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

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Structural and Spectroscopic Investigation of Novel 2D and 3D Uranium Oxo-Silicates/Germanates and Some Statistical Aspects of Uranyl Coordination in Oxo-Salts Haijian Li,†,§ Eike M. Langer,† Philip Kegler,† 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 § Science and Technology on Combustion and Explosion Laboratory, Xi’an Modern Chemistry Research Institute, Xi’an 710065, China

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‡

S Supporting Information *

ABSTRACT: Synthesis, structural and spectroscopic characterization, and topological analysis of five novel uranyl-based silicates and germanates have been performed. The open-framework K4(UO2)2Si8O20·4H2O has been synthesized under hydrothermal conditions and is based upon [USi6] heptamers interconnected via edge-sharing. Its structure is composed of sechser silicate layers with 4-, 8-, and 16-membered rings. The largest 16-membered rings have an average dimension of ∼8.93 × 9.42 Å2. β-K2(UO2)Si4O10 has been obtained by the high-temperature flux growth method. Its 3D framework contains a loop-branched sechser single layer with 4- and 8-membered rings and consists of the same [USi6] heptamers as observed in K4(UO2)2Si8O20·4H2O. Na6(UO2)3(Si2O7)2 has also been synthesized from melted fluxes and represents a 2D layer structure composed by [USi4] pentamers. Two iso-structural compounds A+(UO2)(HGeO4)·H2O (A+ = Rb+, Cs+) were synthesized via the hydrothermal method, and their structures are of the αuranophane type. The 2D layers consist of [U2Ge2] tetramer secondary building units (SBUs). The Raman spectra of all novel phases were collected, and bands were assigned according to the existing oxo-silicate rings and oxo-germanium units. Additionally, we performed a statistical investigation of the local coordination of uranyl ions in all known inorganic structures with different oxo-anions (TOx, T = B3+, Si/Ge4+, P/As5+, S/Se/Te6+, Cr/Mo/W6+, P/As3+, and Se/Te4+). We found a direct correlation between the ionic potential of the central cations T in oxo-anions in their higher oxidation states and the coordination number of uranyl groups.

1. INTRODUCTION Uranium oxy-salt phases exhibit an unusual structural chemical behavior and topologies due to the variety of possible oxidation states (U3+ → U6+) coinciding with rich coordination geometries of uranium cations.1−4 U6+ usually demonstrates three types of coordination polyhedra in oxo-based phases, e.g., oxo-square, -pentagonal, and -hexagonal bipyramids.5 The coordination chemistry of uranyl ions in oxoanion phases has been paid a great deal of attention for more than 60 years due to its presence in natural minerals and technological processes.1,6,7 Among the large group of actinidebearing inorganic oxo-phases, uranium-based oxo-salt compounds are the most common. Among them, uranium silicates and germanates have attracted significant attention in past decades due to their importance in understanding uranium migration in nature and within the corrosion of vitrified spent fuel. Currently, 16 uranyl silicate minerals have been found at different natural oxidation zones and most of them hydrated by OH−, H3O+, and/or H2O.8 Additionally, it has to be © XXXX American Chemical Society

mentioned that dissolution tests performed on U-doped borosilicate glass resulted in th e formation of KNa3(UO2)2(Si4O10)2·4H2O9 with a structure and properties similar to mentioned minerals. Practically all known layers in oxo-silicates demonstrate Si/ O ratios equal to 2/5, as it has been noted by Liebau.10 Three different types of silicate layers have been found up to now in the uranium silicates family. The silicate layer of α-K2(UO2)Si4O10 can be denoted as a loop-branched dreier (three) single layer with 4- and 8-membered silicate rings.11 The nuclear glass alteration product KNa3(UO2)2(Si4O10)2·4H2O9 as described above and the synthetic compound Na2(UO2)Si4O10·2.1H2O12 synthesized via mild hydrothermal conditions both contained an unbranched vierer (four) single layer with 4and 8-membered silicate rings. The tetravalent uranium silicates A2USi6O15 (A = K, Rb)2 have the same unbranched Received: May 23, 2019

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DOI: 10.1021/acs.inorgchem.9b01523 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry Table 1. Crystallographic Parameters for K4(UO2)2Si8O20·4H2O, β-K2(UO2)Si4O10,Na6(UO2)3(Si2O7)2, and A+(UO2)(HGeO4)·H2O (A+ = Rb+, Cs+) Compounds fw (g/mol) space group a (Å) b (Å) c (Å) β (deg) volume (Å3) Z F (000) ρ (g cm−3) GoF R(F) for F02 > 2σ(F02)a wR2 (F02)b

