Influence of Synthetic Conditions on Chemistry and Structural

Article pubs.acs.org/crystal

Influence of Synthetic Conditions on Chemistry and Structural Properties of Alkaline Earth Uranyl Borates Yucheng Hao,†,‡ Vladislav V. Klepov,∥ Gabriel L. Murphy,§ Giuseppe Modolo,† Dirk Bosbach,† Thomas E. Albrecht-Schmitt,⊥ Brendan J. Kennedy,§ Shuao Wang,# 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, Samara National Research University, 443086 Samara, Russia § School of Chemistry, The University of Sydney, Sydney, NSW 2006, Australia ⊥ Department of Chemistry and Biochemistry, Florida State University, 95 Chieftan Way, Tallahassee, Florida 32306-4390, United States # School for Radiologic and Interdisciplinary Sciences (RAD-X) and Collaborative Innovation Center of Radiation Medicine of Jiangsu Higher Education Institutions, Jiangsu 215123, China ‡

S Supporting Information *

ABSTRACT: Four novel alkaline earth metal uranyl borates, namely, A[(UO 2 ) 5 (BO 3 ) 2 O 2 (OH) 2 ]·5H 2 O (SrUBO-1, BaUBO-1) and A[(UO2)2(B2O5)O] (SrUBO-2, BaUBO-2) (A = Sr, Ba), have been prepared; SrUBO-1, BaUBO-1, and BaUBO-2 were synthesized using hydrothermal methods, whereas SrUBO-2 was prepared by a traditional hightemperature solid-state method. The compounds SrUBO-1 and BaUBO-1 were found to be iso-structural and crystallized in the centrosymmetric group C2/c. Their structure features two-dimensional anionic layers {[(UO2)5(BO3)2O2(OH)2]2−}n that are composed of [(UO2)5O2(OH)2]4+ clusters and [BO3]3− polyanions. From a topological point of view, this two-dimensional anionic layer can be described as a new 4-nodal net topological type with a point symbol of {35.45}2{36.46.53}3{37.47.57}2. SrUBO-2 and BaUBO-2 are iso-structural and crystallized in space group C2/m. Their structures are based on a three-dimensional framework {[(UO2)2(B2O5)O]2−}n. Within the three-dimensional framework, two-dimensional U(2)−U(1) = U(3) layers (L1) with cation to cation interactions between [U(3)O2]2+ groups and one-dimensional edge sharing U(1)−U(2) chains are present. The B2O5 dimers corner sharing with U(1)-U2 chains and U(3)O6 tetragonal bipyramids forming two-dimensional layers (L2). L1 further vertically interconnect with L2 through bridging B2O5 dimers, forming a threedimensional framework [(UO2)2(B2O5)O]2−. The synthetic conditions, structural characterization, thermal stability, as well as spectroscopic properties of the alkaline earth metal uranyl borates are reported and discussed. countries.24 In the context of the development of nuclear waste management technologies, an extensive understanding of actinide borates as models for the crystallized portions of vitrified nuclear waste needs to be developed. Since the first actinide borate, K6{UO2[B16O24(OH)8]}(H2O)12,25 was reported in 1985, a plethora of uranyl borate structures have been reported, many of them by Wang et al.16 This includes the well-studied uranyl borates U(BO3)2, (UO2)(B2O4), (UO2)2(B9O14(OH)4), (UO2)2(B13O20(OH)3)(H2O)1.25 and (UO2)(B8O11(OH)4), UB4O8.26−29 Structurally, they range from simple 2D layers to more complex 3D frameworks. More than 20 alkali metal uranyl borates have been described since 1985. Among them, A(UO2) (BO3)(A =

1. INTRODUCTION Inorganic oxo-borate compounds possess a variety of chemical compositions and structural types.1−10 They have a wide range of structure dependent applications including, adsorption, catalysis, ion exchange, luminescence, and nonlinear optics (NLO).5−12 This makes the synthesis and investigation of novel oxo-borate systems an area of increasing interest. Commonly, there are two types of B−O groups, BO3 triangle and BO4 tetrahedron. These two groups can connect with each other through vertex or edge sharing to form numerous kinds of structures.13−15 The addition of oxo-anions (PO43−, AlO45−, SiO44−, GeO44−, SO42−, etc.) allows for a myriad of new borate derived classes of compounds.16−22 Knowledge of actinide borate structures may be used for better understanding of actinides local coordination in vitrified nuclear wastes.23 Since commercial nuclear energy production began in the mid-1950s, it has developed into a key energy source for several © 2016 American Chemical Society

Received: June 30, 2016 Revised: August 10, 2016 Published: September 8, 2016 5923

