Divergent Structural Chemistry of Uranyl Borates ... - ACS Publications

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Article Cite This: Cryst. Growth Des. XXXX, XXX, XXX-XXX

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Divergent Structural Chemistry of Uranyl Borates Obtained from Solid State and Hydrothermal Conditions Yucheng Hao,† Philip Kegler,† Dirk Bosbach,† Thomas E. Albrecht-Schmitt,§ 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 and Biochemistry, Florida State University, 95 Chieftan Way, Tallahassee, Florida 32306-4390, United States ⊥ School for Radiological and Interdisciplinary Sciences (RAD-X) and Collaborative Innovation Center of Radiation Medicine of Jiangsu Higher Education Institutions, Jiangsu 215123, China ‡

S Supporting Information *

ABSTRACT: A series of novel uranyl borates, K4Sr4[(UO2)13(B2O5)2(BO3)2O12], A6[(UO2)12(BO3)8O3](H2O)6 (A = Rb and Cs), and Rb3[(UO2)3(BO3)2O(OH)](H 2 O), were synthesized using conventional conditions. Among them, K4Sr4[(UO2)13(B2O5)2(BO3)2O12] and A6[(UO2)12(BO3)8O3](H2O)6 were obtained through a high-temperature solid-state reaction method, whereas Rb3[(UO2)3(BO3)2O(OH)](H2O) was synthesized via a hydrothermal reaction. All compounds adopt novel two-dimensional (2D) layered structures in which basic building units (BBUs) consist of corner- or edge-sharing UOx (x = 6, 7 and 8) polyhedra linked with planar BO3 triangles or B2O5 dimers. K4Sr4[(UO2)13(B2O5)2(BO3)2O12] is the first mixed alkali−alkaline earth metal uranyl borate. This compound has the most complex 2D anion topology observed thus far in 2D uranyl borates. The fundamental building block (FBB) in this structure, [(UO2)13(B2O5)2(BO3)2O12]12−, consists of 3 UO8 hexagonal bipyramids and 10 UO7 pentagonal bipyramids connected with 2 BO3 triangles and 2 B2O5 dimers. The FBB of Rb6[(UO2)12(BO3)8O3](H2O)6 is [(UO2)6(BO3)4O3]6−, comprised of five edge-sharing UO7 pentagonal bipyramids, one UO6 tetragonal bipyramid, and four BO3 triangles. The simplest FBB, [(UO2)3(BO3)2O2]4−, occurs in Rb3[(UO2)3(BO3)2O(OH)](H2O), where three UO7 pentagonal bipyramids are linked with two BO3 triangles via edge sharing. The aforementioned FBBs are further polymerized into the corresponding infinite uranyl borates layers. The synthetic methods, novel topologies, thermal stability, and spectroscopic properties of these compounds are reported herein. other oxoanions including SiO44−, GeO44−, PO43−, GaO45−, etc., giving rise to new families of materials.17−21 While 3d- and 4f-transition metal borates are well represented, actinides borates are in need of further exploration.22 The primary reason for this is that actinides are radioactive and present experimental challenges, especially beyond uranium.23 Over the past decade, uranium borates have attracted considerable attention because of their diverse structural architectures and importance in fundamental studies of actinide chemistry.24 In aerobic, aqueous conditions, uranium is predominantly found in two oxidations states, namely, uranium(IV) and uranium(VI). U(IV) slowly oxidizes, and thus U(VI)-bearing phases dominate in both natural and synthetic conditions.25 The U6+ cation is usually present as part of a linear or nearly linear uranyl oxo-cation, UO22+. It can be coordinated with four

1. INTRODUCTION Metal borates have been the subject of considerable interest for decades because of their remarkable structural complexity and associated physiochemical properties.1−5 On the basis of the diversity structural topologies, many of which are acentric, borates exhibit a wide range of properties, such as magnetic ordering, luminescence, ion exchange, and second-harmonic generation.6−11 The coordination environment of boron is more complex than other light, main group elements, and two possible borate units occur with high frequency. These are planar BO3 triangle and tetrahedral BO4 groups; both tend to polymerize11 into complex zero-dimensional (0D) structures, infinite one-dimensional (1D) chains, two-dimensional (2D) sheets, and even higher condensed three-dimensional (3D) networks. According to the different countercations, borates can be divided into three groups: main group and transition metal borates, lanthanide and actinide borates, and organic complex templated borates.12−16 Moreover, borates can combine with © XXXX American Chemical Society

Received: July 18, 2017 Revised: October 11, 2017

A

DOI: 10.1021/acs.cgd.7b00997 Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

Article

Table 1. Crystal Data and Structure Refinements for Rb6[(UO2)12(BO3)8O3](H2O)6, Rb3[(UO2)3(BO3)2O(OH)](H2O) and K4Sr4[(UO2)13(B2O5)2(BO3)2O12]a

a

compound

Rb6[(UO2)12(BO3)8O3](H2O)6

Rb3[(UO2)3(BO3)2O(OH)](H2O)

K4Sr4[(UO2)13(B2O5)2(BO3)2O12]

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

4367.66 Pnma 26.3286(7) 14.3683(7) 16.1841(4) 90 90 90 6122.4(4) 4 0.71073 7288 4.738 1.067 0.0446 0.0951

1232.12 P21/c 18.5598(7) 12.6453(3) 13.3695(7) 90 101.140(4) 90 3078.6(2) 8 0.71073 4136 5.317 1.142 0.0521 0.1023

4530.13 P1̅ 6.7530(5) 13.1869(11) 15.9242(12) 76.024(7) 87.248(6) 75.401(7) 1331.54(18) 1 0.71073 1886 5.649 0.956 0.0374 0.1002

