Syntheses and Characterization of a Family of Vanadate Compounds

Article pubs.acs.org/crystal

Syntheses and Characterization of a Family of Vanadate Compounds Ba3M(V2O7)2 (M = Co, Mn, Mg, or Zn) with an Edge-Shared [M2O10] Dimer Structure Nannan Wang,†,‡ Zhangzhen He,*,† Meiyan Cui,† Wenbin Guo,† Suyun Zhang,† and Ming Yang† †

State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002, P. R. China ‡ College of Materials Science and Engineering, Fuzhou University, Fuzhou, Fujian 350108, P. R. China S Supporting Information *

ABSTRACT: A series of vanadate compounds Ba3M(V2O7)2 (M = Co, Mn, Mg, or Zn) are synthesized by a conventional high-temperature solid-state reaction. Four compounds are found to exhibit a similar structure, which all crystallize in the monoclinic system of space group P21/c. V5+ ions are tetrahedrally coordinated, forming corner-shared (V2O7)4− groups, while M2+ ions are octahedrally coordinated, forming edge-shared [M2O10] dimers. Isolated {M2V8O28}∞ clusters composed of [M2O10] dimer and (V2O7)4− groups form a pseudo-one-dimensional chain running along the a-axis. Magnetic measurements confirm that both Ba3Co(V2O7)2 and Ba3Mn(V2O7)2 show antiferromagnetic interaction exchanges in the systems, while luminescence measurements confirm that Ba3Mg(V2O7)2 and Ba3Zn(V2O7)2 have a broad emission band from 400 to 650 nm under 256 nm UV irradiation at room temperature.

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exhibit interesting magnetic properties.17−21 In our recent study, we try to explore new transition metal vanadates through a substitution of V5+ ions for similar ions such as P5+, As5+, Sb5+, or Nb5+ in known compounds. In the work presented here, we have successfully obtained a family of compounds formulated as Ba3M(V2O7)2 (M = Co, Mn, Mg, or Zn) from the quasi-ternary Ba−M−V−O systems, based on the idea using substitution of V5+ ions for Sb5+ ions for members of the Ba3MSb2O9 family (M = Cu, Ni, or Co) that are found to exhibit a spin-liquid ground state.22−25 Herein, we report on their syntheses, structures, and magnetic and luminescence properties.

INTRODUCTION Transition metal oxides with a spin-dimer structure have attracted great interest since the discovery of various interesting quantum phenomena, including the Bose−Einstein condensation of magnons in bilayer−dimer system BaCuSi2O61 and the Wigner crystallization of magnons in orthogonal dimer system SrCu2(BO3)2.2 In some cases, many copper-based oxides with an isolated spin-dimer structure such as CaCuGe2O63 and CuTe2O54 show a nonmagnetic spin-singlet ground state. Besides, a few non-copper dimer compounds such as Ba3Mn2O8,5 Ba3Cr2O8,6 Sr3Cr2O8,7 and Ba2Mn2Si2O98 are also found to exhibit a nonmagnetic spin-singlet ground state. However, almost all dimer compounds such as BiCoPO59 are found to possess an antiferromagnetic ordering in their magnetic ground states. Such different magnetic ground states related to dimer structures of compounds have provided an exciting issue in chemistry and physics. This accelerates continuously the exploration of new dimer compounds with new magnetic properties. Vanadate groups serve as links for transition metal polyhedral units giving rise to a variety of frameworks,10,11 because nonmagnetic vanadate groups can adopt multiple coordination geometries such as the (VO4)3− tetrahedron and (VO5)5− square pyramid. Generally, transition metal vanadates not only provide rich structural chemistry but also exhibit various interesting magnetic behaviors. In the quasi-ternary Ba−M−V− O systems (M is a 3d transition metal), Müller-Buschbaum’s group has synthesized a series of BaM2V2O8 (M = Cu, Ni, Co, or Mn)12−15 and BaCuV2O7 compounds,16 which are found to © 2015 American Chemical Society

