Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
pubs.acs.org/IC
Synthetic Strategies for the Synthesis of Ternary Uranium(IV) and Thorium(IV) Fluorides Vladislav V. Klepov, Justin B. Felder, and Hans-Conrad zur Loye* Department of Chemistry and Biochemistry, University of South Carolina, 631 Sumter Street, Columbia, South Carolina 29208, United States S Supporting Information *
ABSTRACT: A series of new U(IV) and Th(IV) fluorides, Na7U6F31 (1), NaUF5 (2), NaU2F9 (3), KTh2F9 (4), NaTh2F9 (5), (H3O)Th3F13 (6), and (H3O)U3F13 (7), was obtained using hydrothermal and low-temperature flux methods. Mild hydrothermal reactions with uranyl acetate as a precursor yielded 1, 7, and the monoclinic polymorph of NaU2F9, whereas direct reactions between UF4 and NaF led to the formation of 2 and orthorhombic NaU2F9 (3). This highlights an unexpected difference in reaction products when different starting uranium sources are used. All seven compounds were characterized by single-crystal X-ray diffraction, and their structures are compared on the basis of cation topology, revealing a close topological resemblance between fluorides on the basis of the layers observed in NaUF5(H2O). Phase-pure samples of 1, 2, and both polymorphs of NaU2F9 were obtained, and their spectroscopic and magnetic properties were measured. The UV−vis data are dominated by the presence of U4+ cations and agree well with the electronic transitions. Effective magnetic moments of the studied compounds were found to range from 3.08 to 3.59 μB.
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in comparison to oxides.14 In addition, the fluoride anions are highly corrosive, which limits the use of silica tubes for the synthesis of uranium fluorides. Another possible route to new uranium fluoride compounds is the use of hydrothermal techniques.15−18 It has been shown previously that utilizing uranyl acetate UO2(CH3CO2)·2H2O as a precursor serves as a convenient way of obtaining new uranium(IV) fluorides, since both the uranium and the reducing agent are simultaneously present in a single precursor.12 This technique employs an in situ reduction step and often produces phase-pure samples in excellent yields; however, it often requires the use of a copper salt that acts as a catalyst for the formation of complex fluorides, which severely hinders the synthesis of new complex fluoride phases containing sodium due to the preferential formation of the Na4CuU6F30 phase.19 Similar synthetic considerations can be applied to thorium(IV) fluorides, which have been more intensively investigated than their U(VI) counterparts. To date, there have been >80 ternary and 9 quaternary thorium fluoride crystal structures deposited in the ICSD,13 although many of them are structure redeterminations or a series of structure determinations under high pressure.20−22 The primary difference between thorium and uranium is the absence of a higher than +4 oxidation state for thorium, which prevents in situ reduction as a possible strategy for obtaining thorium fluorides; on the other hand, it is
INTRODUCTION Uranium redox and crystal chemistry has attracted significant interest and is widely studied because of its importance to each step of the nuclear fuel cycle as well as to control of the effect of nuclear materials on the environment.1−4 The importance of uranium oxides for nuclear power has made them some of the most studied systems.5−7 Extensive studies of the oxides and oxo salts, including phosphates, silicates, borates, etc., have also been prompted by their potential application as matrices for long-term storage of components of spent nuclear fuel.8−11 Among fluorides, UF6 serves the purpose of uranium enrichment and is therefore the most extensively studied and commonly applied material. Despite the extensive use of UF6, other uranium fluorides, particularly ternary and quaternary, have been studied far less, although the presence of unpaired f electrons coupled with the rich crystal chemistry of U(IV) makes these materials very interesting from the standpoint of magnetic and optical properties.12 To date, there have been about 40 ternary and 20 quaternary uranium fluorides deposited in the ICSD.13 One of the reasons for the scarcity of uranium fluorides is the synthetic challenges that make their preparation via high-temperature reactions quite difficult. A reaction targeted to result in a uranium fluoride has to be performed in the absence of oxygen to prevent oxidation of U(IV) and the formation of stable uranium oxides. In fact, fluoride high-temperature flux reactions need to be carried out under inert atmospheres or under vacuum, which complicates the synthesis of these compounds © XXXX American Chemical Society
Received: March 5, 2018
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DOI: 10.1021/acs.inorgchem.8b00570 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry possible to use oxidative fluxes for thorium crystal growth. In this paper, we report on the synthesis and properties of new uranium(IV) fluorides as well as on the development of hydrothermal and high-temperature synthetic routes leading to thorium(IV) fluorides. We demonstrate that sodium uranium fluorides can be successfully grown under mild hydrothermal conditions, starting with either UF4 or uranyl acetate. Notably, the application of a mild hydrothermal method and of nitrate flux crystal growth were both highly successful for producing thorium fluorides.
