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Cu3(CH3COO)4(OH)2·5H2O: A Novel Isolated Spin-1/2 Diamond Chain Compound Showing Possible Valence-Bond Condensation Meiyan Cui, Nannan Wang, Suyun Zhang, 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, P. R. China

Crystal Growth & Design Downloaded from pubs.acs.org by UNIV OF EDINBURGH on 01/14/19. For personal use only.

S Supporting Information *

ABSTRACT: A novel compound Cu3(CH3COO)4(OH)2· 5H2O was obtained by a hydrothermal method with a special synthesis process. The crystal structure is found to exhibit a unique one-dimensional diamond chain along the c-axis. Magnetic measurements confirmed that this compound exhibits unusual magnetic behaviors of a possible valencebond condensation state at low temperature, where the twothirds of all spins are condensed into spin singlets and one-third of the spins still remain in the paramagnetic state.

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Therefore, the search for spin-1/2 diamond-chain compounds is imperative but still a great challenge. Herein we report a new compound Cu3(CH3COO)4(OH)2· 5H2O (1) which is obtained by a hydrothermal reaction with a special synthesis process. The structure of 1 is found to have a unique 1D spin-1/2 diamond chain structure running along the c-axis. Magnetic measurements indicate that this compound exhibits unusual magnetic behaviors with a possible valence-bond condensation state at low temperature, in which two-thirds of all spins are condensed into spin singlets and one-third of the spins remain in the paramagnetic state. Our results show a new and unusual magnetic phenomenon appeared in a spin-1/2 diamond chain system. Single crystals of 1 were synthesized through a conventional hydrothermal reaction. 2.4 mmol of Cu(CH3COO)2·H2O (3N, 0.4860 g), 0.53 mmol of mineralizer Ba(OH)2·8H2O (3N, 0.1679 g), 4 mL of deionized water, and 3 mL of ammonia were mixed and sealed in an autoclave equipped with a Teflon liner (28 mL). The autoclave was put into a furnace and heated at 190 °C for 4 days under autogenous pressure, and then cooled to room temperature at a rate of ∼4 °C/h for 2 days. The obtained solution was filtered, and then the clarified filtrate was put into a beaker. Further, the beaker was covered with the preservative film, and the filtrate was slowly evaporated at room temperature for two months. Finally a cluster of blue cashmere crystals can be obtained (Figure S1, Supporting Information). The selection and separation of pure phases (small single crystals) were performed under a microscope. 1 crystallizes in a monoclinic system of space group P21/c with a = 12.423(3) Å, b = 14.583(3) Å, c = 10.784 (2) Å, and β = 102.395(5)°. The detailed structural parameters can be

eometrically frustrated compounds have generated great interest in the past several decades, due to the discovery of various exotic magnetic phenomena. Usually geometrical frustration occurs in a special spin motif such as a triangle or tetrahedron spin−lattice, where the antiferromagnetic interactions between spins cannot be satisfied simultaneously, giving rise to a large degeneracy of ground states.1 A classical long-range magnetic ordering may thus be destroyed by strong spin quantum fluctuation and geometrical frustration at low temperature, leading to a quantum disorder ground state. In this respect, current interest is focused on a two-dimensional (2D) spin-1/2 kagomé lattice, one of the most geometrically frustrated spin systems, which is expected to realize the quantum spin-liquid state related to Anderson’s resonating valence bond model.2 ZnCu3(OH)6Cl2 may be one of the most representative examples for 2D spin-1/2 kagomé compounds, showing a nearly prefect kagomé lattice structure3 without any magnetic anomalies above the temperature of 50 mK.4 In contrast to 2D systems, one-dimensional (1D) spin systems can be considered to have more advantage because a precise comparison between experimental and theoretical results becomes possible. The diamond chain lattice is the simplest 1D spin-frustrated system, which has attracted much experimental and theoretical attention. In fact, the scientists have investigated the ground state phase diagrams of diamond chain lattices by various theoretical simulations,5,6 suggesting that the ground state of the diamond chain lattice is usually a dimer−monomer (DM) state. If a spin-1/2 diamond chain is distorted, the ground state is no longer a DM state, but is a gapless spin-liquid state without free spins.7,8 Up to now, only a few compounds have however been found to exhibit a spin1/2 diamond chain structure, showing many exotic magnetic properties. These include the 1/3 magnetization plateau of Cu3(CO3)2(OH)2,9 spin-gap ground state of Cu3Cl6(H2O)2· 2H8C4SO4,10 and spin-liquid state of K3Cu3AlO2(SO4)4.11 © XXXX American Chemical Society

