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A new borophosphate Co3(BPO4)2(PO4)(OH)3 was synthesized by a conventional hydrothermal method. The titled compound crystallizes in the monoclinic ...
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A New One-Dimensional Spin Chain System Co3(BPO4)2(PO4)(OH)3 Showing 1/3 Magnetization Plateau Nannan Wang,† Zhangzhen He,*,† Meiyan Cui,†,§ and Yingying Tang† †

State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002, P. R. China § Graduate School of the Chinese Academy of Sciences, Beijing, 100039, P. R. China S Supporting Information *

ABSTRACT: A new borophosphate Co3(BPO4)2(PO4)(OH)3 was synthesized by a conventional hydrothermal method. The titled compound crystallizes in the monoclinic system with space group Cc, which exhibits a typical spin-chain structure. In the structural framework, Co2+ ions form a zigzag chain via edge-sharing oxygen atoms, and further the zigzag Co-chains are separated by linear [B2P2O8]∞ chains. Magnetic measurements confirm that Co3(BPO4)2(PO4)(OH)3 possesses an antiferromagnetic ordering at TN = 12 K, while a 1/3 plateau is observed in the magnetization curve at 2 K. The possible spin arrangements for antiferromagnetic, ferrimagnetic, and ferromagnetic states are also suggested.

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MBPO4(OH)2 (M = Mn, Fe, Co).20 To explore new transition metal based borophosphates and to further investigate interesting magnetic phenomena, recently we have reinvestigated the CoO-B2O3-P2O5 system and synthesized successfully a new borophosphate Co3(BPO4)2(PO4)(OH)3 using a conventional hydrothermal method. This compound shows a unique chain structure built by magnetic Co2+ ions and fourmembered borophosphate rings. Magnetic measurements confirm that the system exhibits an antiferromagnetic ordering at 12 K, while a 1/3 magnetization plateau with a large hysteresis can be observed in the magnetization curve at 2 K. To the best of our knowledge, this is the first time such steplike magnetization behavior has been observed in borophosphate compounds.

INTRODUCTION Cobalt-based compounds have been studied intensively due to their unique spin−lattices such as spin-cluster,1,2 spin-chain,3 spin-ladder,4 spin-kagome,5 and spin-honeycomb.6 Among cobalt-based compounds, the spin-chain systems have attracted special attention since the discovery of their unusual Ising spin characters. For example, BaCo2V2O8 exhibits a linear chain structure built by CoO6 octahedra along the c-axis, which has been found to show a field-induced order−disorder transition and large magnetic anisotropy,7−9 while CoNb2O6 consists of zigzag chains along the c-axis, which has been found to show complicated magnetic phase diagram and exotic quantum critical behavior.10−13 Besides, many Co-based compounds with one-dimensional (1D) spin-chain structure including Ca3Co2O6,14 CsCoBr3,15 CoCl2·2H2O,16 and CoV2O617 are all found to show a step-like magnetization curve at low temperature. Generally, the magnetic properties of Co-based compounds with a 1D spin-chain structure have been a keen interest in the field of magnetochemistry. Borophosphates usually show rich structural chemistry due to a great variety of connection patterns similar to silicates. In borophosphates, the linkages of PO4 and BO4 or BO3 often exhibit a large amount of anionic partial structures such as oligomeric units, chains, ribbons, and layers, corresponding to the different ratios of borate and phosphate groups (B:P ratios).18 Up to now numerous metal borophosphates have been obtained by means of high-temperature solid-state reaction, hydrothermal, solvothermal, and ionothermal synthesis methods. However, there are quite rare examples for 3d transition-metal based borophosphates with a 1D spin-chain structure such as BiM 2 BP 2 O 10 (M = Co, Ni) 19 and © XXXX American Chemical Society

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EXPERIMENTAL SECTION

Synthesis of Samples. Single crystals of Co3(BPO4)2(PO4)(OH)3 were synthesized by a hydrothermal method from a mixture of CoCl2·7H2O (2 N, 0.0626 g), NH4H2PO4 (2 N, 0.0507 g), Li2O (2 N, 0.0146 g), H3BO3 (2 N, 1.5158 g), and 1 mL of deionized water in a 28 mL autoclave. After the autoclave was sealed, the reaction was carried out at 230 °C under autogenous pressure in a furnace for 7 days and then cooled to room temperature for 2 days. The pink flakeshaped crystals were obtained from this procedure. The purity of the crystals was identified by using powder X-ray diffraction (Figure 1, Supporting Information). Crystallographic Studies. A small crystal of Co3(BPO4)2(PO4)(OH)3 (0.1 mm × 0.1 mm × 0.05 mm) was selected and mounted on Received: July 23, 2016 Revised: September 18, 2016

