Comparison of Uranium(VI) and Thorium(IV) Silicates Synthesized via

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

Comparison of Uranium(VI) and Thorium(IV) Silicates Synthesized via Mixed Fluxes Techniques Haijian Li,†,‡ Philip Kegler,† Vladislav V. Klepov,§ Martina Klinkenberg,† 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 § Samara National Research University, 443086 Samara, Russia ‡

S Supporting Information *

ABSTRACT: Two uranium and two thorium silicates were obtained using high temperature mixed fluxes methods. K14(UO2)3Si10O30 crystallizes in the P21/c space group and contains open-branched sechser (six) single silicate chains, whereas K2(UO2)Si2O6 crystallizes in the C2/c space group and is built of unbranched achter (eight) silicate chains. The crystals of K14(UO2)3Si10O30 and K2(UO2)Si2O6 are related by increasing U/Si molar ratios, and both structures contain the same secondary building units (SBUs), [USi6] heptamers. The triangle diagram for all known A+−UO22+−SiO44− phases demonstrates the high polymerization level of silicate groups in the system, which was compared with the family of A+− UO22+−BO33−/BO45− compounds. For both thorium silicates, the transformation of K2ThSi2O7 to K2ThSi3O9 was found to be a factor of the reaction time. K2ThSi2O7 crystallizes in the C2/c space group and belongs to the Na2SiVISi2O7 structure type. Its 3D framework consists of diorthosilicate Si2O7 group and ThO6 octahedra. Noncentrosymmetric K2ThSi3O9 crystallizes in the hexagonal P63 space group and adopts mineral wadeite-type structure based upon triorthosilicate Si3O9 rings and ThO6 octahedra. The coordination environment of thorium for all existing oxo-anion compounds including B, Si/Ge, P/As, Cr/Mo/W, and S/Se/Te are summarized and analyzed. Additionally, spectroscopic properties of all novel materials have been studied.

1. INTRODUCTION

conditions and mechanism of their formation and structural aspects of actinides bearing silicates. Basically, silicate phases can be built upon complex silica based chains, tubules, sheets, frameworks, and clusters.7 In particular, silicate chains were categorized into unbranched-, open-, and loop-branched single silicate chains and multiple chains based on the classification of Liebau.7 The study of the formation, structures and properties of actinide-bearing silicates have been rapidly expanded over the past decade. Three main methods including hydrothermal, high-temperature (HT), and high-temperature/high-pressure (HT/HP) hydrothermal syntheses have been used for the synthesis of silicates including those with actinide elements. Recently, the zur Loye group has demonstrated that salt-inclusion uranyl silicates can be obtained via using complex halide based fluxes.8−10 Several uranyl silicates with single tetrahedral chains, usually named as monopolysilicates,7 demonstrate three-dimensional (3D) framework structures. This includes α-Cs2(UO2)(Si2O6),11 Rb2(UO2)Si2O6,12 and Cs2(UO2)(Si2O6)(H2O)0.5,11 with achter7 (eight) silicate chains, β-Cs2(UO2)Si2O6,12 Ba(UO2)-

U and Th are the remarkable members of actinide (An) series of elements. The compounds with these elements possess complex chemical and materials properties and can be observed in a form of natural minerals and artificial phases related to process of U and Th migration in nature. Uranium exists in a wide range of oxidation states varying from +2 to +6.1−4 U(VI) usually forms uranyl groups UO22+ and coordinates from four to six oxygen atoms in the equatorial plane, forming square, pentagonal, and hexagonal bipyramidal environment. In some specific cases uranium(VI) demonstrates “exotic” coordination behavior such as nearly symmetrical tetraoxido core (UO4)2−.5 Uranyl-based oxo-anion compounds are widely spread in nature and can be found in the form of silicates, arsenates, phosphates, molybdates, selenites, and other oxo-salts phases. One of the largest groups among the mentioned classes is the family of uranium based silicates. Additionally, long-term corrosion experiments of UO2 pellets in oxidative conditions resulted in the formation of secondary hexavalent uranium silicate phases such as soddyite, uranophane, boltwoodite, and sklodowskite.6 The structural diversity of the U−Si−O minerals encourages us to synthesize novel uranyl silicate phases and study the © XXXX American Chemical Society

Received: April 18, 2018

A

DOI: 10.1021/acs.inorgchem.8b01072 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry Table 1. Crystallographic Data for K14(UO2)3Si10O30, K2(UO2)Si2O6, K2ThSi2O7, and K2ThSi3O9

a

compound

K14(UO2)3Si10O30

K2(UO2)Si2O6

K2ThSi2O7

K2ThSi3O9

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

2118.39 P21/c 12.9022(4) 11.1248(3) 13.9515(4) 90 90.806(3) 90 2002.32(10) 2 0.71073 1940.0 3.514 1.009 0.0344 0.0989

4003.28 C2/c 21.695(3) 14.4156(7) 16.073(3) 90 136.71(3) 90 3447(2) 2 0.71073 3552.0 3.857 1.087 0.0476 0.1540

1913.68 C2/c 10.0633(5) 5.6494(2) 13.4525(7) 90 105.812(5) 90 735.86(6) 1 0.71073 848.0 4.318 1.000 0.0159 0.0403

2154.04 P63 14.318(5) 14.318(5) 10.547(5) 90 90 120 1872.5(18) 2 0.71073 1936.0 3.820 1.055 0.0203 0.0569

R(F) = Σ(FF0| − |Fc||/Σ|F0|. bR(F20) = [w(F20 − F2c )2/Σw(F40)1/2. 17.6). The crucible was then placed into a furnace and held at 980 °C in air for 4 h. After this, the mixture was cooled down slowly to 500 °C with a rate of 6 °C/h, and afterward quenched by shutting down the furnace. Light green-colored crystals of K14(UO2)3Si10O30 (Figure S2) and some colorless amorphous phases were found in the crucible. When the molar ratios of U/Si was increased from 1/7.05 to 1/10.57, green crystals of K2(UO2)Si2O6 (Figure S2) were found after the reaction. Interestingly, the same phase can be obtained as a minor product by excluding of KF from the initial flux and replacing it with 130.7 mg (1.754 mmol) of KCl. 2.1.2. K2ThSi2O7 and K2ThSi3O9. Two thorium based phases were obtained from mixed KF−KCl flux. For K2ThSi2O7, the initial chemicals, including Th(NO3)4·5H2O (50 mg, 0.088 mmol), SiO2(42.15 mg, 0.701 mmol), KF (407.6 mg, 7.016 mmol), and KCl (261.5 mg, 3.508 mmol), were mixed with acetone and then ground in an agate mortar. The ratio of initial components Th/Si/KF/KCl was 1/8/80/40. The mixture was transferred into a Pt crucible and then placed into a furnace. The mixture was held at 900 °C in air for 24 h and then cooled slowly down to 600 °C with a cooling rate of 5 °C/h, and after that, the furnace was shut down. Colorless crystals of K2ThSi2O7 and glassy mass were found after the reaction. The second Th phase in this work, K2ThSi3O9, can only be obtained by decreasing the dwelling time from 24 h to 10 h under the same reaction condition. We found that crystals of K2ThSi2O7 and K2ThSi3O9 can be grown from other fluxes, K2CO3−WO3 and K2CO3−KF−V2O5, respectively. For K2ThSi2O7, initial chemicals with Th/Si/K2CO3/WO3 ratio of 1/ 12/40/20 were held at 1000 °C for 6 h and then cooled to 600 °C with a cooling rate of 6 °C/h. For K2ThSi3O9, ratio of Th/Si/K2CO3/ KF/V2O5 was 1/8/40/20/20 with a similar synthetic procedure. 2.1.3. Synthesis of Pure Phases. The powder samples of K14(UO2)3Si10O30 and K2ThSi2O7 were prepared based on the molar ratios of their chemical composition. The initial chemicals, including K2CO3, UO2(NO3)2(H2O)6 /Th(NO3)4·5H2O and SiO2, were mixed in an agate mortar and placed into ceramic crucibles. Mixtures were heated to 600 °C and held at this temperature for 24 h, and after this, the furnace was shut down. The obtained powders were used for collection of PXRD data. Identical procedure was repeated stepwise with temperature increasing 50 °C/step. The pure phase of K14(UO2)3Si10O30 can be obtained at 900 °C in Figure S3. However, powder samples of K2ThSi2O7 are always impure due to the presence of unreacted ThO2. Moreover, we found that K2ThSi2O7 is thermally unstable and converts to K2ThSi3O9. A detailed description and discussion is provided in section 3.2.6. 2.2. Single-Crystal Structure Determinations. The single crystals of potassium uranium and thorium silicates were selected for data collection and were mounted on glass fibers. The data were