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EXPERIMENTAL SECTION

Synthesis of Ba3Co(V2O7)2. Single crystals of Ba3Co(V2O7)2 were obtained by a standard solid-state reaction method in air using a mixture of high-purity reagents BaCO3 (3 N, 2.9853 g), CoC2O4· 4H2O (3 N, 0.9249 g), and V2O5 (3 N, 1.8352 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 placed in the furnace. After the furnace had been heated to 850 °C and kept at 850 °C for 12 h, the furnace was slowly cooled to 450 °C at a rate of 5 °C/ h and then cooled to room temperature at a rate of 50 °C/h. With this procedure, dark green crystals of Ba3Co(V2O7)2 were obtained by mechanical separation from the crucible. Powdered samples were synthesized using a mixture of BaCO3, CoC2O4·4H2O, and V2O5 with Received: August 28, 2014 Revised: February 10, 2015 Published: March 4, 2015 1619

DOI: 10.1021/cg5012814 Cryst. Growth Des. 2015, 15, 1619−1624

Article

Crystal Growth & Design Table 1. Crystal Data and Structural Refinements for Ba3M(V2O7)2 (M = Co, Mn, Mg, or Zn) 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) a

Ba3Co(V2O7)2

Ba3Mn(V2O7)2

Ba3Mg(V2O7)2

Ba3Zn(V2O7)2

898.71 room temperature 0.71073 P21/c 7.176(2) 13.919(4) 14.039(4) 90 108.824(1) 90 1327.3(7) 4 4.498 1.272 1.150 0.0210, 0.0396 0.0249, 0.0408




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

a molar ratio of 3:1:2. After the mixture had been calcined at 600 °C in air for 40 h with several intermediate grindings, the product was further pressed into pellets and then sintered at 750 °C for 2 days. The quality of the samples was confirmed by powder X-ray diffraction (Figure S1a of the Supporting Information). Synthesis of Ba3Mn(V2O7)2. Single crystals of Ba3Mn(V2O7)2 were obtained by a standard solid-state reaction method using a mixture of high-purity reagents BaO (3 N, 0.5757 g), MnO (2 N, 0.0894 g), and V2O5 (3 N, 0.4555 g) as the starting materials. The mixture was ground carefully and homogenized thoroughly in an agate mortar and then pressed into a pellet. The pellet was further sealed into an evacuated quartz tube, and then the quartz tube was placed in the furnace. After the furnace had been heated to 850 °C and kept at 850 °C for 12 h, the furnace was slowly cooled to 450 °C at a rate of 5 °C/h and then cooled to room temperature at a rate of 50 °C/h. With this procedure, dark red crystals of Ba3Mn(V2O7)2 were obtained by mechanical separation from the quartz tube. Powdered samples were synthesized by a high-temperature solid-state reaction under an Ar atmosphere. A mixture of BaCO3, MnO, and V2O5 with a molar ratio of 3:1:2 was calcined at 600 °C in air for 40 h with several intermediate grindings and further sintered at 750 °C for 2 days. The quality of the samples was confirmed by powder X-ray diffraction (Figure S1b of the Supporting Information). Synthesis of Ba3Mg(V2O7)2. Single crystals of Ba3Mg(V2O7)2 were obtained by a standard solid-state reaction method in air using a mixture of high-purity reagents BaCO3 (3 N, 2.9665 g), MgC2O4· 4H2O (3 N, 1.0885 g), and V2O5 (3 N, 1.8405 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 placed in the furnace. After the furnace had been heated to 950 °C and kept at 950 °C for 12 h, the furnace was slowly cooled to 550 °C at a rate of 5 °C/ h and then cooled to room temperature at a rate of 50 °C/h. With this procedure, yellow crystals of Ba3Mg(V2O7)2 were obtained by mechanical separation from the crucible. Powdered samples were synthesized using a mixture of BaCO3, MgC2O4·4H2O, and V2O5 with a molar ratio of 3:1:2. After the mixture had been calcined at 600 °C in air for 40 h with several intermediate grindings, the product was further pressed into pellets and then sintered at 850 °C for 2 days. The quality of samples was confirmed by powder X-ray diffraction (Figure S1c of the Supporting Information). Synthesis of Ba3Zn(V2O7)2. Single crystals of Ba3Zn(V2O7)2 were obtained by a standard solid-state reaction method in air using a mixture of high-purity reagents BaCO3 (3 N, 2.9614 g), ZnC2O4· 4H2O (3 N, 1.1106 g), and V2O5 (3 N, 1.8384 g) as the starting materials. With the same procedure described for Ba3Mg(V2O7)2,