Received: October 29, 2018 Revised: December 31, 2018 Published: January 10, 2019 A

DOI: 10.1021/acs.cgd.8b01620 Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

Communication

seen in Tables S2−S5 of Supporting Information. Figure 1 shows the three-dimensional (3D) framework of

Figure 2. Diamond-chain spin−lattice built by Cu ions of 1 with different interaction exchanges (J) and coupling distances (d).

Figure 3. Temperature dependence of magnetic susceptibility of 1 and the corresponding inverse magnetic susceptibility. The red line is a fit of the Curie−Weiss law.

Figure 1. Structure of 1 viewed on (a) the a−b plane and (b) the b−c plane, in which (c) the linkage of CuO4 squares is seen. The blue, brown, red, and pink balls represent Cu, C, O, and H atoms, respectively.

of Cu2+ ions is calculated to be 2.255(2) μB, which is much larger than the theoretical value of 1.732 μB with S = 1/2 and g = 2, indicating a large orbital moment contribution of Cu2+ ions in such an oxygen-coordinated environment. This is also seen in Cu5V2O10, due to a large Jahn−Teller distortion of Cu2+ ions.12 A negative Weiss temperature suggests that the dominative interactions between neighboring Cu2+ ions are antiferromagnetic (AF) type in the system. Interestingly, a clear change in the slope of the inverse magnetic susceptibility occurs at ∼50 K, while typical Curie−Weiss behavior is observed below 50 K, giving the Curie constant C′ = 0.639(2) emu·mol−1·K and Weiss temperature θ′ = −0.53(1) K. We note that the value of low temperature C′ is about one-third of the high-temperature C value, showing two-thirds of the spins with a zero spin moment to low-temperature susceptibility. Figure 4 shows the isothermal magnetization M−H curve of 1

Cu3(CH3COO)4(OH)2·5H2O. It is noted that all of the Cu ions are four-oxygen-coordination, forming distorted CuO4 planar squares with Cu−O bonds ranging from 1.83 to 2.07 Å, and further such CuO4 squares connect to each other, forming a 1D diamond chain structure running along the c-axis. Each diamond chain is surrounded by four neighboring chains, and the shortest distance of Cu··Cu between neighboring chains is 7.2316(1) Å, in which (CH3Coo)− and H2O molecules are located between the chains. To check the linkage of CuO4 squares in the diamond chains (Figure 1c), it is noted that Cu2+ ions have three different crystallographic Cu1, Cu2, and Cu3 sites, where Cu1 and Cu3 ions form an edge-sharing [Cu1Cu3O6] dimer via oxygen [O9, O10] atoms, and the dimers connect to each other through Cu2O4 squares (Cu2 ions) with corner-sharing oxygen atoms (O9 or O10). There are five different interaction exchanges (J1, J2, J3, J4, J5) with different coupling distances (d1, d2, d3, d4, d5) and coupling angles in Table 1. A diamond chain spin−lattice built by Cu2+ ions (S = 1/2) for Cu3(CH3COO)4(OH)2·5H2O is shown in Figure 2. Table 1. Cu−Cu Lengths (Å) and Corresponding Cu−O− Cu Bond Angles (deg) in Cu3(CH3COO)4(OH)2·5H2O d1