DOI: 10.1021/acs.cgd.6b00978 Cryst. Growth Des. 2016, 16, 5923−5931

Crystal Growth & Design

Article

Li, Na)30,31 reported by Gasperin and A(UO2) (BO3)(A = K, Rb, Cs)32 reported by Wu were prepared using molten B2O3 as a flux in high temperature solid state reactions. A variety of alkali metal uranyl borates were also reported by Wang et al., 33−35 including Li[UO 2 B 5 O 9 ](H 2 O) 3 , Na[(UO 2 )(B6O10(OH)](H2O)2, Na[(UO2)2(B10O15(OH)5], Na[(UO2)(B10O15(OH)5](H2O)3, K2[(UO2)2B12O19(OH)4)](H2O)0.3, A[(UO 2 ) 2 B 1 0 O 1 5 (OH) 3 ](H 2 O) 0 . 7 )(A = K, Rb), K[(UO 2 ) 2 B 10 O 15 (OH) 5 ], Rb[(UO 2 ) 2 B 10 O 16 (OH) 5 ], Cs[(UO2)2B11O16(OH)6], that were prepared by hydrothermal methods using excess H3BO3 as a flux. These compounds shared a common motif consisting of a linear uranyl core (UO2)2+surrounded by BO3 triangles and BO4 tetrahedra forming a UO8 hexagonal bipyramid. Wu et al. have also reported three potassium uranyl borates prepared using hightemperature/high-pressure (HT/HP) hydrothermal methods.36 Whereas the three aforementioned borates are based on twodimensional (2D) uranyl borate layers, only BO3 triangles exist in the potassium uranyl borates. Four pseudo alkali metal uranyl borates Ag[(UO2)(B5O8(OH)2],33 α,βTl 2[(UO2)2B11O18(OH)3], Tl[(UO2 )2B10O16(OH) 3], and Tl2[(UO2)2B11O19(OH)]34 were also reported by Wang et al. The structures of these resemble the above alkali metal uranyl borates, and are based on a common structural motif. To date, only two alkaline earth metal uranyl borates have been reported that do not contain other oxo-anions, namely, Mg(UO2)B2O537 and Ca[(UO2)2(B2O5)O].38 The structures and physical properties of these are significantly different than those of the alkali metal uranyl borates. Here we report the synthesis of four novel alkaline earth metal uranyl borates, namely, A[(UO2)5(BO3)2O2(OH)2] 5H 2 O (SrUBO-1, BaUBO-1) and A[(UO 2 ) 2 (B 2 O 5 )O] (SrUBO-2, BaUBO-2) (A = Sr, Ba). These have been structurally characterized and investigated using thermogravimetric and differential scanning calorimetry as well as Raman spectroscopy.

BaUBO-1 crystal, as well as partial occupation of barium positions, the refinement is not as good as for other studied phases. Energy dispersive X-ray spectroscopy (EDS) elemental analysis on several single crystals of both compounds gave average molar ratios of Sr:U = 1:4.57 and Ba:U = 1:4.72, respectively, for SrUBO-1 and BaUBO-1, which are in good agreement with those obtained from single crystal X-ray diffraction studies (Figure S1a, S1b). 2.1.2. Synthesis of SrUBO-2. The starting composition of UO2(NO3)2·6H2O (0.0528 g, 0.10 mmol), Sr(NO3)2 (0.0216 g, 0.10 mmol), H3BO3 (0.0619 g, 1.05 mmol), and Li2B4O7 (0.0253 g, 0.15 mmol) with a ratio of U:Sr:B = 1:1:16 for SrUBO-2. All the reactants were thoroughly ground in an agate mortar and then transferred to a platinum crucible. The reaction mixtures were heated up to 965 °C for 10 h in a box furnace and then cooled down to 450 °C at a cooling rate of 5 °C/h before the furnace was switched off. Orange block crystals, SrUBO-2, were obtained, along with yellow plate-shaped crystals which were determined to be LiUBO5.25 Pure polycrystalline samples of SrUBO-2 were synthesized quantitatively by the reaction of a mixture of UO2(NO3)2·6H2O (0.2028 g, 0.40 mmol), SrCO3 (0.0297 g, 0.20 mmol), and H3BO3 (0.0247 g, 0.40 mmol) with a molar ratio of 2:1:2 at 900 °C for 2 days. The obtained product was characterized with X-ray powder diffraction (XRD) and Rietveld refinement. It indicates that SrUBO-2 was obtained with a high purity and the yield is over 96% (Figure 6). EDS elemental analyses on several single crystals of this compound gave an average molar ratio of Sr:U = 1:1.89, which is in good agreement with the proposed chemical compositions (Figure S1c). 2.1.3. Synthesis of BaUBO-2. A mixture of UO2(NO3)2·6H2O (0.0532 g, 0.12 mmol), Ba(NO3)2 (0.0266 g, 0.11 mmol), H3BO3 (0.0316 g, 0.52 mmol), Li2B4O7 (0.0852 g, 0.51 mmol), and deionized water (1.5 mL) in a ratio of U:Ba:B = 1:1:25 was sealed into a Teflonlined stainless steel autoclave (23 mL) and transferred into a programmable furnace, heated to 220 °C, held 36 h, and then cooled to room temperature at a rate of 4 °C/h. The product was washed with boiling water to remove excess boric acid before being rinsed with ethanol. Orange block-shaped crystals were collected. The yield of BaUBO-2 is a little bit lower than the other phases described above, and it is around 39% based on U content. EDS analysis on several single crystals gave an average molar ratio of Ba:U = 1:1.91, which is in good agreement with its proposed chemical compositions (Figure S1d). 2.2. Crystallographic Studies. Single crystal diffraction data for all four compounds were collected on an Agilent Technologies SuperNova diffractometer with Mo Kα radiation (λ = 0.71073 Å) at room temperature. All data sets were corrected for Lorentz and polarization factors as well as for absorption by the multiscan method.39 The structures of all compounds were solved by direct methods and refined by a full-matrix least-squares fitting on F2 by SHELX-97.40 All structures were checked for possible missing symmetry elements using PLATON with the ADDSYM algorithm, and no higher symmetry solutions were found.41 Crystallographic data and structural refinements for four compounds are summarized in Table 1. More information about the important bond distances and angles are listed in Table S1 in Supporting Information. X-ray powder diffraction data were measured on a Bruker-AXS D4 Endeavor diffractometer, 40 kV/40 mA, in Bragg−Brentano geometry. The diffractometer is equipped with a copper X-ray tube and a primary nickel filter producing Cu Kα radiation (λ = 1.54187 Å). A linear silicon strip LynxEye detector (Bruker-AXS) was used. Data were recorded in the range of 2θ = 10−80° with 10 s/step and a step width of 0.02°. The aperture of the fixed divergence slit and the receiving slit was set to 0.2 mm and 8.0 mm, respectively. The discriminator of the detector was set to an interval from 0.16 to 0.25 V. 2.3. SEM/EDS Analysis. For elemental analysis, scanning electron microscopy (SEM) images and EDS measurements were collected on a FEI Quanta 200F Environment Scanning Electron Microscope with a low-vacuum mode at 0.6 mbar. SEM/EDS results are given as Supporting Information (Figure S2). 2.4. Thermal Analysis. The thermal behavior of the dried powder of SrUBO-2, up to 1200 °C, was studied by differential scanning