(Si 2 O 6 ) 1 3 with dreier 7 (three) chains, [Cs 2 Cs 6 F][(UO2)2(Si6O17)]8 with loop branched dreier chains, and (Cs3F)(UO2)(Si4O10)8 with unbranched dreier double chains, and Rb4(UO2)2(Si8O20) with branched dreier double chains.14 Ca((UO2)2(SiO3OH)2(Si3O6))(H2O)7.5 with loop branched sechser7 (six) single chains has a 2D sheet structure.15 Recently, we systemized existing uranyl silicates using a secondary building units (SBUs) method in Figure S1 and demonstrated that synthetic conditions play a very important role in the structure formation of uranyl silicates in respect to the local geometry of U centers coordination.16 In contrast to uranium, thorium is predominantly stable in the +4 oxidation state under air atmosphere. Thorium oxoanion compounds demonstrate a great diversity of structure types, which are not only attributed to the different ligands but also to the rich coordination environments of Th centers ranging from 4 to 15. Recently, Gorden et.al17 investigated the coordination chemistry of Th in the reported organic/metal− organic compounds for the period from 1971 to 2016. In contrast, we were interested in synthesis and study of inorganic Th oxo-anion compounds and analysis of coordination geometries in all known inorganic oxo-salts with thorium. In this work, using mixed flux growth method, we synthesized two framework uranyl silicates K14(UO2)3Si10O30 with open-branched sechser single silicate chains and K2(UO2)Si2O6 with unbranched achter single chains and two thorium silicates K2ThSi2O7 and K2ThSi3O9. We also analyzed coordination geometry in all known Th based oxo-salts and revealed dependences in this series.

2. EXPERIMENTAL SECTION 2.1. Syntheses. Caution! The uranium nitrate and thorium nitrate used in this study are radioactive materials. Therefore, standard precautions were used, and all experiments were conducted in a laboratory dedicated to studies on radioactive elements. 2.1.1. K14(UO2)3Si10O30 and K2(UO2)Si2O6. The crystals of two titled compounds were grown from K2CO3−KF−MoO3 mixed fluxes. For K14(UO2)3Si10O30, the initial chemicals including UO2(NO3)2(H2O)6 (50 mg, 0.099 mmol), SiO2(42.15 mg, 0.701 mmol), K2CO3 (484.8 mg, 3.508 mmol), KF (101.9 mg, 1.754 mmol), and MoO3 (252.4 mg, 1.754 mmol) were mixed and ground with acetone, and then placed into a platinum crucible. The U/Si ratio in this composition was 1/ 7.05, and the flux ratio K2CO3/KF/MoO3 was 2/1/1 (U/flux ratio, 1/ B


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Inorganic Chemistry

Figure 1. (a) Experimental and calculated (calcd) PXRD patterns of K2ThSi2O7 at 900 °C with increasing K2CO3 contents. (b) SEM-BSE image of original ratio (K/Th/Si ratio of 2:1:2) sample. (c) SEM-BSE image of 150% K2CO3 (K/Th/Si ratio of 3:1:2) sample. (d) SEM-BSE image of 200% K2CO3 (K/Th/Si ratio of 4:1:2) sample. (e) SEM-BSE image of 250% K2CO3 (K/Th/Si ratio of 5:1:2) sample. (f) SEM-BSE image of 200% K2CO3 sample after washing 5 times using water. collected using Mo Kα1 radiation (λ = 0.71073 Å) on a Agilent Technologies SuperNova diffractometer. The diffraction data for all the crystals were integrated and reduced by the CrysAlisPro software, and the absorption effects were corrected by the multiscan method. The primary structures are refined by the SHELXL-2018/1 operated on WinGX v2014.1.18 The initial structures were solved by direct methods. The crystallographic data for all compounds are given in Table 1. 2.3. PXRD Analysis. PXRD data of all compounds were collected at room temperature using a Bruker-AXS D4 powder X-ray diffractometer equipped with a Cu tube. The diffraction data were collected in a range from 5 to 80°, and the counting time is 10 s/step for the step width of 0.02°. 2.4. SEM/EDS Measurements. Semiquantitative elemental analysis for the four novel compounds was performed using a FEI Quanta 200F scanning electron microscope (SEM) equipped with an Apollo X silicon drift Detector (EDAX) for energy-dispersive X-ray spectroscopic (EDS) measurements. The working distance was 10 mm, and the accelerating voltage was 20 kV at low-vacuum conditions (60 Pa). The SEM pictures and EDS data are provided in Figures S4− S7 and Tables S1−S4. U/Si and Th/Si ratios are in a good agreement with the results of single crystal structure refinements. 2.5. Raman Spectroscopy. The Raman spectra of four titled compounds were obtained by a Horiba LabRAM HR spectrometer. All the samples were in the form of single crystals. The 632.81 nm line of He−Ne laser is used as the excitation source. The spectral resolution was about 1 cm−1. The Raman spectra were recorded for the region from 200 to 1100 cm−1 for all compounds. 2.6. Bond-Valence Sums Analysis. The bond-valence sums (BVS) analysis for all atoms in the four novel compounds was performed using Ri for UVI−O bonds given by Burns.19 The Ri for ThIV−O, Si−O, and K−O bonds were provided by Brese and O’Keeffe.20,21 The BVS results for all atoms are in good agreement with the expected oxidation states.

3. RESULTS AND DISCUSSION 3.1. Synthesis. The combination of several fluxes enhancing the crystal growth process is well-established in the recent literature.8,22 Different components of these fluxes were found to contribute to different steps of crystal growth process.8,10,23 Combinations of different types of fluxes, such as CsF−CsCl for uranyl silicates8 and PbO-PbO2−PbF2−MoO3 for rare earth silicates,22 are particularly advantageous for the targeted synthesis of novel oxide and oxo-salt compounds. In this work, two uranyl and two thorium silicates were successfully grown from mixed molten fluxes as high-temperature solutions. Both K14(UO2)3Si10O30 and K2(UO2)Si2O6 can be obtained from the same flux K2CO3−KF−MoO3. The alkali metal carbonates can dissolve oxides because of their relatively low melting points. Additionally, crystals can be isolated easily from the flux by dissolving it in water.23,24 MoO3 has been usually employed since it contributed to the growth of larger crystals with good quality.10,22 In combination with alkali metal fluorides it has been used widely in the synthesis of novel inorganic materials.25,26 Halide salts KF and KCl were added in the reactions resulted into K2ThSi2O7 and K2ThSi3O9 to dissolve initial amount of SiO2.10,23 The binary oxides V2O5, MoO3, and WO3 are very effective fluxes because of their volatility at high temperatures. These fluxes are often beneficial to grow large crystals by inducing supersaturation in high temperature solution.23 The powder samples of K2ThSi2O7 were prepared by heating the original mixture (K/Th/Si ratio of 2/1/2) at 900 °C for 24 h. Compared to the calculated PXRD derived from single crystal structure, the sintered powder contains ThO2 as it shown in Figure 1a. We found that peaks of ThO2 disappear almost completely when 200% K2CO3 (K/Th/Si ratio of 4/1/ 2) was added in the starting mixture, whereas higher quantities C


