2(OH)6 with Diluted Kagomé Net Containing Frustrated Corner

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Layered Cu7(TeO3)2(SO4)2(OH)6 with Diluted Kagomé Net Containing Frustrated Corner-Sharing Triangles Wen-Bin Guo,†,‡ Ying-Ying Tang,†,‡ Junfeng 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 § Wuhan National High Magnetic Field Center, Huazhong University of Science and Technology, Wuhan, Hubei 430074, People’s Republic of China S Supporting Information *

ABSTRACT: The half-spin Kagomé antiferromagnet is one of the most promising candidates for the realization of a quantum spin liquid state because of its inherent frustration and quantum fluctuations. The search for candidates for quantum spin liquids with novel spin topologies is still a challenge. Herein, we report a new diluted Kagomé lattice in Cu7(TeO3)2(SO4)2(OH)6, showing a 9/16-depleted triangle lattice, where the corner-sharing triangle units [Cu5(OH)6O8] are separated by CuO2(OH)2. Magnetic measurements show that the title compound does not exhibit long-range antiferromagnetic order down to 2 K, suggesting strong spin frustration with f > 19.

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INTRODUCTION Geometrically frustrated magnets (GFMs) are special materials in which the sublattice of magnetic ions contains triangular or tetrahedral motifs and antiferromagnetic long-range order can be suppressed upon cooling. Exotic physics phenomena hide in GFMs, including heavy Fermion behavior in LiV2O4,1a spin ice behavior in Dy2Ti2O7,1b non-Fermi liquid behavior in FeCrAs,1c and a half-magnetization plateau in CdCr2O4.1d The most fascinating magnetic ground state of GFMs with S = 1 /2 species is the long-sought magnetic state of quantum spin liquids (QSLs) in which long-range order and spontaneously broken symmetry are absent.2 QSLs can exhibit fractional excitation and topological order for potential applications in quantum information technology.3 To illustrate QSL states, great efforts have been made to synthesize half-spin Kagomé antiferromagnets, including the Cu2+-based minerals herbertsmithite ZnCu3(OH)6Cl2,4a barlowite Cu4(OH)6FBr,4b volborthite Cu 3 V 2 O 7 (OH) 2 ·2H 2 O, 4 c and vesignieite BaCu3V2O8(OH)24d and V4+-based complexes [NH4]2[C7H14N][V7O6F18]5a and [C3H5N2][V9O6F24(H2O)2].5b However, the nature of QSLs is still an open question.6 Numerical approaches suggest that a one-dimensional Kagomé strip with corner-sharing triangles separated by a bridge maintains the local symmetry of the Kagomé lattice and reflects the nature of QSLs.7 To maintain the local symmetry of the Kagomé lattice, corner-sharing triangle units seem to be essential motifs. Unfortunately, experimental candidates forming one-dimensional (1D) Kagomé strips are still rare. For chemists, the design and synthesis GFMs with unique topologies of cornersharing triangle units are urgently needed. © 2017 American Chemical Society

During the exploration of GFMs, several unique twodimensional (2D) lattices containing triangle units have been discovered, included star,8 maple leaf,9 triangular Kagomé net,10 and 1/6-depleted triangle11 lattices. For example, a star lattice was found in the complex [Fe3(μ3-O)(μ-OAc)6(H2O)3][Fe3(μ3-O)(μ-OAc)7.5]2·7H2O with large frustration,8 and a 1/6-depleted triangle lattice with alternate triangle and honeycomb strips was reported in Cu5(VO4)2(OH)4.11 This leads to the interesting question of whether corner-sharing triangles connected by two bridges can form a reticular plane that is analogous to the star lattice (Figure S1). A valid strategy for designing and creating new topologies is deleting sites in an orderly manner based on a simple lattice. For example, the maple leaf lattice is a 1/7-depleted triangle lattice,9 and the triangular Kagomé net is a 7/16-depleted triangle lattice.10 Additionally, Kagomé and honeycomb lattices are considered to be 1/4-depleted and 1/3-depleted triangle lattices, respectively.12 If one-half of the corner-sharing triangles are removed from a standard Kagomé lattice (Scheme 1), a novel reticular lattice containing a tetradecagon and corner-sharing triangles will be fabricated. This structure can be considered as a diluted Kagomé lattice because the corner-sharing triangle units are separated by two bridges. Considering that the Kagomé lattice is a 1/4-depleted triangle lattice, this lattice can be accurately considered as a 9/16-depleted triangle lattice (9/16-DTL). In a 9/16-DTL, the topological network containing three types of Received: September 14, 2016 Published: January 30, 2017 1830

