Framework Uranyl Silicates: Crystal Chemistry and a New Route for the Synthesis

To date, uranyl silicates are mostly represented by minerals in nature. However, their synthetic counterparts can be used as ion exchange materials. A new approach for the synthesis of framework uranyl silicates is reported. The new compounds Rb2[(UO2)2(Si8O19)](H2O)2.5 (1), (K,Rb)2[(UO2)(Si10O22)] (2), [Rb3Cl][(UO2)(Si4O10)] (3) and [Cs3Cl][(UO2)(Si4O10)] (4) were prepared at harsh conditions in “activated” silica tubes at 900 °C. The activation of silica was performed using 40% hydrofluoric acid and lead oxide. Crystal structures of new uranyl silicates were solved by direct methods and refined: 1 is orthorhombic, Cmce, a = 14.5795(2) Å, b = 14.2083(2) Å, c = 23.1412(4) Å, V = 4793.70(13) Å3, R1 = 0.023; 2 is monoclinic, C2/m, a = 23.0027(8) Å, b = 8.0983(3) Å, c = 11.9736(4) Å, β = 90.372(3) °, V = 2230.43(14) Å3, R1 = 0.034; 3 is orthorhombic, Imma, a = 15.2712(12) Å, b = 7.9647(8) Å, c = 12.4607(9) Å, V = 1515.6(2) Å3, R1 = 0.035, 4 is orthorhombic, Imma, a = 15.4148(8) Å, b = 7.9229(4) Å, c = 13.0214(7) Å, V = 1590.30(14) Å3, R1 = 0.020. Their framework crystal structures contain channels up to 11.62 × 10.54 Å filled by various alkali metals.


Introduction
The rising interest in studies of hexavalent uranium silicates is underpinned by a variety of reasons, including the mineralogy of the oxidation areas of uranium deposits [1] and the technogenesis of spent nuclear fuel (SNF) [2]. By now, this family hosts 21 mineral species [3], as well as ca. 40 synthetic compounds containing uranium and silicon. Natural uranyl silicates are formed at the earlier formation stages of the oxidation areas of uranium deposits. Due to their ability to exchange cations, as exemplified by boltwoodite, (K,Na)[(UO 2 )(SiO 3 OH)](H 2 O) 1.5 [4] and cuprosklodowskite, Cu[(UO 2 ) 2 (SiO 3 OH) 2 ](H 2 O) 6 [5], uranyl silicates are expected to take an active and essential part in the migration, accumulation, and deposition processes.
Model experiments on oxidation of UO 2 [6] and the hydration of uranium-doped borosilicate glasses [7] have demonstrated the formation of uranyl silicates during SNF oxidation. Structural peculiarities of a KNa 3 [(UO 2 ) 2 (Si 4 O 10 ) 2 ](H 2 O) 4 compound obtained during glass hydration suggest it to be a potential absorber of radionuclides, such as Np V or Am III . The promising properties of uranyl silicates, both natural and synthetic, arise from their highly porous crystal structures wherein the uranyl cation generally coordinates four to five ligands in the equatorial plane (most commonly oxygen atoms from oxyanions, hydroxyl groups, water molecules, and halide anions) with formation of tetra-or pentagonal bipyramids. These polyhedra generally do not share edges or vertices, except for the representatives of the uranophane group [8] and layers in the haiweeite structure [9].
Several approaches to synthesis of framework uranyl silicates are known including "soft" [18] and "hard" [17,42] hydrothermal treatment, as well as salt flux synthesis [40]. Both standard Teflon-lined autoclaves [42], and sealed gold tubes [18,43] have been used for the "soft" and "hard" hydrothermal processes, respectively. In the high-temperature syntheses, use of fluxing agents, mostly molten alkali metal halides, permits obtaining the so-called salt-inclusion structures, which can be classified as microporous zeolite-like frameworks [14,44].
In our experiments, we employed new techniques which permitted preparation of single crystals of four new uranyl silicates:

