Expanding the Structural Diversity of Uranyl ... - ACS Publications

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From Two-Dimensional Layers to Three-Dimensional Frameworks: Expanding the Structural Diversity of Uranyl Compounds by Cation− Cation Interactions Bin Xiao,†,‡ Hartmut Schlenz,† Jakob Dellen,† Dirk Bosbach,† Evgeny V. Suleimanov,§ and Evgeny V. Alekseev*,†,‡ †

Institute of Energy and Climate Research (IEK-6), Forschungszentrum Jülich GmbH, 52428 Jülich, Germany Institut für Kristallographie, RWTH Aachen University, 52066 Aachen, Germany § Department of Chemistry, Lobachevsky State University of Nizhny Novgorod, 603950, Nizhny Novgorod, Russia ‡

S Supporting Information *

ABSTRACT: Two uranyl tungstates, Cs4[(UO2)4(WO5)(W2O8)O2] (CsUW-1) and Cs4[(UO2)7(WO5)3O3] (CsUW2), were obtained via the high-temperature solid-state synthesis method by reacting uranyl nitrate with WO3 in the presence of cesium nitrate. CsUW-1 crystallizes in space group P21/n, adopting a two-dimensional (2D) sheet structure. CsUW-2 forms a three-dimensional (3D) framework constructed by complex 2D sheets linked by cation−cation interactions of UO22+ groups. The cation−cation interactions fragment presented in CsUW-2 involves the 2D → 3D transformation of the uranyl tungstate network and acts as an intermediate part by bridging the structures of CsUW-1 and CsUW-2. We demonstrate that the chemical and structural transformation from CsUW-1 to CsUW-2 is possible via adding a suitable amount of UO3 oxide. In addition, the differential scanning calorimetry-thermogravimetric technique was carried out to gain insight into the thermal behavior of the synthesized compounds. Raman spectra of titled compounds were obtained and analyzed for signature peaks.

1. INTRODUCTION Over the past two decades, the material chemistry and physicochemical properties of actinide compounds have received considerable attention through the recognition of the importance of long-term nuclear waste disposal and nuclear fuel cycle chemistry.1−7 On the basis of this, the structural chemistry of inorganic uranium(VI) has achieved a rapid expansion, both in minerals and synthetic compounds.8 U(VI) is usually observed in the configuration of an approximately linear dioxo uranyl unit (UO22+), adopting equatorial coordination numbers from four to six, yielding tetragonal, pentagonal and hexagonal bipyramids, and in most cases sharing the equatorial ligands with each other or through other oxoanions to result in infinite sheet structures.9−11 In these cases, the uranyl cations are oriented almost perpendicularly to the equatorial plane, with relatively weak interactions between the uranyl oxygen anions and other lower-valence ions located in the interlayer space. Recent studies among the explosive growth of uranium compounds provide some unprecedented results that are competing and expanding our understanding of such typical uranium(VI) geometry.12−14 In particular, the cation−cation interactions (referred to herein as CCIs), a term used to describe direct binding of one actinyl dioxo ion (AO2n+) with another actinide group, have always gained prominence.15−17 © 2015 American Chemical Society

CCIs provide a means of U polyhedral linkage with anomalous connectivity and enabled the development of novel structural features. First found in aqueous solution studies, CCIs became the key feature of solid-state and molecular actinide chemistry and are particularly well-characterized in Np(V) and U(V) compounds.18−20 CCIs, however, are quite uncommon and account only for about 2% of all uranyl crystal structures.21−25 The most prevailing formation of CCIs occurs in two centers where an axial O atom of one actinyl group behaves as an equatorial ligand in order to coordinate with only one other actinide unit.26 The number of known compounds with multicentered CCIs has increased steadily in response to the active research in actinide chemistry during the past decade. Up to now, 15 different types of CCIs among uranyl ions are recognized.17 CCIs can serve as effective routes to diversify the linkages of uranyl groups and increase the dimensional complexity of uranium compounds. Compared to the overwhelming majority of 2D sheet structures of uranyl moieties, the uranyl compounds containing CCIs prefer to adopt 3D frameworks. Received: March 27, 2015 Revised: June 11, 2015 Published: June 29, 2015 3775