2. EXPERIMENTAL SECTION Caution! The UO2(NO3)2·6H2O used in this study contained natural uranium; nevertheless, the standard precautions for handling radioactive materials must be followed. 2.1. Materials and Methods. Uranyl nitrate UO2(NO3)2·6H2O (International Bioanalytical Industries, Inc.), strontium nitrate Sr(NO3)2 (Alfa-Aesar, 99.9%), strontium carbonate SrCO3 (Alfa-Aesar, 99.9%), barium nitrate Ba(NO3)2 (Alfa-Aesar, 99.9%), lithium tetraborate Li2B4O7 (Alfa-Aesar, 99.9%), and boric acid H3BO3 (Alfa-Aesar, 97%) were all used as received. 2.1.1. Syntheses of SrUBO-1 and BaUBO-1. Both compounds were synthesized by a hydrothermal method. The compositions UO2(NO3)2·6H2O (0.0526 g, 0.10 mmol), Sr(NO3)2 (0.0212 g, 0.10 mmol), H3BO3 (0.0621 g, 1.05 mmol), Li2B4O7 (0.0426 g, 0.25 mmol), and deionized water (2 mL), in a ratio of U:Sr:B = 1:1:20 for SrUBO-1; UO2(NO3)2·6H2O (0.0526 g, 0.10 mmol), Ba(NO3)2 (0.0262 g, 0.10 mmol), H3BO3 (0.0496 g, 0.80 mmol), Li2B4O7 (0.0426 g, 0.25 mmol), and deionized water (2 mL) in a ratio of U:Ba:B = 1:1:18 for BaUBO-1 were sealed into Teflon-lined stainless steel autoclaves (23 mL) and then transferred into a box furnace, heated to 220 °C, held 24 h, and then cooled to room temperature at a rate of 3 °C/h. The resulting products were washed with hot water and then rinsed with ethanol to remove excess boric acid. Yellowish parallelogram and triangular pallet shaped crystals (SrUBO-1 and BaUBO-1) were obtained as well as unknown orange needle-shaped crystals with bad qualities. The yields of (SrUBO-1 and BaUBO-1) are 52% and 47%, respectively, based on U content. Fine crystals were collected for further analyses. Due to the nonmerohedral twinning of 5924



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states. BVS of all atoms in the four alkaline earth uranyl borates phases were calculated and agree well with corresponding formal values of oxidation states. The bond-valence parameters for U(VI)-O, Sr/ Ba(II)-O, and B(III)-O were used according to Brese and O’Keeffe.42,43

Table 1. Crystal Data and Structure Refinements for SrUBO1, BaUBO-1, SrUBO-2, and BaUBO-2a compound

