Hydrothermal Synthesis, Study, and Classification of Microporous

Article pubs.acs.org/IC

Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

Hydrothermal Synthesis, Study, and Classification of Microporous Uranium Silicates and Germanates Haijian Li,†,‡ Philip Kegler,† Dirk Bosbach,† 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

‡

S Supporting Information *

ABSTRACT: Four novel uranyl silicates and germanates with framework structures, K4Na2(UO2)3(Si2O7)2·3H2O, K4Na2(UO2)3(Ge2O7)2·3H2O, H3O(UO2)2(HGe2O7)·2H2O, and Na2(UO2)GeO4, have been synthesized by means of the hydrothermal method. The structures of the title compounds were refined by single-crystal X-ray diffraction and characterized by Raman spectroscopy. We used the method of secondary building units (SBUs) for a crystal chemical analysis of the 3D framework and their topologies. The framework of the K4Na2(UO2)3(T2O7)2·3H2O (T = Si, Ge) series exhibits large 14-membered rings and smaller 8membered rings which are built upon [UT4] pentamers. The internal size of the largest pores is approximately 12.39 × 3.33 Å 2 . H 3 O(UO2)2(HGe2O7)·2H2O is based on 10-membered rings with intermediate sized pores. They are built upon [U2Ge2] tetramers with 7-fold-coordinated U. The internal dimension of the pores in H3O(UO2)2(HGe2O7)·2H2O is smaller compared to the K4Na2(UO2)3(T2O7)2·3H2O (T = Si, Ge) series with ∼5.91 × 5.33 Å2. Its topology is similar to several uranium germanate synthetic phases and silicate minerals, especially α- and β-uranophane which are constructed from similar building units. A novel 3D framework type of Na2(UO2)GeO4 with 8-membered rings demonstrates the smallest free volume in the family of porous uranium germanates. It crystallizes in tetragonal symmetry and is built upon corner sharing of [UGe4] pentamers. The size of the channels is ∼6.76 × 4.27 Å2. The vibrational bands in Raman spectra were associated with pyro-(Si2O7)6− and -(Ge2O7)6− groups, with the Ge−OH bond and with H3O+ cations, confirming the results of the X-ray crystallographic structural characterization. We systemized existing uranyl silicates and germanates based on their building units and chemical composition. We found a simple structural dependence between synthetic conditions and chemical composition.

1. INTRODUCTION The study of coordination chemistry and reactivity of uranium in aqueous solutions is of great importance for a deeper understanding of the structural coordination preferences of actinide elements with different types of inorganic ligands.1,2 The uranium(IV) mineral coffinite (USiO4)3 and its synthetic analog UGeO44 have been found more than 50 years ago. The structure of soddyite5 ((UO2)2(SiO4)(H2O)2) and its synthetic germanate analog (UO2)2(GeO4)·H2O6, weeksite7 ((K2(UO2)2(Si2O5)3(H2O)4), α- and β-uranophane8,9 as well as the other minerals have been reported in recent years. Most of them exhibited two-dimensional structures with typical (UO2)2+ cations. The uranyl cation (UO2)2+ is the most stable species of UVI in solid state phases and in aqueous solutions under ambient atmosphere.2 Some exceptions are isotropic and tetraoxo-core coordinations of U in several newly observed oxides and uranium based complexes.10,11 Among the uranium(VI)-bearing compounds, uranium links two oxygen anions to form UO22+ cations with short UO multiple bonds and is coordinated by an additional 4−6 oxygen atoms in an equatorial plane, comprising tetragonal, pentagonal and hexagonal bipyramids. Uranyl groups form a large number of complexes with naturally occurring oxo© XXXX American Chemical Society

ligands, including SiO4 and GeO4. Several minerals and synthetic phases have been found to form in aqueous solutions such as cuprosklodowskite Cu(UO2)2SiO3OH(H2O)6,12 haiweeite Ca[(UO2)2Si5O12(OH)2](H2O)4.5,13 K5(UO2)2[Si4O12(OH)],14 (Cu(H2O)4)((UO2)(HGeO4))2(H2O)2,6 Ag((UO2)2(HGe2O7))(H2O),15 and (NH4)((UO6)2(UO2)9(GeO4)(GeO3(OH))).16 The counter cations, OH groups, and H2O molecules fill the free voids and space in the structures of these materials. Currently, researchers pay great attention on microporous compounds owing to their specific properties in adsorption, catalysis, and ion exchange, including challenges of nuclear waste treatment.17−21 For example, porous titanosilicates have been proposed as a prospective class of compounds for extraction of radionuclides.22 These properties are mainly based on the compatibility between the size and shape of the uranyl ion and the pore size of ETS-10 (Na1.5K0.5TiSi5O13·nH2O).23 Additionally, ETS-10 is suitable for the sorption of some radioactive isotopes, as proved with the uptake of the γ-ray radionuclides Received: February 21, 2018

A

DOI: 10.1021/acs.inorgchem.8b00466 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry Table 1. Crystallographic Data of K4Na2(UO2)3(Ge2O7)2·3H2O, K4Na2(UO2)3(Si2O7)2·3H2O, Na2(UO2)GeO4, and H3O(UO2)2(HGe2O7)·2H2O

a

compound

K4Na2(UO2)3(Ge2O7)2·3H2O

K4Na2(UO2)3(Si2O7)2·3H2O

Na2(UO2)GeO4

H3O(UO2)2(HGe2O7)·2H2O

fw (g/mol) space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) volume (Å3) Z λ (Å) F (000) ρ (g·cm−3) GoF R(F) for F02 > 2σ(F02)a wR2 (F02)b

1574.91 C2/m 13.163(5) 15.533(5) 5.984(5) 90 105.717(5) 90 1177.7(11) 2 0.71073 1372.0 4.441 1.187 0.0246 0.0726

1396.83 C2/m 13.1542(9) 15.1590(9) 5.8240(4) 90 104.644(7) 90 1123.61(13) 2 0.71073 1228.0 4.129 1.073 0.0692 0.1985

452.62 P42/m 5.3893(3) 5.3893(3) 9.7755(5) 90 90 90 283.93(3) 2 0.71073 388.0 5.294 1.170 0.0300 0.0752

853.39 Cmcm 14.0220(6) 10.8275(4) 7.1125(2) 90 90 90 1079.84(7) 2 0.71073 180 5.199 1.172 0.0257 0.0518

R(F) = Σ||F0| − |Fc||/Σ|F0| bR(F20) = [w(F20 − F2c )2/Σw(F40)1/2].