DOI: 10.1021/acs.cgd.5b00427 Cryst. Growth Des. 2015, 15, 3775−3784

Crystal Growth & Design

Article

Pure Phase Synthesis of Cs4[(UO2)7(WO5)3O3] (CsUW-2). CsUW-2 can be synthesized in two different ways: Method 1: Heating the mixture of CsNO3 (66.54 mg), (UO2)(NO3)2(H2O)6 (300 mg), and WO3 (59.36 mg), and with the molar ratio in accordance with the chemical formula of CsUW-2 at 450 °C for 15 h. Afterward, the PXRD technique was carried out to check the product purity. As a next step, the reaction temperature was increased by 50 °C for several steps until the pure phases were obtained (see above). Finally, CsUW-2 could be synthesized as a pure phase at a temperature of around 800 °C (shown in Supporting Information Figure SI1b). Method 2: We found that pure CsUW-2 powder can also be prepared directly from the pure product of CsUW-1. By taking a close look at the chemical formulas of CsUW-1 (can be rewritten as Cs4U4W3O23) and CsUW-2 (can be rewritten as Cs4U7W3O32), one can easily calculate that the formulas of these two compounds only differ by 3 U and 9 O. In this way, the formula’s difference between CsUW-1 and CsUW-2 can be compensated by adding three parts of γUO3. The proposed synthesis method for polycrystalline CsUW-2 is as follows: The as-prepared CsUW-1 precursor and γ-UO3 are mixed in a molar ratio of 1:3 (that is, 300 mg and 107.12 mg for CsUW-1 and γUO3, respectively). After being fully ground in a mortar for 30 min, the mixture is transferred to a Pt crucible and calcined at different temperatures starting from 650 °C with temperature intervals of 50 °C. After each reaction, the homogeneity of CsUW-2 is characterized by PXRD (shown in Figure 4), and the final pure product can be obtained at 800 °C. 2.3. Crystallographic Studies. Crystals of all titled compounds were picked and mounted on an Agilent SuperNova (Dual Source) diffractometer with optical alignment using a digital camera. The crystal data were collected by means of monochromatic Mo−Kα radiation (0.71073 Å), the diffractometer being equipped with microfocus X-ray tube technology, running at 50 kV and 0.8 mA, providing a beam size of approximately 30 μm in diameter. A scan width of 0.75°/ω and an exposure time of 35 s/frame were used for date collection, respectively. Standard CrysAlisPro software was used for calculating the dimensions of the unit cells as well as for controlling data collections. More than a hemisphere of reflections was collected for each crystal. After collection, data were corrected for Lorentz, polarization, absorption,29 and background effects. The SHELXL-97 program was used for the determination and refinement of the structures.30 The detailed crystallographic information is listed in Table 1. The structures were solved by direct methods and refined to R1 = 0.0392 for CsUW-1 and R1 = 0.0463 for CsUW-2, respectively.

While the influence of CCIs on the increase of the uranyl moiety dimensionality has attracted extensive attention, considerably few three-dimensional (3D) frameworks have been found to be directly constructed by low dimensional uranyl compounds through CCIs. A5(UO2)20(UO6)2O16(OH)6(H2O)627 and its NH4 analogue,28 have been reported as rare examples of 3D frameworks constructed via pseudosheets, which are topologically similar to the wellestablished two-dimensional (2D) β-U3O8 structure, formed by CCIs of UO22+ cations. Here, we report the structural and spectroscopic properties of a novel 3D framework, Cs4[(UO2)7(WO5)3O3] (CsUW-2). It consists of a new type of CCIs and can be formed directly by 2D sheets of the uranyl compound Cs4[(UO2)4(WO5)(W2O8)O2] (CsUW-1) via cation−cation interaction of uranyl polyhedra. We will demonstrate a facile synthetic route for new 3D uranium compounds with CCIs and its significant influence on the structural architecture of uranium chemistry.

2. EXPERIMENTAL SECTION 2.1. Synthesis of Single Crystals. While the (UO2)(NO3)2(H2O)6 used in this study contains depleted U, there is really not much compositional dif ference between depleted uranium and natural abundance uranium, and standard precautions for handling radioactive materials should be followed at all times. Crystal Growth of Cs4[(UO2)4(WO5)(W2O8)O2] (CsUW-1). CsUW-1 single crystal samples were prepared by the high-temperature solidstate synthesis method. 100 mg (UO2)(NO3)2(H2O)6 (Merck), 77.63 mg CsNO3 (Alfa-Aesar), and 138.51 mg WO3 (Alfa-Aesar) (molar ratio of U:Cs:W = 1:2:3) were carefully ground and loaded into a platinum crucible. The mixtures were held initially at 1050 °C in air for 5 h in order to get a homogeneous melt. After this, the crystals were cooled down to 400 °C at a rate of 5 °C/h, followed by rapid quenching. The product in the crucible consisted of a yellowish mixture. After the mixture was washed with distilled water, red crystals were separated from the mixture. Crystal Growth of Cs4[(UO2)7(WO5)3O3] (CsUW-2). Single crystals of CsUW-2 were obtained accidently during the process of pure phase synthesis of CsUW-1 (see below). 100.00 mg (UO2)(NO3)2(H2O)6 (Merck), 38.82 mg CsNO3 (Alfa-Aesar), and 34.63 mg WO3 (AlfaAesar) were used. The molar ratio, U: Cs: W = 4:4:3, is the same as for the chemical formula of CsUW-1. The mixture was heated in a platinum crucible at 950 °C for 15 h and was subsequently cooled down to room temperature at a very fast rate (50 °C/h). Again, the product in the crucible consisted of a yellowish mixture. After the mixure was washed with distilled water, red crystals were separated from the mixture. 2.2. Pure Phase Synthesis of Crystalline Powders. Pure polycrystalline powders of all as-grown compounds were prepared by the solid-state reaction method in Pt crucibles in an air atmosphere. The reagents and devices used were the same as for the single crystal growth experiments. Pure Phase Synthesis of Cs4[(UO2)4(WO5)(W2O8)O2] (CsUW-1). 116.45 mg CsNO3, 300 mg (UO2)(NO3)2(H2O)6, and 103.88 mg WO3, respectively, were weighed according to the stoichiometric ratio of CsUW-1 (U:Cs:W = 4:4:3). The mixture was thoroughly ground together before being loaded into the heating furnace. The initial heating temperature was set to 450 °C, and after keeping it for 15 h, Xray powder diffraction data (PXRD) were collected at room temperature in order to analyze the phase content and purity, respectively. The above grinding and heating steps were repeated for a series of different temperatures (temperature intervals of 50 °C), and all samples were checked by PXRD, until the experimental PXRD fitted the theoretical one obtained by single crystal data analysis. Pure polycrystalline powders of CsUW-1 were obtained at 800 °C (see Supporting Information Figure SI1a).