SrUBO-1

BaUBO-1

SrUBO-2

BaUBO-2

FW Space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3) Z λ(Å) F(000) Dc(g cm−3) GOF on F2 R1 wR2

1711.47 C2/c 18.2199(6) 8.9303(2) 16.1231(6) 90 117.502(4) 90 2325.95(13) 4 0.71073 2832 4.807 1.072 0.0468 0.1321

1761.17 C2/c 18.3785(15) 8.8531(3) 17.9181(11) 90 123.823(10) 90 2425.3(3) 4 0.71073 3059 5.232 1.175 0.0661 0.1687

745.30 C2/m 16.7901(5) 8.3533(2) 6.6557(2) 90 98.032(2) 90 924.32(4) 4 0.71073 1248 5.356 0.796 0.0259 0.0699

795.01 C2/m 16.8063(9) 8.3444(4) 6.6524(3) 90 98.084(5) 90 923.66(8) 4 0.71073 1288 5.784 1.100 0.0536 0.1297

3. RESULTS AND DISCUSSION 3.1. Syntheses. Exploration of the A−U−B−O (A = Sr, Ba) system, uncovered four novel alkaline earth metal uranyl borates SrUBO-1, BaUBO-1, SrUBO-2, and BaUBO-2. The compounds SrUBO-1, BaUBO-1, and BaUBO-2 were obtained through mild hydrothermal synthesis at 220 °C. It is noteworthy that the H3BO3 played the role of reagent and flux in these syntheses as previously described by Wang et al.2 The importance of the starting ratio of UO2(NO3)2·6H2O:Sr(NO3)2:H3BO3:Li2B4O7 was examined and reactions with the ratios 1:1:4:2.5, 1:1:10:2.5, and 1:1:16:2.5 were investigated; however only with the ratio 1:1:10:2.5 was SrUBO-1 isolated. It is interesting to note that when the reagent ratio of UO2(NO3)2·6H2O:Ba(NO3)2:H3BO3:Li2B4O7 was changed from 1:1:8:2.5 to 1:1:5:5, a second polymorph of BaUBO-2 was obtained that has a 3D network rather than the 2D arrangement seen in BaUBO-1. SrUBO-2 was synthesized by traditional high-temperature solid-state synthesis using a reagent ratio of UO2(NO3)2·6H2O:Sr(NO3)2:H3BO3:Li2B4O7 of 1:1:10:1.5. We performed a synthesis of SrUBO-2 without Li 2 B 4 O 7 with initial ratios of UO 2 (NO 3 ) 2 ·6(H 2 O):Sr(NO3)2:H3BO3 of 1:1:8, 1:1:12, and 1:1:18 at 965 °C for 4 days. The only crystalline phase was a strontium uranate SrU2O7. Thus, we presumed that Li2B4O7 is an important boron source in the reaction for preparation of SrUBO-2. We attempted to obtain BaUBO-2 via a high temperature reaction replacing strontium nitrate by barium nitrate; however, this was unsuccessful. Thus, it is presumed that both H3BO3 and Li2B4O7 are necessary for the synthesis, where they act as both reagents and reaction medium. The preparation of uranyl borates by traditional hydrothermal syntheses is known to be difficult due to the higher

R 1 = ∑∥F o| − |F c∥/∑|F o|, wR 2 = {∑w[(F o ) 2 − (F c ) 2 ] 2 / ∑w[(Fo)2]2}1/2.

a

calorimetry (DSC) analysis coupled with thermogravimetry (TG) in air at a heating rate of 10 °C/min using a Netzsch STA 449C Jupiter apparatus. The sample (20.5 mg) was loaded in a platinum crucible, which was closed with a platinum cover. During the measurements a constant air flow of 20−30 mL/min was applied. 2.5. Raman Spectroscopy. Unpolarized Raman spectra were recorded with a Horiba LabRAM HR spectrometer using a Peltier cooled multichannel CCD detector. An objective lens 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 single crystals. The incident radiation was produced by a He−Ne laser line at a power of 17 mW (λ = 632.8 nm). The focal length of the spectrometer was 800 mm, and an 1800 gr/mm grating was used. The spectral resolution was approximately 1 cm−1 with a slit of 100 μm. The spectra were recorded in the range of 100−4000 cm−1. 2.6. Bond-Valence Analysis. Bond-valence sums (BVS) are a semiempirical method for the approximate determination of valence

Figure 1. Coordination environments for the uranium and boron sites within SrUBO-1 (a), a 2D uranyl borate layer along [1, 0, −1] (b), view of the structure along the b-axis (c). Uranyl polyhedra and BO3 triangles are shown in yellow and green, strontium and oxygen are shown in blue and red, respectively. 5925