K4(UO2)2Si8O20·4H2O

β-K2(UO2)Si4O10

Na6(UO2)3(Si2O7)2

Rb(UO2)(HGeO4)·H2O

Cs(UO2)(HGeO4)·H2O

1305.18 P 21 14.179(5) 14.179(5) 14.965(5) 90 3008.6(18) 4 2384.0 2.881 0.982 0.0545 0.1786

620.59 C 2/c 17.933(2) 6.7928(3) 11.7443(16) 125.516(19) 1164.5(3) 4 1128.0 3.540 1.001 0.0351 0.0743

1284.39 C 2/m 22.323(5) 7.463(5) 5.777(5) 99.377(5) 949.6(11) 2 1116.0 4.492 1.052 0.0522 0.1213

508.09 P 21/c 6.7778(4) 7.1983(4) 14.2922(8) 100.568(5) 685.46(6) 4 868.0 4.923 1.003 0.0345 0.0862

555.53 P 21/c 6.8044(3) 7.2337(3) 14.8524(7) 101.475(4) 716.44(6) 4 940.0 5.150 1.066 0.0349 0.0872

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

a

SiO2 (fumed powder, particle size ≈ 0.008 μm, Sigma-Aldrich), Na2CO3 (Alfa Aesar, 98%), K2CO3 and Rb2CO3 (Alfa Aesar, 99%), NaF (Alfa Aesar, 99%), C8H20O4Si (TEOS) and C4H12O4Si (TMOS) (Alfa Aesar, 98%), Na2MoO4 (Sigma-Aldrich, 99.9%), NaCl (Alfa Aesar, 99%), WO3 (Alfa Aesar, 99.8%), GeO2 (Alfa Aesar, 99.999%), CsOH·xH2O (15−20% H2O) (Alfa Aesar, 99.9%), NH4Cl (Alfa Aesar, 98%), RbOH (Alfa Aesar, 50% w/w aq. soln., 99.6+%), and KOH (Alfa Aesar, pellets, 85%) were utilized in this research. Caution! The rules for handling radioactive materials must be followed while working with uranyl nitrate and uranium trioxide. K4(UO2)2Si8O20·4H2O was synthesized using mixed mineralizers (WO3−NaCl) under hydrothermal conditions.19 β-K2(UO2)Si4O10 with 2D oxo-silicate layers was synthesized using a mixed flux (K2CO3−WO3) under high temperature (H-T) conditions. The 2D sheet compound Na6(UO2)3(Si2O7)2 was obtained using a mixed flux (NaF−Na 2 MoO 4 ) under H-T conditions. Two compounds, A+(UO2)(HGeO4)·H2O (A+ = Rb+, Cs+), were synthesized by the hydrothermal synthesis in 23 mL Teflon-lined steel autoclaves, and NH4Cl was used as a mineralizer. 2.1.1. K4(UO2)2Si8O20·4H2O. The initial solution was prepared by thoroughly stirring a mixture of UO2(NO3)2·6H2O (0.05 g), TEOS(0.936 mL), KOH (0.995 mL, 4 mol/L), NaCl (0.029 g), and WO3(0.046 g) at a ratio of 1:10:40:5:2. Finally, 5 mL of H2O were filled into the autoclave. The autoclave was placed into a Memmert program controlled oven, which was heated gradually to 220 °C for 2 h, held at that temperature for 48 h, and finally slowly cooled to 80 °C at a rate of 1 °C/h. The oven was shut down, and green crystals of K4(UO2)2Si8O20·4H2O formed in a mixture with previously published Na2(UO2)(Si4O10)·2.1H2O.12 The yield of K4(UO2)2Si8O20·4H2O is around 35% based on uranium content. K4(UO2)2Si8O20·4H2O can be also obtained by a mixture of UO2(NO3)2·6H2O (0.05 g), TMOS (0.371 mL), and KOH (0.597 mL, 4 mol/L) at a ratio of 1:4:24 and 5 mL of H2O. The mixture was heated to 220 °C for 2 h, kept at that temperature for 36 h, and slowly cooled to 60 °C at a rate of 2 °C/h. Under this condition, the yield of K4(UO2)2Si8O20·4H2O is higher and around 60%. 2.1.2. β-K2(UO2)Si4O10. The initial materials were prepared by grinding a mixture of UO2(NO3)2·6H2O (0.05 g), SiO2(0.063 g), K2CO3 (0.485 g), and WO3(0.813 g) with a ratio of 1.15:12:40:40 in an agate mortar. The mixture was transferred into a platinum crucible and placed in a high-temperature furnace, which was heated gradually to 950 °C for 3 h, held at that temperature for 5 h, and slowly cooled to 500 °C at a rate of 5 °C/h. The furnace was shut down, and green crystals within an amorphous phase were found in the crucible. The yield of β-K2(UO2)Si4O10 is around 22% based on uranium content. 2.1.3. Na6(UO2)3(Si2O7)2. The initial materials were prepared by grinding a mixture of UO3(0.025 g), SiO2(0.063 g), Na2CO3 (0.018 g), NaF (1.101 g), and Na2MoO4 (0.27 g) at a ratio of 1:12:2:300:15

dreier (three) layer which has been found in thorium silicate Cs2ThSi6O15.13 The Si6O15 layers in these structures are based upon 4-, 6-, and 8-membered rings and can be found in some nonactinide phases such as CaZrSi 6 O 15 ·3H 2 O 14 and Na3LaSi6O15·2H2O.15 Two main structural groups can be found in the family of uranyl silicate minerals and germanate phases such as boltwoodite and sklodowskite,16 forming in groundwater conditions. The first type is based upon two-dimensional sheets composed of TO4 (T = Si, Ge) tetrahedra and uranyl pentagonal bipyramids. Most of these sheets display T/U ratios of 0.5 or 1 and can be constructed from [U2Si2] tetramers secondary building units (SBUs). Kasolite (Pb(UO2)(SiO4)·H2O), as a typical example of a uraninite alteration product, exhibits an α-uranophane-sheet structure type.17 Similar 2D sheets and generally layered structures are quite unusual for synthetic phases within the U−Si/Ge system. To date, only two compounds were reported with similar structural types, Na3Rb3(UO2)3(Si2O7)2 and Na3K3(UO2)3(Si2O7)2·2H2O,18 which have been obtained from high-temperature, high-pressure (H-T/H-P) hydrothermal reactions. The second typical mineral structures within the U−Si/Ge system are based upon 3D frameworks and have a T/U ratio is 2.5. The framework is composed of [U2Si6] octamers. It should be noted that the 3D frameworks are obviously connected to the structures of 2D sheets, and an increasing Si tetrahedra number from [U2Si2] tetramers to [U2Si6] octamers leads to the structural transformation from 2D sheets to 3D frameworks. Herein, we applied two synthetic methods, namely, high temperature (H-T) and hydrothermal techniques, for the synthesis of two 3D frameworks K4(UO2)2Si8O20·4H2O and βK2(UO2)Si4O10 with new oxo-silicate layers and three 2D phases Na6(UO2)3(Si2O7)2 and A+(UO2)(HGeO4)·H2O (A+ = Rb+, Cs+). These materials have been structurally and spectroscopically characterized. Additionally, we analyzed a statistical distribution of coordination numbers of uranyl cations in inorganic oxo-salts depending on the nature of the central cation of oxo-salt groups.