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DOI: 10.1021/acs.cgd.6b01092 Cryst. Growth Des. XXXX, XXX, XXX−XXX

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RESULTS AND DISCUSSION Structure Description. X-ray analyses indicate that Co3(BPO4)2(PO4)(OH)3 crystallizes in the monoclinic system of a space group Cc with a = 6.405(4) Å, b = 13.600(7) Å, c = 11.668(6) Å, and β = 96.965(6)°. There are three Co atoms, three P atoms, and two B atoms in an asymmetric unit. As shown in Figure S2 (Supporting Information), cobalt atoms have three independent crystallographic sites (Co1, Co2, and Co3), in which Co1 and Co2 sites are coordinated by six oxygen atoms forming distorted CoO6 octahedra with Co−O bond lengths ranging from 1.984(3) Å to 2.372(3) Å and O− Co−O bond angles ranging from 61.27(1)° to 174.74(1)°, while Co3 sites are coordinated by five oxygen atoms forming distorted CoO5 trigonal bipyramids with Co−O bond lengths ranging from 1.955(3) Å to 2.142(3) Å and O−Co−O bond angles ranging from 80.96(1)° to 176.83(1)°. P atoms have three independent crystallographic sites (P1, P2, and P3) and B atoms have two independent crystallographic sites (B1 and B2), which are all coordinated by four O atoms forming distorted PO4 and BO4 tetrahedra. The bond lengths of PO4 tetrahedra range from 1.515(3) to 1.596(3) Å, and the angles of O−P−O bonds are from 102.76(1)° to 115.24(1)°. The bond lengths of BO4 tetrahedra range from 1.430(6) to 1.528(6) Å, and the angles of O−B−O bonds are from 102.10(4)° to 112.90(4)°. The bond valence sum (BVS) calculations of Co1 (1.872), Co2 (2.107), Co3 (1.906), P1 (4.790), P2 (4.697), P3 (4.839), B1 (3.034), and B2 (3.107) confirm that these atoms are very close to their postulated oxidation states. Besides, according to the results of the BVS, the values of O10, O12, and O13 are 1.276, 1.269, and 1.022, respectively, which shows that these O atoms should be connected to H atoms, forming OH groups for charge balancing the formula. As shown in Figure 1a, Co atoms connect to each other via edge-sharing oxygen atoms, forming 1D skew chains running along the [201] direction. Three different distances between neighboring Co atoms are 3.2433(1) Å (Co1−Co2), 3.1043(1) Å (Co2−Co3), and 3.0759(1) Å (Co3−Co1), respectively. As shown in Figure 1b, a four-membered ring [B2P2O8] is built by two PO4 and two BO4 tetrahedra via corner-sharing. Further, a mixed chain [B2P2O8]∞ is seen along the c-axis, in which such [B2P2O8] rings connect to each other through BO4-to-BO4 forming B2O7 groups via corner-sharing oxygen atoms. It should be noted that this novel [B2P2O8]∞ chain with such unique connections of PO4 and BO4 is a new structural type, which has not been pointed out according to the functional building units (FBUs) on borophosphates.24 As shown in Figure 2a, the 3D framework of Co3(BPO4)2(PO4)(OH)3 is composed of 1D skew Co-chains and [B2P2O8]∞ mixed chains, featuring a pseudolayered structure viewed along the b-axis. The layers are built by the parallel 1D skew Co-chains linked by isolated PO4 tetrahedra (Figure 2b), which are separated by the [B2P2O8]∞ linear chains. We note that the structural features of the titled compound are quite similar to those of Cu3(BPO4)2(PO4)(OH)3.25 Since BO45−, PO45−, and (OH)− groups are nonmagnetic, the spin−lattice of Co3(BPO4)2(PO4)(OH)3 is determined by the topological arrangements of magnetic Co2+ ions, showing a skew-like spin chain structure running along the [201] direction as seen in Figure 2c. Magnetic Properties. Figure 3a shows the temperature dependence of magnetic susceptibility (χ) and corresponding reciprocal one (χ−1) of Co3 (BPO 4) 2(PO4)(OH)3. The susceptibility increases with decreasing temperature, while a