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affinity of water, over borate, to coordinate to metal centers.23 A feature of Wang’s investigations33−35 is the use of a large excess of boric acid as flux (B/U > 10) but with relatively small amounts of water (∼0.05 mL), to prepare alkali metal uranyl borates using low-temperature (180 to 220 °C) flux methods. We have explored a similar experimental protocol for the alkaline earth metal uranyl borates system; however, our attempts, to date, have been unsuccessful. When Li2B4O7 and much more water (>1.5 mL) were used as initial reagents and medium in the present work, three alkaline earth metal uranyl borates were synthesized, for the first time, by mild hydrothermal methods at temperatures of around 220 °C. In these compounds the boron exclusively forms BO3 triangles, whereas both BO3 triangles and BO4 tetrahedra are present in the alkali metal uranyl borates.33−35 This indicates that boron tends to lower its coordination number when the relative amount of water is increased in the initial reaction mixture. Wu et al. studied the reactivity of several systems in high-temperature/ high-pressure (HT/HP) hydrothermal conditions (650 ± 5 °C and 200 ± 10 MPa), with minimal dilute KOH solution as medium inside gold capsules. This resulted in the synthesis of the first uranyl aluminoborate UO2[B3Al4O11(OH)]44 and three potassium uranyl borates,36 in which further evidence for the stabilization of U(VI) within a tetraoxo core (UO4O2) was confirmed. This indicates that under hydrothermal conditions, reducing the relative amount of water to borate favors the generation of uranyl borate systems. 3.2. Structure Description. SrUBO-1 and BaUBO-1 are isostructural and crystallized in the monoclinic space group C2/ c. The structure, illustrated for SrUBO-1 in Figure 1, is based on 2D uranyl borate layers on the (1̅01) plane (Figure 1b), the molar ratio of U/B within these layers exceeds 1 with U:B = 5:2. It is tempting to ascribe this to the charge of the alkaline earth metal cation (Sr or Ba), since in the alkali metal uranyl borates described by Wang et al.,30−32 the U/B molar ratios are invariably less than 1. Most of alkali metal uranyl borates feature a common structural motif consisting of linear uranyl, UO 2 2+ , cations surrounded by BO3 triangles and BO 4 tetrahedra to create UO8 hexagonal bipyramids. The borate anions bridge the uranyl units to create a layered structure. There are three crystallographically independent uranium atoms in the SrUBO-1 structure. Two of these, U(2) and U(3), have a distorted pentagonal bipyramid geometry, while the U(1) cation displays the common linear uranyl U(1)O22+ geometry involving two O atoms from [BO3]3− polyanions. The tetragonal bipyramid coordination of U(1) is completed with two μ3-oxogroups to form a tetragonal bipyramid (Figure 1b). The corners and edges of the U(1), U(2), and U(3) polyhedra sharing with each other form five-membered rings (MRs) on (1̅01) plane. Isolated BO3 triangles, contained in the five MRs, share an edge with the U(3) pentagonal bipyramids, corners with the U(2) pentagonal and U(1) tetragonal bipyramids, forming a 2D uranyl layer parallel to (1̅01) . The U−O bond lengths range from 1.782(5) to 1.806(10) Å for the axial O atoms and from 2.306(8) to 2.540(8) Å for the equatorial oxygen atoms. Within the BO3 triangles, the B−O bond lengths are in the range of 1.354(15)−1.409(15) Å and O−B−O bond angles range from 114.3(11)° to 125.4(11)° (Table S1). BVS calculations indicate that three uranium cations are hexavalent with values of 5.86, 5.66, and 5.99 for U(1), U(2), and U(3), respectively. The BVS values suggest that the B cation is 3+ with the B(1) BVS of 2.97, and those for O(1) to O(10) are 1.81, 1.91, 1.90, 1.61, 1.96, 2.11, 1.84, 2.04,

1.82, and 2.05, respectively.42,43 According to the BVS values and the coordination mode of oxygen atoms, together with the charge balance of the structure, we conclude that O(4) atoms are present as hydroxyl (OH−) groups and all others are O2−. The Sr/Ba cations are ninefold coordinated and occupy the interlayer space, balancing the charge of the layers. The Sr−O and Ba−O bond distances are in the range 2.508(1)−2.773(10) Å and 2.61(6)−3.035(19) Å, respectively (Table S1). It is further observed that increasing the size of alkaline earth cation results in an increase in the interlayer distance in these compounds from ∼7.210 to ∼7.681 Å in SrUBO-1 and BaUBO-1, respectively. Thus, the A site cations presumably do not influence the connectivity within the layers; however, their size does determine the interlayer dimension. Along with the cations the interlayer space is also occupied by disordered water molecules. The presence of water is confirmed by Raman spectroscopy (Figure 7). In order to reveal the topology of the layers, we simplified the [(UO2)5(BO3)2O2(OH)2]2− sheets of SrUBO-1 by removal of anions while retaining the connectivity between the cations. This simplified net can be described as a new 4nodal net topological type with a point symbol of {35.45}2{36.46.53}3{37.47.57}2 (Figure S2).45−47 The topology of the compounds can also be described using the anion topology method proposed by Burns et al.48 In this the 2D anion sheet of SrUBO-1 is depicted as pseudo P chains (with edge-sharing pentagons and squares, denoted here as Ps chains, shown in gray and dark gray) connected with modified R chains (based on isolated BO3 triangles, denoted here as Rt chains shown in green) in Figure 2. This demonstrates that the