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

2. EXPERIMENTAL SECTION

to six oxygen anions, forming UO6 tetragonal, UO7 pentagonal, and UO8 hexagonal bipyramids. These UOx (x = 6, 7, and 8) polyhedra can be connected with various borate or polyborate groups and give rise to a variety of chemical compositions and structural chemistry of uranyl borates.1 In 1986, K6{UO2[B16O24(OH)8]}(H2O)12, the first actinide borate, was obtained through a slow evaporation method.26 Subsequently, a number of uranyl borates have been reported, e.g., by Gasperin, Wang, Wu and Hao, etc.27−41 The compounds reported by Gasperin were prepared through high-temperature, solid-state reactions, and these conditions primarily led to BO3 triangles as expected. In contrast, Wang et al. reported the use of what is termed a hydroflux of H3BO3 with low water content under mild heat near 200 °C for the synthesis. The resulting structures share a common structural motif, consisting of a linear uranyl core (UO2)2+ surrounded by BO3 triangles and BO4 tetrahedra, forming a UO8 hexagonal bipyramid, for example, Li[UO2B5O9](H2O)3, Na[(UO2)(B6O10(OH)](H2O)2, Na[(UO2)2(B10O15(OH)5], K2[(UO2)2B12O19(OH)4)](H2O)0.3, Rb[(UO2)2B10O16(OH)5], Cs[(UO2)2B11O16(OH)6], etc.22 Compared to Gasperin and Wang et al., Wu et al. has prepared a series of potassium uranyl borates, K4[(UO2)5(BO3)2O4](H2O), etc. using elevated high-temperature/high-pressure conditions (HT/HP; 650 ± 5 °C and 200 ± 10 MPa).38 The first example of a mixed-valent U(VI)/U(V) borate, K13[(UO2)19(UO4)(B2O5)2(BO3)6(OH)2O5](H2O),42 was prepared by Stritzinger et al. from a supercritical water reaction. The most striking feature in this structure is the tetraoxo core unit, [UO4(OH)2], that contains U(V) with trans hydroxide anions. A series of alkaline earth uranyl borates have been synthesized via different methods that have completely different structural architectures and physiochemical properties than the alkali metal uranyl borates synthesized at similar conditions.41 In this report, new uranyl borate phases were investigated in terms of their syntheses, structural characteristics, and physiochemical properties. Herein, we report the syntheses of four novel uranyl borates, namely, K 4 Sr 4 [(UO 2 ) 13 (B2O5)2(BO3)2O12], Rb3[(UO2)3(BO3)2O(OH)](H2O), and A6[(UO2)12(BO3)8O3](H2O)6 (A = Rb and Cs), their complex structures, thermal stability, and spectroscopic properties.

Caution! The UO2(NO3)2(H2O)6 used in this study contained natural uranium; nevertheless, standard precautions for handling radioactive materials must be followed. 2.1. Materials and Methods. Uranyl nitrate UO2(NO3)2(H2O)6 (International Bioanalytical Industries, Inc.), potassium nitrate KNO3 (Alfa-Aesar, 99.9%), potassium tetraborate K2B4O7(H2O)4 (AlfaAesar, 99.9%), strontium carbonate SrCO3 (Alfa-Aesar, 99.9%), rubidium hydroxide RbOH (50% wt. in aq. Soln., Alfa-Aesar, 99.6%), phosphorous acid H3PO3 (Alfa-Aesar, 99.9%), rubidium nitrate RbNO3 (Alfa-Aesar, 99.9%), cesium nitrate CsNO3 (Alfa-Aesar, 99.9%), and boric acid H3BO3 (Alfa-Aesar, 97%) were all used as received. 2.1.1. Syntheses of A6[(UO2)12(BO3)8O3](H2O)6 (A = Rb and Cs). The compounds A6[(UO2)12(BO3)8O3](H2O)6 (A = Rb and Cs) were synthesized via high temperature solid state methods. The starting chemicals of UO2(NO3)2(H2O)6 (0.1026 g, 0.20 mmol), RbNO3 (0.1166 g, 0.82 mmol), and B2O3 (0.1398 g, 2.05 mmol) were mixed with a ratio of U/Rb/B = 1:4:10. All the mixtures were thoroughly ground in an agate mortar and then transferred into a platinum crucible. The experiments were performed in a box furnace. The temperature was increased to 1000 °C, then kept constant for 5 h, and then slowly cooled down to 600 °C with a rate of 5 °C/h. In a second step it was cooled down to 450 °C at a rate of 10 °C/h followed by quenching to room temperature. Yellow pallet-shaped crystals of Rb6[(UO2)12(BO3)8O3](H2O)6 were isolated. EDS elemental analyses on several single crystals of Rb6[(UO2)12(BO3)8O3](H2O)6 yielded an average molar ratio of U/Rb = 2.0:1.13, which is in good agreement with the proposed chemical compositions determined by single crystal diffraction measurements (see Figure S1a). The replacement of the reactant RbNO3 by CsNO3, using the same experimental conditions with the synthesis route of Rb6[(UO2)12(BO3)8O3](H2O)6, leads to the formations of the isostructural compound Cs6[(UO2)12(BO3)8O3](H2O)6. An average molar ratio of U/Cs = 2.0:1.09 based on the EDS elemental analyses on the single crystals of Cs6[(UO2)12(BO3)8O3](H2O)6 was obtained. This is in good agreement with the structure determined by the refinement of the measured single crystal diffraction pattern (see Figure S1b). 2.1.2. Synthesis of Rb3[(UO2)3(BO3)2O(OH)](H2O). The phase Rb3[(UO2)3(BO3)2O(OH)](H2O) was obtained from a hydrothermal method. UO2(NO3)2(H2O)6 (0.0516 g, 0.11 mmol), H3PO3 (0.0336 g, 0.41 mmol), RbOH (0.4 mL, 1.72 mmol), H3BO3 (0.0943 g, 1.53 mmol), and 0.5 mL of deionized water were mixed in a ratio of U/Rb/ B/H2O = 1:17:15:27. All the chemicals were sealed in a Teflon-lined stainless steel autoclave (23 mL). The autoclave was transferred into a B