2.978(1)

d2 d3 d4 d5

3.069(8) 3.270(1) 3.258(1) 3.182(8)

∠Cu1−O9−Cu3 ∠Cu1−O10−Cu3 ∠Cu2−O10−Cu1 ∠Cu2−O10−Cu3 ∠Cu2−O9−Cu1 ∠Cu2−O9−Cu3

98.25(4) 98.85(4) 102.75(3) 113.07(3) 112.04(3) 107.46(3)

Figure 4. Magnetization (M) vs applied field (H) for 1 at 2 K. The red line is a fit by the Brillouin function with S = 1/2.

at 2 K. The magnetization increases with increasing field and further saturates at above 3 T, while there are not any hysteresis and remnant magnetization near H = 0. Also, the moment of saturated magnetization is found to be ∼0.35 μB/ Cu2+, which approximately corresponds to 1/3 of the full moment of Cu2+ ions (S = 1/2) with 1 μB/Cu2+. Such a slightly large value of spin moment indicates magnetic anisotropy in the system, agreeing with a slightly larger effective moment of

The magnetic susceptibility of 1 (Figure 3) increases with decreasing temperature, and no peaks can be observed down to 2 K, showing a typical paramagnetic behavior at low temperature. It is noted that the magnetic susceptibility above 60 K can be fitted well by the Curie−Weiss law with a Curie constant C = 1.907(2) emu·mol−1·K and Weiss temperature θ = −91.7(3) K. The effective magnetic moment B

DOI: 10.1021/acs.cgd.8b01620 Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

Communication

Cu2+ ions seen in the magnetic susceptibility. Further, the magnetization can be fitted well using the spin-1/2 Brillouin function, confirming one-third of the spins with a paramagnetism at low temperature. The heat capacity data of 1 in Figure 5 does not show any λlike peaks from 300 K down to 2 K, ruling out the appearance

all of interaction exchanges (J) between neighboring Cu ions in the diamond chains are likely antiferromagnetic.15 Since a pair of edge-sharing Cu-polyhedra may be easier to form a spin-dimer and this can be seen in most of Cu2+-based oxides with a spin-singlet state such as CuTe2O5 (dCu‑‑Cu = 3.186 Å, ∠Cu−O−Cu = 96.7°),16 BaCu2V2O8 (dCu‑‑Cu = 2.862 Å, ∠Cu−O−Cu = 93.7°),17 Na2Cu2TeO6 (dCu‑‑Cu = 2.858 Å, ∠Cu−O−Cu = 91.3°),18 and SrCu2(BO3)2 (dCu‑‑Cu = 2.903 Å, ∠Cu−O−Cu = 97.7°),19 it seems to be quite reasonable to consider edge-sharing [Cu1Cu3O6] units as a spin-pair in the system (Figure 6) because the coupling distance and angle of [Cu1Cu3O6] units (dCu‑‑Cu = 2.978 Å, ∠Cu1−O9−Cu3 = 98.2°) are similar and close to those of spin-dimer systems mentioned above. In summary, we have found a new frustrated compound 1 featuring an isolated 1D spin-1/2 diamond chain structure. Magnetic measurements confirmed a possible VBC state in the system, where two-thirds of all spins are condensed into spin singlets and one-third of the spins remain in the paramagnetic state. However, a magnetic phase diagram depicted by a dimer−monomer model indicates that a condensed valencebond state in the system seems to be localized and static singlets. We envisage that the first realization of a possible VBC state in a 1D diamond chain system will enrich lowdimensional magnetism and spin-frustration physics, and further stimulate investigations of experiments and theories for such new magnetic phenomena.