Figure 2. Anion topology observed in SrUBO-1. U(1)O2O4 squares, U(2)O2O5, U(3)O2O5 pentagons, and BO3 triangles are shown in gray, dark gray, and green, respectively.

present compounds have a distorted sayrite [Pb2(UO2)5O6(OH)2·4(H2O)] topology built on alternating square units in the P chains and BO3 triangles, instead of squares, in the R chains, i.e., PsRtPsRtPsRt.49 It is worth comparing the structure of Sr[(UO2)5(BO3)2O2(OH)2]·5H2O (SrUBO-1) with that of K4[(UO2)5(BO3)2O4]·(H2O),36 which also has a U/B molar ratio of 5:2. K4[(UO2)5(BO3)2O4]·(H2O) was obtained by a HT/HP hydrothermal method, and crystallized in the centrosymmetric space group P21/c (No. 14). Like SrUBO-1 it has a 2D-layered structure consisting of two types of uranium polyhedra, namely, UO2O4 square bipyramids and UO2O5 pentagonal bipyramids. These polyhedra are connected with each other in a quite similar manner to that in SrUBO-1, forming five uniform MRs along the a-axis which are occupied by BO3 triangles via a corner or edge sharing motif (Figure S3a). The six, sevenfold coordinated K+ cations are located in the interlayers with bond lengths ranging from 2.639(3) Å to 5926



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Figure 3. Coordination environments for the uranium sites and a B2O5 dimer in SrUBO-2 (a), a 2D uranyl layer L1 along the a-axis (b), a 2D uranyl borate layer L2 along the b-axis (c), and view of the structure along the c-axis (d). Uranyl polyhedra and BO3 triangles are shown in yellow and green, strontium and oxygen atoms are shown in blue and red, respectively.

Note that the axial O(7)/O(8) oxygen atoms of the U(1)O6 tetragonal bipyramids belong to one U(3)O7 pentagonal bipyramid, which means each U(1)O6 polyhedra is connected to one U(3)O7 polyhedra by a cation−cation bond, displaying a two-centered cation−cation bond feature (Figure 4). B(1)O3

3.227(6) Å. The 2D layers in the two structures are built on similar moieties; in K4[(UO2)5(BO3)2O4]·(H2O) these are [(UO2)5(BO3)2O4]4− polyanions whereas in SrUBO-1 they are [(UO2)5(BO3)2O2(OH)2]2−. However, their 2D layered anionic topology and interlayer distance are different. (Figure S3b). The K+ cations, that occupy the interlay space, have coordination numbers of six or seven in the latter and the Sr2+ cations are nine-coordinate in SrUBO-1. The effective ionic radius of seven-coordinate K+ is 1.46 Å and 9-coordinate Sr2+is 1.31 Å. Despite this, the interlayer distance in SrUBO-1 is larger than that in K4[(UO2)5(BO3)2O4]·(H2O), 7.210 vs 6.801 Å. This suggests that changes in the bonding of the interlayer cation are important and it is postulated that the K+ or Sr2+ cations drive changes in the anionic topology that are reflected it the interlayer separation. SrUBO-2 and BaUBO-2 are iso-structural with previously reported Ca[(UO2)2(B2O5)O],38 and crystallize in space group C2/m. This crystal structure is based on a 3D framework that contains uranyl units with cation−cation interactions (CCIs). The CCIs allow one uranyl unit that is coordinated with another uranium metal center by an oxo-atom, to form 1D chain, 2D layer, or 3D framework without the need for subordinate ligands. To date, more than 15 different types of CCIs among uranyl cations (UO22+) have been recognized, demonstrating that CCIs play an important role in determining structural motifs in actinide compounds.50−52 There are three crystallographically unique uranyl units in SrUBO-2, U(1)O6 and U(3)O6 forming tetragonal bipyramids and U(2)O7 having a pentagonal bipyramid geometry (Figure 3a). Two edge-sharing U(2)O7 polyhedra further connect with U(1)O6 tetragonal bipyramids through edge sharing (O6−O4), forming a 1D chain along the c-axis. The U(3)O6 tetragonal bipyramids connect with U(1)−U(2) chains via corner sharing (O7/O8) along the b-axis, forming a 2D uranyl layer (here noted as L1) on the bc-plane. The CCIs effectively turn uranyl polyhedra perpendicular to one another creating corrugated 2D uranyl layers L1 within the structure, as illustrated in Figure 3b.