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shutting down the furnace. The resulting product containing orthorhombic NaU2F9 along with a small amount of an unidentified impurity was filtered and washed with distilled water and acetone. According to PXRD estimates, the impurity comprises less than 5% of the sample, and the yield of NaU2F9 is about 68%. KTh2F9 (4). Although numerous attempts had been made to obtain single crystals of this compound hydrothermally, their small size and/ or low quality did not allow us to determine the structure of this compound. However, single crystals of sufficient quality and size could be obtained by a flux reaction of ThF4 in a LiNO3−KNO3 eutectic. A 0.1500 g portion of ThF4 was mixed with 1.000 g of LiNO3−KNO3 eutectic (34% LiNO3 and 66% KNO3 by mass).23 The mixture was placed in a silver crucible and the temperature ramped up to 250 °C in 1 h, kept at this temperature for 20 h, cooled to 100 °C within 15 h, and then to room temperature by shutting down the furnace. The flux was dissolved in distilled water, and the product was filtered and washed with water and acetone. To carry out the structure determination, small colorless crystals of KTh2F9 were picked directly from the product that contained powder impurities of unreacted thorium fluoride. NaTh2F9 (5). This compound was obtained hydrothermally by a reaction of 0.1500 g of ThF4 and 0.0205 g of NaF in 0.50 mL of 49% HF. The starting reagents were loaded and sealed into a PTFE-lined autoclave and the autoclave ramped up to 200 °C at a rate of 1 °C/ min, kept at this temperature for 20 h, and then cooled to 40 °C at a cooling rate of 0.2 °C/min. The product was found to be ThF4 as the major phase along with small well-faceted crystals of NaTh2F9 as the minor phase (less than 10%). Increasing the dwelling time is likely to improve the yield; however, since obtaining a phase-pure NaTh2F9 sample for magnetic measurements is not of significant interest due to the diamagnetic nature of Th(IV), no further attempts to improve the yield and purity were made. (H3O)Th3F13 (6). This compound was grown under hydrothermal conditions using ThF4 and CuC2O4·0.5H2O as starting reagents. A mixture of ThF4 (0.1500 g, 0.487 mmol), CuC2O4·0.5H2O (0.2346 g, 1.461 mmol), 0.50 mL of 49% HF, and 1.00 mL of CH3OH was sealed in an autoclave, which was placed into a furnace. The furnace was ramped up to 200 °C, held at this temperature for 36 h, and then cooled to room temperature by shutting down the furnace. The resulting solution contained a fine powder of red copper metal, which was separated from the main product by decanting off the mother liquor. The final product was filtered and washed with distilled water and acetone and was found to be a mixture of unreacted ThF4, small amounts of copper metal, and (H3O)Th3F13. The crystals of the last species were separated and picked for single-crystal XRD in silicon oil. No further attempts to obtain a phase-pure product or to improve the yield were made. (H3O)U3F13 (7). This compound was obtained by reduction of uranyl acetate in a mixture of HF and methanol in the presence of copper oxalate. A 0.4240 g portion of UO2(CH3CO2)2·2H2O (1.00 mmol) and 0.4816 g of CuC2O4·0.5H2O (3.00 mmol) were mixed and placed into a PTFE-lined autoclave. The autoclave was then charged with 1.00 mL of methanol and 0.50 mL of 49% HF and placed into a furnace, which was ramped up to 200 °C and held at this temperature for 36 h. After the autoclave was cooled by switching off the furnace, it was unsealed and the mother liquor was decanted from the precipitate, which was then rinsed on a filter with 5 mL of concentrated HNO3 to remove copper metal. The resulting green crystals were washed with water and acetone and were found to be a mixture of (H3O)U3F13 along with previously reported U3F12(H2O).24 Numerous attempts to obtain a phase-pure sample of (H3O)U3F13, e.g. by varying uranyl to copper or HF to methanol ratios as well as via the addition of a strong acid, such as HCl, were unsuccessful. Monoclinic Polymorph of NaU2F9. This compound was obtained hydrothermally using uranyl acetate as a precursor. A 0.4240 g portion of UO2(CH3CO2)2·2H2O was mixed with 0.0336 g of NaF and sealed in a 23 mL autoclave along with 1.00 mL of 49% HF. The autoclave was kept at a temperature of 200 °C for 36 h and then cooled to room temperature by shutting off the furnace. The resulting dark green crystals were filtered and washed with distilled water and acetone. The
EXPERIMENTAL SECTION