Cd2+, 204Hg2+, 60Co2+, and 137Cs+.22,23 Another hot spot in research of recent years, is designing metal organic frameworks (MOFs) as highly porous materials for removing mobile radionuclides, such as uranyl (UO22+) and pertechnetate (TcO4−) ions, from solution.20,21,24 In the past few decades, the chemistry of mixed cationic tetrahedral-based microporous open-framework materials has been extended to actinide materials. One remarkable example is [ThB5O6(OH)6][BO(OH)2]·2.5H2O (NDTB-1) with a supertetrahedral cationic framework which is capable for anion extraction from water solutions.18,19 More importantly, the compound is recyclable due to the exchange of TcO4− from NDTB-1 by higher-charged anions like PO43− and SeO42−. Within the oxo-salt series, uranyl silicates and germanates demonstrate rich structural chemistry including phases with microporous structures. A very diverse connection between UO22+ based bipyramids and mono- or pyro-silicate or pyrogermanate dimers results in the formation of materials exhibiting framework structures with 7-membered rings,25 8-membered rings,26 10-membered rings,15 and 12-membered ring channels.27−29 Among the above-described uranyl silicates and germanates, Cs6[(UO2)3(Ge2O7)2]·4H2O, possesses an extralarge pore and 12-membered ring channels. This phase is stable at temperatures up to 700 °C and water molecules can reenter the framework after a heating−cooling process.28 One of the most interesting properties of actinide compounds with different pores or channels is a potential application as ion exchangers. However, it is very challenging to design and synthesize inorganic compounds with a required dimension and type of pores. Herein, by using hydrothermal methods we synthesized four novel open-framework uranyl silicates and germanates: K4Na 2 (UO2 ) 3(Ge 2O 7 ) 2·3H 2O and K4 Na 2 (UO 2) 3 (Si 2O 7) 2 · 3H 2 O with extra-large 14-membered rings, H 3 O(UO2)2(HGe2O7)·2H2O with medium size 10-membered rings, and Na2(UO2)GeO4 with smaller 8-membered rings structure. In this work, we report the synthesis, crystal structures, properties, and Raman spectroscopy of these synthetic phases as well as a systematization of known uranyl silicates and germanates. 115m

2. EXPERIMENTAL SECTION Caution! Although isotopically depleted uranium was used in this work, standard precautions for working with radioactive materials must be strictly followed when conducting such experiments. 2.1. Crystal Growth. All the chemicals were analytical pure from Alfa Aesar except for uranium nitrate (International Bioanalytical Industries, Inc.). The syntheses of all compounds were performed under hydrothermal conditions at 220 °C. All the syntheses were performed in Teflon-lined steel autoclaves using similar experimental procedures, and the differences were only the initial reactants and their molar ratios, applied temperature and cooling rates. The syntheses of all compounds are reproducible. 2.1.1. K4Na2(UO2)3(Si2O7)2·3H2O and K4Na2(UO2)3(Ge2O7)2·3H2O. 2.1.1.1. Synthesis of K4Na2(UO2)3(Si2O7)2·3H2O. First, 50 mg of UO2(NO3)2·6H2O (0.099 mmol), 226.4 mg of Na2SiO3·9H2O (0.976 mmol), 995.8 μL of 4 M KOH (3.983 mmol), 29.1 mg of NaCl (0.498 mmol), 46.2 mg of WO3 (0.199 mmol), and 5 mL of deionized water were weighed and mixed together. The ratio of elements was U/Si/K/ Na as 1:8:40:21. 2.1.1.2. Synthesis of K4Na2(UO2)3(Ge2O7)2·3H2O. First, 50 mg of UO2(NO3)2·6H2O (0.099 mmol), 220.9 μL of 1.14g/mL germanium ethoxide (0.996 mmol), 995.8 μL of 4 M KOH (3.983 mmol), 29.1 mg of NaCl (0.498 mmol), 46.2 mg of WO3 (0.199 mmol), and 5 mL of deionized water were weighed and mixed together. The resulting ratio of U/Ge/K/Na in the synthetic system was 1:10:40:5. Typical mineralizers including hydroxides KOH and halides NaCl were included in the original reaction mixtures and as an additional mineralizer, WO330,31 was added. The mixtures were stirred thoroughly and filled in 23 mL autoclaves, and then these autoclaves were placed inside a Memmert program controlled furnace. The autoclaves were heated to 220 °C and held at this temperature for 48 h. The oven was then cooled to 80 °C with a rate of 2 °C/h and then shut down. Green rectangular shaped crystals of K4Na2(UO2)3(Si2O7)2·3H2O with a yield of around 20.9% based on uranium content and K4Na2(UO2)3(Ge2O7)2·3H2O with a yield of around 11.6% were obtained. 2.1.2. H3O(UO2)2(HGe2O7)·2H2O. Uranium nitrate solution was prepared by mixing 50 mg of UO2 (0.185 mmol) and 100 μL of HNO3 (68%−70%) in 23 mL autoclaves, then 155 mg of GeO2 (1.481 mmol), 833.2 μL of 4 M KOH (3.333 mmol), and 1 mL of deionized water were weighed and added. This resulted in a U/Ge/K ratio of 1:8:18. The mixed solutions (pH ∼13.96) were stirred thoroughly, and then these autoclaves were placed into a Memmert program controlled oven. They were heated to 220 °C at a rate of 100 °C/h and held there for 24 h. The oven was then cooled to 60 °C with a rate of 3 °C/h and B