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Inorganic Chemistry of additional K2CO3 lead to the increase of ThO2 content in the resulting powder. SEM microphotographs and EDS results for the different syntheses of K2ThSi2O7 powder samples are shown in Figure 1b−f and Table S5. The SEM/EDS of the original ratio sample (K/Th/Si ratio of 2/1/2) exhibits large, bright particles (see Figure 1b, point 2) that contain a larger amount of Th as compared to that in the other parts of the sample (see Figure 1b, point 1 and similar), which allows us to presume that these particles consist of ThO2 covered with various silicates. A similar situation is for the 150% excess of potassium carbonate showed in Figure 1c. Addition of 200% of K2CO3 results in the formation of more fine particles with regular distribution (Figure 1d). After washing the 200% K2CO3 sample we can see the uniform distribution of the fine particles in Figure 1f and some ThO2 on the surface. The sample with 250% excess of K2CO3 also exhibits particles of ThO2 which is in a good agreement with PXRD data. 3.2. Structure Description. 3.2.1. K14(UO2)3Si10O30. This phase crystallizes in the monoclinic P21/c space group. This phase exhibits a three-dimensional (3D) framework structure constructed upon open-branched silicate chains Si10O30, which are connected to each other through UO6 tetragonal bipyramids (Figure 2). The silicate chain consists of a

tetrahedra, which have a stabilizing influence on the branched single chains. Second, soft K+ cations compared to hard cations (Li+, Mg2+) can fit into the branched silicate chain. The coordination numbers of these potassium cations are 7 for K1, K2, and K6, 8 for K3, K4, and K5 and 9 for K7. BVS results for Si and K cations are in a good agreement with the expected 4+ and 1+ oxide states, respectively. An instructive way for further understanding of the K14(UO2)3Si10O30 structure is to analyze the structural geometry of the slabs formed by fundamental chains and its branches. The two slabs are stacked along the c axis in ABAB mode and zip-locked with open-branched [Si10O30] chains into a 3D framework. The slab A in Figure 3a is based upon the [(UO2)Si6O18] clusters, which are regarded as heptamer SBUs [USi6] (1+A+1 type)16 and directed along the a axis. In this SBU, one UO6 tetragonal bipyramid is connected to six SiO4 tetrahedra. Four SiO4 tetrahedra share oxygen atoms to connect other adjacent SBUs, forming a sechser spiral-like single chain [Si6O18] with the sequence (ddduuu)∞ and oval-shaped 12rings along the c axis (Figure 3). Similar single chains with the periodicity of 6 were observed in some minerals, such as gaidonnayite, stokesite, and chkalovite.7 In Figure 3b, infinite chains are comprised by [USi4] pentamers (A type), which link another adjacent chains to form [(UO2)2(Si2O7)2]8− slabs with 4- and 6-rings along the bc plane. Similar slabs and their anion topologies observed in K2Ca4((UO2)(Si2O7)2), Na9F2(UO2)(UO2)2(Si2O7)2, and K8(K5F)U6Si8O40, and are shown in Figure S9. In contrast to K14(UO2)3Si10O30, the SiO4 tetrahedra in the structure of Na9F2(UO2)(UO2)2(Si2O7)2 have the same orientation.27 However, the orientation of adjacent tetrahedra in Si2O7 groups in K8(K5F)U6Si8O409 is different, which is identical to that we observed in novel K14(UO2)3Si10O30. 3.2.2. K2(UO2)Si2O6. This phase crystallizes in the monoclinic C2/c space group. The 3D structure of this phase (Figure 4) is similar to that of α-Cs2(UO2)(Si2O6)11 and Rb2(UO2)Si2O6,12 which were prepared by HT/HP hydrothermal and flux growth methods, respectively. The 3D framework of K2(UO2)Si2O6 consists of four unique U sites, four Si sites, and five K sites. U6+ cations coordinate six O atoms to produce a typical UO6 tetragonal bipyramid with nearly linear OUO fragments, with bond distances ranging from 1.807(6) to 1.838(6) Å. The longer equatorial U−O bonds are in a range from 2.219(5) to 2.276(6) Å. The Si−O bond lengths in four silicate tetrahedra are within the range of 1.588(6)−1.640(6) Å, and O−Si−O angles are in the region of 105.3(4)−114.5(3)°. Each tetrahedra shares two corners to form a single achter silicate chain. The Si−O−Si angles in the achter chain vary from 131.86 to 180° (Figure S8). The silicate chain has a periodicity of 16.07 Å. UO6 tetragonal bipyramids constrict achter silicate chains to create the 3D framework along the c axis. K atoms are located in the voids of the framework and exhibit diverse and complex coordination environments with CN from 6 to 10. 3.2.3. Comparison of Structures and Chemistry of Uranium Silicates. Both title uranyl silicate compounds contain three SBUs, namely, [USi4] pentamers (A type), [USi6] heptamers (1+A+1 type), and [USi4] pentamers (A2 type), as depicted in Figure 5. The classification of SBUs in uranyl silicates has been given in our recent work.16 In the structure of K14(UO2)3Si10O30, the [USi4] pentamers (A type) are interconnected to form the slab [(UO2)2(Si2O7)2]8− with the structure previously observed in K2Ca4((UO2)(Si2O7)2)27 (Figure 5d). Importantly, the [USi 6 ] heptamers in K14(UO2)3Si10O30 create links between corners via two

Figure 2. Polyhedral presentation of the structure of K14(UO2)3Si10O30 along the (a)b axis. (b) Open-branched chains based on [Si10O30]20− unit with chain periodicity, P = 6 (chain parallel to the b axis). The yellow and green polyhedra are UO6 tetragonal bipyramids and SiO4 tetrahedra, respectively, K atoms are mauve.

fundamental fragment Si6O18 with a periodicity of six (P = 6). Each of these Si6O18 fragments is decorated with two disilicate Si2O7 groups, forming an open-branched infinite chain parallel to the b axis. The helix-shaped Si6O18 fragments have a length of ca. 11.13 Å (Figure 2b). The structure of K14(UO2)3Si10O30 contains two unique uranium sites. Their local environment is similar and based on four SiO4 tetrahedra in the equatorial plane. Each UO6 tetragonal bipyramid exhibits two short UO bonds (1.809(6)−1.833(6) Å) and four longer U−O bonds (2.186(6)−2.285(6) Å). BVS analysis yields 5.82 and 5.80 v.u. for U1 and U2 cations, respectively. The Si−O bonds in five unique SiO4 tetrahedra spanning a range of 1.570(7)−1.680(7) Å, and each of the four SiO4 tetrahedra has one short Si−O bond (1.570(7)−1.584(7) Å) involving the nonbridging terminal O atoms. The O−Si−O angles in SiO4 tetrahedra are within a normal range from 108.23 to 116.58°.7 The Si−O−Si angles in the open-branched silicate chains vary from 134.23 to 167.39° (Figure S8). Branched silicate anions between Si1 and Si3 have shortest Si··· Si distances of 3.06 Å, which has low stability compared with unbranched ones.7 The reasons of the formation of branched chains were given by Liebau.7 First, the U6+ cations with high electronegativity reduce the repulsive forces between the SiO4 D


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Inorganic Chemistry

Figure 3. (a, d) View down the a-axis showing the two-dimensional ∞2[(UO2 )(Si6O16 )]6 − slab A and corresponding to anion topology present in K14(UO2)3Si10O30, formed by the corner-sharing of SBUs [USi6] heptamers (1+A+1 type). (b, e) View down the a axis showing the 2D 2 8− slab B and corresponding to anion topology present in K14(UO2)3Si10O30. These slabs are composed of the infinite kröhnkite∞[(UO2 )2 (Si 2O7 )2 ] like chain constructed by [USi4] pentamers (A type). (c) Polyhedral presentation of K14(UO2)3Si10O30 along the c axis. The direction of the noncoordinating oxygen atom of each silicate tetrahedron is represented by using down (d) and up (u) signs.