yellow crystals of Ba3Zn(V2O7)2 were obtained by mechanical separation from the crucible. Powdered samples were synthesized using a mixture of BaCO3, ZnC2O4·4H2O, and V2O5 with a molar ratio of 3:1:2. The mixture was calcined at 600 °C in air for 40 h with several intermediate grindings and further sintered at 850 °C for 2 days. The quality of samples was confirmed by powder X-ray diffraction (Figure S1d of the Supporting Information). X-ray Crystallorgraphy. Small single crystals of Ba3M(V2O7)2 (M = Co, Mn, Mg, or Zn) were 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 graphite monochromated Mo Kα radiation (λ = 0.71073 Å). The structure was determined by direct methods and refined on F2 by a full-matrix least-squares method using the SHELXL/PC programs.26 The final refined structural parameters were checked by PLATON.27 The crystal data and structural refinement information are listed in Table 1. The final refined atomic positions and structural parameters are seen in the Supporting Information (Tables S1−S12). Magnetic Measurements. Magnetic and heat capacity measurements were performed using a commercial Quantum Design Physical Property Measurement System (PPMS). Powdered samples of Ba3M(V2O7)2 (M = Co or Mn) were placed in a gel capsule ample holder that was suspended in a plastic drinking straw. The magnetic susceptibility was measured at 0.1 T from 300 to 2 K (temperature scan of 5 K/min), and magnetization was measured at 2 K with applied fields from −8 to 8 T (field scan of 0.1 T/step). Heat capacity data were measured at zero field by a relaxation method. Photoluminescence Analysis. Photoluminescence analyses of Ba3M(V2O7)2 (M = Mg or Zn) were performed on an FLS920 fluorescence spectrophotometer equipped with a continuous Xe-900 xenon lamp and a F900 microsecond flash lamp at room temperature; 395 and 475 nm high-pass filters were applied when performing the PL determinations.

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RESULTS AND DISCUSSION Structural Descriptions. X-ray analyses indicate clearly that the Ba3M(V2O7)2 compounds (M = Co, Mn, Mg, or Zn) crystallize in the monoclinic system of space group P21/c. Because they are isostructural, Ba3Co(V2O7)2 is selected as a representative for the detailed description of their structures. For Ba3Co(V2O7)2, there are three Ba atoms, one Co atom, and four V atoms in the asymmetric unit. As shown in Figure S2 (Supporting Information), all Co2+ ions are equivalent and 1620


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Crystal Growth & Design

Figure 1. Linkage of (a) the Co−Co dimer, (b) the (V2O7)4− group built by V(1) and V(2), (c) the (V2O7)4− group built by V(3) and V(4), and (d) the [Co2V8O28] cluster.