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Inorganic Chemistry

Figure 1. (a, b, e) Polyhedral presentation of the structure of K4Na2(UO2)3(Ge2O7)2·3H2O along the [001], [010], and [100] crystallographic directions. (c) Polyhedral and ball-and-stick representations of SBUs in K4Na2(UO2)3(Ge2O7)2·3H2O: [UGe4] pentamers closed configuration. (d) Polyhedral representations of SBUs of K4Na2(UO2)3(Ge2O7)2·3H2O: [UGe4] pentamers. The yellow and royal blue polyhedra are UO6 tetragonal bipyramids and GeO4 tetrahedra, respectively. Mauve, cyan, and green denote K cations, Na cations, and H2O molecules, respectively. then shut down. Green needle crystals were acquired with a yield of around 13.3% based on uranium content. 2.1.3. Na2(UO2)GeO4. First, 50 mg of UO2(NO3)2·6H2O (0.099 mmol), 220.9 μL of 1.14g/mL germanium ethoxide (0.996 mmol), 1.991 mL of 2 M NaOH (3.983 mmol), 46.2 mg of WO3 (0.199 mmol), and 4 mL of deionized water were weighed and mixed. The resulting ratio of U/Ge/Na is 1:8:40. The mixtures were stirred thoroughly and filled in 23 mL autoclaves; then, these autoclaves were placed into a Memmert program controlled furnace. They were heated at a rate of 100 °C/h to 220 °C and held there for 24 h. The oven was then cooled to 80 °C with a rate of 2 °C/h and then shut down. Green needle crystals have a yield of around 5% based on uranium content. 2.2. Crystal Structure Studies. U−Si/Ge crystals were picked up and their crystallographic data were collected at 298 K using a Mo Kα1 tube (λ = 0.71073 Å) that operates at 50 kV and 0.8 mA on an Agilent SuperNova diffractometer. The three-dimensional data for the all compounds were processed using the CrysAlisPro software. The Lorentz polarization factors and absorption corrections were performed by using multiscan techniques.32 The crystal structures are determined and refined using the SHELXL-97 program and the software suite WinGX v1.80.05.33 The crystallographic data and the collection condition of data can be found in Table 1. Using direct methods for structure solution, the title crystal structures were solved and refined to R1 = 0.025 for K4Na2(UO2)3(Ge2O7)2·3H2O, R1 = 0.0692 for K4Na2(UO2)3(Si2O7)2·3H2O, R1 = 0.030 for Na2(UO2)GeO4, and R1 = 0.0257 for H3O(UO2)2(HGe2O7)·2H2O, respectively. 2.3. Powder X-ray Diffraction (PXRD). PXRD patterns of the entire product of each reaction were collected from 8 to 80°, with 2 s/ step and a step width of 0.02°. The powder samples were measured on a Bruker-AXS D4 Endeavor powder X-ray diffractometer with Cu Kα radiation. The comparison between the PXRD patterns of the reaction products and the calculated patterns generated from single crystal structures is shown in Figure S1a−d. 2.4. Scanning Electron Microscopy/Energy-Dispersive Spectroscopy (SEM/EDS). The chemical compositions for the obtained U− Si/Ge crystals were collected using a Quanta SEM with EDS capabilities with 30 kV accelerating voltage. The EDS results for the title U−Si/Ge compounds confirm the presence of elements, which is consistent with the desired compositions (see Figure S2 and Table S1). 2.5. Raman Spectroscopy. The unpolarized Raman spectra of the title U−Si/Ge crystals were collected at 298 K by using a Horiba spectrometer. The spectrometer is equipped with an objective lens with

a 50× magnification, which can be able to analyze samples with a dimension of 2 μm. A He−Ne laser generates the incident radiation (λ = 632.81 nm). The resolution of Raman spectra was about 1 cm−1. The Raman spectra of Na2(UO2)GeO4 were collected in the range of 200− 1500 cm−1 and do not exhibit any peaks with wavenumbers in the region above 1500 cm−1. For the Raman spectra of the other compounds, the range of collection was from 100 to 4000 cm−1, which reveals Raman shifts of above 1500 cm−1 derived from OH stretching, H2O bending or hydronium (H3O)+ ions vibrations. 2.6. Bond-Valence Sums (BVS). The BVS of all atoms in the observed uranium silicate and germanate compounds were calculated and analyzed. The formula for BVS calculation was given in the Supporting Information. The parameters Ri for UVI−O bonds,34 and other Si−O, Ge−O, K−O, and Na−O bonds35,36 were taken from literature. The BVS results of all atoms for the title compounds are given in Table S2.