DOI: 10.1021/acs.cgd.7b00997 Cryst. Growth Des. XXXX, XXX, XXX−XXX

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box furnace and heated up to 220 °C for 24 h and then slowly cooled down to 160 °C at a cooling rate of 3 °C/h and further cooling down to room temperature in 24 h before the furnace was switched off. The resulting products were washed with hot water and filtered. Yellow block-shaped crystals Rb3[(UO2)3(BO3)2O(OH)](H2O) were obtained. The yield of Rb3[(UO2)3(BO3)2O(OH)](H2O) is around 46% based on U content. EDS elemental analysis on selected single crystals of Rb3[(UO2)3(BO3)2O(OH)](H2O) showed an average molar ratio of U/Rb = 1.0:1.06, which is in good agreement with the proposed chemical compositions of its single crystal structure solution (see Figure S1c). 2.1.3. Synthesis of K4Sr4[(UO2)13(B2O5)2(BO3)2O12]. K4Sr4[(UO2)13(B2O5)2(BO3)2O12] was prepared via a high temperature solid state method. UO2(NO3)2(H2O)6 (0.1016 g, 0.20 mmol), SrCO3 (0.0298 g, 0.20 mmol), K2B4O7(H2O)4 (0.1241 g, 0.40 mmol), and H3BO3 (0.1282, 2.07 mmol) were mixed in a ratio of U/Sr/K/B = 1:1:2:5. The reactants were ground in an agate mortar thoroughly, and then transferred to a platinum crucible. The reaction mixtures were then heated up to 980 °C in a box furnace. The holding time was 5 h. Then the sample was cooled down to 550 °C at a cooling rate of 5 °C/h before the furnace was switched off. Yellow tablet-shaped crystals of K4Sr4[(UO2)13(B2O5)2(BO3)2O12] were obtained. Pure polycrystalline samples of K4Sr4[(UO2)13(B2O5)2(BO3)2O12] were synthesized quantitatively by the reaction of a mixture of UO2(NO3)2(H2O)6 (0.3035 g, 0.6046 mmol), K2CO3 (0.0131 g, 0.0936 mmol), SrCO3 (0.0278 g, 0.1875 mmol), and H3BO3 (0.0177 g, 0.2801 mmol) with a molar ratio of 13:2:4:6 at 900 °C for 2 days. The product obtained was characterized using powder X-ray diffraction (PXRD), which indicated a high purity (see Figure S2). EDS elemental analyses on several single crystals of this compound gave an average molar ratio of U/K/Sr = 13.0:4.06:4.13, which is in good agreement with the proposed chemical compositions (see 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.43 The structures of all compounds were solved by direct methods and refined by a full-matrix least-squares fitting on F2 by SHELX-97.44 All structures were checked for possible missing symmetry elements using PLATON with the ADDSYM algorithm, and no higher symmetry solutions were found.45 Crystallographic data and structural refinements for four compounds are summarized in Table 1. 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 CuKα1,2 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. Scanning electron microscopy (SEM) images and EDS measurements for elemental analysis 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 (see Figure S1). 2.4. Thermal Analysis. The thermal behavior of dried polycrystalline powder of K4Sr4[(UO2)13(B2O5)2(BO3)2O12] up to 1200 °C was studied by differential scanning calorimetry (DSC) analysis coupled with thermogravimetry (TG) in dry 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 analyses 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) as a semiempirical method was applied for the determination of valence states. BVS of all atoms in the four uranyl borates phases were calculated and in a good agreement with corresponding formal values of oxidation states. The bond-valence parameters for K(I)−O, Rb(I)− O, Sr(II)−O, and B(III)−O were used according to Brese and O’Keeffe,46,47 whereas the bond-valence parameter of U(VI)−O was according to Burns.48

3. RESULTS AND DISCUSSION 3.1. Syntheses. The exploration of alkali and alkali earth metal uranyl borates system yielded in four novel 2D layered structural compounds, A6[(UO2)12(BO3)8O3](H2O)6 (A = Rb and Cs), Rb3[(UO2)3(BO3)2O(OH)](H2O) and K4Sr4[(UO2)13(B2O5)2(BO3)2O12]. Among these four reported phases, A6[(UO2)12(BO3)8O3](H2O)6 (A = Rb and Cs) and K4Sr4[(UO2)13(B2O5)2(BO3)2O12] were prepared from high temperature solid state synthesis methods at around 1000 °C, whereas Rb3[(UO2)3(BO3)2O(OH)](H2O) was synthesized from a hydrothermal reaction at 220 °C. Sr[(UO2)2(B2O5)O] has been previously obtained from the high temperature solid state reaction of UO2(NO3)2(H2O)6, Sr(NO3)2, H3BO3, and Li2B4O7.41 Substitution of Li2B4O7, as a reaction media−flux, by K2B4O7(H2O)4, leads to a completely new layered structure of K4Sr4[(UO2)13(B2O5)2(BO3)2O12]. We presumed that the reaction medium of K2B4O7(H2O)4 is of great importance for the structure formation of K4Sr4[(UO2)13(B2O5)2(BO3)2O12]. We suggest that countercations with larger ionic radii are more preferable for the final structure compared to the countercations with smaller radii. The synthesis of Rb3[(UO2)3(BO3)2O(OH)](H2O) was from the starting ratio UO2(NO3)2(H2O)6/H3PO3/RbOH/H3BO3/H2O of 1:4:17:15:27. Excluding H3PO3 from the reaction with the starting ratios of 1:10:15:27, 1:17:15:27, and 1:20:15:27, the preparation of Rb3[(UO2)3(BO3)2O(OH)](H2O) was unsuccessful. We supposed that H3PO3 is a very important pH value controlling medium in the synthetic process. Compared to Wang’s et al. boric acid flux method,22 we have used more water as reaction medium in hydrothermal synthesis. The final structural motif of Rb3[(UO2)3(BO3)2O(OH)](H2O) is totally different with the phases that prepared by Wang et al.22 The structural topology of Rb3[(UO2)3(BO3)2O(OH)](H2O) is more similar to the structures that are obtained from the solid state synthesis. This probably indicates that the reaction medium (water) is dominated over polymerization of oxo-borate groups from boric acid flux.22 The syntheses of A6[(UO2)12(BO3)8O3]((H2O)6 (A = Rb and Cs) was performed using the following starting chemicals UO2(NO3)2·6H2O, RbNO3/CsNO3, and B2O3 with a ratio of U/Rb/B = 1:4:5. The substitution of RbNO3/CsNO3 by LiNO3, NaNO3, and KNO3, keeping elemental ratios (U/MI/B = 1:4:5) and synthetic conditions identical, leads to a formation of previously described M(UO2)(BO3) (M = Li, Na, K) phases.33 It is obvious that the size of countercations plays a key role in the formation of the final composition of the material and its structural architecture. 3.2. Crystal Structures and Topological Descriptions. 3.2.1. Structure of Rb6[(UO2)12(BO3)8O3](H2O)6. The comC

