Manganese Vanadate Chemistry in Hydrothermal BaF2 Brines

Dec 5, 2016 - Ba3Mn2(V2O7)2F2 crystallizes in space group C2/c and consists of dimers built from edge-sharing MnO4F2 trigonal prisms with linking V2O7...
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Manganese Vanadate Chemistry in Hydrothermal BaF2 Brines: Ba3Mn2(V2O7)2F2 and Ba7Mn8O2(VO4)2F23 Liurukara D. Sanjeewa,† Colin D. McMillen,† Michael A. McGuire,‡ and Joseph W. Kolis*,† †

Department of Chemistry and Center for Optical Materials Science and Engineering Technologies, Clemson University, Clemson, South Carolina 29634-0973, United States ‡ Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States S Supporting Information *

to several new manganese vanadate fluorides with unusual coordination environments.6 The present study describes preliminary results involving BaF2 mineralizers with manganese vanadates, resulting in significantly different chemistry. The present work also provides a foundation for comparison with concurrently studied systems involving BaCl2 and SrCl2 brines, which yield products very different from those of the CsF and BaF2 systems, as well as one another. Such behavior may result from the differing relative solubilities of each brine mineralizer. The initial studies with BaF2 reveal two unusual new compounds [see the Supporting Information (SI), Tables S1−S5], each with some very interesting structural features. One, Ba3Mn2(V2O7)2F29 (I), contains dimers of fluorine-edge-shared Mn2+ trigonal prisms. Compound II, Ba7Mn8O2(VO4)2F23,10 consists of two types of layers, one consisting of corner-shared Mn2+ octahedra and the other of corner/edge-sharing Mn2+/3+, where charges are delocalized to give a an average oxidation state of 2.75 for Mn. The magnetic properties of the latter compound were also studied. Compounds I and II were prepared by a hydrothermal reaction of BaCO3 with Mn2O3 and V2O5 using 5 M BaF2 brine as a mineralizer. High-temperature hydrothermal solutions (550−600 °C) have shown to be excellent reaction media for high-quality large single-crystal growth. Both products were isolated from the same reaction in low-to-moderate yield. Our attempts to increase the yield of compounds I and II were unsuccessful because higher mineralizer concentrations (>5 M) and higher temperatures tend to only produce more BaF2 crystals. Compound I contains only Mn2+ and is yellow, as might be expected, while compound II contains both Mn2+ and Mn3+ and is a much darker brown, also as expected. Although both barium and fluoride play a role in the final structures, there is no evidence of any salt-inclusion-type formation, as is observed in similar reactions involving BaCl2, presumably because of the higher solubility of BaF2. Compound I consists of Mn2+ dimers and V2O7 pyrovanadates as well as lone Ba2+ ions (Figure 1a). The centerpiece of the compound is an unusual dimer of trigonal prisms that share a common fluorine edge, having V2O7 groups chelating the Mn2+ centers across the basal planes of the trigonal prisms (Figure 1b). The overall connectivity between edge-sharing dimers of MnO4F2 trigonal prisms and V2O7 groups forms slabs extending along the b axis (Figure 1c). The Mn2+−O distances average

ABSTRACT: Manganese vanadate fluorides were synthesized using high-temperature hydrothermal techniques with BaF2 as a mineralizer. Ba3Mn2(V2O7)2F2 crystallizes in space group C2/c and consists of dimers built from edge-sharing MnO4F2 trigonal prisms with linking V2O7 groups. Ba7Mn8O2(VO4)2F23 crystallizes in space group Cmmm, with a manganese oxyfluoride network built from edge- and corner-sharing Mn2+/3+(O,F)6 octahedra. These octahedra form alternating Mn2+ and Mn2+/3+ layers separated by VO4 tetrahedra. This latter compound exhibits a canted antiferromagnetic order below TN = 25 K.

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he use of tetrahedral oxyanions is an excellent route to new materials ranging from “simple” to astonishingly complex in structure.1 In addition to the various bonding possibilities, the 3fold axis of the tetrahedron could lead to 3-fold symmetry in the crystal lattice forming frustrated magnetic solids.2 Vanadates are of particular interest because they serve as magnetic linking groups and display a considerable range of unusual magnetic activity.3 They have a wide range of structural flexibility, adopting a variety of coordination modes but also forming oligomeric polyvanadates that dramatically increase the structural possibilities.4 Recently, we found that high-temperature hydrothermal fluids (ca. 600 °C and 150 MPa) are excellent media for the synthesis of new metal vanadates.5 The traditional mineralizer for high-temperature hydrothermal growth of metal oxyanion-containing crystals has been hydroxide. However, because brine systems are very common in natural hydrothermal environments, we started expanding our exploration into the role of halides in forming new metal complexes. Some initial studies using metal fluorides and chlorides, as well as mixed hydroxide−halide fluids, suggest that there is considerable sensitivity to the identity, concentration, and basicity of the brine mineralizer.6 Structurally, the halide ions may be incorporated as ligands on the metal centers, or as a salt inclusion lattice.7 This behavior may be useful in directing unusual coordination environments or templating longrange structural features, further enriching the structural chemistry and magnetic properties.8 The chemical nature of the metal halide mineralizer (fluoride versus chloride, alkali versus alkaline earth, for example) appears to also influence the structure. Previously, we reported that the use of CsF as a mineralizer in a manganese vanadate system leads © XXXX American Chemical Society