3. RESULTS AND DISCUSSION 3.1. Syntheses. Three phases, namely, K4Na2(UO2)3(Ge2O7)2·3H2O, K4Na2(UO2)3(Si2O7)2·3H2O, and Na2(UO2)GeO4, have been obtained with the help of mixed mineralizers (NaOH, NaCl, WO3) under hydrothermal conditions at 220 °C. We found that the presence of WO3 in the starting solutions promoted the crystallinity and reduced the reaction time. NaCl improved the size and quality of crystals. In literature, KF was used as mineralizer for the synthesis of UV silicate and germanate compounds under high-temperature/ high-pressure (H-T/H-P) hydrothermal conditions.37,38 Therefore, we tried different fluorides (LiF, NaF and KF) to replace NaCl as mineralizer, but the reaction products contained only badly crystallized or amorphous materials. K4Na2(UO2)3(Ge2O7)2·3H2O has been synthesized through adding germanium ethoxide (C8H20GeO4) as reacting sources. We tried to synthesize the isotopological silicate compound K4Na2(UO2)3(Si2O7)2·3H2O by using tetraethyl orthosilicate (TEOS, C8H20SiO4) and tetramethyl orthosilicate (TMOS, C4H12SiO4) as starting materials. Surprisingly, all syntheses under the same synthetic conditions were unsuccessful. C


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Inorganic Chemistry

Figure 2. (a) Topology representations of the structure of K4Na2(UO2)3(Ge2O7)2·3H2O along the [001] crystallographic direction. (b) Topology representations of the 14-membered ring tubular units with lateral 4-membered ring and 6-membered ring windows in the structure of K4Na2(UO2)3(Ge2O7)2·3H2O.

Figure 3. Structural comparison between 3D K4Na2(UO2)3(Ge2O7)2·3H2O and 2D Na3K3[(UO2)3(Si2O7)2]·2H2O.39 (a) Topology representation of the structure of K4Na2(UO2)3(Ge2O7)2·3H2O along the [010] crystallographic direction. (b) Topology representation of 2D sheets with 14-membered rings in K4Na2(UO2)3(Ge2O7)2·3H2O. (c) Topology representation of the structure of Na3K3[(UO2)3(Si2O7)2]·2H2O along the [010] crystallographic direction. (d) Topology representation of 2D sheets with 8-membered rings in Na3K3[(UO2)3(Si2O7)2]·2H2O. Yellow circles are UO6 tetragonal bipyramids, and royal blue and green are GeO4 and SiO4 tetrahedra, respectively. Noted that [UT4] pentamers are highlighted using the black and white nodes.

3.2. Structural Descriptions. 3.2.1. K4Na2(UO2)3(T2O7)2· 3H2O (T = Si, Ge) series. K4Na2(UO2)3(T2O7)2·3H2O (T = Si, Ge) are refined in space group C2/m. The K4Na2(UO2)3(Si2O7)2·3H2O crystals were found to be of relatively poor quality because of the nonmerohedral twinning. However, these two structures K4Na2(UO2)3(T2O7)2·3H2O (T = Si, Ge) are isostructural, and here we only discuss the 3D framework of K4Na2(UO2)3(Ge2O7)2·3H2O which was adequately refined. The structure of K4Na2(UO2)3(Ge2O7)2·3H2O is shown in Figure 1. The asymmetric unit consists of two U sites, one Ge site, two K sites, one Na site, six O sites, and two H2O molecules. Both UVI cations are coordinated unsymmetrically by six oxygen atoms forming UO6 tetragonal bipyramids with UOUr bond lengths ranging from 1.833(4) to 1.843(5) Å and remaining longer U−Oeq bond distances from 2.227(3) to 2.241(4) Å. Ge cations are tetrahedrally coordinated by O anions and the Ge−O bond distances range from 1.730(3) to 1.780(2) Å. Two GeO4 tetrahedra share corners forming (Ge2O7)6− dimers, with a bridging oxygen bond angle of 135.74(32)°. The BVS calculations for U(1), U(2), and Ge sites are 5.78, 5.67, and 4.03 v.u., respectively, which agrees with the expected oxidation stares of U6+ and Ge4+. K(1) and K(2) cations are 9-, and 12coordinated by oxygen, with K−O bonds ranging from 2.696(4)

to 3.196 (4) Å, respectively, and Na cations are in six-coordinated oxygen environments with Na−O bonds from 2.315(4) to 2.535(4) Å. The 3D framework of K4Na2(UO2)3(Ge2O7)2·3H2O is composed of two types of secondary building units (SBUs) including [UGe4] pentamers (A type) and [UGe4] pentamers (A2 type), as shown in Figure 1 and later in section 3.3. Each of the UO6 tetragonal bipyramids shares its four equatorial corners with four germanate tetrahedra to form [UGe4] pentamers (A) (Figure 1d). The linkages of two (Ge2O7)6− dimers and UO6 result in the formation of [UGe4] pentamers (A2) (Figure 1c). The layers with 4-membered and 6-membered rings shown in Figure 1e is constructed by the interconnection of [UGe4] pentamers (A) and structurally related to uranium silicates.26,39−41 Adjacent layers are further fused together by [UGe4] pentamers (A2) to form the uranyl germanate framework with ellipse shaped 14-membered tubes. These tubes are approximately 12.39 × 3.33 Å2 based on the shortest O−O distance and directed along [001]. The 8-membered parallelogram-like shaped tubes are smaller and possess a size of 3.61 × 3.33 Å2 and directed along the [010]. K cations and disordered H2O molecules are at the edge and at the center of 14membered ring tubes, respectively, whereas Na cations are on the D


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Inorganic Chemistry

Figure 4. (a, b) Polyhedral presentation of the structure of H3O(UO2)2(HGe2O7)·2H2O along the [001] and [100] crystallographic directions. (c) Polyhedral presentation of uranium−oxygen chain formed by edge-sharing of uranyl pentagonal bipyramids in H3O(UO2)2(HGe2O7)·2H2O. Green and red are H2O molecules and oxygens, respectively.