2. EXPERIMENTAL SECTION 2.1. Materials and Synthetic Methods. UO2(NO3)2·6H2O (International Bioanalytical Industries, Inc.), UO3 (prepared by heating UO2(NO3)2·6H2O in a ceramic crucible at 450° for 4 h), B


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Inorganic Chemistry

Figure 1. (a−c) Polyhedral presentation of the crystal structure of K4(UO2)2Si8O20·4H2O viewed along the [010], [010], and [001] directions. (d) Silicate layer with 4-, 8-, and 16-membered rings in the compound. Key: yellow tetragonal bipyramids, UO6; green tetrahedra, SiO4; mauve atoms, K; and red atoms, H2O molecules. program21 was used for the checking of possible higher symmetries. The crystallographic data of all final structures and the collection conditions for all compounds are given in Table 1. 2.3. Powder XRD. After the reaction products were dried, their PXRD data were recorded at room temperature on a Bruker D4 diffractometer. The diffraction angles 2θ are in the 10∼80° region with a counting time of 2 s at each step. The simulated XRD patterns were generated by using Mercury 3.8 as shown in Figure S1(a, b). 2.4. SEM-EDS. The presence of all heavy elements for the title compounds was confirmed by a Quanta 200F scanning electron microscope and energy-dispersive spectroscopy (SEM-EDS) with an electron beam of 30 kV. The EDS results for all compounds can be seen in Figure S2 and Table S1. 2.5. Raman Spectroscopy. The Raman spectra for the title compounds were obtained from single crystals by a Horiba LabRAM HR spectrometer with the spectral resolution of 1 cm−1 at 296 K. The range of Raman spectra for K4(UO2)2Si8O20·4H2O and A+(UO2)(HGeO4)·H2O (A+ = Rb+, Cs+) was from 100 to 4000 cm−1, and the range for anhydrous phases β-K2(UO2)Si4O10 and Na6(UO2)3(Si2O7)2 was from 100 to 1500 cm−1. 2.6. Bond Valence Sums. Two compounds, A+(UO2)(HGeO4)· H2O (A+ = Rb+, Cs+), contain water molecules and hydroxyl groups, and it was necessary to calculate the valence states of all atoms to support the presence of OH groups using the bond-valence sums (BVS) method. The formula for BVS and bond valence parameters are described in the Supporting Information. The BVS results for all atoms are given in Table S2.

in an agate mortar. The mixture was transferred into a platinum crucible and placed in a high-temperature furnace, which was heated gradually to 750 °C for 2.5 h, kept at that temperature for 12 h, and slowly cooled to 600 °C at a rate of 6 °C/h. The furnace was shut down, and some needle green crystals of Na6(UO2)3(Si2O7)2 with a yield of around 77% based on uranium and an amorphous phase have been found in the crucible. 2.1.4. A+(UO2)(HGeO4)·H2O (A+ = Rb+, Cs+). The initial solution for Rb(UO2)(HGeO4)·H2O was prepared by thoroughly stirring a mixture of UO2(NO3)2·6H2O (0.1 g), GeO2(0.073 g), RbOH (0.806 mL), and NH4Cl (0.009 g) at a ratio of 2.28:8:90:2 with 2 mL of H2O in a 23 mL autoclave. The compound can also be obtained from a mixture of UO2(NO3)2·6H2O (0.05 g), GeO2(0.042 g), RbOH (0.408 mL), and Rb2CO3 (0.184 g), at a ratio of 1:4:40:8 and 1 mL of H2O. The yields of Rb(UO2)(HGeO4)·H2O by the addition of different mineralizers (NH4Cl, Rb2CO3) are similar and around 30%. For Cs(UO2)(HGeO4)·H2O, the initial solution was prepared by thoroughly stirring a mixture of UO3 (0.03 g), GeO2 (0.011 g), CsOH (0.315 g), and NH4Cl (0.006 g) at a ratio of 2:2:40:2 and 2 mL of H2O in a 23 mL autoclave. The autoclaves were sealed and placed into a Memmert program controlled oven, which was heated gradually to 220 °C for 2 h, held at that temperature for 24 h, and slowly cooled to 80 °C at a rate of 2 °C/h. The oven was shut down, and needleshaped green crystals of Cs(UO2)(HGeO4)·H2O with a yield of about 47% were found in the autoclave. 2.2. Crystallographic Studies. Good quality crystals were selected for structural refinement. The data of all compounds were recorded with CrysAlisPro software on an Agilent Oxford Diffraction Super Nova diffractometer with a Mo Kα tube at 296 K. Absorption corrections for all raw data were performed by the multiscan method. The unit cell was determined, and background effects were processed by the CrysAlisPro software. The initial structures for the title compounds were refined through the SHELXL-2018 of WinGX (v1.80.05) software,20 and the ADDSYM algorithm of the PLATON

3. RESULTS AND DISCUSSION 3.1. Crystal Structures. 3.1.1. K4(UO2)2Si8O20·4H2O. The 3D framework K4(UO2)2Si8O20·4H2O crystallizes in the monoclinic space group P21 (Table 1). It has to be noted that unit cell parameters of this phase are very close to C


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Inorganic Chemistry

Figure 2. (a) Polyhedral presentation and (b) topology of a silicate layer with a [46.8.16] ring type in K4(UO2)2Si8O20·4H2O viewed parallel to the (001) plane. The green polyhedra are SiO4 tetrahedra.

