WANG Nan-Nan HE Zhang-Zhen②CUI Mei-Yan GUO Wen-Bin ZHANG Su-Yun YANG Ming TANG Ying-Ying

a (College of Materials Science and Engineering, Fuzhou University, Fuzhou 350108, China)

b (State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002, China)

1 INTRODUCTION

Transition-metal oxohalogenides have attracted much attention due to their rich structural features and interesting magnetic properties. Oxohalogenide compounds have been found to exhibit various structural frameworks, including zero-dimensional dimer[1], one-dimensional (1D) chain[2-5], and twodimensional[6](2D) kagomé network[7]. For example,Na2Co5(MoO4)4Cl4[8]is found to exhibit a 2D kagomé-like structure while Sr2Cu(SeO3)2Cl2[9]shows a typical 1D-chain structure. Moreover,transition-metal oxohalogenides have also been found to exhibit exotic magnetic phenomena. For example, ZnCu3(OH)6Cl2with a kagomé structure is a promising candidate for the realization of a quantum spin liquid proposed by Anderson[10,11].Cu2Te2O5Br2with a tetrahedral spin-cluster structure shows a spin-singlet ground state behavior while isostructural Cu2Te2O5Cl2exhibits an antiferromagnetic transition at low temperature[12-14]. The correlation between magnetic properties and structural features of oxohalogenides has given an exciting issue in the fields of structural chemistry and material physics.

Recently, we have successfully obtained a new oxohalogenide compound Ba7CoV6O21Cl4. Its structure is found to exhibit an interesting quasi-1D chain built by CoO4Cl2octahedra and (V2O7)4-groups. In this paper, we report its crystal structure and magnetic properties.

2 EXPERIMENTAL

2.1 Synthesis of Ba7CoV6O21Cl4

Single crystals of Ba7CoV6O21Cl4were synthesized by a high-temperature solid-state reaction from a mixture of BaCO3(3 N, 6.0433 g), BaCl2(3 N,9.8000 g), V2O5(3 N, 3.8808 g) and Co(C2O4)2·4H2O (3 N, 1.7794 g) as the starting materials. The mixture was ground carefully and homogenized thoroughly in an agate mortar, and then packed into an alumina crucible. The crucible was capped with a cover and then was put into the furnace. After the furnace was heated up to 830 ℃and kept at 830 ℃ for 24 h, the furnace was slowly cooled to 450 ℃ at a rate of 5 ℃/h and then cooled to room temperature at a rate of 50 ℃/h. With this procedure, green crystals of Ba7CoV6O21Cl4were obtained by mechanical separation from the crucible.

2.2 X-ray ccrystallography

Small single crystals of Ba7CoV6O21Cl4were selected and mounted on glassy fibers for X-ray diffraction (XRD) measurements. Data collections of single crystals were performed at room temperature on a Rigaku mercury CCD diffractometer equipped with a graphite-monochromated MoKα radiation(λ = 0.71073 ?). The structure was solved by direct methods and refined on F2by full-matrix leastsquares method using the SHELXL/PC programs[15].The final refined structural parameters were checked by the PLATON program[16]. The selected bond distances and bond angles for Ba7CoV6O21Cl4are listed in Table 1.

Table 1. Selected Bond Lengths (?) and Bond Angles (°)

2.3 Magnetic measurements

Magnetic measurements were performed using a commercial Quantum Design Physical Property Measurement System (PPMS). A powdered sample of Ba7CoV6O21Cl4(17.30 mg) prepared by crushing single crystals was placed in a gel capsules ample holder which was suspended in a plastic drinking straw. Magnetic susceptibility was measured at 0.1 T from 80 to 2 K (temperature scan of 5 K/min) and magnetization was measured at 2 K in an applied field from 0 to 8 T (field scan of 0.1 T/step).