Figure 4. (a) Polyhedral presentation of the structure of K2(UO2)Si2O6 along the c axis. (b) Achter single chains with a length of 16.07 Å.

[SiO4] tetrahedra, resulting in a [(UO2)2(Si6O16)]6− slab with 12-membered rings and sechser silicate chains (Figure 5e), whereas the same heptamers in the structures of K2(UO2)Si2O6 share equatorial edges to form achter chains extending along the b axis (Figure 5f). The [USi4] pentamers (A2 type) are interconnected to form a 2D slabs with corner-sharing 10membered rings in K 2 (UO 2 )Si 2 O 6 . The parts of K14(UO2)3Si10O30 and K2(UO2)Si2O6 structures can be described using the same SBUs [USi6] heptamers as it is shown in Figure 6. It should be noted that K14(UO2)3Si10O30 contains infinite sechser fundamental chains with the sequence (ddduuu)∞ while in K2(UO2)Si2O6 the achter chains have a (dddduuuu)∞ sequence. It is necessary to note that we observed a quite rare phenomenon in which the periodicity of silicate chain increases practically linear from 6 to 8 via only increasing molar ratios of U/Si from 1/7.05 to 1/10.57 in synthetic mixture. The major difference between the two structures is the morphology of the silicate chains. The open-branched silicate chains in K14(UO2)3Si10O30 is composed of secondary and tertiary SiO4 tetrahedra with the Si/O atomic ratio of 1:3. In contrast, the unbranched silicate chains in K2(UO2)Si2O6 are

Figure 5. (a) SBUs: [USi4] pentamers (A type). (b) SBUs: [USi6] heptamers (1+A+1 type). (c) SBUs: [USi4] pentamers (A2 type). (d, e, h) Procedure for the topological connection of [USi4] pentamers (A type) and USi6 heptamers to K14(UO2)3Si10O30. (f, g, i) Procedure for the topological connection of USi6 heptamers and [USi4] pentamers (A2 type) to K2(UO2)Si2O6. Some of the rings characterizing each topology are evidenced in color.

built only from tertiary tetrahedra with the same Si/O oxygen ratio with that observed in K14(UO2)3Si10O30. To date, all known single chain uranyl silicates have a 1:3 ratio which is identical to that observed in uranyl polyphosphates U(PO3)428 with achter single phosphate chains and K[(UO2)(P3O9)]29 with zwölf (12) chains. To analyze the chemistry of the existing uranyl silicates, we plotted a triangle diagram of chemical composition for all known phases in A+− UO22+− SiO44− family (Figure 7a). This diagram demonstrates a range of chemical stability of all known E


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Inorganic Chemistry

3.2.4. K2ThSi2O7. This phase crystallizes in the monoclinic system with the C2/c space group. This compound contains single Th, Si, and K crystallographically unique sites. Th atoms are coordinated by six O atoms creating ThO6 octahedra with an average Th−O bond distance of ∼2.31 Å. The observed Si− O bond lengths (1.605(3)−1.656(2)Å, average 1.625 Å) and O−Si−O bond angles (104.33(19)−113.21 (20)°) in silicate tetrahedra are within typical values.7 The connection of two SiO4 tetrahedra in the structure of K2ThSi2O7 leads to the diorthosilicate Si2O7 groups. These groups are linked with the isolated ThO6 octahedra via corners sharing, resulting into a 3D framework (Figure 8). K cations balance the negative charge of the framework, and K−O bonds lengths are in the range from 2.743 (4) to 3.408 (4). BVS analysis for Th, Si, and K cations are 4.10, 4.01, and 0.90 v.u., respectively. The structure of K2ThSi2O7 is a close analogue of the high pressure phase Na2SisixSi2O7 (C2/c),34 and closely related to the isoformular minerals khibinskite K2ZrSi2O7 (P21/b),35 and parakeldyshite Na2ZrSi2O7 (P1̅)36 (see Figure S10). Despite the fact that all the mentioned phases crystallize in different space groups, they all share to the same A2MT2X7 structure motif.37 K2ThSi2O7 and Na2SisixSi2O7 have different alkali cations and octahedra (ThO6 and SiO6, respectively), but the Si−O−Si angles of diorthosilicate groups in both structure are very close to 132° (132.07° for K2ThSi2O7 and 132.81° for Na2SiVISi2O7, respectively). The structure of novel K2ThSi2O7 has larger volume of unit cell and the β angle than that of Na2SiVISi2O7, which is a result of the larger Th−O bond lengths. 3.2.5. K2ThSi3O9. The 3D framework of K2ThSi3O9 adopts the mineral wadeite38 type and crystallizes in the hexagonal system with the P63 space group (Figure 9). Similar to K2ThSi2O7, its structure is based upon isolated ThO6 octahedra and SiO4 tetrahedra. The Th−O bond lengths are in the range from 2.276(8) to 2.310(8) Å. BVS calculations yield 4.25 and 4.05 v.u. for Th1 and Th2 cations, respectively. The Si−O bond distances are ranging from 1.554 (8) to 1.649 (9) Å, and the O−Si−O angles are within the range from 105.3(5) to 116.5(5)°. SiO4 tetrahedra are connected together to form three-membered Si3O9 rings with Si−O−Si angles between 131.6(5) and 134.4(6)°. The K cations fit into the voids of the 3D framework structure, exhibiting rich coordination geometry with the CN′ varying from 6 to 12. The structure of K2ThSi3O9 is similar to those of Cs2ThSi3O9 and Rb2ThSi3O9, which were prepared by a HT/HP hydrothermal method; however, K2ThSi3O9 was not encountered under the HT/HP conditions.39 The analysis of A+2ThSi3O9 (A+ = K, Rb, and Cs) wadeite structures show that the size of alkali metal cation has a strong influence on the Th−O−Si and Si−O−Si angles. These angles increase with an increase of A+ radius. Thus, K2ThSi3O9 possesses the smallest Th−O−Si and Si−O−Si angles (average 139.97 and 133.05°, respectively) and CsThSi3O9 the highest (147.49 and 134.32°, respectively), as it can be seen from diagram in Figure S11. Compared to wadeite, K2ZrSi3O9, the angles in Th analog are smaller.38 Speculatively, it may suggest that the larger Th4+ cation with less ionic potential reduces stress in the silicate chains in wadeite-like structures. 3.2.6. Structural Comparison and Chemical Transformation between K2ThSi2O7 and K2ThSi3O9. In both structures, the Th atoms are six-coordinated and surrounded by six different SiO4 tetrahedra, forming [ThSi6] heptamers. These fragments can be used as fundamental building blocks for description of K2ThSi2O7 and K2ThSi3O9 structures. The similarities between