Figure 3. (a) Topological representation of K4(UO2)2Si8O20·4H2O along the [001] direction. (b) Nodal representation of two uranyl silicate cages based upon [USi6] heptamers (1 + A + 1 type) highlighted by in red along the [010] direction. Key: yellow and black circles, UO6 tetragonal bipyramids; green and white, SiO4 tetrahedra.

topology parallel to (001) are shown in Figure 2. The 16membered rings within the silicate layers are ∼8.93 × 9.42 Å2 as measured from the centers of the nearest oxygen atoms. The size of smaller 8-membered rings is ∼5.99 × 5.92 Å. Silicate layers are linked together by uranyl ions to create a novel three-dimensional open-framework with an ABAB stacking sequence. The voids of the framework are filled by K atoms and H2O molecules. In previous work, we classified diverse SBUs for U−Si/Ge oxo-compounds.22 The cationic topology of K4(UO2)2Si8O20· 4H2O along [001] is depicted in Figure 3a. It is unique and can be considered as stacking of uranyl silicate cages. The peachshaped cage is constructed from edge-sharing [USi6 ] heptamers (1 + A + 1 type) highlighted by the red color in Figure 3b. Each peach-shaped cage contains one 16- and two 8-membered rings. Additionally, the cage exhibits lateral 6-ring and 8-ring windows on the wall and has a large interior space of 10.55 × 10.66 × 14.97 Å.3 In the Experimental Section, we described that the reaction products contained two compounds, K4(UO2)2Si8O20·4H2O and previously reported Na2(UO2)(Si4O10)·2.1H2O,12 due to the application of metal hydroxide (KOH) and mineralizer (NaCl) in the synthetic experiment. Interestingly, the 3D frameworks of both compounds contain silicate layers and can be built upon the same [USi6] heptamer SBUs, as shown in Figure S3.

tetragonal; however, the structural model of this phase is monoclinic and cannot be converted to any tetragonal space group. Thus, we conclude a pseudo tetragonality of K4(UO2)2Si8O20·4H2O. Different initial materials (TEOS or TMOS) and experimental conditions were used for the synthesis of this compound. However, in most of the syntheses the crystals of K4(UO2)2Si8O20·4H2O were of bad quality as a result of nonmerohedral twinning. The better crystals were grown from a mixture of UO2(NO3)2·6H2O, TEOS, and KOH by using WO3 andNaCl as mineralizers. As illustrated in Figure 1, the structure of K4(UO2)2Si8O20·4H2O is based upon four isolated UO6 tetragonal bipyramids, 16 SiO4 tetrahedra, 10 K atoms, and 10 water molecules. The typical UO 6 tetragonal bipyramids exhibit the UOUr bond lengths ranging from 1.74(5) to 1.86(5) Å and UOeq bonds (in the equatorial plane) ranging from 2.18(4) to 2.41(4) Å.8 The OUO bond angles are between 173(2)° and 176(3)°. The SiO distances in SiO4 tetrahedra are beyond the required normal range because of artifacts of the nonmerohedral twinning and range from 1.48(4) to 1.77(4) Å. SiO4 tetrahedra are linked together via sharing of three corners to create a loop-branched sechser single silicate layer with 4-, 8-, and 16-membered rings (Figure 1d). The SiOSi angles have a wide range from 129(2)° to 174(2)°. The structure of the silicate layer and its D


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Inorganic Chemistry

Figure 4. (a, b) Polyhedral representation of the crystal structure of β-K2(UO2)Si4O10 viewed along the [001] and [010] directions. Key: yellow tetragonal bipyramids, UO6; green tetrahedra, SiO4; mauve atoms, K; and red atoms, H2O molecules.

Figure 5. (a) Silicate layers with [4.8] rings and its topology (b) in the structure of β-K2(UO2)Si4O10. (c) Silicate layers and its topology (d) in the structure of α-K2(UO2)Si4O10. Green polyhedra are SiO4 tetrahedra.

3.1.2. β-K2(UO2)Si4O10. The structure of β-K2(UO2)Si4O10 is based upon a 3D framework (Figure 4) and crystallizes in the monoclinic space group C2/c. The previously reported αK2(UO2)Si4O1011 was obtained by the high-temperature−highpressure (H-T/H-P) hydrothermal method. The most obvious distinction between both structures are the silicate layers. The α-type compound has a loop-branched dreier (three) single layer, whereas the novel β-type contains a loop-branched sechser (six) single layer, as it is depicted in Figure 5. The 3D framework of β-K2(UO2)Si4O10 contains two symmetrically independent oxo-silicate tetrahedra, which have normal SiO interatomic distances ranging from 1.567(6) to 1.620(6) Å (mean value of 1.589 Å for Si(1), 1.596 Å for Si(2)) and the OSiO bond angles range from 106.7(4)° to 114.7(4)°. Three vertices of each tetrahedron are linked forming a silicate layer with 4- and 8-membered rings. SiOSi angles range from 139.2(7)° to 180°. The isolated uranyl ions are located between silicate layers and form UO6 tetragonal bipyramids with linear OUO bonds. The structure of β-K2(UO2)Si4O10 contains two identical UOUr bonds with 1.800(6) Å