Reagents. UF4 (International Bio-Analytical Industries, ACS grade), UO2(CH3CO2)2·2H2O (International Bio-Analytical Industries, ACS grade), Th(NO3)4·5H2O (Baker Analyzed), NaF (Alfa Aesar), CuC2O4·0.5H2O (Strem Chemicals), LiNO3 (99+%, Acros Organics), KNO3 (99+%, Alfa Aesar), methanol (99.9%, SigmaAldrich), and 49% hydrofluoric acid solution (EMD) were used as received. Caution: Hydrof luoric acid is toxic and corrosive and must be handled with extreme caution and the appropriate protective gear! If contact with the liquid or vapor occurs, proper treatment procedures should immediately be followed. Caution: Both thorium and uranium, although the uranium precursor used in this synthesis contains depleted uranium, require that proper procedures for handling radioactive materials are observed. All handling of radioactive materials was performed in laboratories specially designated for the study of radioactive actinide materials. ThF4 was obtained by dissolving thorium nitrate (1.50 g) in 5 mL of distilled water in a 23 mL PTFE liner and adding 1 mL of 49% hydrofluoric acid. A PTFE liner with the mixture was sealed in an autoclave and placed into a programmable furnace at a temperature of 125 °C. After the mixture was kept at this temperature for 24 h, the resulting fine powder of ThF4 was filtered, washed with distilled water and acetone, and then dried for 5 h at a temperature of 140 °C in a drying oven. The phase purity of the resulting ThF4 was confirmed by powder X-ray diffraction. Syntheses. Na7U6F31 (1). This compound was produced via the mild hydrothermal route by a reaction of uranyl acetate with sodium fluoride in hydrofluoric acid. A 0.4240 g portion of UO2(CH3CO2)· 2H2O was mixed with 0.0756 g of NaF (molar ratio 1:1.8) and sealed in a PTFE-lined autoclave with 1.00 mL of 49% hydrofluoric acid. The autoclave was placed into a furnace, which was ramped up and held at 200 °C for 36 h. After the furnace was cooled to room temperature by shutting it down, the resulting bright green crystals were filtered and washed with distilled water and acetone. The phase purity of the sample was confirmed by PXRD. The yield (based on U) is 74%. NaUF5 (2). This compound was obtained by a reaction of UF4 with NaF. A 0.2500 g portion of UF4 was mixed with 0.0361 g of NaF, corresponding to a 1:1.08 ratio, in a 23 mL PTFE liner. A 0.25 mL portion of 49% HF and 0.5 mL of distilled water were added to the reaction mixture, and the liner was sealed in an autoclave, which was placed into a furnace and kept at 200 °C for 36 h. The furnace then was turned off and the resulting product, once cool, was filtered and washed with distilled water and acetone. Powder X-ray diffraction showed that the product was NaUF5 with a small, about 5%, impurity of UF4. A single crystal for structure determination was picked directly from the product, which was then thoroughly ground with an additional 0.0030 g of NaF and sealed in the PTFE-lined autoclave with a fresh portion of 0.25 mL of 49% HF and 0.50 mL of distilled water. The autoclave was kept at 200 °C for another 24 h, affording an almost phase pure sample of NaUF5. The yield is about 60%. NaU2F9 (3). Samples of 0.1500 g of UF4 and 0.0108 g of NaF, corresponding to a 2:1.08 ratio, were mixed with 0.25 mL of 49% HF and 1.50 mL of distilled water in a PTFE liner. The liner was sealed in an autoclave, which was placed into a programmable furnace. The reaction temperature was ramped up to 200 °C, held at this temperature for 36 h, and then cooled to room temperature by B
DOI: 10.1021/acs.inorgchem.8b00570 Inorg. Chem. XXXX, XXX, XXX−XXX
chem formula formula wt cryst syst space group, Z a, Å b, Å c, Å β, deg V, Å3 ρcalcd, g/cm3 radiation (λ, Å) μ, mm−1 T, K cryst dimens, mm3 2θ range, deg no. of rflns collected no. of data/params/restraints Rint goodness of fit R1 (I > 2σ(I)) wR2 (all data)
Na7U6F31 2178.11 trigonal R3̅, 3 14.7342(3) 14.7342(3) 9.7973(2) 90 1842.00(8) 5.891 Mo Kα (0.71073) 39.773 302(2) 0.04 × 0.04 × 0.01 2.62−27.49 12555 942/73/0 0.0287 1.141 0.0118 0.0251
1
Table 1. Crystallographic Data for 1−7
48.224 300(2) 0.05 × 0.03 × 0.03 3.44−27.47 6280 816/59/0 0.0311 1.096 0.0132 0.0311
44.608 300(2) 0.04 × 0.02 × 0.01 3.06−27.50 10923 899/73/0 0.0318 1.075 0.0136 0.0275
3 NaU2F9 670.05 orthorhombic Pnma, 4 8.6362(2) 11.1390(3) 6.9996(2) 90 673.35(3) 6.610
2 NaUF5 356.02 orthorhombic Pnma, 8 8.6866(3) 8.0733(2) 10.4014(3) 90 729.45(4) 6.484
4
40.832 302(2) 0.06 × 0.04 × 0.02 3.33−27.50 10227 885/58/0 0.0330 1.080 0.0121 0.0284
KTh2F9 674.18 orthorhombic Pnma, 4 8.8479(13) 11.6515(17) 7.1757(11) 90 739.75(19) 6.053
5
41.451 300(2) 0.04 × 0.03 × 0.03 3.37−27.50 13841 828/60/0 0.0365 1.071 0.0089 0.0200
NaTh2F9 658.07 monoclinic C2/c, 4 11.7673(3) 7.1226(2) 8.8122(2) 102.9863(8) 719.69(3) 6.073
6
42.469 300(2) 0.12 × 0.10 × 0.08 3.61−27.50 7685 1292/89/1 0.0280 1.154 0.0130 0.0319
(H3O)Th3F13 962.14 orthorhombic Pmc21, 2 8.1856(2) 7.4372(2) 8.6411(2) 90 526.05(2) 6.074
7
48.653 300(2) 0.06 × 0.04 × 0.02 3.67−27.50 7989 1215/90/1 0.0361 1.087 0.0104 0.0239
(H3O)U3F13 980.11 orthorhombic Pmc21, 2 8.0358(2) 7.3446(2) 8.4697(2) 90 499.88(2) 6.512
Inorganic Chemistry Article
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DOI: 10.1021/acs.inorgchem.8b00570 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry crystal structure of NaU2F9 was previously reported;15 however, no property measurements accompanied the structural description. Single-Crystal X-ray Diffraction. Single-crystal X-ray diffraction data were collected at 300(2)−302(2) K on a Bruker D8 QUEST diffractometer equipped with an Incoatec IμS 3.0 microfocus radiation source (Mo Kα, λ = 0.71073 Å) and a PHOTON II area detector. The crystals were mounted on a microloop with immersion oil. The raw data reduction and absorption correction were performed using the SAINT and SADABS programs.25,26 Initial structure solutions were obtained with SHELXS-2017 using direct methods. Full-matrix leastsquares refinements against F2 were performed with SHELXL software.27 All of the structures were checked for missing symmetry with the Addsym program