Experimental
Caution! Although the uranium precursors used contain depleted uranium, standard safety measures for handling radioactive substances must be followed. 5 . Yellow platelets of 1 ( Figure 3a) were obtained in a high-temperature synthesis. The starting compounds were 135 mg U 3 O 8 (Vecton, Russia, 99.7%), 24 mg of RbCl (Vecton, 99.7%), 67 mg of PbO (Vecton, 99.7%). As some previous experiments have indicated that the reaction between uranium and silicon oxides requires harsh conditions, the reagents were additionally activated. The mixture was transferred into a silica tube (which served also as the source of silicon), then 30 µL of 40% hydrofluoric acid was injected. After one minute, the tube was attached to a vacuum line, evacuated, and sealed. The tube was heated to 900 • C at a rate of 100 • C/h, annealed for 50 h, and cooled to room temperature at the rate of 10 • C/h. Reaction with the tube walls produced the target crystals. Tiny drops of solidified Pb metal were also found in the sample indicating oxidation of U 3 O 8 by the lead oxide during reaction.

Synthesis
), which are described below.

Experimental
Caution! Although the uranium precursors used contain depleted uranium, standard safety measures for handling radioactive substances must be followed.

Synthesis
Rb2[(UO2)2(Si8O19)](H2O)2.5. Yellow platelets of 1 ( Figure 3a) were obtained in a hightemperature synthesis. The starting compounds were 135 mg U3O8 (Vecton, Russia, 99.7%), 24 mg of RbCl (Vecton, 99.7%), 67 mg of PbO (Vecton, 99.7%). As some previous experiments have indicated that the reaction between uranium and silicon oxides requires harsh conditions, the reagents were additionally activated. The mixture was transferred into a silica tube (which served also as the source of silicon), then 30 µL of 40% hydrofluoric acid was injected. After one minute, the tube was attached to a vacuum line, evacuated, and sealed. The tube was heated to 900 °С at a rate of 100 °С/h, annealed for 50 h, and cooled to room temperature at the rate of 10 °С/h. Reaction with the tube walls produced the target crystals. Tiny drops of solidified Pb metal were also found in the sample indicating oxidation of U3O8 by the lead oxide during reaction. were pre-dried at 80 °C and annealed in "activated" silica tubes and processed as described above.

Crystal Structure Determination
Single crystals of 1-4 selected for X-ray diffraction analysis were attached onto glass fibers and mounted on a Rigaku XtaLAB Synergy-S diffractometer (Tokyo, Japan) were pre-dried at 80 • C and annealed in "activated" silica tubes and processed as described above.

Crystal Structure Determination
Single crystals of 1-4 selected for X-ray diffraction analysis were attached onto glass fibers and mounted on a Rigaku XtaLAB Synergy-S diffractometer (Tokyo, Japan) equipped with a PhotonJet-S detector (Tokyo, Japan) operating with MoKα radiation at 50 kV and 1 mA. More than a hemisphere of data was collected in each case with a frame width of 0.5 • in ω, and counting time of 10 s. The data were integrated and corrected for absorption applying a multiscan type model using the Rigaku Oxford Diffraction programs CrysAlis Pro (Rigaku OD, 2015) (Tokyo, Japan). The experiments were performed with cooling to 150 K. The unit cell parameters were calculated by the least-squares method. The structures were solved u direct methods using WinGX (Glasgow, UK) [45] and Olex2 (Regensburg, Germany) [46] software. The main parameters of the experiment and refinement are collected in Table 1. The final solutions include the coordinates and anisotropic thermal parameters of atoms. Selected interatomic distances are collected in Tables S2-S5.