length. In the equatorial plane, the two UOeq bonds have lengths of 2.231(6) Å and another two bonds have lengths of 2.244(6) Å. Analysis of BVS displays that valence states of U(VI) and Si(IV) are in good agreement with the expected oxidation states. Potassium cations are 7-fold coordinated with KO bond distances ranging from 2.776(6) to 3.315(2) Å. The structure of β-K2(UO2)Si4O10 parallel to (011) is shown in Figure 6. It can be seen that the β-polymorph can be represented by [USi6] heptamers (1 + A + 1 type) in Figure 6c, whereas the structure of α-K2(UO2)Si4O1011 is made of [USi4] pentamers, shown in Figure 6f. The different SBUs in both structures result in the formations of distinct 2D slab types (Figure 6b and d), generating diverse and unique frameworks. Previously, we found a direct relation between Si/ U ratio and SBUs types for uranyl silicate/germanate oxocompounds.22 Here, we can notice that β-K2(UO2)Si4O10 and K4(UO2)2Si8O20·4H2O have the same Si/U ratio of 4, and their structures are based on the same SBUs. However, the interconnection type of SBUs within both structures is very distinct. Two [USi6] heptamers are connected by utilizing two E


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Inorganic Chemistry

Figure 6. (a) Polyhedral representation of the crystal structure of β-K2(UO2)Si4O10 in the bc plane. (b) A slab of β-K2(UO2)Si4O10 consisting of [USi6] heptamers (1 + A + 1 type) (c). (d) Polyhedral representation of the crystal structure of α-K2(UO2)Si4O10 in the bc plane. (e) A slab of αK2(UO2)Si4O10 consisting of [USi4] heptamers (A type) (f). The yellow polyhedra and black circles are UO6 tetragonal bipyramids, and green polyhedra and white circles are SiO4 tetrahedra.

Figure 7. (a, b) Polyhedral representation of the two-dimensional structure of Na6(UO2)3(Si2O7)2 projected along the [010] and [001] directions. (c) A slab of Na6(UO2)3(Si2O7)2 based upon [USi4] pentamers (A type) (d) and [USi4] pentamers (A2 type) (e).

common tetrahedra constructing K4(UO2)2Si8O20·4H2O. In other words, the SBUs assemble by edge-sharing to create

K4(UO2)2Si8O20·4H2O, whereas the SBUs assemble by cornersharing to comprise β-K2(UO2)Si4O10. F


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Inorganic Chemistry

Figure 8. Genetics of uranyl based 0D, 1D, 2D, and 3D structures. (a) 0D [(UO2)(MoO4)4]6− clusters in the structure of Rb6(UO2)(MoO4)4. It represents SBU [USi4] pentamers (A type) in the system of uranyl silicates. (b) 1D chain in the structure of K2Ca4(UO2)(Si2O7)2. (c) 2D sheets of K2Ca4(UO2)(Si2O7)2. (d) The 2D layer structure of Na6(UO2)3(Si2O7)2 consisting of (c) 2D sheets and SBU [USi4] pentamers (A2 type). Similar situation occurs in the 3D framework structure of K4Na2(UO2)3(Si2O7)2 (e). (f) 2D layer of Ba(UO2)(Si2O6) is constructed by cross-linkage of double 1D sheets.

Figure 9. (a, c) Polyhedral representation of the crystal structure of Cs(UO2)(HGeO4)·H2O along [010] and [001] directions. (b) Zipped uranyl chain in the structure of Cs(UO2)(HGeO4)·H2O. (d) [U2Ge2] tetramers (7-coordinated U) (D type) SBUs in Cs(UO2)(HGeO4)·H2O and its topology (e). Key: yellow pentagonal bipyramides, UO7; dark teal tetrahedra, GeO4; pink and small red circles, Cs and O atoms, respectively.

3.1.3. Na6(UO2)3(Si2O7)2. Na6(UO2)3(Si2O7)2 exhibits a 2D structure and crystallizes in the monoclinic system in the C2/m space group. The structure has the same anoinic network with the mixed cationic hydrate Na3K3[(UO2)3(Si2O7)2]·2H2O18 (space group P1̅), which was synthesized under H-T/H-P

hydrothermal conditions. Two unique uranyl ions in the structure of Na6(UO2)3(Si2O7)2 occur as tetragonal bipyramids. The UO and UO bond lengths range from 1.80(2) to 1.83(2) Å and from 2.21(1) to 2.24(1) Å, respectively. The SiO bonds in silicate tetrahedra are in the range of 1.57(1) G


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Inorganic Chemistry

Figure 10. Statistics of the coordination geometries of uranyl ions in the family uranyl oxo-anion compounds (B3+, Si/Ge4+, P/As5+, S/Se/Te6+, and Cr/Mo/W6+). The total number of phases is given alongside each part of pie charts. Distinct coordination environments of uranyl ions in each phase are presented by using different colors.