implemented into PLATON software, and no higher symmetry was found.28 The crystallographic data and results of the diffraction experiments are summarized in Table 1. Powder X-ray Diffraction. Powder X-ray diffraction (PXRD) data for phase purity confirmation were collected on polycrystalline samples ground from single crystals (Figures S1−S7). Data were collected on a Bruker D2 PHASER diffractometer utilizing Cu Kα radiation. The data were collected over the range from 10 to 65° in 2θ with a step size of 0.02°. Energy-Dispersive Spectroscopy (EDS). EDS was performed on product single crystals using a Tescan Vega-3 SEM instrument equipped with a Thermo EDS attachment. The SEM was operated in low-vacuum mode. Crystals were mounted on an SEM stub with carbon tape and analyzed using a 20 kV accelerating voltage and an 80 s accumulating time. The results of EDS confirm the presence of elements found by single-crystal X-ray diffraction (Table S1). Optical Properties. UV−vis spectra were recorded using a PerkinElmer Lambda 35 UV/visible scanning spectrophotometer used in the diffuse reflectance mode and equipped with an integrating sphere. Diffuse reflectance spectra were recorded in the 300−900 nm range. Reflectance data were converted to absorbance using the Kubelka−Munk function. All optical measurements were performed on polycrystalline powders obtained by grinding the product single crystals. UV−vis reflectance data (Figure S8) for Na7U6F31, NaUF5, and the polymorphs of NaU2F9 share the same features with the spectra of previously reported uranium(IV) compounds.12 The spectra contain several prominent absorption bands with maxima in the ranges of 434−452, 505−514, and 554−566 nm, corresponding to f−f transitions in the U4+ cations. Magnetism. Magnetic property measurements were performed on a Quantum Design MPMS 3 SQUID magnetometer. Zero-field-cooled magnetic susceptibility measurements were performed from 2 to 300 K in an applied field of 0.1 T. The raw data were corrected for radial offset and sample shape effects according to the method described in the literature.29 All magnetic data were collected on polycrystalline powders obtained by grinding the product single crystals. Topological Analysis and Crystal Chemical Calculations. Crystal structure analysis was performed using the TOPOS 4.0 software package.30,31 The method of intersecting spheres was employed for coordination number determination using the AutoCN program.32 Dirichlet and ADS programs were employed for Voronoi− Dirichlet polyhedra construction and topological analysis, respectively. The standard structure simplification procedure was employed to obtain the underlying nets of the compounds.33
using a ratio of 0.3 resulted in the formation of U3F12(H2O) mixed with both polymorphs of NaU2F9. Increasing the ratio gradually decreases the yield of U3F12(H2O) and increases the amount of the NaU2F9 polymorphs until, at a ratio of 0.6, no U3F12(H2O) is detected by PXRD. Any additional increase of the ratio favors the formation of the monoclinic polymorph over the orthorhombic form. The former can be obtained as a phase-pure sample at a ratio of 0.8, indicating that the polymorph formation under these conditions is a function of the sodium concentration. Although the formula of this compound allows one to assume that a sodium to uranium ratio of 0.5 should be sufficient for the formation of this phase, it is noteworthy that the formation of the phase-pure sample of the monoclinic polymorph is observed in a 60% excess of sodium fluoride. Any additional increase in the NaF content for the initial reaction mixture introduces a new product in the system, Na7U6F31, which can be obtained as a phase-pure sample when a ratio of 1.8 is used. It is worth noting that this ratio is ∼54% higher than that required by the formula of this compound, similar to what was observed in the case of monoclinic NaU2F9. The second route for obtaining sodium uranium fluorides is a direct reaction between UF4 and NaF which, surprisingly, enables the formation of different product compositions and is somewhat more straightforward from the point of view of choosing ratios for a desired product. For example, the formation of almost phase pure NaUF5 can be achieved by the reaction between NaF and UF4 in a ratio of 1.08:2, which employs only 8% excess of NaF in comparison to the 60% excess required for reactions utilizing uranyl acetates. It is interesting that a direct reaction between UF4 and NaF in dilute HF (0.25 mL of 49% HF diluted by 1.50 mL of water) results in a phase-pure sample of orthorhombic NaU2F9, whereas the in situ reduction of UO2(CH3CO2)·2H2O in 49% HF produces the monoclinic polymorph. This suggests that the formation of these polymorphs is affected by the amount of water present in the reaction mixture. Indeed, a direct reaction between UF4 and NaF in 0.50 mL of HF and 1.50 mL of methanol produces phase-pure monoclinic NaU2F9. However, the orthorhombic polymorph could not be obtained from uranyl acetate as a precursor because the reduction step requires either the presence of a copper salt that leads to the formation of the quaternary copper compound Na4CuU6F30 or a higher concentration of hydrofluoric acid. The reaction between ThF4 and copper(II) oxalate that results in the formation of a hydronium