DOI: 10.1021/acs.inorgchem.6b02209 Inorg. Chem. 2017, 56, 1830−1834

Article

Inorganic Chemistry

measurements. Data collection was performed with a Rigaku Mercury CCD diffractometer with graphite-monochromated Mo Kα radiation (λ = 0.71073 Å) at 293 K. The data sets were corrected for Lorentz and polarization factors and for absorption by the Multiscan method.16 The structure was solved by direct methods and refined by full-matrix least-squares fitting on F2 using SHELX-97.17 All non-hydrogen atoms were refined with anisotropic thermal parameters. S1, O8, O9, and O10 each split into two sites as a result of position disorder. H1, H2, and H3 were refined with isotropic thermal parameters. The final refined structural parameters were checked by the PLATON program.18 Crystallographic data, selected bond lengths, and bond valence sum (BVS) calculations for 1 are summarized in Tables S1 and S2. The quality of the powdered samples was confirmed by powder XRD studies (Figure S3). Magnetic Measurements. Magnetic measurements were performed using a commercial Quantum Design Physical Property Measurement System (PPMS). A powdered sample of 1 (10.1 mg) was placed in a gel capsule sample holder that was suspended in a plastic drinking straw. The dc magnetic susceptibility was measured at 0.1 T from 300 to 2 K, and the low-temperature susceptibility was measured at 0.1 T from 2 to 150 K (at a heating rate of 5 K min−1) with the zero-field-cooling (ZFC) and field-cooling (FC) regimes using a commercial Quantum Design MPMS-XL superconducting quantum interference device (SQUID) magnetometer. Magnetization was measured at 2 K in applied fields ranging from 0 to 8 T (at 0.1 T/ step), and ac magnetic susceptibilities were measured at an amplitude of 3 Oe with different frequencies from 500 to 5000 Hz using the Quantum Design PPMS. Heat capacity was measured at zero field by a relaxation method using a pellet sample (2.4 × 2.6 mm2, ∼16.3 mg). Diamagnetic corrections were estimated using Pascal constants and background correction obtained by experimental measurements on sample holders. High-field magnetization was measured using a coaxial pickup coil by an induction method in pulsed magnetic fields. During the pulse shot, dM/dt signals from the sample were collected using a high-speed data acquisition card and integrated as a function of magnetic field. The curve was subsequently calibrated by comparison with the low-field data using a 7-T SQUID magnetometer. This induction method has a high sensitivity to magnetic transitions because of the high sweeping rate of the pulsed fields. In this case, we employed a pulsed magnet with a very short duration of ∼7 ms, which improved the field-sweeping rate to near 20kT per second.

Scheme 1. Topological Evolution between Kagomé and Diluted Kagomé Lattices, Including a Total of 16 Sites, Where the Blue Corner-Sharing Triangles Indicate the Removed Sites

sites is represented by the notation (3,14,3,14)(14,14)(3,14,14) from Grünbaum and Shephard.13 Based on a persistent exploration of the tellurite sulfate system, we have successfully constructed and synthesized some interesting lattices related to the Kagomé network, including a 1D Kagomé strip14a and a 2D striped Kagomé net.14b Herein, the distorted 9/16-DTL was synthesized successfully in another layered tellurite sulfate, namely, Cu7(OH)6(TeO3)2(SO4)2 (1), which exhibits paramagnetic behavior down to 2 K as a result of strong spin frustration.