Figure 6. Slabs in the structures of K14(UO2)3Si10O30 and K2(UO2)Si2O6 are derived from the same assemblage of [USi6] heptamers (SBUs). (a) SBUs of the uranyl silicates: USi6 heptamers (1+A+1 type). (b) Topological description of a 2D ∞2[(UO2 )(Si6O16 )]6 − slab A constructed by [USi 6 ] heptamers in the structures of K14(UO2)3Si10O30, (c) The topological description of a 2D slab constructed by USi6 heptamers in the structures of K2(UO2)Si2O6. The backslash and black circles are pointing down and up of SiO4 tetrahedra, respectively. Yellow are UO6 tetragonal bipyramids.

phases. The molar fraction of (UO2)2+ is in the range from 7.7 to 25% among all analyzed phases. The SiO4 content exceed the content of uranium in almost all phases, and resulting Si molar fraction is from 25 to 76.9%. The A+ cations vary in a wide range from 15.4 to 51.9% which shows their accomplishing role for phase formation. Such behavior can be explained by high tendency of polymerization for SiO4 anions. In contrast, the family of uranyl borates demonstrates more diverse chemistry with a strong dependence from synthetic conditions. In the Figure 7b, we plotted a triangle diagram of chemical composition for the family of A+−UO22+−BO33−/ BO45− compounds. From the diagram, it is clear that these compounds can be separated into two groups. In the first (I) a molar fraction of (UO2)2+ is very low ranging from 4.35 to 18.2%. This is similar to the silicate systems and exhibits a high polymerization level of borates groups for example complex polyborate sheet in Rb2[(UO2)2B13O20(OH)5].30 In opposite, the second group demonstrates high uranium fraction from 33.3 to 55.6%. In this group, the oxo-borate unites are not polymerized or forming B 2 O 5 dimers except for K10((UO2)16(B2O5)2(BO3)6O8)(H2O)10,2 but the uranyl polyhedra are condensed and form complex polymers and coordination geometries. For example, the 2D structure of K15[(UO2)18(BO3)7O15]31 is composed of uranyl square and pentagonal and hexagonal bipyramids with single BO3 units. Comparing both diagrams, for Si and B based systems, we can see that the synthesis conditions are playing an extremely important role in the resulting chemical composition in uranyl borates family. Most of the phases in the first group are obtained from a low temperature flux, but practically all phases in the second group are formed under high-temperature or high-temperature/high-pressure conditions. Unlike the borates, the silicates do not show such extreme dependence, and the composition of the materials is not that diverse. Speculatively, it can be explained by a significantly lower volatility of SiO2 comparing to BO3.32,33 We suggest that understanding of chemical trends can help us in predicting of the possible composition ranges of other oxo-silicate based actinide bearing compounds such as phases with Np(VI)/Np(V) and Pu(VI). F


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Inorganic Chemistry

Figure 7. Triangle diagrams of chemical composition for the families of (a) A+−(UO2)2+−(SiO4)4− and (b) A+−(UO2)2+−(BO3)3−/(BO4)5− compounds.

formula NapZrSiqOz·nH2O (q = 2, 2.5, 3, 4, and 6).37 As it has been mentioned in Experimental Section, we found that a decrease of the reaction time from 24 to 10 h leads to the formation of different crystals K2ThSi2O7 and K2ThSi3O9, respectively. Surprisingly we found that K2ThSi2O7 transforms to K2ThSi3O9 between 900 and 1000 °C. The transformation can be clearly seen from the change of the diffraction pattern of the powder samples, as it is shown in Figure 11a. ThO2 always exists as an impurity and its amount is increasing along with the temperature. Therefore, based on the composition of both phases we can write the following reaction of thermal decomposition:

K2ThSi2O7 and K2ThSi3O9 can be better presented using black and white nodal representation showed in Figure 10. [ThSi6] heptamers are connected to create only the 4-membered rings in K2ThSi2O7, sharing one vertex between all three heptamers; whereas in the structure of K2ThSi3O9 these heptameters form 3- and 6-membered rings, respectively, by connection between the silicate groups of the heptamers. This connection reflects the presence of additional silicate group in the structure of K2ThSi3O9 (Figure 10, parts 1−4 and 4−7). K2ThSi2O7 is of considerable importance for understanding the crystallization processes in alkali thorium silicate family, which includes K2ThSi2O7, A+2ThSi3O9 (A+ = K+, Rb+, and Cs+), K2ThSi4O10F2, and Cs2ThSi6O15.39 The system is similar to a series of Na−Zr silicate compounds with the general

3K 2ThSi 2O7 → 2K 2ThSi3O9 +ThO2 +K 2O G

(I)


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Inorganic Chemistry

Figure 8. (a) Polyhedral presentation of the structure of K2ThSi2O7 along the b axis. (b) 2D slab of K2ThSi2O7 structure constructed by [ThSi6] heptamers.

Figure 11. Experimental PXRD patterns demonstrating K2ThSi2O7 to K2ThSi3O9 transformation in 3K2ThSi2O7 → 2K2ThSi3O9 + ThO2 + K2O reaction at temperatures from 700 to 1100 °C, and calculated (calcd) PXRD patterns of K2ThSi3O9 and K2ThSi2O7. Shift of some peaks from K2ThSi2O7 to K2ThSi3O9 with increasing temperature happened and is highlighted by different color dash.

Figure 9. (a) Polyhedral presentation of the structure of K2ThSi3O9 along the b axis. (b) 2D slab of K2ThSi3O9 structure constructed by [ThSi6] heptamers.

Figure 12. Coordination environments of thorium for the system of inorganic thorium oxo-anion phases, and pie charts show the quantities of thorium compounds with different composition and structural types. Key: Different colors stand for coordination number (CN) of Th in each pie chart. Figure 10. Structures of K2ThSi2O7 and K2ThSi3O9 are derived from the same assemblage of [ThSi6] heptamers. (fundamental building blocks). (1−3) Procedure for the topological connections of [ThSi6] heptamers leads to the formation of K2ThSi2O7. (4) [ThSi6] heptamers. (5−7) Procedure for the topological connections of [ThSi6] heptamers leads to the formation of K2ThSi3O9.

which have different chemical composition and structural types. These phases were separated into 5 systems based on the chemistry of these elements. Thorium borates are very rare, and there are only 5 Th−B−O compounds have been described in the literature. Similar scarcity is observed in the Si/Ge based systems that both contain only 12 phases known to date. The other three are more representative with over 30 phases for each. All five systems show a very small number of sixcoordinate thorium compounds and are almost exclusively concentrated in the Si/Ge-based systems. Si/Ge oxo-anions show the smallest average value of ionic potential in Table S6 and Figure 13, and thorium in Si/Ge oxo-compounds exhibits the lowest average coordination number (∼7.5). The average coordination environment of Th is about 8.4 for the families of P/As and Cr/Mo/W based oxo-compounds. The average values of ionic potential for P/As and Cr/Mo/W are also very similar, 0.120 and 0.113, respectively (Table S6), which leads to

This is very unusual that after the thermal decomposition a ratio between An and TO4 increases in the new phase. Usually, such phases decompose on An oxides and counter cationic based oxo-salts or oxides. In this example we can see that both phases are not only structurally similar but also can be chemically derived one from another. 3.2.7. Analysis of Coordination Numbers of Th in OxoAnion Phases. Th(IV) oxo-anion compounds display unique flexible coordination environments ranging from 6 to 12. We analyze a distribution of Th CNs depending on chemical nature of the central atom of oxo-salt group. The pie charts shown in Figure 12 illustrate 126 known thorium oxo-salts compounds, H


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Inorganic Chemistry

Figure 13. Average values of coordination number of thorium as a function of the ionic potential of central cations in oxo-anions.