Table 1. Crystallographic Data and Structure Refinement Information for Cs4[(UO2)4(WO5)(W2O8)O2] (CsUW-1) and Cs4[(UO2)7(WO5)3O3] (CsUW-2), Respectively

a

3776

compound

CsUW-1

CsUW-2

formula weight space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3) Z λ (Å) F(000) ρcalcd (g cm−3) R(F) for F02 > 2σ(F02)a Rw(F02)b

2403.28(8) P21/c 8.1990(4) 32.8343(10) 10.7529(6) 90 117.594(4) 90 2565.5(2) 4 0.71073 3976 6.243 0.0392 0.0673

3261.32(5) P21/c 8.6864(4) 41.8958(15) 10.8213(7) 90 116.467(4) 90 3525.4(3) 1 0.71073 5368 6.173 0.0463 0.1480

R(F) = Σ||F0| − |Fc||/Σ|F0|. bR(F20) = [Σw(F20 − F2c )2/Σw(F40)]1/2. DOI: 10.1021/acs.cgd.5b00427 Cryst. Growth Des. 2015, 15, 3775−3784


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Table 2. Selected Bond Distances and Angles for CsUW-1 U(1)O(13) U(1)O(6) U(1)−O(14) U(1)−O(19) U(1)−O(7) U(1)−O(1) U(1)−O(2)

1.805(11) 1.820(10) 2.196(9) 2.368(9) 2.376(8) 2.392(8) 2.437(8)

U(2)O(17) U(2)O(23) U(2)−O(3) U(2)−O(16) U(2)−O(5) U(2)−O(8) U(2)−O(1)

1.797(11) 1.801(12) 2.231(8) 2.321(9) 2.336(9) 2.433(8) 2.479(8)

U(3)O(9) U(3)O(22) U(3)−O(14) U(3)−O(11) U(3)−O(3) U(3)−O(4) U(3)−O(19)

1.808(10) 1.814(11) 2.228(9) 2.320(9) 2.368(8) 2.378(9) 2.552(10)

U(4)O(15) U(4)O(18) U(4)−O(14) U(4)−O(8) U(4)−O(3) U(4)−O(7) U(4)−O(2)

1.810(10) 1.821(11) 2.189(9) 2.323(9) 2.353(8) 2.386(8) 2.519(8)

W(1)−O(12) W(1)−O(2) W(1)−O(8) W(1)−O(7) W(1)−O(1)

1.724(11) 1.874(8) 1.897(9) 1.905(9) 1.915(8)

W(2)−O(20) W(2)−O(5) W(2)−O(19) W(2)−O(10) W(2)−O(4)

1.714(12) 1.795(9) 1.832(10) 1.956(9) 1.966(9)

W(3)−O(21) W(3)−O(11) W(3)−O(16) W(3)−O(10) W(3)−O(4)

1.727(11) 1.794(9) 1.797(9) 1.911(10) 2.118(9)

O(13)−U(1)−O(6) O(14)−U(1)−O(19) O(7)−U(1)−O(1) O(14)−U(1)−O(2) O(7)−U(1)−O(2)

176.9(4) 67.5(3) 64.6(3) 68.9(3) 78.9(3)

O(17)−U(2)−O(23) O(3)−U(2)−O(16) O(16)−U(2)−O(5) O(3)−U(2)−O(8) O(5)−U(2)−O(1) O(8)−U(2)−O(1)

177.0(4) 82.4(3) 71.8(3) 72.3(3) 72.1(3) 62.2(3)


177.6(4) 70.1(3) 85.8(3) 80.8(3) 63.8(3) 60.3(3)


174.3(4) 71.0(3) 72.2(3) 87.3(3) 67.4(3) 63.5(3)