thorium fluoride, (H3O)Th3F13, can shed light on the role of the copper salts in the reactions with uranyl acetate.19 We attempted to obtain this composition by using ThF4 as a starting material and by varying the solution composition, its acidity, and the dwelling time; however, none of the reactions without a copper salt resulted in single crystals of (H3O)Th3F13. This suggests to us that the reduction of a copper salt introduces nucleation centers to the reaction which facilitate the crystal growth of a thermodynamically more stable phase for a given set of reaction conditions. Ultimately, however, no final conclusion can be drawn on this without a more thorough and systematic investigation of the pertinent solution chemistry of this system, which is beyond the scope of this work. Another notable synthetic feature of this work is the formation of KTh2F9 single crystals in a molten nitrate eutectic flux. This synthesis establishes a new approach for the synthesis of thorium fluorides and, in addition, benefits from both the
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RESULTS AND DISCUSSION Synthesis. The synthesis of different uranium fluorides reported herein was performed by two different routes. The first route, mild hydrothermal synthesis, includes the hydrothermal reaction of uranyl acetate with sodium fluoride in 1 mL of 49% hydrofluoric acid. We performed a series of reactions varying the sodium to uranium ratios in steps of 0.1 from 0.3 to 1.8, the latter being the ratio at which the synthesis of phasepure Na7U6F31 was achieved. In each case the mass of the uranyl acetate sample was 0.4240 g (1 mmol). The first reaction D
DOI: 10.1021/acs.inorgchem.8b00570 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry
whereas the 12 fluorine atoms that bind these 6 uranium atoms are reminiscent of a cuboctahedron. There are 8 possible positions for the disordered fluorine atoms inside the cluster, each of them located below one of the triangular faces of the octahedral cluster. The positions of the disordered fluorine atoms form a slightly distorted cube, which is dual to the uranium octahedron. Each position of the disordered fluorine atoms, therefore, functions as “capping” one of the inside faces of the octahedral cluster. The occupancies of all eight positions add up to one fluorine atom per octahedral cage, and only one position can be occupied by an atom at a time. It is possible, therefore, to consider that the fluoride anion inside the octahedral cluster is centered on one of the uranium octahedron faces, most likely dynamically, since there are no apparent obstacles preventing it from changing between the different locations. As shown in Figure 1d, the overall connectivity of the octahedral clusters exhibits the primitive cubic pcu topology.34 NaUF5 (2). This compound crystallizes in the orthorhombic space group Pnma (No. 62) and contains one uranium, two sodium, and seven fluorine atoms in the asymmetric unit. Both unique sodium sites and four fluorine sites lie on a mirror plane, whereas the rest of the atoms, including a single crystallographic U1 atom, occupy general positions. The structure exhibits a framework built of uranium polyhedra, which are connected through vertex and edge sharing, and contains channels in which the sodium atoms reside (Figure 2). The underlying net of the framework, which consists exclusively of U nodes and the connectivity between them, corresponds to the bnn topology (Figure 2e). Uranium atoms form UF9 coordination polyhedra in the shape of a monocapped tetragonal antiprism (Figure 2c). The U−F bond distances span a range of 2.134(2)−2.486(2) Å, consistent with the reported values for uranium(IV) fluorides.19 Each UF9 polyhedron shares three edges and two vertices with the neighboring units, forming chains of edge-sharing polyhedra running along the b axis (Figure 2d), which in turn are connected via vertex sharing with four other chains. Na1 and Na2 atoms are connected to six and nine fluorine atoms, respectively, with Na−F distances of 2.324(5)−2.894(4) Å. NaU2F9 (3) and KTh2F9 (4). These compounds crystallize in the orthorhombic space group Pnma (No. 62) and are isostructural with the previously reported compounds KU2F9, RbU2F9, and RbTh2F9.17,19,35 The asymmetric unit contains one U or Th, one Na or K, and five fluorine atoms. The structure is a framework built up of vertex- and edge-sharing uranium or thorium polyhedra with channels occupied by the alkali atoms (Figure 3). The framework has the osc topology.34 Each actinide atom is bonded to nine fluorine atoms to form MF9 coordination polyhedra with M−F bond lengths ranging from 2.24775(17) to 2.418(2) Å and from 2.3252(4) to 2.4311(19) Å for M = U, Th, respectively, the decrease being consistent with the actinide contraction.36,37 MF9 polyhedra have the shape of a tricapped trigonal prism and share three edges and three vertices each with the neighboring units. The edge-shared polyhedra form layers in the ac plane, which are connected to each other through vertex sharing to leave channels in the interlayer space, which are occupied by the alkali cations. The alkali cations form six short, 2.417(3)− 2.432(3) and 2.655(2)−2.778(2) Å, and four longer bonds, 2.805(5)−2.855(5) and 3.155(2)−3.292(2) Å, for Na−F and K−F, respectively.