Received: September 27, 2016

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DOI: 10.1021/acs.inorgchem.6b02355 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 1. (a) Structure of I along the b axis. (b) MnO4F2 trigonal prisms and a chelating V2O74− unit. (c) Mn−F−O−V slabs run along the b axis by chelated vanadates connecting MnO4F2 trigonal prisms dimerized via F(1).

Figure 2. Projected view of II along the c axis showing the discrete Mn2+ and Mn2+/3+ layers alternating along the b axis. These layers are interconnected by VO4 tetrahedra (yellow polyhedra) along the b axis.

Mn−F−Mn ring repeating units (Figure 3a). These layers link to the adjacent Mn2+/3+ layers via vanadate groups along the b axis.

2.1778(15) Å, typical for Mn2+. The shared edges are formed by bridging fluorides with Mn−F distances of 2.1817(11) Å. There are two unique Ba2+ ions that occupy the interstices between the manganese vanadate slabs, with Ba(1) forming an 11-coordinate environment with nine Ba−O and two Ba−F interactions and Ba(2) having 12 Ba−O interactions. The V−O distances in the two unique pyrovanadate groups are typical, ranging from 1.668(2) to 1.815(2) Å, with the bridging V−O−V distances being the longest and the terminal unbound V−O distances the shortest, as expected. The chelating V2O7 groups are bent considerably to accommodate bonding across the basal edges of the trigonal prisms [O−V−O angle = 111.78(10)°]. The trigonal-prismatic environment is much less common than the octahedral environment for transition metals, especially among the first-row transition-metal oxides. Typically, the environment is observed in second- and third-row d-block metals and more commonly among soft ligands such as sulfur and selenium.11 There have been several Mn2+ trigonal prisms reported in the literature,12 but they are usually constrained somewhat by a specialized multidentate ligand and often show somewhat of a distortion.13 In compound I, there appear to be neither any restrictions on the environment nor any significant distortions from the ideal. It may be that the chelating behavior of the V2O7 group and the geometry needed to bridge the dimers into the slabs assist in enforcing the trigonal-prismatic geometry. Interestingly, it appears that Mn2+ has the greatest tendency of the classical first-row d-block complexes to form trigonal prisms because of a combination of zero ligand-field stabilization energy (versus other metals in Oh) and a favorable ionic radius.14 The other material isolated from the reaction, Ba7Mn8(VO4)2O2F23 (II), contains a series of alternating layers of edge- and corner-shared octahedra. One is a well-ordered Mn2+ layer, while the other contains mixed-valent Mn2+/3+ sites, both in slightly distorted octahedral environments. The lone VO43− tetrahedron buttresses the layers, and the Ba2+ ions occupy various interstices between the layers (Figure 2). The Mn2+ layers consist of trans-corner-shared octahedra tilted relative to each other to create an irregular corrugated surface. The Mn(1)F6 and Mn(2)O2F4 octahedra are all bridged via F(1) to form Mn(1)−F(1)−Mn(2) linkages with eight-membered

Figure 3. (a) Layers of Mn2+−(O,F)−Mn2+ in the (010) plane of II. (b) Layers of Mn2+/3+−(O,F)−Mn2+/3+ in the (010) plane. Characteristic Mn8 rings are highlighted in the SI, Figure S1.

In addition, the Mn2+ octahedra all have typical Mn2+−O and Mn2+−F distances, with the Mn−F distances averaging 2.128(6) Å and a Mn−O distance of 2.163(2) Å. Bond-valence-sum (BVS) calculations for Mn(1) and Mn(2) are 1.99 and 1.94 (see the SI, Table S4). The layers containing mixed-valent Mn2+ and Mn3+ also consist of two different types of octahedra, Mn(3)O4F2 and Mn(4)OF5, respectively. The Mn(3)O4F2 octahedra edge-share via O(3) and F(5) to form infinite chains along the c axis (Figure 3b). These bridging edge-shared oxides are not associated with the VO4 groups. Parallel chains are connected to one another in the overall Mn2+/3+ layer through dimers made of Mn(4)OF5 that are corner sharing through F(3). These dimers intersect with the neighboring Mn(3) chains via the edge-shared O(3), creating a B