2.4. Powder XRD Characterization. X-ray powder diffraction patterns for all pure phases were carried out on a Bruker-AXS D4 Endeavor diffractometer in Bragg−Brentano geometry with a copper tube, equipped with a primary nickel filter providing Cu Kα radiation (λ = 1.54187 Å) at room temperature. Using a voltage equal to 40 kV and an electric current equal to 40 mA (1.6 kW), the one-dimensional silicon strip LynxEye detector (Bruker-AXS) was adopted to collect data in the range of 2θ from 10° to 80° (total counting time =10 s/ step with the step width of 0.02°). The aperture of the fixed divergence slit was set to 0.2 mm and of the receiving slit to 8.0 mm. The discriminator of the detector was set to an interval 0.16−0.25 V. The collected data were compared to calculated ones derived from single crystal data using the Mercury software.31 2.5. Bond-Valence Analysis. Using the data from Burns,9 the bond-valence sum calculations for uranium confirm that all uranium centers are unambiguous U(VI). The bond-valence parameters for Cs(I)−O and W(IV)−O were used according to Brese and O’Keeffe.32 The results of BVS for all other ions are in good agreement with the expected values. 2.6. Thermal Analysis (TG-DSC). The thermal behavior of the polycrystalline powders of both tungstates was studied from roomtemperature up to 1250 °C by differential scanning calorimetry analysis (DSC) coupled with thermogravimetry (TG) in air atmosphere. This experiment was carried out using a Netzsch STA 449C Jupiter apparatus with a heating rate of 10 °C/min. For each experiment, 20 mg of sample was loaded in a Pt crucible which was closed with a platinum cover. A constant air flow of 20−30 mL/min was applied during the measurements (Figure 5).

2.7. Raman and IR Spectroscopy. The unpolarized Raman spectra were recorded using a Horiba LabRAM HR spectrometer with a Peltier cooled multichannel CCD detector. An objective with a 50× magnification was linked to the spectrometer, allowing the analysis of samples as small as 2 μm in diameter. All the samples were in the form of polycrystalline powders. The incident radiation was performed by a He−Ne laser at a power of 17 mW (λ = 632.81 nm). The focal length of the spectrometer was 800 mm and a 1800 gr/mm grating was used. The spectral resolution was around 1 cm−1 with a slit of 100 μm. The spectra were recorded in the range of 100−1100 cm−1. No photoluminescence (PL) was observed (Figure 6). IR experiments were carried out using a Bruker Equinox spectrometer, and the KBr pellet technique was applied. Approximately 200 mg of KBr and nearly 2 mg for each sample were carefully mixed. Then, a pressure of 10 tons was applied and held constant for 3 min to prepare each pellet. The IR spectra were recorded in the range from 400 to 1050 cm−1. The blank sample of pure KBr pellet was prepared under the same experimental conditions. The results of the infrared spectra for CsUW-1 and CsUW-2 are part of the Supporting Information (Figure SI3). 2.8. Scanning Electron Microscopy/Energy-dispersive Spectroscopy (SEM/EDS). Scanning electron microscopy images and energy-dispersive spectroscopy (SEM/EDS) data were collected on a FEI Quanta 200F environment scanning electron microscope. The EDS results are provided in the Supporting Information (Figure SI2 and Table SI2). 3777



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Figure 1. View of the crystal structures of CsUW-1 and CsUW-2. (a) Polyhedral representation of 2D CsUW-1 projected along the [001] direction. (b) Polyhedral representation of 3D CsUW-2 projected along the [001] direction. (c) View of sheets in CsUW-1. (d) View of sheets without CCIs ions in the structure of CsUW-2. Note that the direct transformation from the crystal structure of CsUW-1 to CsUW-2 is satisfied by cutting CsUW1 along the red dash line and merging the resulting two parts with additional CCIs fragment. Legend: UO7 and UO6 polyhedra are shown in yellow and blue, respectively; WO5 polyhedra units in green; and Cs+ cations in blue nodes.

3. RESULTS AND DISCUSSION 3.1. Crystal Structure of Cs4[(UO2)4(WO5)(W2O8)O2] (CsUW-1). CsUW-1 crystallizes in the monoclinic system with space group P21/c. Selected bond distances and angles are listed in Table 2. The crystal structure is based upon 2D sheets constructed from four symmetrically independent linear UO22+ molecules and three W6+ sites. All UO22+ ions, with UO bond distances in the range of 1.80(1)−1.82(1) Å, are coordinated by five O atoms in each equatorial plane with U−O bond lengths between 2.20(1) and 2.55(1) Å, respectively, resulting in the formation of slightly distorted UO7 pentagonal bipyramids. Four of such UO7 pentagonal bipyramids connect together via sharing equatorial edges to form a tetramer [U4O21] that has a width of two polyhedra, as shown in Figure 1c. Adjacent tetramers are bridged by sharing common O corners from both sides, generating infinite chains that are oriented perpendicular to the b-axis. The resulting uranium chains are connected to [(UO2)4(WO5)(W2O8)O2]4− corrugated sheets parallel to the b-axis, by edge-shared W2O8 dimers on the one side and by WO5 square pyramids on the other side (Figure 1c). The alkali metal cations, Cs+, are disorderedly located in the interlayers to balance the charge, creating overall 2D layered structures. The bond lengths for all three W−O, from 1.71(1) to 2.12(0) Å, are in good agreement with previous results concerning well-refined uranyl tungstates,33−36 with a clear difference displayed between bridging W−O (around 1.86 Å on average) and terminal W−O bond lengths (around 1.72 Å on average). 3.2. Crystal Structure of Cs4[(UO2)7(WO5)3O3] (CsUW2). CsUW-2 was also refined in space group P21/c, containing seven crystallographically different U sites, six of which are 7-