Figure 5. The heat capacity data of 1 obtained at H = 0 T. The inset shows C−T curve.

of a long-range magnetic ordering and supporting paramagnetic behavior at low temperature. We note that a upturn in the C/T−T curve below 5 K may be due to a multilevel Schottky-like anomaly from the paramagnetism of the spins.13 As a result, the magnetic and heat capacity measurements have confirmed that 1 shows an unusual magnetic behavior of possible valence-bond condensation, where the system is paramagnetic with a random arrangement of the spins at high temperature, while two-thirds of all spins are condensed into spin singlets with a cooling temperature down below T = 50 K, and one-third of the spins still remain in the paramagnetic state. The image of magnetic phase diagram can be depicted in Figure 6.

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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.cgd.8b01620. A detailed experimental process, crystallography data, image of single crystals, the bond valence sum (BVS) calculation, and XRD pattern (PDF) Accession Codes

CCDC 1848255 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 [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

Figure 6. Image of magnetic phase diagram of 1. The red arrows indicate the spins and elliptical circles indicate a pair of spin-dimer.

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The valence-bond condensation (VBC) behavior was first suggested in the 2D frustrated cluster compound LiZn2Mo3O8 featuring spin-1/2 Mo3O13 clusters with an equilateral triangle lattice.13 This compound shows a geometrically frustrated effect between spin-1/2 Mo3O13 clusters, while two-thirds of all spins are condensed into spin singlets below T = 96 K and one-third of the spins still remain in the paramagnetic state.13 We note that magnetic and heat capacity results of 1 are quite similar to those observed in LiZn2Mo3O8,13 supporting a possible VBC state in the system. Interestingly and surprisingly, unlike LiZn2Mo3O8, a condensed valence-bond state composed of two-thirds of all spins in 1 seems to be localized and static singlets, reminiscent of valence-bond solid phenomenon. Here, we discuss the spin-pairs of Cu1−Cu3 sites in 1. It is well-known that the sign and the magnitude of the exchange constant J are related to the bonding geometry on the basis of the Goodenough rules.14 As seen in Table 1, considering all of coupling angles larger than 98°, it is reasonable to suppose that

AUTHOR INFORMATION

Corresponding Author

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

Suyun Zhang: 0000-0001-9394-4390 Zhangzhen He: 0000-0002-8496-1532 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the National Natural Science Foundation of China (NSFC) (No. 21573235; U1632159; 21875249) and the Chinese Academy of Sciences (ACS) under Grant No. KJZDEW-M05 for financial support.

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REFERENCES

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DOI: 10.1021/acs.cgd.8b01620 Cryst. Growth Des. XXXX, XXX, XXX−XXX