DOI: 10.1021/acs.cgd.7b00997 Cryst. Growth Des. XXXX, XXX, XXX−XXX

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B(1)O3, B(3)O3, and B(4)O3 triangles are further linked with U(1)O7, U(2)O7, U(3)O7, U(4)O6, and U(5)O7 polyhedra through sharing edges or corners, creating the FBB. Each FBB shares vertexes or edges with seven neighboring ones and polymerized into the novel 2D uranyl borate sheet {[(UO2)12(BO3)8O3]6−}n parallel ac-plane (see Figure 1a). Within the structure, only O(5) is μ2-O shared by U(4)O6 tetragonal bipyramids and B(3)O3 triangles. The rest of the equatorial oxygens are μ3-Os shared by three uranyl or two uranyl polyhedra and one borate triangle (see Figure S4). The Rb+ cations are located in the interlayers as well as the water molecules. We suppose that the water molecules residing between the interlayers are absorbed from air during the slow cooling process. This assumption is based on the synthetic method that we applied (the temperature of the synthesis was 980 °C). It is obvious that water molecules cannot exist in the structure at such a high temperature and ambient pressure. Therefore, the only possible mechanism of water to enter the interlayers is absorption of from air during the process of crystal cooling.49 The presence of water in molecular form was confirmed by the broad weak vibrational peak around 3000 cm−1 in the Raman spectrum (see Figure 8). The topology of Rb6[(UO2)12(BO3)8O3](H2O)6 is relatively complex. It can be described using the anion topology method proposed by Burns.50 Two different basic fragments can be separated within the 2D anion uranyl borate sheets as shown in Figure 2a,b. The first fragment is composed out of two centrosymmetric S-shaped finite P-chains as shown in Figure 2c. Each chain contains five pentagons (shown as dark gray). Two BO3 triangles are encircled by the two P-chains (denoted as P1, P2 chains). The second fragment can be designated as Rt clusters. They are shown as two gray squares sharing two BO3 triangles and linked to four additional ones in the rest of the corners. The stacking mode of Rb6[(UO2)12(BO3)8O3](H2O)6 can be described as the anion topology of [P1P2Rt]n (see Figure 2c). In order to see the new topology of the uranyl borate sheets more clearly, we simplified the 2D layers of Rb6[(UO2)12(BO3)8O3](H2O)6 by omitting the oxygen anions and keeping the B and U links. The simplified cationic net is a 10-nodal net topology51−53 (32.43.5) {(33.43)(34.45.54.62)(35.45.55)(35.45)2(36.46.53)(37.47.57)3 (see Figure S5). From the point symbol view, the topology of this uranyl borate is quite complex and comparable with the reported phases previously. 3.2.2. Structure of Rb 3 [(UO 2 ) 3 (BO 3 ) 2 O(OH)](H 2 O). Rb3[(UO2)3(BO3)2O(OH)](H2O) crystallizes in the monoclinic centrosymmetric space group P21/c. There are three uranium, two boron, and three rubidium atoms in the asymmetric unit, all of them showing a standard crystallographic environment. The uranyl UO bond distances range from 1.771(19) to 1.85(2) Å for all the distorted pentagonal bipyramids, whereas for the equatorial interactions range from 2.224(16) to 2.544(17) Å. The OUO bond angles are in the range of [175.5(8)−178.4(8)°]. The B−O bond lengths for the BO3 triangles are in the range of [1.24(4)−1.40(3) Å], and the O−B−O bond angles are from 108(2)° to 128(3)°. The Rb−O bond distances are observed to range from 2.78(2) to 3.62(2) Å. The BVS of U(1)−U(6), B(1)−B(4), and Rb(1)− Rb(6) are 6.09 v.u., 5.94 v.u., 6.12 v.u., 5.98 v.u., 5.91 v.u., 6.01 v.u., 3.02 v.u., 3.06 v.u., 2.93 v.u., 2.97 v.u., and 1.07 v.u., 0.97 v.u., 0.99 v.u., 1.02 v.u., 0.91 v.u., 1.05 v.u., respectively, indicating that the U cations are 6+, B cations are 3+, and Rb cations are 1+ oxidation states.46−48

pounds A6[(UO2)12(BO3)8O3](H2O)6 (A = Rb and Cs) were obtained using high temperature solid state methodology. Because the nonmerohedral twinning, the structure of Cs6[(UO2)12(BO3)8O3](H2O)6 is not publishable, but it is a full analogue of Rb6[(UO2)12(BO3)8O3](H2O)6. We have picked several crystals from different syntheses. Unfortunately, all of the thin pallet-shaped crystals contained twinning components that do not allow a refinement of the structure to an R1 value below approximately 10%. Rb6[(UO2)12(BO3)8O3](H2O)6 crystallizes in the orthorhombic space group Pnma and adopts a novel uranyl borate layered structure. It contains six crystallographically unique U and four B atoms in the asymmetric unit. Uranium atoms exhibit two different oxygen coordination environments, which are distorted UO6 tetragonal bipyramids and UO7 pentagonal bipyramids. The axial UO bond distances for the uranyl cations in the UO7 pentagonal bipyramids are from 1.75(2) Å to 1.784(19) Å. The equatorial bond lengths of Ueq−O are in the range of [2.170(18)−2.579(14) Å]. The OUO bond angles range from 177.0(8)° to 178.7(8)°, whereas the UO bond lengths of the UO6 tetragonal bipyramids are slightly shorter ranging from 1.75(2) to 1.78(2) Å, the Ueq−O bond distances are in the range of [2.170(18)−2.419(16) Å], and the OUO bond angles are 177.5(9)°. The B−O bond lengths range from 1.32(3) to 1.42(3) Å, and the O−B−O bond angles are from 111(2)° to 131(3)°. The Rb−O bond distances are in the range of [2.70(4)−3.47(2) Å]. The bond valence sum (BVS) of U(1)−U(6), B(1)−B(4), and Rb(1)−Rb(8) are 6.11 v.u., 5.99 v.u., 5.96 v.u., 5.91 v.u., 6.01 v.u., 6.06 v.u., 3.07 v.u., 3.10 v.u., 2.95 v.u., 2.98 v.u. and 1.13 v.u., 0.96 v.u., 0.97 v.u., 1.05 v.u., 0.98 v.u., 1.09 v.u., 1.06 v.u., and 0.99 v.u., respectively, which implies that the U cations are 6+, B cations are 3+, and Rb cations are 1+ oxidation states.46−48 The FBB, namely, [(UO2)12(BO3)8O3]6−, of the negatively charged uranyl borate layers is shown in Figure 1c,d. U(1)O7− U(6)O7 polyhedra is sharing corners or edges forming a U6hexamer, in which a 5U-ring can be observed on the ac-plane. The B(2)O3 triangles are reside in the 5U-rings via shared edges or corners with U(3)O7, U(4)O6, and U(6)O7 polyhedra.