fold coordinated, forming UO7 pentagonal bipyramids, and the remaining one is 6-fold coordinated, forming UO6 tetragonal bipyramids. The selected bond distances and angles are listed in Table 3. The variation of UO distances is appreciable, from 1.76(3) to 1.99(2) Å, and the U−O bonds occur from 2.09(2) to 2.53(2) Å. The 5-fold coordinated environment of the W sites observed in CsUW-2 is quite similar to that of CsUW-1 with the W−O bond distances differing from 1.73(3) to 2.05(2) Å, and the average being 1.84 Å. CsUW-2 is based on a 3D framework and is structurally directly related to CsUW-1. The former is constructed by 2D corrugated uranium-tungstate layers, which are further interlinked by additional uranium polyhedra as CCIs, shown in Figure 1b,d. Similar to CsUW-1, the Cs+ counteractions playing the role of maintaining charge neutrality are also found resided between the adjacent layers of CsUW-2. The interlayer behaviors are different between these two compounds. The additional uranium polyhedra between adjacent sheets in CsUW-2 result in larger interlayer distances (roughly 8.6 Å) in comparison to that of CsUW-1 (roughly 8.2 Å) Structural Relationships between CsUW-1 and CsUW-2. The crystal structures of CsUW-1 and CsUW-2 differ only in one important respect, the containment of complex U(VI) CCIs fragments. The uranium CCIs fragments among neighboring sheets consist of six UO7 pentagonal bipyramids and two UO6 tetragonal bipyramids. Figure 1 highlights such uranium CCIs fragments and the associated structural transition from 2D CsUW-1 to 3D CsUW-2. In CsUW-2, two U(7)O7 pentagonal bipyramids share a common edge to form a pentagonal dimeric unit that is further surrounded by two U(4)O6 tetragonal bipyramids in an edge-sharing manner, leading to a uranium tetramer [U4O21] with the direction 3778



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Table 3. Select Bond Distances and Angles for CsUW-2 U(1)O(18) U(1)O(8) U(1)−O(15) U(1)−O(6) U(1)−O(22) U(1)−O(3) U(1)−O(9)

1.80(3) 1.79(3) 2.23(2) 2.37(2) 2.37(3) 2.39(2) 2.47(2)

U(2)O(28) U(2)O(16) U(2)−O(29) U(2)−O(15) U(2)−O(17) U(2)−O(11) U(2)−O(12)

1.76(3) 1.76(2) 2.19(3) 2.26(2) 2.35(2) 2.48(2) 2.51(3)

U(3)−O(7) U(3)−O(4) U(3)−O(13) U(3)−O(2) U(3)−O(1) U(3)−O(21) U(3)−O(1)

1.78(2) 1.81(2) 2.32(2) 2.34(2) 2.38(2) 2.40(2) 2.47(2)

U(4)O(32) U(4)O(1) U(4)−O(14) U(4)−O(19) U(4)−O(2) U(4)−O(27)

1.79(4) 1.99(2) 2.09(2) 2.11(3) 2.12(2) 2.36(3)

U(5)O(26) U(5)O(30) U(5)−O(29) U(5)−O(3) U(5)−O(15) U(5)−O(20) U(5)−O(24)

1.76(3) 1.78(3) 2.15(3) 2.31(2) 2.33(2) 2.32(2) 2.53(2)

U(6)O(10) U(6)O(25) U(6)−O(29) U(6)−O(20) U(6)−O(12) U(6)−O(9) U(6)−O(24)

1.78(3) 1.81(3) 2.22(3) 2.34(2) 2.36(3) 2.42(2) 2.42(2)

U(7)O(5) U(7)O(23) U(7)−O(19) U(7)−O(19) U(7)−O(11) U(7)−O(14) U(7)−O(17)

1.77(3) 1.79(3) 2.27(3) 2.33(3) 2.37(2) 2.39(2) 2.44(2)

W(1)−O(6) W(1)−O(21) W(1)−O(12) W(1)−O(11) W(1)−O(2)

1.75(2) 1.78(3) 1.82(3) 1.87(2) 2.00(2)

W(2)−O(31) W(2)−O(24) W(2)−O(3) W(2)−O(9) W(2)−O(20)

1.73(3) 1.83(2) 1.86(2) 1.89(2) 1.93(2)