− 1.63(1) Å. Each tetrahedron shares one vertex with another, creating the pyro-silicate Si2O7 unit, which connects the isolated uranyl ions into a 2D layer of Na6(UO2)3(Si2O7)2 (Figure 7). The layers are linked with interstitial uranyl groups into the double layer aggregates. Further, 6- and 8-membered rings can be observed within this double layer along [010] and [001], respectively. Na atoms reside within 8-rings and between the double layers. The 2D layer structure of Na6(UO2)3(Si2O7)2 consists of two SBUs, namely, A and A2 types of [USi4] pentamers.22 Interestingly, [UMo4] pentamers found as 0D clusters within Rb6(UO2)(MoO4)4 have the same topology as [USi4] pentamers (A type).23 This cluster can be used as a basic genetic fragment for the construction of existing A-type oxosilicate structures as it is shown in Figure 8. These 0D clusters are linked through O atoms of silicate tetrahedra producing the 1D chain in K2Ca4(UO2)(Si2O7)2.24 Further, these 1D chains interconnect to create a typical 2D sheet within the U−Si family (Figure 8c). Three structures containing 2D sheets are presented in Figure 8d−f. The structure discussed above of Na6(UO2)3(Si2O7)2 can be constructed by these 2D sheets. A similar situation occurs in the 3D framework structure of K4Na2(UO2)3(Si2O7)2 which is also based on 2D fragments observed before.22 The cross-linkage of idealized sheets through Si−O bonds which are perpendicular to the sheets leads to the formation of the condensed double layer in the structure of Ba(UO2)(Si2O6).25 3.1.4. A + (UO 2 )(HGeO 4 )·H 2 O (A + = Rb + , Cs + ). The compounds A+(UO2)(HGeO4)·H2O (A+ = Rb+, Cs+) exhibit similar 2D sheet structures. They crystallize in the monoclinic crystal system with a P21/c space group. Interestingly, these materials are close analogs of naturally occurring uranyl silicate minerals such as α-26 and β-uranophane.27 The structure of Cs(UO2)(HGeO4)·H2O is presented in Figure 9. Uranium atoms are 7-fold coordinated with O creating UO7 pentagonal

bipyramids, which share edges producing zipper shaped chains. UOUr bond lengths in the UO7 pentagonal bipyramids are 1.783(8) and 1.784(7) Å, and UOeq bond lengths are in the range from 2.288(6) to 2.482(6) Å. The OUO bond angle is 179.2(3)°. Further, GeO4 tetrahedra with an average of 1.751 Å of GeO distances are connected with uranyl chains by edge-sharing to form 2D layers of the α-uranophane type.26 Two adjacent uranyl germanate layers are connected by Cs cations and H2O molecules. Cs cations are 10-coordinated, whereas Rb cations are involved in a 9-fold coordination environment. Each UO7 pentagonal bipyramid in the structure of Rb(UO2)(HGeO4)·H2O presents two short UOUr bonds (1.774(6) and 1.792(6) Å) and four longer UOeq bonds (the distances from 2.307(7) to 2.491(6) Å). GeO bond lengths in each GeO4 tetrahedra are in the range from 1.632(7) to 1.813(7) Å. The results of BVS calculations for both compounds demonstrate that O6 atoms have only 1.041 v.u. for Cs(UO2)(HGeO4)·H2O and 0.926 v.u. for Rb(UO2)(HGeO4)·H2O. Therefore, GeO4 tetrahedra include an OH group forming HGeO4 fragments in both compounds. Because the distance between water molecule OW1 and O6 is about 2.701 Å, a net of hydrogen bonds occurs in the interlayer space.27,28 3.2. Statistical Analysis of Coordination Geometries of Uranyl Groups in Oxo-Anion Bearing Phases. Although the system of U6+ oxo-anion phases presents only three coordination geometries of uranyl ions ranging from 4 to 6 in the equatorial plane, the combination of differently coordinated uranyl polyhedra, the presence of uranyl cation− cation interactions (CCIs), and the precence of tetroxido cores in some phases further expand the variety of structural architectures. For example, a uranyl borate phase, K12((UO2)19(UO4)(B2O5)2(BO3)6(BO2OH)O10)(H2O)9.5,3 obtained by the high-temperature/high-pressure (H-T/H-P) technique, contains not only uranyl tetragonal and pentagonal H