Figure 5. (a) Polyhedral representations of SBUs in H3O(UO2)2(HGe2O7)·2H2O: [U2Ge2] tetramers (7-coordinated U). (b,c) Polyhedral representations of slab A with 6-membered rings and slab B with 10-membered rings, both slabs are produced from the connection of the same SBUs: [U2Ge2] tetramers (highlighted by red lines in slab A and green lines in slab B, respectively). (d) Direct condensation of two slabs leads to the formation of H3O(UO2)2(HGe2O7)·2H2O.

pentamers (A), and each 8-membered ring of Na3K3[(UO2)3(Si2O7)2]·2H2O based only upon two [UT4] pentamers (A). In both structures, [UT4] pentamers (A) are connected together to form 2D sheets with infinite kröhnkite-like chains (Figure S4). The terminal oxygen atoms in GeO4 tetrahedra in K4Na2(UO2)3(Ge2O7)2·3H2O have opposite direction on both sides of kröhnkite-like chain. Two remaining oxygen atoms of the pyrogermanate group are positioned “up” and “down” in respect to UO6 tetragonal bipyramids forming the 3D framework. However, SiO4 tetrahedra in Na3K3[(UO2)3(Si2O7)2]·2H2O have the same orientation on both sides of the kröhnkite-like chain, which results in the 2D layerered structure. 3.2.2. H3O(UO2)2(HGe2O7)·2H2O. H3O(UO2)2(HGe2O7)· 2H2O crystallizes in the orthorhombic Cmcm space group and possesses a 3D framework structure. It contains only one crystallographically independent position of both, U and Ge cations. The U6+ cations are bonded to 7 oxygen anions, forming UO7 pentagonal bipyramids with a nearly linear [178.73°(43)] OUO bond and UOUr bond lengths of 1.766(8) and 1.767(8) Å. The equatorial U−Oeq bond distances range from 2.298(5) to 2.464(5) Å. The bond lengths agree well with previously reported uranium minerals and synthetic compounds.8,42 The Ge cations are tetrahedrally coordinated with Ge−O bonds from 1.733(5) to 1.763(5) Å. The BVS analysis for O(5) is 1.508 v.u. (cp. Table S2d), which indicates the presence of OH− supported by Raman spectra (see section 3.4). Thus, two

center of 8-membered ring tubes. The topology of K4Na2(UO2)3(Ge2O7)2·3H2O along the [001] direction is shown in Figure 2a. Thus, the complex structure of both silicate and germanate is fully adopted for incorporation of cations and molecules with different size and shape. The framework is formed by a ABABAB stacking sequence. The internal structure of the large 14-membered ring tubes contains lateral 4membered rings and 6-membered ring windows as shown in Figure 2b. The structure of K4Na2(UO2)3(Ge2O7)2·3H2O is closely related to the structures of Na3K3[(UO2)3(Si2O7)2]·2H2O,39 and the U(V,VI) silicate compound Na 9 F 2 (U V O2)(UVIO2)2(Si2O7)2,26 which were obtained by the H-T/H-P hydrothermal method. The latter two phases crystallize in 2D structural types and possess the topology illustrated in Figure 3c,d. All mentioned structures consist of the common SBUs, [UGe4] pentamers (A) and [UGe4] pentamers (A2), and exhibit similar 8-membered ring tubes along the [010] direction. Moreover, all mentioned phases exhibit identical U/Si ratio of 0.75. A comparison of the 14-membered rings in K4Na2(UO2)3(Ge2O7)2·3H2O and the 8-membered rings in Na3K3[(UO2)3(Si2O7)2]·2H2O is given in Figure 3b,d. Obviously, the reduction of ring size from 14 to 8 is due to the different number of SBUs. Additionally, the ratio between larger K and smaller Na cations in K4Na2(UO2)3(Ge2O7)2·3H2O and Na3K3[(UO2)3(Si2O7)2]·2H2O is different. Each 14-membered ring of K4Na2(UO2)3(Ge2O7)2·3H2O contains four [UT4] E


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Inorganic Chemistry

Figure 6. (a, b) Polyhedral presentation of the structure of Na2(UO2)GeO4 with 4- and 8-membered rings along the [100] and [010] crystallographic directions. (c, d) Polyhedral and topologic representations of the structure of Na2(UO2)GeO4 with 8-membered rings; tubular units highlighted by red color along the [001] crystallographic directions and to the topological structure, correspondingly.

the [U2Ge2] tetramers along the [001] direction (Figure 5b,c). Some other uranium silicate minerals and germanate synthetic phases, such as boltwoodite,42 kasolite,44 sklodowskite,45 cuprosklodowskite,12 haiweeite,46 and (Cu(H2O)4)((UO2)(HGeO4))2(H2O)2,6 are also built upon [U2T2] tetramers. According to the analysis of anion topology, the anionic motif of slab A in H3O(UO2)2(HGe2O7)·2H2O is a full analog to that in α- and β-uranophane formed by [U2T2] tetramers (Figure S5). The obvious difference between slab A and two uranophane sheets is the orientation of the tetrahedra relative to the layer planes. The tetrahedra between adjacent U−O chains in slab A have identical orientation with a sequence DDDDDD. The SiO4 tetrahedra in α- and β-uranophane are oriented differently.8,9 3.2.3. Na2(UO2)GeO4. The Na2(UO2)GeO4 phase crystallizes in the tetragonal space group P42/m. This is a novel type of 3D uranyl germanium framework with a relatively small volume of free voids. It is constructed by corner-sharing of [UGe4] pentamers (A) which play a role of SBUs in this structure (Figure 6). It contains one GeO4 tetrahedron and one UO6 tetragonal bipyramid as structural elements. In this crystal structure, UO6 exhibits two UOUr bonds [1.823(8) Å] to create a straight uranyl [OUO]2+ unit and 4 U−Oeq bonds [2.252(5) Å] in the equatorial plane. The Ge−O bond lengths [1.772(5) Å] are within the normal ranges.47 As it is shown in Figure 6, each UO6 shares four equatorial corners with four GeO4 tetrahedra. Correspondingly, each GeO4 tetrahedron is linked to four UO6 tetragonal bipyramids to form a 3D uranyl germanate framework. The structure of Na2(UO2)GeO4 is relatively complex and based on a group of 4-membered and 8-membered channels. There are two sorts of 8-membered channels with a size of 6.76 × 4.27 Å2 along the [100] direction and with a size of 2.76 × 2.76 Å2 along the [001] direction. The uranyl germanium framework is charge-balanced by Na cations which lie in the center of 8-membered rings. A part of the tubular structure is