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(2) Anderson, P. W. The resonating valence bond state in La2CuO4 and superconductivity. Science 1987, 235, 1196−1198. (3) Shores, M. P.; Nytko, E. A.; Bartlett, B. M.; Nocera, D. G. A structurally perfect S = 1/2 kagomé antiferromagnet. J. Am. Chem. Soc. 2005, 127, 13462−13463. (4) Helton, J. S.; Matan, K.; Shores, M. P.; Nytko, E. A.; Bartlett, B. M.; Yoshida, Y.; Takano, Y.; Suslov, A.; Qiu, Y.; Chung, J.-H.; Nocera, D. G.; Lee, Y. S. Spin dynamics of the spin-1/2 Kagome lattice antiferromagnet ZnCu3(OH)6Cl2. Phys. Rev. Lett. 2007, 98, 107204. (5) Takano, K.; Kubo, K.; Sakamoto, H. Ground states with cluster structures in a frustrated Heisenberg chain. J. Phys.: Condens. Matter 1996, 8, 6405−6411. (6) Takano, K. Dimer-monomer ground state for extended spin-1/2 diamond chain. J. Phys. Soc. Jpn. 2017, 86, 093705. (7) Okamoto, K.; Tonegawa, T.; Takahashi, Y.; Kaburagi, M. The ground state of an S = 1/2 distorted diamond chain - a model of Cu3Cl6(H2O)2·2H8C4SO2. J. Phys.: Condens. Matter 1999, 11, 10485− 10498. (8) Okamoto, K.; Tonegawa, T.; Kaburagi, M. Magnetic properties of the S = 1/2 distorted diamond chain at T = 0. J. Phys.: Condens. Matter 2003, 15, 5979−5995. (9) Kikuchi, H.; Fujii, Y.; Chiba, M.; Mitsudo, S.; Idehara, T.; Tonegawa, T.; Okamoto, K.; Sakai, T.; Kuwai, T.; Ohta, H. Experimental observation of the 1/3 magnetization plateau in the diamond-chain compound Cu3(CO3)2(OH)2. Phys. Rev. Lett. 2005, 94, 227201. (10) Ishii, M.; Tanaka, H.; Hori, M.; Uekusa, H.; Ohashi, Y.; Tatani, K.; Narumi, Y.; Kindo, K. Gapped ground state in the spin-12 trimer chain system Cu3Cl6(H2O)2·2H8C4SO2. J. Phys. Soc. Jpn. 2000, 69, 340−343. (11) Fujihala, M.; Koorikawa, H.; Mitsuda, S.; Hagihala, M.; Morodomi, H.; Kawae, T.; Matsuo, A.; Kindo, K. Spin-liquid ground state in the spin 1/2 distorted diamond chain compound K3Cu3AlO2(SO4)4. J. Phys. Soc. Jpn. 2015, 84, 073702. (12) He, Z.; Lin, C.; Cheng, W.; Okazawa, A.; Kojima, N.; Yamaura, J.; Ueda, Y. Unusually large magnetic anisotropy in a CuO-based semiconductor Cu5V2O10. J. Am. Chem. Soc. 2011, 133, 1298−1300. (13) Sheckelton, J. P.; Neilson, J. R.; Soltan, D. G.; McQueen, T. M. Possible valence-bond condensation in the frustrated cluster magnet LiZn2Mo3O8. Nat. Mater. 2012, 11, 493−496. (14) Goodenough, J. B. Magnetism and the Chemical Bond; John Wiley and Sons: New York, 1963. (15) Geertsma, W.; Khomskii, D. Influence of side groups on 90° superexchange: A modification of the Goodenough-KanamoriAnderson rules. Phys. Rev. B: Condens. Matter Mater. Phys. 1996, 54, 3011−3014. (16) Deisenhofer, J.; Eremina, R. M.; Pimenov, A.; Gavrilova, T.; Berger, H.; Johnsson, M.; Lemmens, P.; Krug von Nidda, H.-A.; Loidl, A.; Lee, K.-S.; Whangbo, M.-H. Structural and magnetic dimers in the spin-gapped system CuTe2O5. Phys. Rev. B: Condens. Matter Mater. Phys. 2006, 74, 17442. (17) He, Z.; Kyomen, T.; Itoh, M. BaCu2V2O8: Quasi- onedimensional alternating chain compound with a large spin gap. Phys. Rev. B: Condens. Matter Mater. Phys. 2004, 69, No. 220407(R). (18) Xu, J. X.; Assoud, A.; Soheilnia, N.; Derakhshan, S.; Cuthbert, H. L.; Greedan, J. E.; Whangbo, M. H.; Kleinke, H. Synthesis, structure, and magnetic properties of the layered copper(II) oxide Na2Cu2TeO6. Inorg. Chem. 2005, 44, 5042−5046. (19) Kageyama, H.; Yoshimura, K.; Stern, R.; Mushnikov, N.; Onizuka, K.; Kato, M.; Kosuge, K.; Slichter, C. P.; Goto, T.; Ueda, Y. Exact dimer ground state and quantized magnetization plateaus in the two-dimensional spin system SrCu2(BO3)2. Phys. Rev. Lett. 1999, 82, 3168−3171.

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DOI: 10.1021/acs.cgd.8b01620 Cryst. Growth Des. XXXX, XXX, XXX−XXX