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Inorganic Chemistry

Si/Ge to P/As. Two uranyl systems containing hexavalent central cations (S/Se/Te6+, Cr/Mo/W6+) present quite similar coordination distributions (Figure 10 and Table S3), which leads to similar average CN values for uranyl groups in both systems. Moreover, the great majority of the phases contain uranyl pentagonal bipyramids, and no mixing coordination environment phases were found in our analysis. By comparison of both systems (U−S/Se/Te6+−O and U−Cr/Mo/W6+−O), we can conclude that ionic potentials of hexavalent central cations have a weak influence on the average CNs of uranyl ions in these systems. The uranyl-oxo-anions systems also include the phases with reduced central cations such as P/As3+ and Se/Te4+. The crystal structures of all U−P3+−O phases contain phosphite (HPO32−) tetrahedra. Most of these phases have U pentagonal bipyramids and two in 11 known phases involve 4- and 5coordinated uranyl groups (Figure 12). Interestingly, phosphite and phosphate tetrahedra coexist in the structures of Cs4((UO2)8(HPO4)5(HPO3)5)(H2O)46 and Tl3((UO2)2(H1.5PO4)(H0.5PO4)(H1.5PO3)2).29 For the U− As 3 + −O system, only two phases, (Ni 0 . 6 Mg 0 . 4 )(UO2)2(AsO3)2(H2O)730 and Mg((UO2)(AsO3)0.7(AsO4)0.3)2(H2O)7,31 were reported, and they all have 5-coordinated uranyl groups. The latter contains mixing valence-state As3+ and As5+. The average CN of uranyl ions in the U−P/As3+−O family is ∼4.88. The U−Se/Te4+−O family also shows a similar pie chart with U−P/As3+−O, and each structure contains HSeO3− trigonal pyramids and/or SeO32− pseudo-tetrahedra. Only one phase (H3O)UO2 (SeO4)(SeO2(OH))32 involves hexavalent selenate and tetravalent selenite. Almost half of the structures for the U−Se/Te4+−O system have 5-coordinated uranyl, and structures of 10 phases possess mixed 4- and 5-coordinated and 5- and 6-coordinated uranyls. Recently, we found that high-temperature/highpressure conditions increase the diversity of U−Se/Te4+−O frameworks due to the variation of coordination geometries of Te4+ and U6+.33,34 The average CN of U for U−Se/Te4+−O family (5.04) is close to that for U−P/As3+−O, which is attributed to the same ionic potential (0.06) for both systems. By analyzing the coordination environments of uranyl ions in all known U6+−T−O phases, we found that the ionic potential of central cations T in oxo-anion groups in their higher oxidation states correlates to the CNs of uranyl ions in these materials. Uranyl oxo-borate groups is an exemption from this correlation due to the strong polymerization tendency of oxo-borate fragments. This study improves our understanding of structural flexibility of uranyl-based phases and gives us a predictive tool for structures of unknown phases from the uranyl oxo-salt family. 3.3. Raman Analysis. The Raman spectrum of K4(UO2)2Si8O20·4H2O in the 100−3600 cm−1 region is shown in Figure 13 and exhibits a strong peak at 781 cm−1, which corresponds to the symmetric ν1 mode of (UO2)2+ cations (Table S4).35 Another shoulder at 751 cm−1 is also attributed to the ν1(UO2)2+ vibrations.35,36 The ν3 modes of (UO2)2+ ions can be found at 813 cm−1,36,37 and the ν2(UO2)2+ modes are located at 223 and 297 cm−1.38 The Raman spectrum in the range of 400−700 cm−1 and high frequency range of 950−1100 cm−1 illustrates a large number of peaks owing to the existing of complex oxo-silicate fragments in the structure. First, the weak band at 1091 cm−1 corresponds to the ν3 modes of (SiO4)4− units.39 Two bands at 952 and 961 cm−1 correspond to the ν1(SiO4)4−

bipyramids but a uranyl tetraoxido core as well. Another interesting and uncommon structure is the 3D framework of Na2Li8[(UO2)11O12(WO5)2],4 which exhibits all three types of uranyl coordination environments and CCIs. In this part, we counted and analyzed the coordination geometries of uranyl ions for 404 different crystal structures of known U6+−T−O (where T = B3+, Si/Ge4+, P/As5+, S/Se/ Te6+, Cr/Mo/W6+, P/As5+, and Se/Te4+) compounds without uranyl CCIs, as illustrated in Table S3. Pie charts are used to present the coordination environment distributions of uranyl ions including mixed coordination environments (Figure 10). More than half of uranyl borate structures contain uranyl hexagonal bipyramids, indicating the preferred CN of 6 in the U−B3+−O system; here and in the future, CN refers to the number of oxygen donor atoms in the equatorial plane of uranyl group ((UO2)2+) but not U6+ cations. We did not find structures that only contain uranyl tetragonal bipyramids. Seven phases involve 4- and 5-, 5- and 6-, and 4-, 5-, and 6coordination of uranyl groups, and the average CN of these mixed-coordination U ranges from 4.67 to 5.29. These facts result in the highest average coordination geometries (∼5.32) of U for the U−B3+−O system compared to other oxo-anion systems. It can be explained by significant polymerization of oxo-borate groups in these phases with simultaneous settling of uranyl groups inside of oxo-borate rings. In the family of U− Si/Ge4+−O compounds, the structures only contain 4- and 5coordinated uranyl groups. The mixed coordination environment of uranyl groups are not reported to date in this family (Figure 10). Thus, the average CN of uranyl groups for this system is the smallest among others with ∼4.48. It corresponds to the low ionic potential of central cations (Si/Ge4+) in oxoanions, as plotted in Figure 11.

Figure 11. Plot showing the relationship between the average values of CN of uranyl ions and the ionic potential of central cations (Si/ Ge4+, P/As5+, S/Se/Te6+, and Cr/Mo/W6+) in oxo-anions.

There are 113 different U−P/As5+−O structures reported in the literature and in the Inorganic Crystal Structure Database (ICSD). Twenty-nine phases demonstrate a mixing coordination environment, and they have a wide average CNs ranging from 4.5 to 5.67. The average CN of uranyl groups for the U− P/As5+−O system is higher than for the silicate/germanate family and around 4.77. The ionic potential also increases from I


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Inorganic Chemistry

Figure 12. Statistics of the coordination geometries of uranyl ions in the family of all known uranyl anion (P/As3+ and Se/Te4+) oxo-compounds.

Figure 13. Raman spectra in the 100−1200 cm−1 region for K4(UO2)2Si8O20·4H2O, β-K2(UO2)Si4O10, and Na6(UO2)3(Si2O7)2, and Raman spectrum in the region from 2800 to 3600 cm−1 for K4(UO2)2Si8O20·4H2O.