Figure 1. (a) A 2D uranyl borate sheet of compound Rb6[(UO2)12(BO3)8O3](H2O)6 on ac-plane; (b) view of the structure down the baxis; (c) the FBB [(UO2)6(BO3)4] of the 2D UB sheet of Rb6[(UO2)12(BO3)8O3](H2O)6 and the ball-and-stick fashion (d). Uranyl polyhedra and BO3 triangles are shown in yellow and green, rubidium and oxygen are shown in blue and red, respectively. D

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Figure 2. Anion topology observed in Rb6[(UO2)12(BO3)8O3](H2O)6. UO2O5 pentagons, UO2O4, and BO3 triangles are shown in dark gray, gray, and green, respectively.

within the unique FBBs. The FBB is connected with the six neighboring ones and further polymerized into the 2D infinite uranyl borate layers parallel to the ab-plane. Note that O(20), O(22), O(23), O(26), O(27), O(28), O(29), O(36), and O(38) are μ3-O’s, whereas the rest are μ2-O’s. Compared to K4Sr4[(UO2)13(B2O5)2(BO3)2O12] no μ4-O’s are present (see Figure S4). Rb+ cations are resided in the interlayers for charge balance. One can separate the 2D uranyl borate layers of Rb3[(UO2)3(BO3)2O(OH)](H2O) in two different fragments using anion topology method,50 as shown in Figure 4. The first fragments are typical P-chains composed of edge sharing linked UO7 pentagonal bipyramids (shown as dark gray pentagon). The second fragments can be designated as quasi-U(D) chains. They are denoted as Ut or Dt chains, shown as one triangle linking each pentagon on both sides of U or D chains. The stacking mode of Rb3[(UO2)3(BO3)2O(OH)](H2O) can be described as distorted protasite54 Ba(UO2)3O3(OH)2(H2O)3 sheet anion topology with additional BO3 triangle linkers on U(D) chains, i.e., ...PUt PDtPUt PDt.... (see Figure 4). For the purpose of describing the new topology of the sheets more clearly, we simplified the uranyl borate layers of Rb3[(UO2)3(BO3)2O(OH)](H2O) by omitting the oxygen anions and only kept the connectivity between the B−U cations. The simplified B−U net was observed as a 10-nodal net topology51−53 (3 2 .4.5 3 )(3 2 .4 2 .5 2 )(3 3 .4 2 .5 3 .6 2 ) 2 (3 3 .4 2 .5) 2 (3 3 .4 3 .5 3 .6) (35.44.54.62)(35.45.55)(37.47.57) (see Figure S5). From the point symbol view, we can observe that the cationic topology of Rb3[(UO2)3(BO3)2O(OH)](H2O) is not as complex as that in the structure of Rb 6 [(UO 2 ) 12 (BO 3 ) 8 O 3 ](H 2 O) 6 and K4Sr4[(UO2)13(B2O5)2(BO3)2O12]. It is worth to compare the structural relationships of the rubidium uranyl borates family. There are only three known synthetic phases, Rb2[(UO2)2B13O20(OH)5], Rb[(UO2)2-

The structure is based on novel 2D uranyl borate layers {[(UO2)3(BO3)2O2]4−}n, with a U/B ratio 3:2, is composed of UO7 pentagonal bipyramids and BO3 triangles. The FBB, [(UO2)3(BO3)2O2]4− of Rb3[(UO2)3(BO3)2O(OH)](H2O) is provided in Figure 3c. This FBB is not as complex as that in

Figure 3. (a) A basic 2D uranyl borate sheet of compound Rb3[(UO2)3(BO3)2O(OH)](H2O) on the ab-plane; (b) view of the structure along the b-axis; (c) a FBB: [(UO2)3(B2O7)] of the 2D UBsheet and the ball-and-stick mode (d). Uranyl polyhedra and BO3 triangles are shown in yellow and green, rubidium and oxygen are shown in blue and red, respectively.

Rb6[(UO2)12(BO3)8O3](H2O)6. U(1)O7, U(4)O7, and U(6)O7 or U(2)O7, U(3)O7, and U(5)O7 pentagonal bipyramids are sharing common edges or corners with each other and are connected into U3 trimers (see Figure 3c). Two BO3 triangles further bridged the U3 trimers via shared edges or corners

Figure 4. Anion topology observed in Rb3[(UO2)3(BO3)2O(OH)](H2O). UO2O5 pentagons and BO3 triangles are shown in dark gray and green, respectively. E

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B10O16(OH)3](H2O)0.7, and Rb(UO2)(BO3).33,36 The first two compounds were obtained from low temperature synthesis with an excess boric acid flux in which uranium only exists as one independent UO8 hexagonal bipyramid in the structure. The UO8 polyhedra are surrounded by three BO3 triangles and six BO4 tetrahedra forming the common uranyl borate structural motif. The uranium atoms in both Rb(UO2)(BO3) and Rb3[(UO2)3(BO3)2O(OH)](H2O) are in the form of UO7 pentagonal bipyramids. But only one crystallographically unique U-site in Rb(UO2)(BO3) encircled by four equal BO3 triangles, and three unique U-sites in Rb3[(UO2)3(BO3)2O(OH)](H2O), corner or edge sharing linked with three different planar BO3 triangles, is present. These facts make the anion topology of Rb3[(UO2)3(BO3)2O(OH)](H2O) is the most complex one in this uranyl borates series. 3.2.3. Structure of K4Sr4[(UO2)13(B2O5)2(BO3)2O12]. The compound K4Sr4[(UO2)13(B2O5)2(BO3)2O12] crystallizes in the P1̅ space group. This is the first example of mixed alkali and alkali-earth metal uranyl borate, which has a completely different structural arrangement compared to previously reported phases.1,22 The structure of K4Sr4[(UO2)13(B2O5)2(BO3)2O12] is based upon seven uranium, three boron, two potassium, and three strontium atoms in an asymmetric unit. The axial bond lengths of UO for the uranyl polyhedra are in the range of [1.802(11)−1.847(10) Å], whereas for the equatorial U−O bond distances range from 2.189(10) Å to 2.993(7) Å. The OUO bond angles are in the range of [174.9(5)−179.999(1)°]. The B−O bond lengths for the BO3 triangles are in the range of [1.33(2)−1.40(2) Å], and the O− B−O bond angles are from 117.4(14)° to 126.5(14)°. K−O bond distances range from 2.661(12) Å to 3.190(12) Å, whereas the Sr−O bond lengths are in the range of [2.578(13)−2.967(12) Å]. The BVSs of U(1)−U(7) and B(1)−B(3) are 5.96 v.u., 5.93 v.u., 6.05 v.u., 5.91 v.u., 5.99 v.u., 6.07 v.u., 5.95 v.u., and 3.05 v.u., 2.97 v.u., 3.03 v.u., respectively, suggesting that the U cations are 6+ and B cations are 3+ oxidation states.46−48 K4Sr4[(UO2)13(B2O5)2(BO3)2O12] features a complex 2D layered structural topology, which is based on the uranyl borate layers parallel to the (111) plane (see Figure 5a,b). The uranyl borate sheets are composed of distorted UO8 hexagonal, UO7 pentagonal bipyramids and coplanar BO3 triangles. In order to present the layered structure more clearly, the FBB, namely, [(UO2)13(B2O5)2(BO3)2O12]12‑ of its uranyl borate sheet was provided in Figure 5c. Within the FBB, the U(6)O8 polyhedra is the inversion center and is surrounded by two centrosymmetric subgroups [(UO2)6(B2O5)(BO3)O5]7− (see Figure 5c). The subgroups are constructed by six UO7 polyhedra, one B2O5 dimer, and one B(1)O3 triangle are sharing corners or edges with each other. The two subunits are further bridged by the U(6)O8 hexagonal bipyramid via edge sharing (see Figure S4). The FBBs are polymerizing, forming the unique 2D infinite uranyl borate layers [(UO2)13(B2O5)2(BO3)2O12]12− parallel to the (111) plane (see Figure 5c,d). O(6), O(9), O(13), O(16), O(20), O(21), O(22), and O(25) are μ3-Os, whereas O(24) is a μ4-O (see Figure S4a). K+ and Sr2+ cations are located in the interlayer space for charge balance. The topology of the 2D layers in K4Sr4[(UO2)13(B2O5)2(BO3)2O12] was observed for the first time in the uranyl borates family.2 The topology of the 2D uranyl borate sheets can be described using the anion topology method proposed by Burns et al. (1996)50 (see Figure 6a,c). The sheet anion topology of K4Sr4[(UO2)13(B2O5)2(BO3)2O12] is exceedingly complex. It