coordinated by six oxygen atoms, forming distorted CoO6 octahedra with Co−O bond lengths ranging from 2.012(3) to 2.370(3) Å and O−Co−O bond angles of the CoO 6 octahedron ranging from 84.39(1)° to 173.46(1)°. V atoms have four independent crystallographic sites [V(1)−V(4)]. All V5+ ions are tetrahedrally coordinated by four oxygen atoms, forming a nearly regular VO4 tetrahedron with V−O distances ranging from 1.642(3) to 1.842(3) Å and O−V−O angles in the range of 102.62(1)−117.09(1)°. Barium atoms have three independent crystallographic sites [Ba(1)−Ba(3)], in which Ba(1) and Ba(3) sites are surrounded by nine oxygen atoms with Ba−O bond lengths ranging from 2.634(3) to 3.179(3) Å, while the Ba(2) site is surrounded by 10 oxygen atoms with Ba−O bond lengths ranging from 2.744(3) to 3.207(3) Å. As shown in Figure 1, two CoO6 octahedra connect to each other via edge-sharing oxygen atoms [O(5)−O(5)], forming [Co2O10] dimers with a Co−Co distance of 3.185(7) Å and a Co−O−Co bond angle of 94.96(8)°. It is noted that all V sites are not isolated VO4 tetrahedra, in which V(1) and V(2) sites connect to each other via corner sharing [O(4)], forming (V2O7)4− groups with a V(1)−V(2) distance of 3.277(2) Å and a V(1)−O−V(2) bond angle of 127.68(3)°, while V(3) and V(4) sites via corner sharing [O(11)] form (V2O7)4− groups with a V(3)−V(4) distance of 3.163(7) Å and a V(3)−O−V(4) bond angle of 120.11(3)°. The building unit [Co2V8O28] is composed of a dimer [Co 2 O 10 ] and four (V 2 O 7 ) 4− pyrovanadate anionic groups with edge-sharing oxygen atoms. As shown in Figure 2, Ba3Co(V2O7)2 features an interesting three-dimensional framework. There are four isolated [Co2V8O28] clusters in a unit cell, in which such clusters are surrounded by Ba2+ cations. The [Co2V8O28] clusters form [Co2V8O28]∞ chains running along the a-axis, in which the building [Co2V8O28] units connect to each other through VO4 tetrahedra, showing Co−O−V−O−Co routes in such pseudoone-dimensional chains. It is noted that two types of (V2O7)4−

Figure 2. Crystal structure of Ba3Co(V2O7)2 with [Co2V8O28] clusters on (a) the b−c plane and along (b) the a-axis.

pyrovanadate anionic groups play different roles in the linkage of polyhedra, in which (V2O7)4− groups built by V(1) and V(2) provide V(2)O4 tetrahedra as a bridge with corner sharing for the linkage between [Co2O10] dimers, while the other (V2O7)4− groups built by V(3) and V(4) are dangling. We note that a similar compound formulated as Ba3PbV4O14 exhibits a dimer 1621


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Crystal Growth & Design

Figure 3. (a) Temperature dependencies of the magnetic susceptibility (χ) and corresponding reciprocal value (χ−1) for (a) Ba3Co(V2O7)2 and (b) Ba3Mn(V2O7)2. The red line is a fit by a modified version of the Van Vleck equation.30

Figure 4. Isothermal magnetization as a function of applied field (M − H) at 2 K for (a) Ba3Co(V2O7)2 and (b) Ba3Mn(V2O7)2. The red line is a fit by the Brillouin function with S = 3/2.

Figure 5. (a) Heat capacity data of Ba3M(V2O7)2 (M = Co, Mn, or Zn) measured at zero field. (b) Magnetic contribution of Ba3Co(V2O7)2 and Ba3Mn(V2O7)2.

structure built by edge-sharing PbO7 polyhedra.28 However, the linkages of polyhedra between PbO7 and (V2O7)4− groups are completely different from those of Ba3Co(V2O7)2 with Co−O− V−O−Co routes. Magnetic Properties. Figure 3a shows the temperature dependences of the magnetic susceptibility (χ) and corresponding reciprocal value (χ−1) of Ba3Co(V2O7)2. The susceptibility increases with a decrease in temperature, and no any anomalies can be observed down to 2 K. A typical Curie−Weiss behavior is observed above 30 K, giving a Curie constant C of 3.97(5) emu mol−1 K and a Weiss temperature θ of −21.9(4) K. The effective magnetic moment of Co2+ ions in the system is calculated to be 5.63(5) μB using the equation μeff2 = 8C, which is larger than the theoretical spin value of 3.873 μB for Co2+ (S = 3/2; g = 2) ions obtained from the equation μeff2 = gS(S + 1), indicating a large orbital moment contribution of Co2+ in the