GeO4 tetrahedra are connected together to form a corner-sharing (HGe2O7)5− dimer with a Ge−O−Ge angle of 127.56°(60). The (HGe2O7)5− dimer was also found in the structure of the U−Ge compound (Cu(H2O)4)((UO2)(HGeO4))2(H2O)2.6 The uranyl pentagonal bipyramids share equatorial edges to form uranium−oxygen chains parallel to the [001] direction (Figure 4c), which in turn share polyhedra edges with (HGe2O7)5− dimers to construct an open framework with 10-membered rings with intermediate pore sizes of approximately 5.91 × 5.33 Å2, as shown in Figure 4a. Generally, this framework is similar to Ag[(UO2)2(HGe2O7)](H2O).15 H2O molecules in the structure of H3O(UO2)2(HGe2O7)·2H2O reside in the center of 10membered ring channels within the uranyl germanate framework. The anions O(w1) and O(w2) are defined to be water molecules. Some strong hydrogen bonds (O···O shorter than 2.7 Å) exist between water molecules (O(w1)···O(w2) 2.324 Å) and between water molecules and oxygen anions of the U−Ge framework ((O(w1)···O(4) 2.516 Å, (O(w1)···O(2) 2.676 Å). It forms a hydrogen-bond network and indicates the existence of hydronium (H3O)+ cations with three strong H-bonds.9,43 This is supported by prominent vibrational modes of (H3O)+ ions in Raman spectra of H3O(UO2)2(HGe2O7)·2H2O. Additionally, there is a weak H-bonding net between O(w1) water molecules (O(w1)···O(w1), 2.793 Å).43 Upon heating to 383 K, H3O(UO2)2(HGe2O7)·2H2O loses its crystallinity due to the loss of water and hydronium cations. This shows that the existence of the water molecules is crucial and significant for the stability of the uranyl germante framework in this phase. The structure of H3O(UO2)2(HGe2O7)·2H2O is based on [U2Ge2] tetramers with 7-coordinated U (D type), as shown in Figure 5. The interconnection of [U2Ge2] tetramers by cornersharing leads to the formation of slab A with 6-membered rings and slab B with 10-membered rings in H3O(UO2)2(HGe2O7)· 2H2O. Slab A can be converted into slab B via rotation of 180° of F


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Inorganic Chemistry

SBUs were extracted from all of the reported uranyl silicates compounds and are listed in Figure 9. Two out of these SBUs are five-connected U-based clusters, and the linkages between U polyhedra can be found in four SBUs. Uranyl silicate compounds in particular show quite complex 3D frameworks by the combination of different SBUs. In comparison to usually tetrahedrally coordinated silicon, germanium exhibits three types of coordination including GeO4 tetrahedra, GeO5 trigonal bipyramid, and GeO6 octahedra,52 which link UO22+ cations forming more complex frameworks. However, the number of reported uranyl germanate compounds is very limited, only 5 SBUs are reported, and they are illustrated in Figure 10, among which only one contains GeO5 trigonal bipyramids. All SBUs for the U−Si system presented in the Figure 9 can be separated into four central groups: A→ 4-connected U-centers, B→ 4-connected U with [(UO2)2O] dimer, C→ 4-connected U with uranyl poly chain, and D→ 5-connected U. We named each SBUs using these simplified letters based on the connectivity and number of Si-tetrahedra, as it is shown in Figure 9. For the family of uranyl silicates, the classification of different SBUs in relation to the chemical composition, namely Si/U ratio, is shown in Figure 11a and Table S3. The Si/U ratios, observed in the phases based on group A SBUs, demonstrate a wide range from 1 to 10. Among these, the compounds with the Si/U ratios from 1.33 to 2.28 are constructed from two or three types of SBUs (see Table S3). Three unusual phases can be seen in the series of uranyl silicates, A2USiO6 (A = Rb, Cs),53 which were obtained by the flux growth method and based on group C SBUs. Another phase synthesized by a similar method, K8(K5F)U6Si8O40,40 is based on a complex composition of groups D and A. Surprisingly, all U−Si minerals, independently from the Si/U ratio, are constructed upon in group D. Speculatively, it can be a confirmation that the slow kinetics of crystallization affects the local coordination of uranium in forming phases. Besides this, most of the synthetic U−Si materials crystallize in structures with A types of local coordination U-centers. These materials were obtained from different synthetic conditions but still have similar structures. Potentially, it can be an effect of relatively fast formation processes which take place in synthetic reactions in U−Si−O systems. In the U−Ge series, which is significantly less representative, no B and C groups were found. An additional SBU containing 5coordinated Ge can be found and denominated as the E group. The chemical composition of known U−Ge compounds exhibits a narrow range for the Ge/U ratio only from 0.5 to 2 (cp. Figure 11b). All these materials were obtained from hydrothermal and H-T/H-P hydrothermal conditions and two are minerals. Despite the small number of phases in the Ge based system, we can observe that most of these crystallize in A type groups, similar to that of the Si based system. 3.4. Raman Spectral Analysis. In the past decade, many studies have been devoted to the vibrational spectroscopy of uranyl oxo-anion minerals, particularly Raman microprobe spectra.54−56 The application of vibrational spectroscopy, including Raman spectroscopy, is part of our ongoing work, which allows to understand structural short-range ordering as well as indicating the presence of OH− and H3O+ groups, which can usually not be detected from XRD. The Raman spectra of the uranyl group, (UO2)2+, in the four studied uranium phases can be separated into three specific regions: (1) uranyl antisymmetric stretching vibrations ν3(UO2)2+ between 800 and 920 cm−1; (2) uranyl symmetric

shown in Figure 7. To further understand the interconnection in this fragment, we unfold it onto a 2D plane as shown in Figure 7b.