displays three ν2(UO2)2+ bands at 232, 276, and 362 cm−1. The frequency of ν3(SiO4)4− band at 1026 cm−1 is lower than that of above observed uranyl silicates with silicate layers. The typical band at 1026 cm−1 is present in systems of silicate compounds with Si2O7 units.46−48 The weak bands at 679 and 487 cm−1 correspond to the ν1(Si−O−Si) of diorthosilicate units.44,47 Both compounds A+(UO2)(HGeO4)·H2O (A+ = Rb+, Cs+) are iso-structural, and their Raman spectra present similar shapes, as plotted in Figure 14. When Cs+ is substituted for Rb+, most of the bands shift slightly to the high-frequencies range. The ν1(UO2)2+ modes in the spectra of Cs(UO2)(HGeO4)·H2O are described in the bands at 735, 751, and 788 cm−1 (Table S5). Only one band at 855 cm−1 is attributed to the ν3(UO2)2+ modes. A large number of peaks have been found in the range of 200−380 cm−1, which can be assigned to the ν2(UO2)2+ modes. The Raman spectra of 2D layer structures for uranyl oxo-anions such as arsenates49 and borates50 also exhibit a large number of bands in the low frequency region. The ν3(GeO4)4− and ν4(O−Ge−O) bands appear at 680 and 429 cm−1,45,51 respectively. The O−H vibrations of water appear at around 2871 and 2925 cm−1 for both compounds.35,36,52 After the band positions of ν1(UO2)2+ and ν3(UO2)2+ modes were identified from the Raman spectra of each phase, UO bond lengths were calculated using the empirical expression that was found by Bartlett and Cooney.53 The calculated results for the UO lengths are compared with those in the crystal structures, as shown in Table S6, and the

modes.35,40 In the low frequency region, three bands at 576, 605, and 656 cm−1 can be assigned to the ν4 bend modes of (SiO4)4− units.35,39 The bands at 520 cm−1 could be attributed to the ν1(Si−O−Si) of 4-membered silicate rings.41,42 The low frequenciey bands at 492, 467, and 429 may be related to the higher-membered silicate rings.41 The ν3(SiO4)4− modes are observed at 403 cm−1.35,36 The band at 179 cm−1 may be attributed to the translations of K cations or external modes of (SiO4)4− units.43 The Raman spectrum of K4(UO2)2Si8O20· 4H2O shows three strong peaks at 2869, 2934, and 3462 cm−1 in the range of 2800−3600 cm−1, which correspond to the antisymmetric stretching vibrations of H2O molecules22,35 For β-K2(UO2)Si4O10, the Raman spectrum presents two high intensity bands at 762 and 779 cm−1, which correspond to the ν1(UO2)2+ modes.35,36 No obvious peak for the ν3(UO2)2+ modes can be seen in the 800−930 cm−1 region, whereas more weak peaks for ν2(UO2)2+ modes are found at 217, 275, 294, and 345 cm−1. The vibrations of (SiO4)4− units at 961 and 1078 cm−1 are analog to that of K4(UO2)2Si8O20·4H2O. The band at 686 cm−1 corresponds to the ν1(Si−O−Si) modes.44 The vibrations of silicate rings can be seen at 439 and 478 cm−1, which agree well with the theoretical analysis of Raman spectra for vitreous silica (V-SiO2).41 The bands at 115 and 173 cm−1 may correspond to the K translations or phonon vibrational modes. 4 5 In the Raman spectrum of Na6(UO2)3(Si2O7)2, the ν1(UO2)2+ bands appear at 719, 742, and 761 cm−1, and the ν3(UO2)2+ bands can be found at 827, 898, and 924 cm−1.35,36 Additionally, the Raman spectrum J


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Inorganic Chemistry

PXRD patterns, SEM/EDS images, BVS results, structural figures, statistical tables for coordination environments of uranyl ions, and Raman shifts and proposed band assignments for the titled compounds (PDF) Accession Codes

CCDC 1918156−1918160 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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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Evgeny V. Alekseev: 0000-0002-4919-5211 −1

Notes

Figure 14. Raman spectra in the 100−1200 cm region with an excerpt of the 2600 to 3200 cm−1 range for A+(UO2)(HGeO4)·H2O (A+ = Rb+, Cs+).

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the DFG aunder grant AL1527/3-1. The authors thank Dr. Martina Klinkenberg (IEK-6, Forschungszentrum Jülich) for help with SEM/EDX measurements and Dr. Hartmut Schlenz for help with Raman measurements. H.J. is grateful to the financial support from the Chinese Scholarship Council.

former presents slightly longer average UO lengths for each structure compared to the bond lengths observed from X-ray crystallography.

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4. CONCLUSIONS In this work, five uranyl-based silicate and germanate oxocompounds have been prepared using two methods, i.e., hightemperature melting flux growth and hydrothermal techniques. Two novel silicate layers and three 2D structures were described. A 3D framework K4(UO2)2Si8O20·4H2O and its byproduct Na2(UO2)(Si4O10)·2.1H2O12 have been synthesized under hydrothermal conditions, and both structures are composed of the same [USi6] heptamer SBUs. Therefore, the concept of SBUs is helpful to build the relationship between particularly different compounds. Additionally, the polymorphs of α- and β-K2(UO2)Si4O10 have different silicate layers and building units, indicating that synthesis can be tuned to deliver the desired structure of the product. Most importantly, a statistical analysis of equatorial plane coordination geometries of hexavalent uranium for existing U6+−T−O phases shows that a larger ionic potential of the central cation T in oxo-anions in their higher oxidation states results in a higher CN of U6+. Speculatively, the larger ionic potential of T cations in their higher oxidation state withdraws more electron density from their bound oxygen which leaves less available for donation to the uranyl unit, thereby increasing the need for more oxygen donors and, therefore, yielding a higher coordination number. The valence states of central cations (P/As5+ and P/As3+, S/Se/Te6+ and Se/Te4+) have critical influences to the coordination environments of U6+. The study facilitates the understanding of structural flexibility of uranyl oxo-phases.

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REFERENCES

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Inorganic Chemistry

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