low melting points of the nitrate fluxes and the fact that thorium in ThF4 is in its highest oxidation state. The main advantage of using these low-temperature fluxes for the crystal growth of thorium fluoride systems is that they prevent the formation of thorium oxide, making it possible to perform these reactions in air. Structure Description. Na7U6F31 (1). This compound crystallizes in the trigonal space group R3̅ (No. 148) and is isostructural with the potassium and ammonium analogues K7U6F31 and (NH4)7U6F31, as well as with the sodium zirconium fluoride Na7Zr6F31.16,19 There is one U site at a general position with a multiplicity of 18, two sodium sites with C1 and C3i site symmetries, and seven F sites, two of which (F6 and F7) are partially occupied. Edge- and vertex-shared uranium coordination polyhedra build a framework. The structures of the two isostructural fluorides K7U6F31 and (NH4)7U6F31 have been described in detail previously;19 however, a better crystal quality (R1 = 1.18%) allowed us to find an improved model for the fluorine site disorder. The single crystallographic uranium atom forms a coordination polyhedron in the shape of a square antiprism, which is either capped or uncapped depending on the position of the disordered fluorine atoms F6 and F7. The coordination number of the uranium atoms therefore can be either 8 or 9. The U−F bond lengths are in the range of 2.190(2)−2.358(2) Å for the eight bonds with ordered F atoms, whereas the disordered F atoms form considerably longer bonds of 2.67(1)−2.78(3) Å. A possible reason for this elongation is that the disordered fluorine atoms connect three uranium atoms each, whereas the other F atoms are either shared by two U atoms or connected to a single U atom. The uranium fluoride framework can therefore be described as consisting of [(UF8)6F] vertex sharing uranium clusters (Figure 1). The uranium atoms within the clusters are arranged in an almost regular octahedral fashion,
Figure 1. (a) Octahedral cluster in the structure of Na7U6F31 and (b) its simplified representation emphasizing the disorder of the fluorine atom inside the cluster. (c) Arrangement of the octahedral clusters and (d) their simplified net with the pcu topology.34 Uranium and fluorine atoms are shown in dark and light green, respectively. E
DOI: 10.1021/acs.inorgchem.8b00570 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry
Figure 2. View of the structure of NaUF5 (1) along the b and c axes ((a) and (b), respectively). (c) UF9 monocapped tetragonal-antiprismatic coordination polyhedron and (d) the chains built by edge sharing. (e) Simplified net of the [UF5]− framework with the bnn topology.34 Uranium and fluorine atoms are shown in dark and light green, respectively, and sodium atoms are shown in blue.
thorium and fluorine atoms form a framework with channels in which the sodium cations reside. The coordination polyhedron of the thorium atoms is a capped square antiprism, ThF9, with bond lengths spanning a range of 2.29077(16)−2.4350(19) Å. The ThF9 polyhedra share three edges and two vertices each to form pseudolayers in the bc plane (Figure 4),15 which are connected into a framework via vertex sharing. The layer cation topology exhibits the tts topology; the connectivity between the layers is shown in Figure 4c. The disordered sodium cations reside in the interlayer space. In either of two possible positions of the sodium cation, their coordination number equals 6, with Na−F distances ranging from 2.445(4) to 2.529(4) Å.
Figure 3. View of the structure of orthorhombic NaU2F9 (3) along the a and b axes. The uranium, fluorine, and sodium atoms are shown in dark green, light green, and blue, respectively.
NaTh2F9 (5). This compound was found to crystallize in the monoclinic space group C2/c (No. 15) and to be isostructural with the previously reported monoclinic NaU2F9.15 The structure contains one crystallographically unique Th atom, one half-occupied site of Na atoms, and five additional crystallographic sites for the fluorine atoms. All atoms except F5, which lies on a 2-fold axis, occupy general positions. The
Figure 4. (a) View of a pseudolayer, (b) its simplified net, and (c) the connectivity between the layers in the structure of NaTh2F9 (5). Thorium, sodium, and fluorine atoms are shown in gray, blue, and green, respectively. F
DOI: 10.1021/acs.inorgchem.8b00570 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry (H3O)Th3F13 (6) and (H3O)U3F13 (7). These compounds are the first structurally characterized thorium and uranium hydronium-containing fluorides and adopt the same structural type as previously reported for RbU3F13.19 Both compounds crystallize in the noncentrosymmetric space group Pmc21 and adopt a framework structure with channels occupied by hydronium cations (Figure 5). The asymmetric unit contains
Table 2. Characteristics of O1 Atomic Voronoi−Dirichlet Polyhedra in the Structure of (H3O)U3F13 (7)a atom
d(O1···A), Å
Ω(O1···A), %
F6 F8 F5 F5 F8 F4 F4 F9 F9 F2 F2 F9 F9 other contacts
2.763 2.828 2.863 2.863 2.958 3.161 3.161 3.193 3.193 3.360 3.360 3.630 3.630 >4.00
12.2 11.0 11.5 11.5 9.7 5.7 5.7 6.1 6.1 6.1 6.1 2.9 2.9 2.5
Ω is the corresponding solid angle expressed in percent of the full solid angle of the 4π steradian.
a
Figure 5. View of the crystal structure of (H3O)U3F13 (7) along the c (top) and b (bottom) axes. Uranium and fluorine atoms are shown in dark and light green, respectively, and oxygen atoms in red.
