by the orientation of S1O4 tetrahedra serving as bridging units, while the interlayer Cu··· C

DOI: 10.1021/acs.inorgchem.5b01889 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry at low magnetic field, evidencing the appearance of ferromagnetic moments, and further increases linearly to ∼0.5 μB at 8 T. A clear kink at the critical field of H = 0.15 T is seen in the dM/dH curve, showing a field-induced magnetic transition. For 2, the magnetization increases linearly with increasing field, while a rapid jump is seen at H = ∼4 T, also indicating a field-induced magnetic transition. Magnetic moment also reaches ∼0.5 μB at 8 T. In both cases, the magnetization increases with increasing field even above the critical field, indicating a spin-flop-like transition, in which the antiferromagnetic spin direction becomes vertical to the field direction at the critical field and then the angle of antiferromagnetic spins (180°) gradually closes toward ferromagnetic spin alignment. Meanwhile, the low-temperature magnetization was measured at different magnetic fields. For 1, a sharp peak in the susceptibility is observed at around 2.4 K under low applied magnetic fields of ≤0.05 T and completely disappeared while a magnetic field of ≥0.5 T is applied (Figure 4a), showing field-induced magnetic transition. For 2, a peak is observed at applied fields of ≤4 T and disappeared at H ≥ 5 T (Figure 4b), also indicating field-induced magnetic transition. Therefore, it may be suggested that compound 1 is a canted antiferromagnet together with a spin-flop-like transition due to the existence of ferromagnetic moments, while compound 2 is an antiferromagnet with a spin-flop-like transition. The different Néel temperatures of magnetic orderings and different critical fields of spin-flop-like transitions may arise from different magnitudes of magnetic interactions between Cu2+ ions in a kagomé strip lattice due to different radii of Na+ and K+. As shown in Figure 2c, there are five main magnetic exchange interactions numbered as J1, J2, J3, J4, and J5 in a distorted kagomé strip lattice. It is well known that the Hamiltonian for exchange interaction between any two spins (S1 and S2) can be expressed as H = −2JS1·S2, where the magnitude and the sign of the exchange constant J are determined by the bonding geometry according to the Goodenough rules.26 It has been suggested that an empirical measure for spin frustration by defining the value of f = |θCW|/TN, where θCW is the Weiss temperature and TN is the Néel temperature. The value of f > 10 indicates a strong frustration effect in magnetic systems.27 We note the f value of ∼2.54 (θ = −6.1(8) K; TN = 2.4 K) and ∼2.67 (θ = −13.9(4) K; TN = 5.2 K) for 1 and 2, respectively, ruling out spin frustration in the systems. According to the simplest possible interpretation in the framework of Goodenough−Kanamori−Anderson rules, J1 may be ferromagnetic because the corresponding Cu···Cu distance of 2.84 Å is the shortest (Figure 2c), which requires a 90°-like superexchange geometry (87.3° in 1 and 87.9° in 2). Further, due to the negative Weiss temperatures obtained from the magnetic susceptibilities, it is reasonable to suggest that dominant antiferromagnetic interactions between Cu atoms are antiferromagnetic in the systems. Therefore, the mixture of antiferromagnetic and ferromagnetic Cu−Cu couplings may exist in both compounds, leading to quite small Weiss temperatures. Thermal Analysis. To confirm the stability of two compounds, the thermal behaviors are shown in Figure 5. Both compounds are found to show two main steps of weight loss, indicating 1 and 2 are stable up to ∼400 °C. For 1, the remarkable loss of the weight starts at ∼400 °C, while a plateau is seen in the temperature range of 500−700 °C, corresponding to release of 2 mol of H2O from the dehydration of OH− groups. The observed weight loss is 4.34%, close to the

Figure 5. Thermogravimetric curves for A2Cu5(TeO3)(SO4)3(OH)4 (A = Na, K).

theoretical one, 4.02%. Further, a rapid drop of weight loss above 700 °C indicates the further decomposition of these compounds. For 2, the loss of the weight also occurs at ∼400 °C, while a plateau is seen in the temperature range of 450− 650 °C, corresponding to the release of 2 mol of H2O from dehydration of OH− groups. The observed weight loss is 3.89%, close to the theoretical one, 3.90%. The weight loss above 650 °C indicates further decomposition of this compound. The total weight loss reaches ∼45.6% and ∼44.1% for 1 and 2, respectively.

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CONCLUSIONS Two new tellurite−sulfates A2Cu5(TeO3)(SO4)3(OH)4 (A = Na, K) have been synthesized by a conventional hydrothermal method. Both compounds are isostructural, which crystallize in the monoclinic system with a space group of P21/m, featuring a distorted 1D kagomé strip structure built by CuO6 octahedra. Magnetic measurements confirmed that the titled compounds possess an antiferromagnetic ordering at low temperature, while a field-induced spin-flop-like transition can be observed at critical field. We envisage that the first preparation of new 1D kagomé strip compounds will stimulate further theoretical and experimental studies of such unique spin−lattice related to the kagomé structure.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.5b01889. Selected bonds and angles, additional structural plots, PXRD (PDF) (CIF) (CIF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] or [email protected]. Notes

The authors declare no competing financial interest. D

DOI: 10.1021/acs.inorgchem.5b01889 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

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(25) He, Z. Z.; Lin, C. S.; Cheng, W. D.; Okazawa, A.; Kojima, N.; Yamaura, J.; Ueda, Y. J. Am. Chem. Soc. 2011, 133, 1298−1300. (26) Goodenough, J. B. Magnetism and the Chemical Bond; John Wiley and Sons: New York, 1963. (27) Schiffer, P.; Ramirez, A. P. Condens. Matter Phys. 1996, 10, 21.

ACKNOWLEDGMENTS This work was financially supported by the National Basic Research Program of China (No. 2012CB921701), National Natural Science Foundation of China (No. 21573235; 21403234), Fund of the Wuhan National High Magnetic Field Center (PHMFF2015006), and Chinese Academy of Sciences under Grant No. KJZD-EW-M05.

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DOI: 10.1021/acs.inorgchem.5b01889 Inorg. Chem. XXXX, XXX, XXX−XXX