Figure 1. Chain structure built by (a) magnetic Co2+ ions and (b) four-membered borophosphate rings. Different sites of Co, P, and B atoms are seen. glassy fibers for single crystal X-ray diffraction (XRD) measurements. Data collections were performed on a Rigaku Mercury CCD diffractometer equipped with a graphite-monochromated 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.21 The structure was solved by direct methods and refined by full-matrix least-squares fitting on F2 by SHELX-97.22 All non-hydrogen atoms were refined with anisotropic thermal parameters. The hydrogen atoms were located at calculated positions and refined with isotropic thermal parameters. The final refined structural parameters were checked by the PLATON program.23 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−S3).

Table 1. Crystal Data and Structure Refinements for the Co3(BPO4)2(PO4)(OH)3 formula

Co3(BPO4)2(PO4)(OH)3

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)

534.32 room temperature 0.71073 Cc 6.405(4) 13.600(7) 11.668(6) 90 96.965(6) 90 1008.9(9) 4 3.498 0.5456 1.178 0.0273, 0.0621 0.0288, 0.0629

Article

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

a

Magnetic Measurements. A powdered sample (∼53.4 mg) of Co3(BPO4)2(PO4)(OH)3 was prepared by crushing single crystals and placed in a gel capsule sample holder suspending in a plastic drinking straw. Magnetic susceptibility was measured under an applied field of 1000 Oe from 300 to 2 K using a commercial magnetic property measurement system (MPMS). Magnetization was measured at 2 K in applied field from 0 to 8 T using the Quantum Design physical property measurement system (PPMS). B

DOI: 10.1021/acs.cgd.6b01092 Cryst. Growth Des. XXXX, XXX, XXX−XXX

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ment.26,27 The negative Weiss constant indicates the dominant antiferromagnetic interaction between Co2+ ions in nature. Figure 3b shows the isothermal magnetization of a powdered sample as a function of the applied field (M−H) at 2 K. The magnetization increases linearly with increasing field, while a rapid jump is observed at ∼3 T, indicating a field-induced magnetic transition. With further increasing field, the magnetization appears the second jump at ∼3.5 T and gradually shifts to saturate at H = 7 T. It is noted that a 1/3 magnetization plateau with a large magnetic loop is observed between increasing and decreasing field regimes, in which the critical fields of 1/3 magnetization plateau for increasing and decreasing field regimes are 2−2.3 T and 2.5−3.0 T, respectively. Figure 3c shows the M−H curve of a pellet sample at 2 K. A rapid jump is observed at ∼2 T, while a slight magnetic loop is observed at 2−3 T between increasing and decreasing field regimes, showing a size-effect of bulk samples. Such size-dependent magnetization behaviors can also be seen in the Ising-spin chain systems Ca3Co2O614 and CoV2O6.28 Here we discuss possible spin arrangements for the ground state and field-induced transitions of Co3(BPO4)2(PO4)(OH)3. It is well-known that the 1/3 magnetization plateau is usually associated with a classical collinear up−up−down type of spin arrangement, which is predicted for spins on a two-dimensional triangular lattice or a quantum state with this classical analogue. For Co-chain compounds, the nature of 1/3 magnetization plateau has been found to have two styles: (i) the 1/3 magnetization plateau in hexagonal Ca3Co2O629,30 and CsCoBr331 may arise from magnetic frustration due to antiferromagnetic interaction between the ferromagnetic Cochains arranged on a triangular lattice; (ii) the 1/3 magnetization plateau in monoclinic CoCl2·2H2O32,33 and CoV2O634 may correspond to a ferrimagnetic state originating from the occurrence of single spin-flip transition in two-sublattices, due to the weak antiferromagnetic interchain interactions between strong ferromagnetic Co-chains. We note that the Neel temperature (TN = ∼20 K) observed in Co3(BPO4)2(PO4)(OH)3 is quite larger than the Weiss temperature (θ = −4.5 K), indicating a weak ferromagnetic interaction inside Co-chains and a strong antiferromagnetic interaction between the nearest neighboring Co-chains. Further, we also note that monoclinic Co3(BPO4)2(PO4)(OH)3 with a skew chain structure does not show any ingredient for geometrical frustration, which is similar to CoCl2·2H2O and CoV2O6. Therefore, the possible spin arrangements under different magnetic fields may be suggested in Figure 4. Below 20 K, the system enters into an antiferromagnetic ground state (Figure 4a) where the spins in ferromagnetic Co-chains are antiparallel between the nearest neighboring chains. As a large external magnetic field is applied,