W(3)−O(27) W(3)−O(22) W(3)−O(13) W(3)−O(17) W(3)−O(14)

1.73(3) 1.76(3) 1.77(2) 1.89(2) 2.05(2)


177.3(12) 76.7(9) 74.4(9) 72.5(8) 74.9(7) 62.1(7)


176.4(14) 70.1(9) 96.6(7) 68.7(8) 64.5(10) 60.2(8)


176.8(11) 74.7(8) 63.9(8) 157.4(7) 64.1(7) 68.3(8)


166.6(14) 76.8(11) 83.7(10) 107.8(11) 92.4(10)


175.1(14) 69.5(9) 72.4(7) 88.1(7) 67.3(9) 63.2(8)


177.2(13) 66.8(10) 64.8(7) 68.5(9) 80.3(8)


177.4(13) 68.7(12) 93.1(10) 135.7(10) 69.1(8) 62.1(8)

vertices of two U(3)O7 pentagonal bipyramids, that is to say, each U(4)O6 donors a cation−cation bond to two U(3)O7 polyhedra, exhibiting a three-centered cation−cation bond feature. For this reason, the resulting linked configuration between these four-membered uranium polydedra can be considered as a conformation of doubled three-centered CCIs, which is revealed for the first time in uranyl chemistry (see Figure 2b). Such remarkable configuration is satisfied by the essentially extended U(4)−O(1) bond distance of 1.99(2) Å. This distance is not common to the uranyl oxo-atoms without involving CCIs, but comparable to that in other three-centered

corresponding to the sheet plane. Two such tetramers sit on the upper and lower sheets, respectively, each sharing one edge with a central uranium dimer [U(3)2O12] in order to create such CCIs fragments. The most significant part of this CCIs fragment, as illustrated in Figure 2a, is a four-membered uranyl unit which connects the adjacent sheets. Having cation−cation bonds above and below the layered structure effectively forces U(4)O6 and U(3)O7 to be perpendicular to each other, forming two mutual vertical sheets and thereby resulting in a 3D architecture. Note that the uranyl O(1) of U(4)O6 tetragonal bipyramid simultaneously belongs to the equatorial 3779



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Figure 3. Anion topology describing structural transformation among three cesium uranium tungstates. (a) Anion topology of sheets in Cs2U2WO10. (b) Polyhedral view of pseudosheets in CsUW-2. (c) Anion topology of sheets in CsUW-1. (d) Anion topology of sheets in CsUW-2 without the CCIs fragment. Note that panels (c) and (d), having exactly the same topology, only differ by the orientation of the labeled WO5 tetragonal pyramids.

pyramids along the joint line are fundamentally distinct. The reason for the deviation is the following: in both structures, each WO5 square pyramid shares four corner O atoms in the equatorial plane with surrounding polyhedra, leaving one nonshared apical O atom that can be oriented either “up” and “down”, corresponding to the plane of the sheet without involving intrasheet polymerization. In CsUW-1, two WO5 square pyramids are oriented in opposite directions composing a W2O8 dimer, whereas the corresponding dimer in a reformed CsUW-2 sheet is linked by two WO5, pointing to the same direction. It is worth mentioning that such structural fragments are commonly observed in uranyl tungstates, such as Rb6[(UO2)7(WO5)2(W3O13)O2]41 and Cs2U2WO10.42 The structural motif of Cs2U2WO10 only differs between CsUW-1 and reformed CsUW-2 sheets by the presence of W2O8 dimers. The anion topologies among these three sheets can be easily converted to each other by the simple topological transformation procedure shown in Figure 3. Direct Chemical Transformation from CsUW-1 into CsUW-2 via UO3 Intercalation. On the basis of the above analysis, it is reasonable to expect that CsUW-2 can be synthesized directly through CsUW-1 by adding the required amount of γ-UO3, since these two compounds only differ by CCIs units of uranium polydedra. Therefore, a series of synthesis experiment was carried out for CsUW-2 in order to get a deeper insight into the phase evolution from CsUW-1 to CsUW-2 associated with the CCIs. The detailed procedure is described in the Experimental Section. As a first step, pure CsUW-1 was synthesized and was subsequently used as a starting material for the formation of CsUW-2 via hightemperature solid-state treatment under different temperature conditions, following the sequence

Figure 2. (a) Local coordination geometry of the cation−cation interaction distances in the structure of CsUW-2. The uranyl bonds are highlighted by red sticks. (b) Schematic view of UO22+ groups in CsUW-2. (c, d) The process of topological reconstruction from CCI fragments observed in CsUW-2 to those of γ-UO3. The U and O ions are depicted in black and white nodes, respectively.