Figure 5. (a) A basic 2D uranyl borate sheet of K4Sr4[(UO2)13(B2O5)2(BO3)2O12]; (b) view of the structure along the [1 1̅ 0] direction; (c) FBB: [(UO2)13(B2O5)2(BO3)2] of the 2D UB sheet with polyhedra (c) and ball-and-stick modes (d). Uranyl polyhedra, [(UO2)O6] hexagons and [(UO2)O5] pentagons and BO3 triangles in anion topology are shown in cyan, yellow, and green; potassium, strontium and oxygen are shown in blue, light blue, and red, respectively.

Figure 6. Anion topology representation of the layer observed in K4Sr4[(UO2)13(B2O5)2(BO3)2O12]. UO2O6 hexagons, UO2O5 pentagons, and BO3 triangles are shown in dark gray, gray, and green, respectively.

can be separated into two different fragments based on pentagon and hexagon parts, as shown in Figure 6c. The first fragment is based on distorted and corrugated infinite pentagon chains from edge or corner-shared UO7 pentagonal bipyramids (shown in gray in Figure 6a). These chains can be designated as Pd-chains according to Burns et al.50 The hexagon fragment is based on infinite edge or corner-shared UO8 hexagonal bipyramids, which can be designated as Ht chains (shown as dark gray in Figure 6a). These two fragments are edge sharing with each other and further linked with BO3 triangles or B2O5 dimers and can be described as ...Pd Ht Pd Ht Pd Ht...55 For the purpose of analyzing the new topology, the simplified cationic net of [(UO2)13(B2O5)2(BO3)2O12]12− was shown in Figure S6 by omitting the oxygen anions, and only the links between B and U cations are kept. The simplified net was observed as a new 10-nodal net topological type with a point symbol of (310.410.57.6)(33.43)2(35.45)2(36.44)2 (36.46.53)6(36.46.56.63)2(37.47.56.6)2(37.47.57)2 (see Figure S6). F

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Table S1). In total, seven different geometries can be observed in [(UO2)13(B2O5)2(BO3)2O12]12− (see Figure S7a). The geometry of U(1) is the simplest one (one UO7 polyhedra and one BO3 triangle). Such geometry also can be observed in the structures of K12[(UO2)19(UO4)(B2O5)2(BO3)6(BO2OH)O10](H2O) and K4[(UO2)5(BO3)2O4]·H2O38 (see Figure S7a). The geometry that is composed of two groups (one UO7 pentagonal bipyramid and one B2O5 dimer) is uncovered for the first time (see Figure S7a2).2,22 Clusters based on four groups (UO7, UO8 polyhedra, BO3 triangles, and B2O5 dimers) occur in five different configurations (see Figure S7a3−7). To the best of our knowledge, these configurations are uncovered for the first time and have not been observed in any known inorganic uranium compounds.2,22 The rich local geometrical configuration of the uranium centers observed in K4Sr4[(UO2)13(B2O5)2(BO3)2O12], making its structural topology very rare, is one of the most complex one in the 2D uranyl borates family to date. It is noteworthy to compare the structural topology of Rb6[(UO2)12(BO3)8O3](H2O)6 and Rb3[(UO2)3(BO3)2O(OH)](H2O) with roubaulite, Cu2[(UO2)3(CO3)2O2(OH)2](H2O)4,57 due to the same molar ratios of U/B and U/C and similar triangular geometries of BO3 and CO3 groups. The FBB of roubaulite is [(UO2)3(CO3)2O4]6−, and it contains two uranyl pentagonal and one hexagonal bipyramids and two carbonate triangles. These FBBs are further connected to each other by edge sharing and is forming a 1D structural topology in Cu2[(UO2)3(CO3)2O2(OH)2](H2O)4 (see Figure S8). In contrast, the FBB in Rb6[(UO2) 12(BO3)8O3](H2O)6 is [(UO2)6(BO3)4O3]6−. It is composed of five UO7, one UO6 polyhedra, and four BO3 triangles. These FBBs are further edge and vertex sharing with the neighboring ones, connected into the infinite 2D layered structures. The FBB in Rb3[(UO2)3(BO3)2O(OH)](H2O) is [(UO2)3(BO3)2O2]4−, and it is based upon three UO7 polyhedra and two BO3 triangles. These FBBs are further interlinked into a 2D infinite uranyl borates layers. A comparison of FBBs and their linkage in studied borates and roubaulite indicates that structural topology of uranyl borates is more complex than in carbonates. However, it has to be noted that the number of complex uranyl carbonates is limited compared to the recent achievements in uranyl borates chemistry. 3.4. Thermal Analysis. The thermogravimetric (TG) and differential scanning calorimetry (DSC) measurement of a powdered sample of K4Sr4(UO2)13(B2O5)2(BO3)2O12 in the temperature range from 50 to 1200 °C is illustrated in Figure 7. The TG curve demonstrates that there is no obvious weight loss. The DSC curve shows a strong endothermic peak at around 1130 °C, which corresponds to the melting of the compound (see Figure 7). A large endothermic peak located at around 1170 °C corresponds to the decomposition of the sample. The TG curve at around 1170 °C shows a small weight loss, which implies an incongruent melting.17 To confirm this, we have calcinated the powder sample at 1190 °C for 10 h, resulting in the formation of a black solid powder. The PXRD pattern of the calcinated sample is different from the initial material and mostly presented by U3O8 (see Figure S3). 3.5. Raman Analyses. Raman spectra of the four new uranyl borates were measured in a range of 100−4000 cm−1 as shown in Figure 8. All four compounds are constructed from linear uranyl groups (UO2)2+ and BO3 triangles or B2O5 dimers, and thus their vibrational modes are quite close to each other as observed in the figures Raman bands in the lower