two actinide, one oxygen, and nine fluorine sites. Both actinide sites in the structures of 6 and 7 form AnF9 coordination polyhedra in the shape of a tricapped trigonal bipyramid with An−F bonds ranging from 2.302(7) to 2.474(4) Å and from 2.260(6) to 2.460(3) Å for An = Th, U, respectively. The actinide polyhedra, via vertex and edge sharing, connect into pseudolayers in the bc plane, which are then further connected into a framework structure via edge sharing between the layers. Even though the R1 values for both structural models are quite low (1.30 and 1.04% for 6 and 7, respectively), the hydrogen atoms of the hydronium cations could not be located in the difference Fourier maps because of the presence of significantly heavier elements, Th and U. However, we can assume that the hydronium cations interact with the framework by means of hydrogen bonding along with electrostatic interactions between the negatively charged [An3F13]− framework and the positively charged H3O+ cations. To reveal possible hydrogen bonds, we built Voronoi−Dirichlet polyhedra of O1 atoms and calculated their characteristics (Table 2).32 According to the literature,38 the O···O contacts that correspond to solid angles (Ω) between 10% and 18% can be assigned to possible moderate H bonds, and contacts with Ω 18% correspond to weak and strong H bonds, respectively. The possible H bonds with O1 atoms therefore can be divided into two groups: five moderate H bonds with d(O1···F) ranging from 2.763 to 2.958 Å and six weak bonds with d(O1···F) = 3.161−3.360 Å (Figure 6). Given
Figure 6. Voronoi−Dirichlet polyhedron of the hydronium oxygen atom in the crystal structure of (H3O)U3F13. The fluorine atoms (light green) shown in the figure are possible acceptors for moderate and weak hydrogen bonds.
the fact that a hydronium cation requires only three H-bond acceptors to fulfill its H-bond capacity, there is more than one possible set of H bonds in the structures of 6 and 7. Due to this, the hydrogen atoms of the hydronium atoms are most likely disordered, which makes locating them in the difference Fourier map even more challenging, if even possible. Topological Relations between Polymorphs of NaU2F9. The structure of 3 (NaU2F9) is an orthorhombic polymorphic modification of the monoclinic structure of NaU2F9 that was previously reported.15 In the structures of both polymorphs the uranium atoms are 9-fold coordinated and form the same coordination polyhedron. It is worth pointing out that in both cases the coordination polyhedra have the same local environment as well: that is, each uranium coordination polyhedron shares three edges and two vertices with five neighboring polyhedra that lie in approximately one plane and one vertex with a uranium polyhedron located above that plane, as illustrated in Figure 7a,b. Given that fact, the polymorphs differ in their structural topology rather than in their local environment. To reveal this difference, we performed topological analyses for both polymorphs. G
DOI: 10.1021/acs.inorgchem.8b00570 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry
previously reported NaUF5(H2O).39 The isolated layers in the latter compound have the same topology and can therefore be considered as the “parent” structure for both polymorphs of NaU2F9. The connectivity between the pseudolayers in NaU2F9 is shown in Figure 7e and can be described as alternating chains of U nodes in both cases. In the structure of the orthorhombic polymorph the layers are arranged strictly one above another, i.e. in a “eclipsed” mode, whereas in the monoclinic polymorph one layer is offset ∼4 Å from the next, resulting in only partial overlap of the nodes in comparison to the orthorhombic polymorph (Figure 7h). This distinct layer arrangement leads to a symmetry change as well as to subtle variations in the topology of the compounds. The latter can be illustrated with coordination sequences of the nodes, which are summarized in Table 3 and detail the number of metal atoms (i.e., nodes for a simplified net) in the nth coordination sphere.40−42 Both the first and second coordination spheres are the same for the polymorphs, and a subtle difference starts only with the third coordination sphere, where the orthorhombic polymorph has 42 metal atoms versus 41 atoms for the monoclinic polymorph. It is interesting, but perhaps not too surprising, that the layers in the structures of the orthorhombic and monoclinic polymorphs are related to the layers observed in RbU3F13, (H3O)U3F13, and (H3O)Th3F13. The layer topology of RbU3F13 can be derived from the layers of the NaU2F9 polymorphs by splitting half of the nodes into two, as shown in Figure S10, which is equivalent to the addition of one UF4 group to the initial layer of NaU2F9. The connectivity between the layers also changes, and instead of alternating chains of nodes connecting the other layers, it is only the “split” nodes in the structure of RbU3F13 that are responsible for interlayer connectivity. Magnetic Properties. The magnetic susceptibility data for Na7U6F31, NaUF5, and the monoclinic and orthorhombic polymorphs of NaU2F9, were collected over the temperature range of 2−300 K (Figure 8). All four materials exhibit paramagnetic behavior at temperatures above 150 K, where they follow the Curie−Weiss law. At temperatures below 150 K, in all cases except Na7U6F31, transitions from a triplet to a singlet state, which is characteristic of U(IV) compounds, are observed, accompanied by a decrease in the molar susceptibility. Similar to the case for the previously reported potassium analogue K7U6F31, Na7U6F31 exhibits paramagnetic behavior and shows no apparent transition to a nonmagnetic ground state. This observation for Na7U6F31 confirms our previous suggestion that the U(IV) transition to a nonmagnetic state is a function of the local coordination environment of the U(IV) cation.19 In the structures of NaUF5 and both polymorphs of NaU2F9 all crystallographically unique uranium atoms have a 9fold environment, whereas Na7U6F31 exhibits 8-fold coordination for the uranium atoms, although there is an additional disordered fluorine atom, which serves as a “capping” unit for half of the uranium atoms in the structure, fulfilling their 9-fold coordination.