Characterization
Powder X-ray diffraction patterns (PXRD) were recorded on a Rigaku R-AXIS RAPID diffractometer (Tokyo, Japan) utilizing CoKα radiation operating at 50 kV and 10 mA. Simulated PXRD patterns were calculated from single-crystal data using the Diamond program (Bonn, Germany) ( Figures S1 and S2). The infrared (IR) spectra were measured on a Bruker vertex 70 spectrometer (Ettlingen, Germany) in the range of 4000-400 cm −1 from samples pressed into KBr pellets ( Figure S3). Microprobe analysis was performed on a Hitachi S-3400N SEM (Tokyo, Japan) with analytical devices: with analytical chamber: EBSD-AzTec HKL Channel 5 Advanced, quantitative EDX-AzTec Energy 350, quantitative WDS-INCA 500 and using the standards listed in Tables S6-S9.

Rb 2 [(UO 2 ) 2 (Si 8 O 19 )](H 2 O) 2.5 (1)
In the crystal structure of 1, the uranium atom forms a uranyl cation (<U-O ap > = 1.806 Å) which is coordinated, in the equatorial plane, by five oxygen atoms (<U-Oeq> = 2.370 Å). Four symmetrically independent silicon atoms are tetrahedrally coordinated with <Si-O> = 1.603-1.617 Å. Two symmetry independent rubidium cations are coordinated by oxygen atoms, including those from four water molecules (<Rb-O> = 2.908, 2.938 Å). To analyze the coordination environment and estimate the valence states, bond valence calculations were performed according to [47]. Full details of the bond valence model can be found in [48]. The bond valence sums are 5.91, 4.16, 4.07, 4.19, 4.22 for U1, and Si1-Si4, respectively (Table S10). The slight overbonding for the silicon atoms is rather commonly observed among the structures of uranyl silicates (Table S11) [23], the BVS for the silicon atoms line is in a broad range of 3.90-4.53. According to the formula given in [49], these values strongly and almost linearly depend on the bond distances in the SiO 4 tetrahedra. Among uranyl silicates, the distribution of these values is close to normal with the maximum at 1.  According to the formula given in [49], these values strongly and almost linearly depend on the bond distances in the SiO4 tetrahedra. Among uranyl silicates, the distribution of these values is close to normal with the maximum at 1.  In the structure of 1, the UO7 polyhedra condense to form ∞ [UO5] 4− chains ( Figure  5а) which share vertices and edges with the SiO4 tetrahedra (Figure 5b). The latter associate into 6-membered rings (Figure 5c) and further into 2 ∞ [Si8O19] 6− layers, which are stitched by the uranium polyhedra into a microporous framework (Figure 5d). The channels (9.15 × 7.31 Å) host the rubidium cations and water molecules.  (13) vs. 4789.06(13)Å 3 ), the differences being mostly due to the replacement of a larger Cs + by a smaller Rb + and the presence of water molecules. The framework topology in both compounds is nearly identical; the most pronounced differences concern the positions and coordination of the alkali metal cations ( Figure 6).   (13) vs. 4789.06(13)Å 3 ), the differences being mostly due to the replacement of a larger Cs + by a smaller Rb + and the presence of water molecules. The framework topology in both compounds is nearly identical; the most pronounced differences concern the positions and coordination of the alkali metal cations ( Figure 6).
Two more topologies of the ∞ [Si8O19] 6− chains are known, both comprised of the [Si6O18] 12− hexameric "building blocks". The former is found in the structures of Cs2Cu2(Si8O19) [50] and Rb2Cu2(Si8O19) [51], while the second, is found in the structure of Na6(Si8O19) [52]. In the structures of the copper silicates, the silicate tetrahedra share vertices to form the ∞
In contrast to 1, in 2 the uranium polyhedra do not condense but share their equatorial vertices only with the silicate tetrahedra SiO 4 . The channels, of 7.69 × 4.38 Å width, contain alkali metal cations.  In both structures, double ∞ [Si10O22] 4− layer can be dissected into ribbons; in 2, they are aligned one against other as dictated by a mirror plane, so one ribbon is completely covered by another on the corresponding projection (Figure 9а). In the meantime, in the structure of Cs2[(UO2)(Si10O22)] the ribbons are shifted due to the rotations of SiO4 tetrahedra (Figure 9d), resulting in a decrease of the overall symmetry.