Figure 2. 3D structural framework of Co3(BPO4)2(PO4)(OH)3, where (a) a layered feature is seen along the b-axis and (b) the layers are built by the parallel 1D Co-chains linked by isolated PO4 tetrahedra, while (c) a spin−lattice of Co2+ ions is also seen.

Figure 3. (a) The temperature dependence of the magnetic susceptibility and the reciprocal one measured in an applied field of 0.1 T. The isothermal magnetization as a function of applied field (M− H) at 2 K for (b) powdered and (c) pellet samples.

sudden downturn is observed below 12 K, indicating the onset of antiferromagnetic ordering. A typical Curie−Weiss behavior is observed above 20 K, giving the Curie constant C = 9.188(6) emu·mol−1 and Weiss temperature θ = −4.5(3) K. The effective magnetic moment of Co2+ ions is calculated to be 4.94(9) μB, which is larger than the theoretical spin value of 3.873 μB for Co2+ (S = 3/2, g = 2) ions. This shows a large orbital moment contribution of Co2+ ions in such oxygen octahedral environ-

Figure 4. Possible spin arrangements of Co3(BPO4)2(PO4)(OH)3 for (a) antiferromagnetic ground state, (b) 1/3 magnetization plateau, and (c) saturated magnetization, corresponding to different magnetic phases (antiferomagnetic, ferimagnetic, and ferromagnetic states) under different applied fields. C

DOI: 10.1021/acs.cgd.6b01092 Cryst. Growth Des. XXXX, XXX, XXX−XXX

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it shows the first metamagnetic transition (one-spin flip transition) to ferrimagnetic spin structure at a critical field, which corresponds to the appearance of a 1/3 magnetization plateau (Figure 4b). With increasing field, the second metamagnetic transition induces the spins into parallel arrangements, corresponding to ferromagnetic state at highfield (Figure 4c). This implies that the Zeeman energy of the spins becomes comparable to the interchain exchange energy, hence making it energetically favorable for the spins of chains to reorient.

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CONCLUSION A new borophosphate Co3(BPO4)2(PO4)(OH)3 has been synthesized by a conventional hydrothermal method. This compound crystallizes in a monoclinic system with a skew chain structure which does not show any ingredient for geometrical frustration. However, our experimental results confirmed that Co3(BPO4)2(PO4)(OH)3 possesses a long-range antiferromagnetic ordering at ∼20 K, while a 1/3 magnetization plateau is observed in the magnetization curve at 2 K. We note that structural features and magnetic behaviors of the titled compound are quite similar to those of CoCl2·2H2O and CoV2O6. Therefore, the possible spin arrangements for antiferromagnetic, ferrimagnetic, and ferromagnetic states of Co3(BPO4)2(PO4)(OH)3 under different fields were suggested based on the model of one-spin flip transition in two-sublattice antiferromagnet.

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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.cgd.6b01092. The final refined atomic positions and structural parameters, PXRD (PDF) Accession Codes

CCDC 1505312 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_ [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

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

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Basic Research Program of China (No. 2012CB921701), National Natural Science Foundation of China (No. 21573235), the Opening Project of Wuhan National High Magnetic Field Center (2015KF08), and the Chinese Academy of Sciences under Grant No. KJZD-EW-M05.

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REFERENCES

(1) Yang, Q.; Zhang, X.-F.; Zhao, J.-P.; Hu, B.-W.; Bu, X.-H. Cryst. Growth Des. 2011, 11, 2839. (2) Liu, S.-J.; Xue, L.; Hu, T.-L.; Bu, X.-H. Dalton Trans. 2012, 41, 6813. D

DOI: 10.1021/acs.cgd.6b01092 Cryst. Growth Des. XXXX, XXX, XXX−XXX