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EXPERIMENTAL SECTION

Synthesis. In contrast to the previously used experimental conditions,15 high-quality crystals of 1 (Figure 1a) were prepared in

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RESULTS AND DISCUSSION Structural Description. The title compound features distorted {Cu7(OH)6O10}∞ layers built by edge- or cornersharing Cu(OH)2O2 squares, which are further separated by TeO32− and SO42− anionic groups, as shown in Figure S4. The shortest interlayer spacing is 3.85(4) Å. Each asymmetrical unit contains five Cu atoms, one Te atom, one S atom, and 10 O atoms, of which O1, O2, and O3 are in OH− groups according to bond valence calculations (Table S2). As shown in Figure S5, TeO3 adopts a trigonal-pyramidal geometry with 5s2 lonepair electrons and sulfur atoms form the distorted tetrahedra. Each Cu atom forms a distorted square geometry of the form CuO2(OH)2 with an average Cu−O distance of 1.970(1)Å. As shown in Figure 1b, Cu2O2(OH)2 and two Cu1O2(OH)2 groups constitute an edge-sharing (O2, O6) trimer of Cu3(OH)4O4 with a Cu1···Cu2 distance of 2.92(6) Å, whereas Cu5 also shares edge (O3, O5) with two nearest-neighbor Cu4 atoms, forming a linear trimer of Cu3(OH)4O4 with Cu4···Cu5 distances of 2.86(1) Å. The trimeric ribbons share corners through O2, forming trimeric chains of {Cu3(OH)4O4}∞, in which the corner-sharing Cu5(OH)6O8 triangle units are bridged by Cu5(OH)2O2 squares (Figure 1b). It is noted that the Cu···Cu distances in the μ3-O2H Cu3 triangles are 2.92(6) Å (Cu2···Cu1), 3.09(7) Å (Cu2···Cu4), and 3.45(4) Å (Cu4···Cu1), and the Cu−O2−Cu angles are 94.734(1)°

Figure 1. (a) Image of as-grown single crystals of 1, (b) trimeric chain with a corner-sharing-triangles unit of Cu5(OH)6O8, (c) distorted layer of {Cu7(OH)6O10}∞ on the {3̅, 4̅, 4} plane, and (d) corresponding topology. The Cu5(OH)6O8 unit, Cu3, and Cu5 are depicted in bright green, green, and blue, respectively, as guides to the eye. this work by a conventional hydrothermal method. A mixture of 1.5 mmol of CuSO4·5H2O (3 N, 0.3745 g), 0.3 mmol of K2TeO3 (3 N, 0.0761 g), and 10 mL of deionized water was sealed in an autoclave equipped with a Teflon liner (28 mL). The autoclave was placed into a furnace, which was heated at 210 °C for 5 days under autogenous pressure and then cooled to room temperature at a rate of ∼3 °C/h for 2 days. Dark green bulk crystals were obtained, and energydispersive X-ray spectroscopy analysis of 1 (Figure S2) yielded an average value for the Cu/Te/S ratio of about 6.93(4):1.95(2):2.02(3). X-ray Crystallographic Studies. A single crystal of 1 was selected and mounted on glassy fibers for single-crystal X-ray diffraction (XRD) 1831

DOI: 10.1021/acs.inorgchem.6b02209 Inorg. Chem. 2017, 56, 1830−1834

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Inorganic Chemistry

= |θ|/TN,20 where TN is the Néel temperature, or the magnetic ordering temperature, the value of f is at least 19 [θ = −38.8(9) K, TN ≤ 2 K], suggesting strong geometric frustration in 1. The frustration might originate from competing magnetic interactions in corner-sharing triangles within the distorted layers. The super−super exchange interactions through the TeO3 or SO4 groups between the neighboring layers might be much weaker than the intralayer superexchange interactions. Such weak interlayer interactions and competing magnetic interactions within the layers could lead to the absence of antiferromagnetic long−range ordering. It is well-known that, when the same spins occupy an isolated triangle with the same antiferromagnetic interaction energies, strong frustration occurs.21 The number of degenerate ground states increases with increasing number of triangles. For example, in a corner-sharing triangle with the uniform antiferromagnetic interaction of Ising spins, there are five frustrated states (Figure S8) and 18 spin arrangements, which are much greater than the corresponding numbers in a triangle (Figure 3). In these spin states, one or three net magnetic