donor O atoms, such as around 1.94 Å in Li4[(UO2)10O10(Mo2O8)]26 and 1.967(9) Å in Sr5(UO2)20(UO6)2O16(OH)6(H2O)6,27 respectively. The bond-valence sum calculations for U(3) and U(4) that participate in CCIs are 5.85 and 5.95 valence units, respectively, which are in good agreement with the expected value for U(VI). As mentioned above, apart from the CCIs fragment, the remaining structural part of CsUW-2 shows an equivalent coordination geometry compared to that in CsUW-1. For the purpose of understanding the linkage relationship of U and W polyhedra within these two structures, the approach of aniontopology37 is adopted to compare the sheet of CsUW-1 with the single-layered sheet extracted from CsUW-2 (without regard to the CCIs fragment, hereafter called a reformed CsUW-2 sheet). This method is especially convenient for the representation of complex 2D uranium architectures.38−40 As shown in Figure 3c,d, both sheets share the same topology, where the pentagons and squares are occupied by UO7 and WO5, respectively. Therefore, one can assume that the 2D → 3D transformation only involves “cut” and “glue” procedures, without any reconstruction of the polyhedral connectivity among the sheets. However, further analysis of the polydedral diagrams shown in Figure 3c,d indicates that the orientations of WO5 square

4CsNO3 + 4(UO2 )(NO3)2 (H 2O)6 + 3WO3 → Cs4[(UO2 )4 (WO5 )(W2O8)O2 ] 3780

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Figure 4. Experimental PXRD diagrams for the temperature evolution from CsUW-1 to CsUW-2. For comparison, the theoretical pattern of CsUW2 is also included. The diffraction patterns were obtained after grinding and heating mixtures of pure polycrystalline powders of CsUW-1 and UO3 with the molar ratio of 3:1 at different temperatures from 650 to 800 °C.

Figure 5. DSC and TG curves for (top) Cs4[(UO2)4(WO5)(W2O8)O2] (CsUW-1) and (bottom) Cs4[(UO2)7(WO5)3O3] (CsUW-2), respectively.

can be prepared directly from CsUW-1 by adding the corresponding molar weight of γ-UO3. It has to be noted that the CCIs fragments observed in the structure of CsUW-2 reveal a dramatic similarity to those found in γ-UO3, the latter also containing three-centered CCIs fragments.43 The detailed examination of these structural fragments shows that they can be transformed into one another by simple “cut” and “glue” topological reconstruction (see Figure 2). The suggestion may be confirmed that the reaction pathway includes the most stable fragments of both CsUW-1 and γ-UO3, resulting in the structure of CsUW-2 as a superposition. It is quite similar to that of classical syntheses of organic molecular materials.

Cs4[(UO2 )4 (WO5 )(W2O8)O2 ] + 3γ ‐UO3 → Cs4[(UO2 )7 (WO5 )3 O3]

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The measured PXRD patterns show the evolution course from mixed CsUW-1 and UO3 to the single CsUW-2 phase (Figure 4). The first observed PXRD pattern after annealing at 650 °C shows overlapping peaks of CsUW-1 and γ-UO3. With increasing temperature, the PXRD patterns develop more and more to that of CsUW-2 pattern. Finally, the PXRD peaks observed at 800 °C are totally consistent with those expected for CsUW-2. This indicates that CsUW-2 is the only phase synthesized by reaction II at 800 °C, concluding that CsUW-2 3781



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Figure 6. Raman spectra of Cs4[(UO2)4(WO5)(W2O8)O2] (CsUW-1) and Cs4[(UO2)7(WO5)3O3] (CsUW-2), respectively.

overlapping modes of the U and W polydedra,48,49 respectively, and because hardly any reference data for vibrational modes of WO5 polyhedra are available in the literature, here we abstain from any further specific assignment of frequencies. The differences between the structures of CsUW-1 and CsUW-2 (Figure 6) have an obvious impact on the vibrational properties of these compounds. The peaks obtained for the compound CsUW-2 at 828(2) cm−1, 871(1) cm−1, and 879(1) cm−1 are almost absent for CsUW-1, which can be presumed to result from the vibrations of U−O inside the CCIs unit. The antisymmetrical stretching mode of (UO2)2+ shows a shift to lower frequencies from CsUW-1 to CsUW-2 in the region 930−960 cm−1. This deviation might be explained as a consequence of the different U−O terminal bond distances.50,51 Owing to CCIs, the U−O bond distances are more abnormal in CsUW-2 (U−Oterminal ranging from 1.80(1) Å to 1.82(1) Å for CsUW-1 whereas, from 1.76(2) Å to 1.99(2) Å for CsUW-2). The peaks between 750 and 800 cm−1 for both compounds could be assigned to the symmetric stretching (v1) of UO22+.52 The stretching modes for W2O8 dimers (with W−O−W connection) in CsUW-1 increase up to 1002(6) cm−1. In CsUW-2 where there is no such W2O8 dimers geometry, and therefore there are no stretching modes beyond 955(7) cm−1. This phenomenon is also demonstrated in many lanthanide tungstate compounds, as the bridging of WOx (x = 4, 5, 6) polydedra in the structure will result in a wider distribution of vibrational frequencies.53