It is interesting to compare the structure of K4Sr4[(UO2)13(B2O5)2(BO3)2O12] with structures of K4[(UO2)5(BO3)2O4](H2O) and Sr[(UO2)5(BO3)2O2(OH)2](H2O)5,41 because all of them are layered uranyl borates and have close U/B ratios and chemical compositions. Two different coordination geometries of U atoms can be observed in all three phases but these are UO6 and UO7 polyhedra in K4[(UO2)5(BO3)2O4](H2O) and Sr[(UO2)5(BO3)2O2(OH)2](H2O)5, and UO7 and UO8 polyhedra in K4Sr4[(UO2)13(B2O5)2(BO3)2O12]. Besides, in K4Sr4[(UO2)13(B2O5)2(BO3)2O12] borate groups are not only presented by isolated BO3 triangles. B2O5 dimers are also existing in its structure. The various coordination environments of uranium atoms, and complex borate groups in K4Sr4[(UO2)13(B2O5)2(BO3)2O12], making its 2D uranyl borate sheet topology much more complex than that in K4[(UO2)5(BO3)2O4](H2O) and Sr[(UO2)5(BO3)2O2(OH)2](H2O)5. 3.3. Comparison of the Structural Complexity in the 2D Uranyl Borates Family Based on the FBBs. Uranyl borates present extremely rich structural chemistry based on the variety of uranium coordination chemistry and borates groups.2,22,56 However, because of the equatorial coordination nature of the uranyl unit (UO2)2+, uranyl borates mostly adopt the less condensed 2D layered structures, which are typical for the most uranyl oxo-salts. All the borates units that were reported in this work exhibited planar BO3 triangles. Thus, the coordination chemistry of uranyl polyhedra has dominated the structural complexity of the 2D layered topology. Here, we derive topologically and metrically possible finite clusters (FBBs) to describe the complexity of the infinite uranyl borate layers. The different FBBs of the previously reported 2D uranyl borates and those in this work are listed in the Table S1. After comparison, we can observe that compounds of the UO2B2O4 and M(UO2)(BO3) (M = Li, Na, K, Rb, Cs)31−33 series have the simplest topologies of the layer with FBB [(UO2)(BO3)]−, which only contains one uranyl polyhedra and one borate group. The FBB of Rb3[(UO2)3(BO3)2O(OH)](H2O) is [(UO2)3(BO3)2O2]4−, which has the second simplest topological geometry based upon three uranyl polyhedra and two borates units. Two different local groups can be observed in this structure. One is based on a single UO7 pentagonal bipyramid and three BO3 triangles, and the second one is based on one UO7 and two BO3 groups (see Figure S7b). The abovementioned local configurations also can be found in the structures of Sr[(UO 2 ) 5 (BO 3 ) 2 O 2 (OH) 2 ](H 2 O) 5 41 and K12[(UO 2) 19 (UO 4)(B 2O 5 ) 2(BO3 ) 6 (BO 2OH)O 10 ](H 2O) n .42 [(UO2)6(BO3)4O3]6−, the FBB of Rb6[(UO2)12(BO3)8O3](H2O)6, is as complex as the FBBs in K4[(UO2)5(BO3)2O4](H 2 O), K 10 [(UO 2 ) 16 (B 2 O 5 ) 2 (BO 3 ) 6 O 8 ](H 2 O) 7 and Sr[(UO2)5(BO3)2O2(OH)2](H2O)5 uranyl borates.38,41 Their FBBs are composed of three different basic building units (BBUs), which are UO7, UO6 polyhedra, and BO3 triangles. As shown in Figure S7c, the uranium configuration of [(UO2)6(BO3)4O3]6− can be observed in the structure of Rb 3 [(UO 2 ) 3 (BO 3 ) 2 O(OH)](H 2 O), but it is absent in K4Sr4[(UO2)13(B2O5)2(BO3)2O12]. Notably, clusters based on four groups (one UO7 pentagonal bipyramid and three BO3 triangles) are uncovered first in the 2D uranyl borates family2,22 (in Figure S7c3). The FBB of K4Sr4(UO2)13(B2O5)2(BO3)2O12 is one of the most complex one compared to K13[(UO2)19(UO4)(B4O10)(BO3)6(OH)2O5](H2O) and K15[(UO2)18(BO3)7O15]38 (see G

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Rb3[(UO2)3(BO3)2O(OH)](H2O) and A6(UO2)12(BO3)8O3((H2O)6 (A = Rb, Cs). Raman bands with very weak peaks in the range of 990−1200 cm−1 have been attributed to the asymmetric and symmetric stretching (υ1, υ3) modes of the B− O−B bonds in the BO3 triangles, with bond lengths around 1.30 Å. These assignments are consistent with the previously reported works.41,58,59 As described above, all the structures are based upon uranyl polyhedra UOx (x = 6, 7, 8) and BO3 planar triangles. Because of the flexibility of the U−O−B connection, BBUs of UOx and BO3 are combined differently exhibiting diverse structural forms and different bond lengths in geometrically similar units. For example, the longest B−O bond distance in Rb3[(UO2)3(BO3)2O(OH)](H2O) is 1.40(3) Å and is shorter than that of the other three uranyl borates, such as 1.42(3) Å in AI6[(UO2)12(BO3)8O3](H2O)6 and 1.42(2) Å in K4Sr4[(UO2)13(B2O5)2(BO3)2O12]. The difference in bond lengths can be also seen from the band positions in Raman spectra. A trend between bond length and Raman frequencies in oxo-borates has been proposed by Chryssikos60 and shown as eq 1. With this equation, the B−O bond lengths can be calculated as a function of Raman frequencies υ3.