(Cu2−O2−Cu1), 103.557(2)° (Cu4−O2−Cu2), and 120.112(2)° (Cu4−O2−Cu1). Further, the trimeric chains are bridged by Cu3O2(OH)2 squares through the dangling Cu−(O1H) bonds at the apex, forming a distorted layer of {Cu7(OH)6O10}∞ with the large Cu14 ring (Figure 1c). The Cu1···Cu3 distance is 3.189(1)Å, and the Cu3−O1−Cu1 angle is 113.718(2)°. In each Cu14 ring, two TeO32− groups and four SO42− groups share corners with CuO2(OH)2, which enhances the stability of the skeleton (Figure S6). The Cu2, Cu3, and Cu5 atoms act as inversion centers in the distorted layers, corresponding to the special Wyckoff site (−1). In the topological geometry of {Cu7(OH)6O10}∞ (Figure 1d), the corner-sharing triangles are separated by Cu3 (green sphere) and Cu5 (blue sphere). Each tetradecagon is surrounded by four corner-sharing triangles, and four tetradecagons encircle one corner-sharing triangle. In this distorted 9/16-DTL, Cu3 and Cu5 sit on the (14,14) site, whereas Cu1 and Cu4 sit on the (3,14,14) site , leaving Cu2 at the (3,14,3,14) site. Compared with the symmetry of the perfect Kagomé lattice, the symmetry of the distorted 9/16-DTL is broken owing to the deleted cornersharing triangles. However, the local symmetry remains, with Cu2, Cu3, and Cu5 as inversion centers. Magnetic Properties. The magnetic susceptibility was found to increase continuously with decreasing temperature until 2 K, as shown in Figure 2a. Typical Curie−Weiss behavior

Figure 2. (a) Magnetic susceptibility and corresponding reciprocal and (b) specific heat data C/T obtained in zero field. Inset: Specific heat as a function of temperature from 2 to 300 K. Figure 3. Spin arrangements in (a) an isolated equilateral triangle5 and (b) a corner-sharing triangle with antiferromagnetic interaction of Ising spins. Red lines denote parallel spins.

was well fitted above 70 K, giving a Weiss temperature of θ = −38.8(9) K and an effective magnetic moment of μeff = 1.83(4) μB/Cu2+. The effective magnetic moment was calculated to be close to the theoretical value of 1.732 μB/Cu2+ (S = 1/2, g = 2), and the negative Weiss temperature suggests a strong antiferromagnetic correlation between the neighboring Cu2+ ions. Moreover, neither a frequency dependence of the ac magnetic susceptibilities nor a magnetic irreversibility between the zero-field-cooling and field-cooling regimes was observed in 1, ruling out the spin-glass and spin-canted transitions (Figure S7). Notably, the observed Weiss temperature is almost equal to the value of −38.52 K for [C3H5N2][V9O6F24(H2O)2] with Kagomé-like slabs.5b As shown in Figure 2b, no sharp peak was observed in either specific heat as a function of temperature or specific heat divided by temperature (CT−1) as a function of temperature. Obviously, 1 does not undergo a transition to long-range magnetic order above 2 K. A similar upturn in CT−1 was also observed for LiZn2Mo3O8, where 2/3 spins form exotic condensed valence-bond ground states and 1/3 spins remain in the paramagnetic state.19 According to the frustration index, f