3.3. Thermal Analysis. The DSC-TG curves of CsUW-1 and CsUW-2 powders in the temperature range from 300 to 1200 °C are shown in Figure 5. The endothermic onset around 914 °C observed in the DSC measurement of CsUW-1 corresponds to its melting point. A small peak located at 964(3) °C beside it demonstrates the sample decomposition. Accompanied by a small weight loss in the TG curve in this range, this reveals an incongruent melting behavior of CsUW-1. As for the case of CsUW-2, the thermal decomposition could be divided into two steps. The first step, starting at a temperature of around 840(8)°C up to 870(8)°C, but not obvious in the TG curve as a mass loss, shows a strong endothermic effect, which is related to sample melting. This is confirmed by heating bulk polycrystalline CsUW-2 powders across the corresponding peak (855(3) °C). The PXRD technique shows the dark black melting products are dominated by U3O8. At higher temperatures, the second endothermic peak at around 1125(3) °C can be explained by the evaporation of the thermal decomposition products. 3.4. Raman Spectroscopic Analysis. The Raman spectra for as-synthesized uranium tungstates measured in the range of 100−1100 cm−1 region are displayed in Figure 6. It is obvious to coarsely divide the spectra into two frequency parts: a low frequency part between 100 and 250 cm−1 where the modes are contributed from the vibrations of the “lattice skeleton”, and a high-frequency part from 300 to 1100 cm−1, which are are dominated by modes from the uranium and tungstate polyhedra. Jones demonstrated that the uranium unit can be simplified as a linear triatomic molecule (UO2)2+ with D∞h symmetry owing to the lack of coupling between the vibrational frequencies of equatorial ligands and those of the uranyl moiety.29 In aqueous solution, the characteristic vibrations of (UO2)2+ are three normal modes, that is, v1 symmetric stretching mode (from 860 to 880 cm−1), v3 antisymmetrical stretching (from 930 to 960 cm−1), and the v2 bending mode (from 199 to 210 cm−1).44,45 Both compounds only contain W ions that are 5-fold coordinated (WO5 square pyramids). Some of the modes for (UO2)2+ and WO5 mentioned above can be recognized in Figure 6. However, band positions will shift and split associated with a symmetry change specific for the respective the crystalline environment.46,47 Because of the

4. CONCLUSION CsUW-1 and CsUW-2 are rare in actinides chemistry, since they are associated with each other via introducing of a CCIs based on UO22+. The CCI topology observed in CsUW-2, to our knowledge, is revealed for the first time in uranyl chemistry. It is composed by four-membered uranium polyhedra with the connection that can be considered as a conformation of doubled three-centered CCIs. The structural transformation from 2D CsUW-1 to 3D CsUW-2 indicates the role of CCIs in forming a 3D framework by the unusual uranium connection geometry. Overall, the sharing of axial O atoms between uranyl polyhedra in general demonstrates that CCIs can provide 3D linkages between 2D sheets of uranyl compounds. This leads to 3782



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more structural flexibility of complexation compared to the exclusive sharing of equatorial O atoms, enhancing the possibility of more diversifying structural themes. CsUW-2 contains structural discrete units which are commonly observed among uranium compounds including CsUW-1. The question is what is the mechanism of CsUW-2 formation in reaction between CsUW-1 and γ-UO3? We suppose that, at the high temperature region, complex structural recombination occurred between U−W layers of CsUW-1 and CCIs fragments of γ-UO3. We expect that U−W fragments which were observed in both CsUW-1 and CsUW-2 are the most stable aggregations of uranyl and oxo-tungstate groups at elevated temperatures. Ideologically this process is quite similar to the reactions between organic molecules where different ligands or functional groups bonded together to form a new material. In this way, both U−W fragments from CsUW1 and CCIs fragments from γ-UO3 can be considered as “functional groups” which are combined together to form CsUW-2. This speculation is also supported by the existence of numerous uranyl compounds with similar U−W fragments obtained from similar temperature conditions.41,42 From a topological point of view, CsUW-2 can be considered as being composed from known structural pieces by an approach similar to a “jigsaw puzzle”. The pieces of the “puzzle”, represented by the layered fragments of CsUW-1 and CCI parts of γ-UO3, are locked together to complete the structure of CsUW-2 at suitable temperature. Of course, more studies are needed to be done before one can make a solid conclusion regarding on the generality of this method. Nonetheless, this report exhibits a potential way that is helpful for designing of actinyl compounds via introducing of CCI fragments at elevated temperatures. All in all, given the dominance of 2D sheets appearing in the crystal chemistry of U(VI), the ability of CCIs in increasing the dimensionality of uranium structures is remarkable, and there is a high possibility that it can originate novel U(VI) topologies.

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ASSOCIATED CONTENT

S Supporting Information *

Powder XRD patterns for pure phase syntheses. EDS results of obtained crystals and IR spectra. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.cgd.5b00427.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful to the Helmholtz Association for funding within the VH-NG-815 Grant. REFERENCES

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