Figure 7. Local environment of UF9 polyhedra in the structures of the (a) orthorhombic and (b) monoclinic polymorphs of NaU2F9. (c) Pseudolayers and (d) their simplified net (cation topology) in the structure of the orthorhombic polymorph. (e, f) Despite the same topology of the layers and similar connectivity between the pseudolayers, the framework topologies (g, h) are different. In the orthorhombic polymorph (g) the pseudolayers are “eclipsed”, and in the monoclinic polymorph (h) the layers are offset with respect to each other. Uranium and fluorine atoms are shown in dark and light green, and topologies are shown in stick mode.
The structures of the polymorphs are based on the pseudolayers that are connected by vertex sharing and can be equated with the tts topology (Figure 7c ,d). It is noteworthy that the same layers are observed in the crystal structure of the
Table 3. Coordination Sequences for the Orthorhombic and Monoclinic Polymorphs of NaU2F940 coordination sphere no. of atoms orthorhombic NaU2F9 monoclinic NaU2F9
d
1st
2nd
3
4th
5th
6th
7th
8th
6 6
19 19
42 41
72 70
112 110
160 157
218 212
284 278
H
DOI: 10.1021/acs.inorgchem.8b00570 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry
A topological analysis of the structures of these compounds demonstrated that both polymorphs of NaU2F9 and (H3O)U3F13 are based on the same pseudolayer with tts topology, which derives from the structure of the layered NaUF5(H2O) compound. UV−vis data are consistent with the +4 oxidation state for uranium in Na7U6F31, NaUF5, and NaU2F9. Magnetic studies revealed paramagnetic behavior for Na7U6F31, NaUF5, and both polymorphs of NaU2F9. All except Na7U6F31 exhibit transitions to a nonmagnetic singlet state; the absence of the transition in Na7U6F31 is most likely due to the 8-fold coordination environment of the U(IV) in this crystal structure.
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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.inorgchem.8b00570. PXRD patterns, SEM images, and elemental composition of 1−7 (PDF)
Figure 8. Temperature dependence of the molar susceptibility for Na7U6F31, NaUF5, and monoclinic and orthorhombic NaU2F9.
Accession Codes
The inverse susceptibility data for the temperature range of 150−300 K were fitted to the Curie−Weiss law to determine the Weiss constants and the effective magnetic moments (Table 4). The moments for Na7U6F31 and NaU2F9 span a
CCDC 1827658−1827664 contain 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.
Table 4. Weiss Constants and Effective Magnetic Moments per U4+ Atom for Na7U6F31, NaUF5, and Both Polymorphs of NaU2F9 compound
Θ (K)
μeff/μB per U4+
Na7U6F31 NaUF5 monoclinic NaU2F9 orthorhombic NaU2F9
−196 −27.7 −110 −112
3.59 3.08 3.47 3.44
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AUTHOR INFORMATION
Corresponding Author
*E-mail for H.-C.z.L.:
[email protected]. ORCID
Hans-Conrad zur Loye: 0000-0001-7351-9098 Notes
The authors declare no competing financial interest.
range of 3.44−3.59 μB and agree well with the calculated value of 3.58 μB derived from Russell−Saunders coupling for the 3H4 ground state, whereas NaUF5 exhibits a decreased moment of 3.08 μB.43 It is notable that the stoichiometrically analogous RbUF5 exhibits a similar decreased moment of 3.01 μB, although these compounds crystallize in completely different structure types; unlike NaUF5, which is based on a uranium fluoride framework, RbUF5 crystallizes in a layered structure. The Weiss constants vary over a wide range from −27.7 to −196 K, indicating the presence of antiferromagnetic interactions in all four cases.
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ACKNOWLEDGMENTS Research supported by the US Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering under award DE-SC0008664. We are grateful to Dr. Gregory Morrison for his help in collecting magnetic data.
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REFERENCES
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CONCLUSION In this report we report on the synthesis and properties of four new uranium(IV) ternary fluorides and three new thorium ternary fluorides. Two mild hydrothermal synthetic routes were used for the synthesis of new U(IV) fluorides and resulted in different compositions depending on the method employed. In situ reduction reactions with uranyl acetate as a starting material produced Na7U6F31, (H3O)U3F13, and monoclinic NaU2F9, whereas the direct reaction between UF4 and NaF led to the formation of orthorhombic NaU2F9 as well as a new structure type: NaUF5. The direct reaction between ThF4 and KF resulted in a polycrystalline product with no single crystals suitable for X-ray diffraction; however, the use of a LiNO3− KNO3 nitrate eutectic flux yielded crystals of this compound in low yield. I
DOI: 10.1021/acs.inorgchem.8b00570 Inorg. Chem. XXXX, XXX, XXX−XXX
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J
DOI: 10.1021/acs.inorgchem.8b00570 Inorg. Chem. XXXX, XXX, XXX−XXX