moments are present in a corner-sharing triangle (Figure 3b). In the title compound, it seems to be difficult to compare the strength of the superexchange interactions in the distorted triangle of μ3-O2H, based on the simplest interpretation within the framework of Goodenough−Kanamori−Anderson rules.22 To explain the unanticipated frustration in 1, we assumed strong magnetic interactions within Cu5(OH)6O8. Under this assumption, the unsteady spins in frustrated corner-sharing triangles could lead to a plateau-like magnetization under a high field. To demonstrate this hypothesis, isothermal magnetization was measured in applied fields of up to 30 T. Figure 4 shows the isothermal magnetization as a function of applied field at 1.7 K. The magnetization initially rose rapidly below 10 T and then increased linearly to 3.328 μB/mol at 30 T. After subtraction of the linear component (blue dotted line), the magnetization moment remained constant at 1.946 μB/mol (bright green curve). This value is nearly equal to two Cu2+ magnetic moments, suggesting a possible 2/7 saturated 1832

DOI: 10.1021/acs.inorgchem.6b02209 Inorg. Chem. 2017, 56, 1830−1834

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Inorganic Chemistry

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X-ray crystallographic file for compound Cu7(OH)6(TeO3)2(SO4)2 (CIF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Wen-Bin Guo: 0000-0002-2467-0952 Author Contributions ‡

W.-b.G. and Y.-y.T. contributed equally.

Figure 4. High-field magnetization curve (black circle) of 1. The bright green curve was obtained from experimental data by subtracting the linear components (blue dotted line), which can be roughly fitted by a Brillouin function (red dotted line).

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (NSFC) (Nos. 21573235, 21403234, 11574098), the Joint Fund of Research Utilizing Large-scale Scientific Facilities under a cooperative agreement between NSFC and CAS (No. 1632159), the Chinese Academy of Sciences (CAS) under Grant KJZD-EW-M05, and the Opening Project of Wuhan National High Magnetic Field Center (2015KF08).

magnetization per formula. Moreover, the experimental magnetization increased somewhat more slowly than expected from the Brillouin function (dashed red curve), also suggesting weak antiferromagnetic interactions between neighboring Cu5(OH)6O8 units. Considering the ratio of types of Cu atoms (Cu1/Cu2/Cu3/Cu4/Cu5 = 2:1:1:2:1), the unfrozen moments might come partially from unsteady spins in cornersharing triangles and partially from Cu3 between corner-sharing triangles because of the poor Cu1−O1H−Cu3 orbital overlap. The linear magnetization behavior under high field originates from Van Vleck paramagnetism23a or a gapless spin liquid ground state.23b On the basis of the high-field magnetization, compound 1 also shows paramagnetic behavior, which coincides with the heat capacity.

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CONCLUSIONS In summary, a novel 9/16-DTL was fabricated by tailoring corner-sharing triangles in a Kagomé lattice, realized in the layered tellurite sulfate Cu7(OH)6(TeO3)2(SO4)2. Strong geometric frustration in corner-sharing Cu5(OH)6O8 triangles is crucial to the suppression of the magnetic ordering in 1. As a candidate QSL, the title compound shows two characteristics: First, long-range order above 2 K is absent because of the unexpectedly strong frustration, and second, the relevance between the Kagomé lattice and the sublattice of copper atoms in 1 is close. Our work not only illustrates the importance of corner-sharing triangle units but also opens an avenue for tailormade topologies in the family of frustrated lattices.

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REFERENCES

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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.6b02209. Star lattice and reticular topologies; energy-dispersive Xray spectrum of 1; calculated and experimental powder XRD patterns of 1; arrangements of layers in 1; coordination geometries of S, Te, and Cu atoms; connection of SO42− and TeO32− groups in the distorted layers; ac magnetic susceptibilities from 500 to 5000 Hz, low-temperature susceptibility with zero-field-cooling and field-cooling regimes below 0.1 T, and (c) magnetization at 2 K with fields ranging from 0 to 8 T; and five possible frustrated arrangements in cornersharing triangles with antiferromagnetic interactions (PDF) 1833

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