the similar distribution of pie charts of Th−P/As and Th−Cr/ Mo/W oxo-compounds. The higher average ionic potential of S/Se/Te favors the formation of the larger nine-coordinated geometry in Th−S/Se/Te system. Therefore, in the system of thorium based oxo-anion compounds, the coordination geometry of Th is strongly affected by nature of central atom in oxo-salts groups. The larger ionic potential of these atoms results in the higher coordination number of Th in Figure 13. Th−B system is very narrow and is excluded from analysis. 3.3. Raman Spectral Analysis. The Raman spectra of uranyl silicate minerals such as uranophane, sklodowskite, cuprosklodowskite, and so on have been interpreted in detail and the vibration bands of (UO2)2+ units and (SiO4)4− tetrahedra in these structures were carefully assigned.40−42 Nevertheless, the numbers of studies involved in the Raman spectra characterization of synthetic uranyl silicates is very low. We collected the Raman spectra for K14(UO2)3Si10O30 and K2(UO2)Si2O6 in the range from 200 to 1100 cm−1 (Figure 14). The Raman spectra of K14(UO2)3Si10O30 and K2(UO2)Si2O6 show the strong bands in the 700−800 cm−1 region which correspond to the ν1 modes of (UO2)2+ units. The weak bands in between 800 and 900 cm−1 correspond to the ν3 modes of (UO2)2+ and the low-frequency bands in the range from 200 to 340 cm−1 correspond to the ν2 modes of (UO2)2+. These results are in good agreement with the values reported in the literature, including those for uranyl silicate minerals.40,41 Interestingly, the Raman spectrum of K14(UO2)3Si10O30 shows more (SiO4)4− stretching vibrations in the range 350−700 cm−1 as compared to those in K2(UO2)Si2O6 (Table S7). The appearance of two extra bands at near 584 and 680 cm−1 in K14(UO2)3Si10O30 is mainly associated with the deformations of the silicate chain,43 including the symmetrical Si−O−Si stretching/bending modes.44 Most researchers identify a Raman band near 1000 cm−1 as interconnected with silicate chains.45,46 The Raman vibrational studies of ThSiO4 (thorite), ThSiO4 (huttonite), and (Ca0.5Na0.5)2NaThSi8O20 have been recently reported in the literature.47,48 The Raman spectra for the novel K2ThSi2O7 and K2ThSi3O9 are presented in Figure 15. The bands in the range of 100−350 cm−1 for both spectra are mainly attributed to the coupling modes among O−Th−O bends and lattice vibrations (Table S8). Bands between 400−

Figure 14. Raman shifts from 200 to 1100 cm−1 for K2(UO2)Si2O6 and K14(UO2)3Si10O30, respectively.

Figure 15. Raman shifts from 200 to 1100 cm−1 for K2ThSi3O9 and K2ThSi2O7, respectively.

800 cm−1 are generally the symmetric bending vibrations of the Si−O−Si bridging oxygen atoms, which reflect the connectivity between adjacent silicon−oxygen tetrahedra.49,50 The bands between 850 and 1100 cm−1 correspond to the symmetric Si− O stretching vibrations of SiO4 groups with nonbridging, terminal oxygen atoms.49−51 It is noteworthy that the highest intensity band at 948 cm−1 in K2ThSi2O7 is assigned to the νs(Si−O) vibrations of the diorthosilicate Si2O7 groups, in I


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structural figures of four titled compounds, and tables of coordination environments of thorium in the reported thorium oxo-anion phases and Raman shifts, and proposed band assignments for four titled compounds (PDF)

agreement with other silicate compounds containing disilicate groups.52−54 The band at 694 cm−1 in K2ThSi2O7 corresponds to the νs(Si−O−Si) of Si2O7 groups, and the similar band at 701 cm−1 is also detected in the Raman spectra of the high pressure phase Na2SiVISi2O7.55 For K2ThSi3O9, there are totally 360 normal vibrational modes (60A + 60B + 602E2 + 601E1 + 601E2 + 602E1) predicted by the factor group analysis, with some of them being acoustic modes.56 It has 59A + 60B + 602E2 + 591E1 + 601E2 + 592E1 optical modes and A + 1E2 + 2E1 acoustic modes. The Ramanactive modes of K2ThSi3O9 occur at the 59A + 602E2 + 591E1 + 601E2 + 592E1 species and infrared-active modes occur at the 59A + 591E1 + 592E1 species. An isolated three-membered Si3O9 ring with point group symmetry C3h has 20 internal vibrational modes: 6A′ + 6E′ + 4A″ + 4E″, where A′ and E″ are Raman active modes, A″ is infrared active optic modes, and E′ is both Raman and infrared active optic modes.57 The band of Si3O9 rings in wadeite-type K2ThSi3O9 locates at frequencies above 800 cm−1, and intermediate frequency bands, between 350 and 800 cm−1, are assigned to the Si−O−Si displacements as well as ring breathing and ring deformation motions.58 In the literature, the intermediate frequency of Raman spectra for each wadeite type mineral is unique and represents a mixture of idealized three-membered ring modes. For example, the highest intensity band at 992 cm−1 in K2ZrSi3O958 is assigned to the ν17 mode in the 20 vibrations of above-described isolated Si3O9 ring, and the bands at 499 and 517 cm−1 in wadeite-type K2SiVISi3O959 are assigned to the ν1 + ν19 and ν3 + ν8 stretching modes of the Si3O9 ring, respectively, very similar to that observed for K2ThSi3O9.

Accession Codes

CCDC 1838176−1838179 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]. ORCID

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

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The work is funded by the Helmholtz Association Project VHNG-815. We are also grateful to Dr. Schlenz for Raman spectra. H.J.L. would like to thank for financial support from the Chinese Scholarship Council.

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REFERENCES

(1) Gorden, A. E. V.; Xu, J. D.; Raymond, K. N.; Durbin, P. Rational Design of Sequestering Agents for Plutonium and Other Actinides. Chem. Rev. 2003, 103, 4207−4282. (2) Stritzinger, J. T.; Alekseev, E. V.; Polinski, M. J.; Cross, J. N.; Eaton, T. M.; Albrecht-Schmitt, T. E. Further Evidence for the Stabilization of U(V) within a Tetraoxo Core. Inorg. Chem. 2014, 53, 5294−5299. (3) Lin, C. H.; Chen, C. S.; Shiryaev, A. A.; Zubavichus, Y. V.; Lii, K. H. K3(U3O6)(Si2O7) and Rb3(U3O6)(Ge2O7): A Pentavalent-Uranium Silicate and Germanate. Inorg. Chem. 2008, 47, 4445−7. (4) Chen, C. S.; Lee, S. F.; Lii, K. H. K(UO)Si2O6: a pentavalenturanium silicate. J. Am. Chem. Soc. 2005, 127, 12208−9. (5) Wu, S. J.; Ling, J.; Wang, S. A.; Skanthakumar, S.; Soderholm, L.; Albrecht-Schmitt, T. E.; Alekseev, E. V.; Krivovichev, S. V.; Depmeier, W. Uranium(VI) Adopts a Tetraoxido Core. Eur. J. Inorg. Chem. 2009, 2009, 4039−4042. (6) Wronkiewicz, D. J.; Bates, J. K.; Gerding, T. J.; Veleckis, E.; Tani, B. S. Uranium Release and Secondary Phase Formation during Unsaturated Testing of UO2 at 90 °C. J. Nucl. Mater. 1992, 190, 107− 127. (7) Liebau, F. Structural Chemistry of Silicates. Springer-Verlag,: Berlin,, 1985. (8) Morrison, G.; Smith, M. D.; zur Loye, H. C. Understanding the Formation of Salt-Inclusion Phases: An Enhanced Flux Growth Method for the Targeted Synthesis of Salt-Inclusion Cesium Halide Uranyl Silicates. J. Am. Chem. Soc. 2016, 138, 7121−7129. (9) Morrison, G.; Tran, T. T.; Halasyamani, P. S.; zur Loye, H. C. K8(K5F)U6Si8O40: An Intergrowth Uranyl Silicate. Inorg. Chem. 2016, 55, 3215−7. (10) Morrison, G.; zur Loye, H. C. Flux Growth of [NaK6F][(UO2)3(Si2O7)2] and [KK6Cl][(UO2)3(Si2O7)2]: The Effect of Surface Area to Volume Ratios on Reaction Products. Cryst. Growth Des. 2016, 16, 1294−1299. (11) Chen, C. S.; Chiang, R. K.; Kao, H. M.; Lii, K. H. Hightemperature, High-pressure Hydrothermal Synthesis, Crystal Structure, and Solid-state NMR Spectroscopy of Cs2(UO2)(Si2O6) and