Figure 7. (a) Representation of the 8-membered ring tubular units in the structure of Na2(UO2)GeO4 along the [001] crystallographic directions. (b) Unrolled tubular wall built from [UGe4] SBU pentamers. (c) Black and white representation of atubular unit. Note that the [UGe4] pentamers are highlighted with blue rectangles. (d) Idealized tubular unit obtained by folding the corresponding letters together at the corresponding opposite ends.

We utilized the black and white nodal representation to describe and analyze the topology of unfolded tubular units. The UO6 and GeO4 are substituted by black and white nodes and are connected via corner-sharing to form [UGe4] pentamers (SBUs) as it is highlighted by blue rectangles in Figure 7d. Each [UGe4] pentamer shares corners with four other [UGe4] pentamers to build a wall. The idealized unit is obtained by folding and gluing the corresponding letters a−a′, b−b′, c−c′, and so on in Figure 7c. We attempted to simplify the topology of the structure based on the concept of SBUs and characterize the inter linkages between them. The 8-membered tubular units are polymerizing and building the topology of Na2(UO2)GeO4 as it is shown in Figure 8a,b. The idealized 3D framework of Na2(UO2)GeO4 is packed by the tubular units consisting of [UGe4] pentamers in Figure 8c,d. 3.3. Classification of Secondary Building Units (SBUs) and Structure−Composition Relation in U−Si/Ge OxoCompounds. In recent years, the theory of secondary building units (SBUs) has been widely applied for understanding and predicting topologies of large-pore structures, such as MOF’s and zeolites.48,49 A great number of uranyl silicate minerals and synthetic phases have been found and reported, and the uranyl silicate family shows rich topologies.50,51 We have found that UO22+ cations can link to SiO44+ tetrahedra to form the 4- and 5connected U-based clusters that we refer to as SBUs. Twelve G


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Inorganic Chemistry

Figure 8. (a, b) Topology representations of Na2(UO2)GeO4. (c, d) The idealized 3D topological framework of the packing of tubular units constructed by the square [UGe4] pentamers (SBUs) in Na2(UO2)GeO4.

Figure 9. Polyhedral and ball-and-stick representations of the SBUs extracted from all reported uranyl silicate compounds.

stretching vibrations ν1(UO2)2+ between 700 and 800 cm−1; and (3) uranyl symmetric bending vibrations ν2(UO2)2+ between 200 and 380 cm−1.56−58

For K4Na2(UO2)3(Ge2O7)2·3H2O, the two strong peaks at 728 and 748 cm−1 and the shoulder at 697 cm−1 in Figure 12 can correspond to the ν1(UO2)2+. The ν3(UO2)2+ vibrations are not H


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Inorganic Chemistry

Figure 10. Polyhedral and ball-and-stick representations of the SBUs extracted from all reported uranyl germanate compounds.

Figure 12. Raman spectra of K 4 Na 2 (UO 2 ) 3 (Ge 2 O 7 ) 2 ·3H 2 O, K4Na2(UO2)3(Si2O7)2·3H2O, H3O (UO2)2(HGe2O7)·2H2O, and Na2(UO2)GeO4 in the range of 100−1500 cm−1 and Raman spectra of K4Na2(UO2)3(Ge2O7)2·3H2O, K4Na2(UO2)3(Si2O7)2·3H2O in the range of 2600−3100 cm−1.

Figure 11. (a) Classification of different types of SBUs in relation to the Si/U ratio in the family of uranyl silicate compounds. (b) Classification of different SBUs in relation to the Ge/U ratio in the family of uranyl germanates.

Ge) of (Ge2O7)6− groups, respectively. Similar bands are also detected in the Raman spectra of Sr 2 MgGe 2 O 7 and Sr2ZnGe2O7.60 The low frequency band at 181 cm−1 corresponds to the mixing vibrations of the (Ge2O7)6− groups and the K and Na cations.61 The Raman spectra of K4Na2(UO2)3(Si2O7)·3H2O shows that the ν1(UO2)2+ vibrations are located in the shoulder peak at 731 and 766 cm−1,56 and the ν3(UO2)2+ vibrations are presented on 849 and 880 cm−1.56,57 The low frequency bands at 210, 232, 272, 322, and 366 cm−1 correspond to the ν2(UO2)2+.58 In case of diorthosilicate (Si2O7)6− groups, two low intensity bands at 946 and 968 cm−1 observed in Raman spectra of K4Na2(UO2)3(Si2O7)·3H2O can correspond to the ν1(Si−O)

present in the obtained spectra. Raman bands of ν2(UO2)2+ are observed at 321, 288, 261, and 217 cm−1. The group theory describes the reducible representations of the vibrations of an isolated Si2O7 or Ge2O7 group with D3d symmetry as 3A1g + 1A1u + 3A2u + 3Eg + 4Eu, of 14 internal vibrational modes, there are 6 Raman internal active modes (3A1g + 3Eg), 7 infrared internal active modes (3A2u + 4Eu) and one inactive mode (1A1u).59 Two weak peaks from the Raman spectra are found at 448 and 507 cm−1, which are attributed to the symmetric bending vibrations ν2(Ge−O−Ge) and the symmetric stretch vibration ν1(Ge−O− I