Figure 4. Illustration of the CCI in the structure of SrUBO-2. The axial uranyl bonds are shown as red cylinders and the equatorial coordinating uranium oxygen linkages are shown as orange cylinders.

and B(2)O3 triangles connect with each other forming B2O5 dimers through corner sharing involving (O2) (Figure 3a). The B−O bond distances are in the range 1.325(18)−1.409(14) Å (Table S1). On the ac-plane, B2O5 dimers corner share with U(1)−U(2) chains through equatorial oxygen atoms (O1, O4, O11) and corner share with U(3)O6 tetragonal bipyramids via axial oxygen atoms (O5), forming a 2D uranyl borate layer (here noted as L2) (Figure 3c). L1 and L2 vertically connect with each other through B2O5 dimers resulting in the 3D uranyl borate framework [(UO2)2(B2O5)O]2−. Strontium cations are contained within the voids of the 3D framework structure (Figure 3d). The bond lengths of Sr−O are in the range of [2.488(10)−2.666(8) Å] (Table S1). The uranyl axial U−O distances for U(2) and U(3) within the L1 layers are normal and range 1.786(10)−1.809(8) Å. The equatorial U−O distances range 2.189(9)−2.450(9) Å. The uranyl unit for U(1) has slightly longer U−O bond lengths of 1.871(6)−1.879(9) Å (Table S1). These U−O distances are not typical for uranyl oxo-atoms (O7/O8) contacts under 5927



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CCIs.48,49 The two atoms that are bridging to the uranyl units containing U(3) have typical bridging distances of 2.300(1) and 2.302(5) Å (Figure 4). BVS calculations indicate that these three uranium cations are hexavalent with values of 6.02, 6.03, and 5.96 for U(1), U(2), and U(3), respectively. The BVS values for B1 and B2 are 3.02 and 2.96 and of the oxygen anions O1 to O9 are 1.89, 1.96, 2.01, 1.98, 2.05, 1.96, 1.92, 2.12, and 2.06, typical of the expected valence states of +3 and −2, respectively.42,43 3.3. Local Geometrical Configurations of Uranium Centers. The diversity of the uranium coordination leads to the structural complexity observed in the uranyl borates.16−23 Boron atoms are threefold coordinated in both structures with planar BO3 triangles. Consequently the local configuration of uranium is the main source of the difference between SrUBO-1 and SrUBO-2. Both these compounds have three crystallographically distinct uranyl units, which exist as UO6 tetragonal bipyramids and UO7 pentagonal bipyramids. The geometries found in these two structures are shown in Figure 5. There are

Figure 6. Observed (black markers), calculated (red line), and difference (green line) profile for SrUBO-2 recorded using XRD. The lower vertical markers indicate the allowed Bragg reflections according to the space group. For clarity the background has been removed.

Table S2). The somewhat larger than typical atomic displacement parameters are believed to be a consequence of use of the domed sample holder, necessary to contain the radioactive U cations. A small number of weak unindexed peaks were observed at low 2θ angles; nevertheless, the XRD analysis demonstrated the bulk sample was predominantly SrUBO-2. 3.5. Thermal Analysis. The thermogravimetric (TG) and differential scanning calorimetry (DSC) curves of SrUBO-2 powders in the temperature range of 300 to 1200 °C are illustrated in Figure S3. The DSC measurements show a strong endothermic peak at around 1115 °C, which corresponds to the melting of the compound. A large peak located at around 1156 °C corresponds to sample decomposition. There is a small amount of weight loss in the TG curve over this temperature range suggesting incongruent melting.21 To confirm this a powder sample of SrUBO-2 was calcinated at 1170 °C for 10 h, resulting in the formation of a black solid. The XRD powder pattern for the calcinated sample is different from that unheated materials and reveals the formation of U3O8 (Figure S4). 3.6. Raman Analysis. The characteristic vibrations of uranyl (UO2)2+ in aqueous solution are three normal modes, namely, the υ1 symmetric stretch (approximately 860−880 cm−1), υ2 bending mode (around 199−210 cm−1), and υ3 antisymmetrical stretch (from 930 to 960 cm−1).53,54 The BO3 unit has four internal modes, the symmetrical stretching (ν1) mode at around 900−1000 cm−1, the B−O bending (ν2) mode at around 650−800 cm−1, the doubly degenerated asymmetrical stretching (ν3) mode in the range 1250−1450 cm−1, and the doubly degenerated in-plane O−B−O bending (ν4) mode at around 590−680 cm−1.55,56 Raman spectra of the new uranium borates were measured in a range of 100−4000 cm−1. For convenience, we can divide the spectra into two sections, a low frequency part 100−1000 cm−1 and a high-frequency region 1000−4000 cm−1. There are more scattering peaks with stronger intensity in the first section (100−1000 cm−1), which is dominated by contributions from the modes of the uranium polyhedra (Figure 7); only small variations can be observed between the Raman spectra of SrUBO-1 and BaUBO-1 as shown in Figure 7, reflecting the iso-structural relationship of these. For these two compounds, the Raman spectra show strong and sharp bands around 840 cm−1 due to