4. CONCLUSIONS Two novel uranyl monopolysilicates K14(UO2)3Si10O30 and K2(UO2)Si2O6 have been synthesized using flux growth method by increasing U/Si molar ratios (1/7.05 → 1/10.57) for each phase, respectively. K14(UO2)3Si10O30 with openbranched sechser single chains and K2(UO2)Si2O6 with unbranched achter chains are constructed from a variety of SBUs. The analysis of the triangle diagrams of A+−U−Si and A+−U−B systems shows the crystallization ranges of U−Si and U−B phases and significant difference between uranyl bearing silicates and borates. Furthermore, the Raman spectra of both uranyl silicates show the difference of vibration bands of openbranched and unbranched- single chains. Two novel thorium silicates, K2ThSi2O7 of the high pressure phase Na2SiVISi2O7type, and K2ThSi3O9 of the wadeite-type, are target grown by complex mixed fluxes. Comparison of packing modes of the [ThSi6] heptamers in these two thorium silicates is helpful for understanding the crystallized processes of rare thorium compounds. Statistical analysis of coordination geometries of thorium in five Th-based oxo-anion systems exhibits a strong dependence between the preferred coordination environments of Th and the ionic potential of the central cation in the oxoligands.

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S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b01072. SBUs extracted from all reported uranyl silicates, single crystal pictures, PXRD patterns, SEM/EDS results, and J


Article

Inorganic Chemistry Variable-temperature Powder X-ray Diffraction Study of the Hydrate Phase Cs2(UO2)(Si2O6)·0.5H2O. Inorg. Chem. 2005, 44, 3914−8. (12) Morrison, G.; Smith, M. D.; Zur Loye, H. C. Flux versus Hydrothermal Growth: Polymorphism of A2(UO2)Si2O6 (A = Rb, Cs). Inorg. Chem. 2017, 56, 1053−1056. (13) Plaisier, J. R.; Ijdo, D. J. W.; de Mello Donega, C.; Blasse, G. Structure and Luminescence of Barium Uranium Disilicate (BaUO2Si2O6). Chem. Mater. 1995, 7, 738−743. (14) Huang, J.; Wang, X.; Jacobson, A. J. Hydrothermal Synthesis and Structures of the New Open-framework Uranyl Silicates Rb4(UO2)2(Si8O20) (USH-2Rb), Rb2(UO2)(Si2O6)·H2O (USH-4Rb) and A2(UO2)(Si2O6)·0.5H2O (USH-5A; A = Rb, Cs). J. Mater. Chem. 2003, 13, 191−196. (15) Plasil, J.; Fejfarova, K.; Cejka, J.; Dusek, M.; Skoda, R.; Sejkora, J. Revision of the Crystal Structure and Chemical Formula of Haiweeite, Ca(UO2)2(Si5O12)(OH)2·6H2O. Am. Mineral. 2013, 98, 718−723. (16) Li, H.; Kegler, P.; Bosbach, D.; Alekseev, E. V. Hydrothermal Synthesis, Study, and Classification of Microporous Uranium Silicates and Germanates. Inorg. Chem. 2018, 57, 4745−4756. (17) Tutson, C. D.; Gorden, A. E. V. Thorium coordination: A comprehensive review based on coordination number. Coord. Chem. Rev. 2017, 333, 27−43. (18) Sheldrick, G. M. A. Short History of SHELX. Acta Crystallogr., Sect. A: Found. Crystallogr. 2008, 64, 112−122. (19) Burns, P. C.; Ewing, R. C.; Hawthorne, F. C. The Crystal Chemistry of Hexavalent Uranium: Polyhedron Geometries, Bondvalence Parameters, and Polymerization of Polyhedra. Can. Mineral. 1997, 35, 1551−1570. (20) Brese, N. E.; O'Keeffe, M. Bond-Valence Parameters for Solids. Acta Crystallogr., Sect. B: Struct. Sci. 1991, 47, 192−197. (21) Brown, I. D.; Altermatt, D. Ctca Crystallogr. Acta Crystallogr., Sect. B: Struct. Sci. 1985, 41, 244−247. (22) Wanklyn, B. M.; Wondre, F. R.; Ansell, G. B.; Davison, W. Flux Growth of Rare-Earth Silicates and Aluminosilicates. J. Mater. Sci. 1974, 9, 2007−2014. (23) Bugaris, D. E.; zur Loye, H. C. Materials Discovery by Flux Crystal Growth: Quaternary and Higher Order Oxides. Angew. Chem., Int. Ed. 2012, 51, 3780−3811. (24) Macquart, R. B.; Smith, M. D.; zur Loye, H. C. Crystal growth and single-crystal structures of RERhO3 (RE = La, Pr, Nd, Sm, Eu, Tb) orthorhodites from a K2CO3 flux. Cryst. Growth Des. 2006, 6, 1361− 1365. (25) Kolitsch, U.; Tillmanns, E. Synthesis and crystal structure of a new microporous silicate with a mixed octahedral-tetrahedral framework: Cs3ScSi8O19. Mineral. Mag. 2004, 68, 677−686. (26) Kolitsch, U.; Tillmanns, E. The structural relation between the new synthetic silicate K2ScFSi4O10 and narsarsukite, Na2(Ti,Fe3+)(O,F)Si4O10. Eur. J. Mineral. 2004, 16, 143−149. (27) Chang, Y. C.; Chang, W. J.; Boudin, S.; Lii, K. H. Hightemperature, High-pressure Hydrothermal Synthesis and Characterization of A Salt-inclusion Mixed-valence Uranium(V,VI) Silicate: [Na9F2][(U(V)O2)(U(VI)O2)2(Si2O7)2]. Inorg. Chem. 2013, 52, 7230−7235. (28) Yu, N.; Klepov, V. V.; Neumeier, S.; Depmeier, W.; Bosbach, D.; Suleimanov, E. V.; Alekseev, E. V. Further Insight into Uranium and Thorium Metaphosphate Chemistry and the Effect of Nd 3+ Incorporation into Uranium(IV) Metaphosphate. Eur. J. Inorg. Chem. 2015, 2015, 1562−1568. (29) Alekseev, E. V.; Krivovichev, S. V.; Depmeier, W.; Knorr, K. Complex topology of uranyl polyphosphate frameworks: Crystal structures of alpha-, beta-K(UO2)(P3O9) and K(UO2)2(P3O10). Z. Anorg. Allg. Chem. 2008, 634, 1527−1532. (30) Wang, S. A.; Alekseev, E. V.; Stritzinger, J. T.; Depmeier, W.; Albrecht-Schmitt, T. E. Crystal Chemistry of the Potassium and Rubidium Uranyl Borate Families Derived from Boric Acid Fluxes. Inorg. Chem. 2010, 49, 6690−6696. (31) Wu, S. J.; Wang, S. A.; Polinski, M.; Beermann, O.; Kegler, P.; Malcherek, T.; Holzheid, A.; Depmeier, W.; Bosbach, D.; Albrecht-