3 RESULTS AND DISCUSSION

3.1 Structural description

X-ray analysis indicates that Ba7CoV6O21Cl4crystallizes in the orthorhombic system of space group Cmcm with a = 16.726(4), b = 10.568(3), and c = 14.808(4) ?. An asymmetric unit contains three Ba atoms, one Co atom, and two V atoms. Ba atoms have three independent crystallographic sites Ba(1),Ba(2), and Ba(3), in which Ba(1) is surrounded by eight O atoms and three Cl atoms while Ba(2) and Ba(3) are coordinated by eight O and two Cl atoms.All of the Co2+ions are equivalently coordinated by four O and two Cl atoms, forming an elongated CoO4Cl2octahedron with the Co–O bond in 2.098(5)? and Co–Cl in 2.550(3) ?. Four O atoms of the CoO4Cl2octahedron are located at a plane with the O–Co–O bond angles of 89.500(3)° and 90.500(3)°,while the Cl–Co–O bond angles range from 84.190(1)° to 95.810(1)°. Vanadium atoms have two independent crystallographic sites V(1) and V(2),which are coordinated by four oxygen atoms forming a slightly distorted VO4tetrahedron with the V–O distances ranging from 1.671(5) to 1.824(2) ? and the O–V–O angles falling in the 104.500(2)～118.300(4)° range. It is noted that all V sites are not isolated VO4tetrahedra, in which the V(1) sites are connected to each other via corner-sharing (O(1)),forming (V2O7)4-groups with the V(1)–V(1)distance of 3.327(3) ? and the V(1)–O–V(1) bond angle of 134.451(7)°, while V(2) via corner-sharing(O(4)) form (V2O7)4-groups with the distance of V(2)–V(2) (3.513(2) ?) and the bond angle of V(2)–O–V(2) (148.756(6)°).

Fig. 1 shows the 3D structural framework of Ba7CoV6O21Cl4. There are five isolated[Co2V4O18Cl4] clusters and four isolated (V2O7)4-groups in a unit cell, in which the Ba2+cations are located inside the vacancies. The [Co2V4O18Cl4]clusters form a chain running along the c-axis, in which the [CoO4Cl2] octahedron connects to each other via (V2O7)4-groups (V(2) sites) with cornersharing O atoms. It should be noted that the (V2O7)4-groups of Ba7CoV6O21Cl4play two different roles, in which the (V2O7)4-groups built by V(2) act as a bridge for the linkage of CoO4Cl2octahedron, while the other (V2O7)4-groups built by V(1) are isolated and dangling, which gives an electrostatic interaction to Ba2+cations.

3.2 Magnetic properties

Fig. 1. View of the 3D framework on the ab-plane, in which [Co2V4O18Cl4] clusters form a quasi-1D chain running along the c-axis. (V2O7)4- groups built by V(1) or V(2) are also seen

Fig. 2 shows the temperature dependence of the magnetic susceptibility (χ) and corresponding reciprocal one (χ-1) of Ba7CoV6O21Cl4. Magnetic susceptibility increases with decreasing temperature and not any anomalies can be observed down to 2 K,indicating a paramagnetic behavior. A typical Curie-Weiss behavior can be observed above 20 K,giving the Curie constant C = 3.1 emu·mol-1·K and the Weiss temperature θ = -5.5(8) K. The effective magnetic moment of Co2+ions in the system is calculated to be 4.979 μBusing the equation of μeff2= 8C, which is larger than the theoretical spin value of 3.873 μBfor Co2+(S = 3/2, g = 2) ions obtained by μeff2= gS(S + 1), indicating a large orbital moment contribution of Co2+in such octahedral environment[17,18]. Also, the negative Weiss temperature indicates that the dominative interactions between magnetic Co2+ions are antiferromagnetic in nature. Fig. 3 shows the isothermal magnetization as a function of applied field (M – H) at 2 K. The magnetization increases rapidly in the low field region and saturates nearly at H > 2 T, while no hysteresis and remnant magnetization can be observed near H = 0. Also, the magnetization can be fitted well by the Brillouin function with S = 3/2,supporting a paramagnetic behavior in the system.This is in good agreement with the result of magnetic susceptibility.

Fig. 2. Temperature dependence of magnetic susceptibility and corresponding reciprocal one for Ba7CoV6O21Cl4

Fig. 3. Isothermal magnetization as a function of applied field (M–H) at 2 K.Red line is a fit with the Brillouin function with S = 3/2

4 CONCLUSION

In summary, a new oxohalogenide compound Ba7CoV6O21Cl4has been synthesized by a hightemperature solid state reactions. The title compound shows a quasi-1D linear chain structure, in which the CoO4Cl2octahedron connects with each other via two (V2O7)4-groups with corner-sharing along the c-axis. Magnetic measurements confirmed that Ba7CoV6O21Cl4exhibits a paramagnetic behavior down to 2 K.

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