Figure 5. Geometries of uranyl group (UO2)2+ center with BO3 or B2O5 groups in SrUBO-1 (a, b, c) and SrUBO-2 (d, e, f).

six different geometries in total across the two SrUBO-1 and SrUBO-2 structures, whereas in the alkali metal uranyl borates, obtained by Wang et al.,23 there is only one pattern of borate environment for the uranium atoms. The first three geometries of the present compounds (Figure 5a−c) are based on three groups (one uranyl polyhedron and two borate triangles), and are present in the structure of SrUBO-1. These resemble that observed in the structure of K4[(UO2)5(BO3)2O4](H2O),36 but the geometry presented in Figure 5c is absent leading to a different 2D anion topology (Figures 2, S3). The coordination type shown in Figure 5c can also be found in K 12 ((UO 2 ) 19 (UO 4 )(B 2 O 5 ) 2 (BO 3 ) 6 (BO 2 H)O 10 )(H 2 O) 9.5 . 36 The geometry types for SrUBO-2, shown in Figure 5d−f, are quite rare for uranyl borates compounds. All of them include B 2 O 5 dimer groups and a single uranyl group. The configuration of Figure 6f shows the uranyl group projected parallel to the same plane as the two B2O5 dimers, which is uncommon in inorganic uranyl compounds. 3.4. XRD Analysis. The structure of SrUBO-2 was refined against a monoclinic structure in the space group C2/m (Figure 6). The refined values of the lattice parameters and atomic coordinates for the uranium and strontium sites were found to agree well with the single crystal diffraction solution (Table 1, 5928



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Figure 7. Raman spectra of the studied compounds in 4000−100 cm−1 region.

the symmetric vibration υ1 mode of the uranyl ion with short uranyl U−O bond lengths of 1.782(4)−1.806(6) Å.57 Raman bands with very weak peaks around 1309 and 974 cm−1 have been assigned to the asymmetric and symmetric stretching υ1, υ3 modes of the B−O bonds in BO3. The vibrational modes from coordinated water molecules with weak peaks are observed at around 3500 and 3000 cm−1, respectively. For the iso-structural compounds SrUBO-2 and BaUBO-2, the peaks in the range of 720−890 cm−1 could be assigned to the symmetric stretching υ1 mode of the uranyl ion with the CCIs units inside. The Raman bands within 970−1200 cm−1 can be attributed to asymmetric and symmetric stretching υ1, υ3 modes of the B−O−B bonds in B2O5 dimers. These assignments are consistent with previously reported works.58

cationic layer in SrUBO-1 and BaUBO-1 is a new 4-nodal net topological type with a point symbol of {35.45}2{36.46.53}3{37.47.57}2. Changing the synthetic conditions resulted in the formation of another two new compounds that feature a 3D framework constructed with two vertical layers, containing CCIs groups, uranyl layers U(2)−U(1) = U(3) L1 and U(1)−U(2)−B(1)(B(2)−U(3) uranyl borate layers L2. L1 and L2 are further vertically connected with each other through bridging B2O5 dimers, leading to a 3D uranyl borate framework with the formula {[(UO2)2(B2O5)O]2−}n. This indicates that the ability of CCIs in increasing the dimensions of uranyl borate structure is exceptional. Additionally, it is noteworthy to consider which role has water in inducing the formation of such crystal structures that may contain important radionuclides, for instance, Sr90 and Ba137m.

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4. CONCLUSIONS The first four examples of strontium and barium uranyl borates, namely, A[(UO 2 ) 5 (BO 3 ) 2 O 2 (OH) 2 ]·5H 2 O (SrUBO-1, BaUBO-1) and A[(UO2)2(B2O5)O] (SrUBO-2, BaUBO-2) (A = Sr, Ba) have been synthesized and structurally characterized. SrUBO-1, BaUBO-1, and BaUBO-2 are the first examples of alkaline earth metal uranyl borates synthesized by mild hydrothermal methods. SrUBO-2 was obtained through high-temperature solid-state synthesis. We suggest that this approach will allow the preparation of more alkaline earth metal uranyl borates. SrUBO-1 and BaUBO-1 are isostructural compounds with a novel layered structure, compared to structures reported for alkali metal uranyl borates. The 2D

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.cgd.6b00978. Selected bond lengths and angles, EDS measurement, topological figures, XRD Rietveld refinement, and TGDSC curves (PDF) Accession Codes

CCDC 1489017−1489020 contains 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 5929



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emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12, Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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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 Authors are grateful to Helmholtz Association for funding within VH-NG-815 project. The authors are also grateful for the United Uranium scholarship granted by the United Uranium Trust Fund. This material is based upon work supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, Heavy Elements Chemistry Program under Award Number DE-FG02-13ER16414. S.W. has been supported by National Natural Science Foundation of China (91326112, 21422704). Yucheng Hao is grateful to China Scholarship Council for the support.

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