Schmitt, T. E.; Alekseev, E. V. High Structural Complexity of Potassium Uranyl Borates Derived from High-Temperature/HighPressure Reactions. Inorg. Chem. 2013, 52, 5110−5118. (32) Garrett, D. E. Borates: Handbook of Deposits, Processing, Properties, and Use. Academic Press, San Diego, CA, 1998. (33) Fergusson, J. E. Inorganic Chemistry and the Earth. Pergamon, New York, 1982. (34) Fleet, M. E.; Henderson, G. S. Sodium Trisilicate - a New HighPressure Silicate Structure (Na2Si[Si2O7]). Phys. Chem. Miner. 1995, 22, 383−386. (35) Chernov, A. N.; Maksimov, B. A.; Ilyukhin, V. V.; Belov, N. V. Crystalline Structure of Monoclinic Modification of K, ZrDiorthosilicate-K2ZrSi2O7. Dokl. Akad. Nauk SSSR 1970, 193, 1293− 1296. (36) Voronkov, A. A.; Shumyatskaya, N. G.; Pyatenko, Y. A. Crystal Structure of A New Natural Modification of Na2Zr(Si2O7). J. Struct. Chem. 1971, 11, 866−867. (37) Ilyushin, G. D. Modeling of self-organization processes in crystal-forming systems. Structural mechanism of self-assembly of zirconosilicate Na2ZrSi2O7 from suprapolyhedral clusters. Russ. Chem. Bull. 2004, 53, 1446−1458. (38) Ilyushin, G. D.; Dem’yanets, L. N. Hydrothermal synthesis of K2ZrSi6O15, K2ZrSi3O9, K2ZrSi2O7, and ZrSiO4 in the system KOHZrO2-SiO2-H2O. Inorg. Mater. 2002, 38, 612−616. (39) Mann, J. M.; McMillen, C. D.; Kolis, J. W. Crystal Chemistry of Alkali Thorium Silicates Under Hydrothermal Conditions. Cryst. Growth Des. 2015, 15, 2643−2651. (40) Biwer, B. M.; Ebert, W. L.; Bates, J. K. The Raman-Spectra of Several Uranyl-Containing Minerals Using a Microprobe. J. Nucl. Mater. 1990, 175, 188−193. (41) Frost, R. L.; Cejka, J.; Weier, M. L.; Martens, W.; Kloprogge, J. T. A Raman and Infrared Spectroscopic Study of the Uranyl Silicates Weeksite, Soddyite and Haiweeite. Spectrochim. Acta, Part A 2006, 64, 308−315. (42) Frost, R. L.; Cejka, J.; Weier, M. L.; Martens, W. Molecular Structure of the Uranyl Silicates - a Raman Spectroscopic Study. J. Raman Spectrosc. 2006, 37, 538−551. (43) Depla, A.; Verheyen, E.; Veyfeyken, A.; Van Houteghem, M.; Houthoofd, K.; Van Speybroeck, V.; Waroquier, M.; Kirschhock, C. E. A.; Martens, J. A. UV-Raman and 29Si NMR Spectroscopy Investigation of the Nature of Silicate Oligomers Formed by Acid Catalyzed Hydrolysis and Polycondensation of Tetramethylorthosilicate. J. Phys. Chem. C 2011, 115, 11077−11088. (44) Mernagh, T. P.; Hoatson, D. M. Raman spectroscopic study of pyroxene structures from the Munni Munni layered intrusion, Western Australia. J. Raman Spectrosc. 1997, 28, 647−658. (45) Halasz, I.; Kierys, A.; Goworek, J.; Liu, H. M.; Patterson, R. E. Si-29 NMR and Raman Glimpses into the Molecular Structures of Acid and Base Set Silica Gels Obtained from TEOS and Na-Silicate. J. Phys. Chem. C 2011, 115, 24788−24799. (46) Brawer, S. A.; White, W. B. Raman-Spectroscopic Investigation of the Structure of Silicate-Glasses I. The Binary Alkali Silicates. J. Chem. Phys. 1975, 63, 2421−2432. (47) Clavier, N.; Szenknect, S.; Costin, D. T.; Mesbah, A.; Poinssot, C.; Dacheux, N. From thorite to coffinite: A spectroscopic study of Th1‑xUxSiO4 solid solutions. Spectrochim. Acta, Part A 2014, 118, 302− 307. (48) Jin, G. B.; Soderholm, L. Solid-state syntheses and single-crystal characterizations of three tetravalent thorium and uranium silicates. J. Solid State Chem. 2015, 221, 405−410. (49) Arroyabe, E.; Kaindl, R.; Kahlenberg, V. Structural and Raman Spectroscopic Investigations of K4BaSi3O9 and K4CaSi3O9. Z. Anorg. Allg. Chem. 2009, 635, 337−345. (50) Kahlenberg, V.; Kaindl, R.; Tobbens, D. M. Synthesis, Rietveld analysis and solid state Raman spectroscopy of K4SrSi3O9. Z. Anorg. Allg. Chem. 2006, 632, 2037−2042. (51) Kahlenberg, V.; Kaindl, R.; Sartory, B. On the existence of a second modification of K4SrSi3O9 - X-ray single crystal diffraction, K


Article

Inorganic Chemistry Raman spectroscopical and high temperature studies. Solid State Sci. 2007, 9, 65−71. (52) You, J. L.; Jiang, G. C.; Hou, H. Y.; Chen, H.; Wu, Y. Q.; Xu, K. D. Quantum Chemistry Study on Superstructure and Raman Spectra of Binary Sodium Silicates. J. Raman Spectrosc. 2005, 36, 237−249. (53) Wierzbicka-Wieczorek, M.; Tobbens, D. M.; Kolitsch, U.; Tillmanns, E. Simultaneous Presence of (Si3O10)8‑and (Si2O7)6‑ Groups in New Synthetic Mixed Sorosilicates: BaY4(Si2O7)(Si3O10) and Isotypic Compounds, Studied by Single-crystal X-ray Diffraction, Raman Spectroscopy and DFT Calculations. J. Solid State Chem. 2013, 207, 94−104. (54) Frost, R. L.; Bouzaid, J. M.; Reddy, B. J. Vibrational spectroscopy of the sorosilicate mineral hemimorphite Zn4(OH)2Si2O7 ·H2O. Polyhedron 2007, 26, 2405−2412. (55) Fleet, M. E.; Henderson, G. S. Structure-composition relations and Raman spectroscopy of high-pressure sodium silicates. Phys. Chem. Miner. 1997, 24, 345−355. (56) Rousseau, D. L.; Bauman, R. P.; Porto, S. P. S. Normal Mode Determination in Crystals. J. Raman Spectrosc. 1981, 10, 253−290. (57) McKeown, D. A.; Bell, M. I.; Kim, C. C. Raman spectroscopy of silicate rings: Benitoite and the three-membered ring. Phys. Rev. B: Condens. Matter Mater. Phys. 1993, 48, 16357−16365. (58) McKeown, D. A.; Nobles, A. C.; Bell, M. I. Vibrational analysis of wadeite K2ZrSi3O9 and comparisons with benitoite BaTiSi3O9. Phys. Rev. B: Condens. Matter Mater. Phys. 1996, 54, 291−304. (59) Chang, L. L.; Liu, X.; Liu, H.; Kojitani, H.; Wang, S. C. Vibrational mode analysis and heat capacity calculation of K2SiSi3O9wadeite. Phys. Chem. Miner. 2013, 40, 563−574.

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