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Inorganic Chemistry vibrations.62,63 The Raman spectra in the range from 380 to 700 cm−1 show three low intensity bands. The band at 671 cm−1 corresponds well to the ν1(Si−O−Si) vibrational modes,60,62 and the band at 544 cm−1 corresponds to the ν4 mode of the (SiO4)4−.56 The lower frequency band at 480 cm−1 can correspond to the bending motions of the (Si2O7)6− units.60,64 Finally, the bands at 2869, 2930, and 2973 cm−1 in the Raman spectra of K4Na2(UO2)3(Ge2O7)2·3H2O and K4Na2(UO2)3(Si2O7)2·3H2O can correspond to the OH stretching vibrations of H2O molecules.56,65 The H3O(UO2)2(HGe2O7)·2H2O shows 108 zone-center vibrational modes (16Ag + 9Au + 17B1g + 12B1u + 10B2g + 17B2u + 11B3g + 16B3u) obtained by factor group analysis.66 96 of these vibrations are optical modes containing 54 Raman-active modes (ΓR = 16Ag + 17B1g + 10B2g + 11B3g) and 42 IR-active modes (ΓIR = 11B1u + 16B2u + 15B3u). In the Raman spectra of H3O(UO2)2(HGe2O7)·2H2O, the strongest intensity band at 823 cm−1 corresponds to the ν3(UO2)2+ mode, similar to the strongest band at around 800 cm−1 seen in α-uranophane.56,67 The vibrational mode of ν3(UO2)2+ can also be seen at 877, 890, and 911 cm−1.58,68 The ν1(UO2)2+ are found at 719 and 766 cm−1.56 The ν1(Ge−O−Ge) is seen at 564 cm−1 in the Raman spectra of H3O(UO2)2(HGe2O7)·2H2O. The low intensity bands at 376 and 427 cm−1 correspond to the ν2(Ge−O−Ge) modes.60,69 A strong, sharp peak at 150 cm−1 may be attributed to the bending motions of the (Ge2O7)6− groups.61,70 The Raman spectrum of H3O(UO2)2(HGe2O7)·2H2O in the range from 1000 to 4000 cm−1 is shown in Figure 13. O(w1)···

The structure of Na2(UO2)GeO4 has two formula units per unit cell. Three Na cations are situated at three nonequivalent sites (2g, 1d, 1c), and two Ge cations are at two nonequivalent sites (1b, 1a). U cations occupy the 2g sites; four O anions are at the 4h sites. Therefore, factor group analyses predict a total of 60 zone-center vibrational modes with mechanical representation: Γ = 11A + 15B + 171E + 172E. Three of these vibrations are 3 acoustic modes (Γacoustic = B + 1E + 2E) and 57 are optical modes (Γoptic = 11A + 14B + 161E + 162E).66 The Raman-active modes of Na2(UO2)GeO4 occur at the 11A + 14B + 161E + 162E species, and its infrared-active modes occur at the 14B + 161E + 162E species. In the Raman spectra of Na2(UO2)GeO4, two strong bands at 759 and 840 cm−1 correspond to the ν1(UO2)2+ and ν3(UO2)2+ vibrations, respectively. Two low frequency bands at 204 and 275 can be seen clearly, which correspond to the ν2(UO2)2+ vibrations.55 Some bands are present within the 380− 700 cm−1 region. The highest intensity bands at 688 cm−1 in the middle frequency region correspond to the ν3 mode of the GeO44− units and are in good agreement with literature.61 The peaks at 416 and 388 cm−1 can be attributed to the ν4 mode of the GeO44− and the ν2 mode of O−Ge−O, respectively.61,69 The lower frequency band at 141 cm−1 corresponds to ν2(O−Ge−O) and Na translations.61

4. CONCLUSIONS We have synthesized and investigated four novel porous phases from the uranium(VI) based systems. Our work revealed that structure formation and crystal growth in uranium silicates/ germanates systems is of strong dependence from mineralizers nature as effective additives. Our results demonstrate a significant variation of channels size and local coordination environment of uranium in these phases. We demonstrated that in the family of uranyl silicates and germanates a local structure of SBUs is directly dependent on chemical composition of the phases. A variation of SBUs connections leads to a significant change in the materials structures. For example, different connectivity of [UT4] pentamers results in the structural transformation from 3D K4Na2(UO2)3(Ge2O7)2·3H2O to 2D Na3K3[(UO2)3(Si2O7)2]· 2H2O. This work enhances the knowledge on the methods of hydrothermal synthesis of uranyl silicates and germanates with porous structures.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b00466. X-ray crystallographic files in CIF format, PXRD patterns, EDS Results, BVS analysis, structural figures, and tables of known uranyl silicates and germanates (PDF)

Figure 13. Raman spectra of H3O (UO2)2(HGe2O7)·2H2O in the range of 1000−4000 cm−1.

O(w2) distance of 2.324 Å indicates the existence of (H3O)+ ions. Four bands at 1043, 1067, 1624, and 3529 cm−1 are consistent with the vibrational modes of the (H3O)+ ions.70,71 The two lower frequency bands at 1043 and 1067 cm−1 correspond to ν2(H3O)+ mode,71 and the peaks at 1624 and 3529 cm−1 correspond to the ν4 mode and ν3 mode, respectively.70,71 The (HGe2O7)5− dimers indicated by BVS analysis indicate weak hydrogen bonds. Several low intensity peaks are able to be seen in the spectrum between 1150 and 1500 cm−1. They correspond to the stretching vibrations of Ge−OH bonds.67 Bands in the range within 2800 and 3500 cm−1 are attributed the O−H vibrations of water.55,56,67

Accession Codes

CCDC 1825915−1825917 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.

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Corresponding Author

*E-mail: [email protected]. J


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Evgeny V. Alekseev: 0000-0002-4919-5211 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Research is supported by the Helmholtz Association for financial support (VH-NG-815). We thank Dr. Martina Klinkenberg (IEK-6) for EDX measurements and Dr. Schlenz (IEK-6) in Raman data collection. H.J. is grateful to the financial support from the Chinese Scholarship Council.

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