Figure 7. TG-DSC curves of K2Sr5[(UO2)13(B2O5)2(BO3)2O12].

frequencies from 190 to 210 cm−1 are originated from the contribution of the uranyl υ2 bending mode. The doubly degenerated in-plane O−B−O bending υ4 mode in the BO3 triangles is at around 510−700 cm−1, the doubly degenerated asymmetrical stretching mode is in the range of 1250−1500 cm−1, and the corresponding B−O bond lengths are from 1.32(3) to 1.42(3) Å. The Raman spectra of all phases show strong and sharp peaks in the range of 810−840 cm−1. This can be attributed to the symmetric vibration υ1 mode of the uranyl (UO2)2+ cation with short uranyl U−O bond lengths of around 1.80 Å, while the relatively weak peaks at 3543 cm−1 (see Figure 8b), 2800−3000 cm−1 (see Figure 8c), and 3156 cm−1 (see Figure 8d) come from the vibrational modes of coordinated water molecules in the structures of

RB − O =

a−v b

(1)

Equation 1 is an empirical correlation between bond lengths and Raman frequencies in borate structures, and least-squares fitting gives a = 16.9 × l03 cm−1 and b = 11.6 × l03 cm−1 Å−1. From the Raman spectra of our materials, the B−O bond stretching modes v3 of Rb3[(UO2)3(BO3)2O(OH)](H2O) start from higher wavenumbers of (560−650 cm−1) compared to that of Rb6[(UO2)12(BO3)8O3](H2O)6 (480−620 cm−1) and

Figure 8. Raman shifts of all four compounds: K4Sr4[(UO2)13(B2O5)2(BO3)2O12] (a), Rb3[(UO2)3(BO3)2O(OH)](H2O) (b), and A6[(UO2)12(BO3)8O3](H2O)6 [A = Rb (c) and Cs (d)]. H

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K4Sr4[(UO2)13(B2O5)2(BO3)2O12] (518−693 cm−1). On the basis of these data and eq 1, we can calculate the longest B−O bond lengths of 1.40 Å for Rb3[(UO2)3(BO3)2O(OH)](H2O), 1.42 Å for Rb6[(UO2)12(BO3)8O3](H2O)6 and 1.41 Å for K4Sr4[(UO2)13(B2O5)2(BO3)2O12], respectively, which is in good agreement with the structure refinement results.

SEM-EDS measurement, PXRD patterns, topological figures (PDF) Accession Codes

CCDC 1562924−1562926 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.

4. CONCLUSIONS A series of novel 2D layered uranyl borates, namely, A6[(UO2)12(BO3)8O3](H2O)6 (A = Rb and Cs), Rb3[(UO2)3(BO3)2O(OH)](H2O), and K4Sr4[(UO2)13(B2O5)2(BO3)2O12] have been prepared and characterized. Among them, A 6 [(UO 2 ) 12 (BO 3 ) 8 O 3 ](H 2 O) 6 (A = Rb and Cs) and K4Sr4[(UO2)13(B2O5)2(BO3)2O12] were obtained from high temperature solid state method, whereas Rb3[(UO2)3(BO3)2O(OH)](H2O) was obtained from a mild hydrothermal synthesis. The preparation of Rb3[(UO2)3(BO3)2O(OH)](H2O) has a similar reaction temperature with Wang’s et al. syntheses,5 but we have used more water medium in the reactions. The final structure is totally different with the uranyl borates reported by Wang et al.5 However, the structure feature of Rb3[(UO2)3(BO3)2O(OH)](H2O) is equal to the phases obtained from solid state syntheses.2,22 This demonstrated that water medium has played a key role for the structure formation of uranyl borates in hydrothermal synthesis. It seems that the more water in the reactions, the resultant phases tend to be 2D layered structure and similar to that prepared from solid state syntheses. K4Sr4[(UO2)13(B2O5)2(BO3)2O12] is the first mixed alkali− alkaline earth metal uranyl borate. Its FBB [(UO2)13(B2O5)2(BO3)2O12]12− is the most complex one, in which UO8 hexagonal and UO7 pentagonal bipyramids are connected by BO3 triangles and B2O5 dimers. The various uranium local configurations resulted in the most complex 2D layered uranyl borate topology. We presumed that the mixed cations (K+ and Sr2+) make a vital contribution to the overall structure. Both Rb 3 [(UO 2 ) 3 (BO 3 ) 2 O(OH)](H 2 O) and Rb 6 [(UO 2 ) 12 (BO3)8O3](H2O)6 have an identical U/B molar ratio of 3:2; however, they possess different FBBs within their layered structures. The FBB of Rb3[(UO2)3(BO3)2O(OH)](H2O) is [(UO2)3(BO3)2O2]4− and is composed of UO7 polyhedra and isolated BO3 triangles, whereas the FBB for Rb6[(UO2)12(BO3)8O3](H2O)6 is [(UO2)6(BO3)4O3]6−, consisting of UO6, UO7 polyhedra, and BO3 triangles. The rich coordination chemistry of uranium in Rb6[(UO2)12(BO3)8O3](H2O)6 makes its structure more complex than that of Rb3[(UO2)3(BO3)2O(OH)](H2O). In this study, three new FBBs of uranyl borates have been found in the A−U−B−O system for the first time. For comparison, the FBBs of the two high temperature solid state phases are more complex than the one from hydrothermal synthesis. We supposed that the coordination chemistry of uranium favored to be more complex at high temperature. This study also shed light on the structural flexibility and variety of uranyl borates at different reaction conditions. More importantly, this report will lead us to a better and deeper understanding of actinides chemistry in oxo-borate systems.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Thomas E. Albrecht-Schmitt: 0000-0002-2989-3311 Shuao Wang: 0000-0002-1526-1102 Evgeny V. Alekseev: 0000-0002-4919-5211 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful to the Helmholtz Association for funding within the VH-NG-815 project. Support for TEAS is provided through the Center for Actinide Science and Technology (CAST), an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences under Award Number DE-SC0016568. S.W. is supported by a grant from the National Natural Science Foundation of China (21422704). The authors are very thankful to Dr. Martina Klinkenberg for the EDS measurements, to Dr. Schlenz for Raman measurements, and to Dr. Modolo for the TGA-DSC measurements.

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