DOI: 10.1021/acs.inorgchem.6b02355 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry μ3-O atom. The BVS calculations for Mn(3) and Mn(4) are 2.89 and 2.40, respectively, showing a delocalized Mn2+/3+ layer. The Mn(3) chain sublattice thus primarily exhibits Mn3+ character, while the abundance of F− around Mn(4) results in more Mn2+ character. The Mn−O and Mn−F distances of Mn(3)O4F2 and Mn(4)OF5 are correspondingly shorter than the Mn2+ distances in Mn(1)F6 and Mn(2)O2F4 because of the presence of Mn3+. The Mn2+/3+−F distances average 2.0024(5) Å, and the Mn2+/3+−O distances range from 1.925(7) to 2.099(4) Å, with the shortest distance being to O(3), which is the trigonalcoordinated bridging atom. The vanadate group has an average V−O bond distance of 1.717(5) Å. The Ba2+ ions occupy space between the layers and also interstices between the dimers within the Mn2+/3+ layers. Despite its complexity, the structure has relatively high Cmmm symmetry, with all of the atoms except two F atoms sitting on special positions. The results of magnetization measurements on II are shown in Figure 4. At high temperature, Curie−Weiss-like behavior is

quickly saturating component of the magnetization present at 2 K, which is absent at 300 K. The saturated portion of the moment at 2 K is relatively small, only about 0.5 μB/fu, and the magnetization continues to increase with increasing field. The simplest model for the magnetic structure is likely a canted antiferromagnetic-like state that develops below a Neel temperature of TN = 25 K, consistent with the negative Weiss temperature. However, the magnetic structure is expected to be complex because of the complex crystal structure and the presence of charge-disordered Mn2+ and Mn3+ ions. Interestingly, the susceptibility measured in a field of 10 kOe (Figure 4a) continues to increase in a Curie−Weiss-like fashion upon cooling below TN. Fits to this low-temperature data give a Curie constant of 5.4 cm3·K/mol·fu, corresponding to 1.2 paramagnetic Mn2+ ions or 1.8 paramagnetic Mn3+ ions per formula unit. This may be related to the charge disorder in the Mn2+/3+ layer, and neutron diffraction experiments would be required to gain further insight. The results described here highlight the rich chemistry of metal vanadates in high-temperature hydrothermal solutions, with new products containing interesting structural features. Compound I contains very unusual edge-sharing dimers of trigonal-prismatic Mn2+, with F− acting as the bridging atoms. Compound II is a complex structure with an alternating series of layers of manganese octahedra, with one layer containing Mn2+ and the other delocalized Mn2+/3+. In both cases, the fluoride ions play an important structural function in the transition-metal networks, in addition to their role as mineralizers. This work highlights the extension of mineralizers to high concentrations of halides, which can lead to interesting new solids. The combinations of halides, alkalinity, and alkali/alkaline-earth counterions lead to a broad matrix of new phases accessible through hydrothermal brines.

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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.inorgchem.6b02355. Experimental details, interatomic distances and angles, and table of elemental analyses (PDF) X-ray crystallographic data in CIF format (CIF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected].

Figure 4. Magnetic properties of II. (a) Inverse susceptibility (H/M) versus temperature at 10 kOe. The line shows a Curie−Weiss fit with parameters given on the plot. (b) Susceptibility (M/H) versus temperature at 100 Oe. (c) Isothermal magnetization curves at 300 and 2 K.

ORCID

Colin D. McMillen: 0000-0002-7773-8797 Notes

The authors declare no competing financial interest.

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observed (Figure 4a). The fit shown on the figure gives a Curie constant of C = 26(3) cm3·K/mol·fu (fu = formula unit) and a Weiss temperature of θ = −58(3) K. The Curie constant is similar to, but somewhat smaller than, the value of 30.9 cm3·K/ mol·fu, as expected for five Mn2+ (S = 5/2) and three Mn3+ (S = 2) per formula unit. The negative Weiss temperature indicates predominantly antiferromagnetic interactions. Evidence for a magnetic ordering transition is observed at 25 K. This is most clearly seen in the low-field data shown in Figure 4b. The sharp increase upon cooling through this temperature indicates a ferromagnetic component to the magnetic order. Magnetization versus applied field data (Figure 4c) support this, revealing a

ACKNOWLEDGMENTS The authors thank the National Science Foundation (Grant DMR-1410727) for financial support. Magnetic studies (M.A.M.) were supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, Materials Sciences and Engineering Division.

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DOI: 10.1021/acs.inorgchem.6b02355 Inorg. Chem. XXXX, XXX, XXX−XXX