oxygen octahedral environment.29 Also, the negative Weiss temperature indicates that the dominant interactions between magnetic Co2+ ions are antiferromagnetic in nature. To estimate the spin−spin coupling between Co2+ ions, the susceptibility is fitted by the isolated spin-3/2 dimer model using a modified version of the Van Vleck equation:30 χ = [NμB2g2(2e−2x + 10e−6x + 18e−12x)]/[3kBT(1 + 3e−2x + 5e−6x + 7e−12x)], where x = J/kBT. The best fit gives a J/kB of −0.117(1) K and a g of 1.837(7). Figure 3b shows the temperature dependencies of the magnetic susceptibility (χ) and corresponding reciprocal value (χ−1) of Ba3Mn(V2O7)2. The magnetic susceptibility increases with a decrease in temperature, while an anomaly is observed at ∼5 K, indicating the onset of the antiferromagnetic transition. A typical Curie− Weiss behavior is observed above 50 K, giving a Curie constant C of 4.91(6) emu mol−1 K and a Weiss temperature θ of 1622


Article

Crystal Growth & Design −24.5(6) K. The effective magnetic moment of Mn2+ ions in the system is calculated to be 6.27(1) μB, which is larger than the theoretical spin value of 5.916 μB for Mn2+ (S = 5/2; g = 2) ions, showing Mn2+ ions with a high-spin state. The negative Weiss temperature also indicates the dominant antiferromagnetic interactions between magnetic Mn2+ ions. Like that of Ba3Co(V2O7)2, the susceptibility above 10 K is fit well by the isolated spin-5/2 dimer model using a modified version of the Van Vleck equation:30 χ = [NμB2g2(e−2x + 5e−6x + 14e−12x + 30e−20x + 55e−30x)]/[3kBT(1 + 3e−2x + 5e−6x + 7e−12x + 9e−20x +11e−30x)], where x = J/kBT, giving a J/kB of −2.009(5) K and a g of 1.995(1). Figure 4a shows the isothermal magnetization as a function of applied field (M − H) at 2 K for Ba3Co(V2O7)2. The magnetization increases rapidly in the low-field region and is nearly saturated at H > 1 T. Further, no hysteresis or remnant magnetization can be observed near H = 0. We note that the magnetization at H < 1 T can be fit well by the Brillouin function with S = 3/2, suggesting paramagnetic behavior in the system. Figure 4b shows the isothermal magnetization as a function of applied field at 2 K for Ba3Mn(V2O7)2. The magnetization shows an increase in its nearly linear manner and does not saturate even in 8 T, agreeing with the antiferromagnetic ground state in the system. Figure 5a shows heat capacity data measured at zero field for Ba3Co(V2O7)2, Ba3Mn(V2O7)2, and Ba3Zn(V2O7)2. Because Ba3Zn(V2O7)2 is a nonmagnetic insulator, its total heat capacity arises mainly from the lattice contribution (Cl). For Ba3Co(V2O7)2 and Ba3Mn(V2O7)2, their heat capacity data consist of the magnetic contribution (Cm) and the lattice contribution (Cl). It is clear that heat capacity data of Ba3Co(V2O7)2 and Ba3Mn(V2O7)2 at high temperatures are mainly due to the lattice contribution. Because Ba3M(V2O7)2 (M = Co, Mn, Mg, or Zn) are isostructural, the magnetic contribution (Cm) of Ba3Co(V2O7)2 and Ba3Mn(V2O7)2 can be obtained by subtraction of the lattice contribution (Cl) from total heat capacity [Cm = Cp − Cl(Zn)]. Figure 5b shows the magnetic contribution of Ba3Co(V2O7)2 and Ba3Mn(V2O7)2. A slight anomaly of heat capacity is observed at ∼4 K for Ba3Mn(V2O7)2, indicating the appearance of a magnetic transition, while a broad peak is observed at ∼5 K for Ba3Co(V2O7)2, which may be a tail of Schottky-type heat capacity of paramagnetic impurity. In addition, the entropy integrated over magnetic anomaly of Ba3Mn(V2O7)2 is estimated to be

Syntheses and Characterization of a Family of Vanadate Compounds

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