[Rb3Cl][(UO2)(Si4O10)] (3) and [Cs3Cl][(UO2)(Si4O10)] (4)
The crystal structures of 3 and 4 contain a single uranium site forming a uranyl cation (<U-Oap> = 1.801 and 1.805 Å for 3 and 4, respectively). As in the previous case, these species coordinate four oxygen atoms in equatorial planes (<U-Oeq> = 2.252 and 2.261 Å for 3 and 4). A unique silicon site centers a tetrahedron (<Si-O> = 1.605 and 1.609 Å for 3 and 4). The bond valence sums are 5.94, 4.18, and 5.94, 4.20 for U1, Si1 in the structure of 3 and 4, respectively (Table S13, S14). The SiO4 tetrahedra share vertices to form ∞ [Si4O10] 4− chains aligned along b (Figure 10a) and linked into framework by the uranium polyhedra ( Figure  10b,c). The length of the equatorial edge of the UO6 bipyramid nearly coincides with the distance between the oxygen atoms in the silicate chain, which permits these bipyramids to reside at the bending points of the zigzag chains (Figure 10b). The channels of framework with the interior size of 11.62 × 10.54 Å are aligned in the ас plane and contain chloride anions and alkali metal cations (Rb + in 3 and Cs + in 4). Such salt-inclusion structures are rather common among uranyl silicates [44,53], with halide anions either being   (Table S13, S14). The SiO4 tetrahedra share vertices to form ∞ [Si4O10] 4− chains aligned along b (Figure 10a) and linked into framework by the uranium polyhedra ( Figure  10b,c). The length of the equatorial edge of the UO6 bipyramid nearly coincides with the distance between the oxygen atoms in the silicate chain, which permits these bipyramids to reside at the bending points of the zigzag chains (Figure 10b). The channels of framework with the interior size of 11.62 × 10.54 Å are aligned in the ас plane and contain chloride anions and alkali metal cations (Rb + in 3 and Cs + in 4). Such salt-inclusion structures are rather common among uranyl silicates [44,53], with halide anions either being The length of the equatorial edge of the UO 6 bipyramid nearly coincides with the distance between the oxygen atoms in the silicate chain, which permits these bipyramids to reside at the bending points of the zigzag chains (Figure 10b). The channels of framework with the interior size of 11.62 × 10.54 Å are aligned in the ac plane and contain chloride anions and alkali metal cations (Rb + in 3 and Cs + in 4). Such salt-inclusion structures are rather common among uranyl silicates [44,53], with halide anions either being coordinated to the uranyl cations [54] or filling the channels. Booth 3 and 4 are isostructural to the fluoride silicate [Cs 3 F][(UO 2 )(Si 4 O 10 )] [21]. It is noteworthy that despite essential difference in the ionic radii of F − and Cl − and more reactive bonding of the former to uranyl cations, both halide anions contribute to isostructural compounds.

Discussion
Consider now the graphs of the frameworks in the structures of 1-4, which are produced by omitting the alkali metal cations, halide anions, and oxygen atoms, including those from water molecules, and joining the nodes formed by uranium and silicon atoms whose polyhedra share vertices or edges. The results reflect the connectivity modes ( Figure 11). The channel dimensions, measured as the internodal distances, are relatively close. The graphs for 1 (Figure 11a) contain six-and eight-membered rings. The graph topology for 1 are rather close to those reported for the structure of zeolite merlinoite, K 6 [21]. It is noteworthy that despite essential difference in the ionic radii of F − and Cl − and more reactive bonding of the former to uranyl cations, both halide anions contribute to isostructural compounds.

Discussion
Consider now the graphs of the frameworks in the structures of 1-4, which are produced by omitting the alkali metal cations, halide anions, and oxygen atoms, including those from water molecules, and joining the nodes formed by uranium and silicon atoms whose polyhedra share vertices or edges. The results reflect the connectivity modes (Figure 11). The channel dimensions, measured as the internodal distances, are relatively close. The graphs for 1 (Figure 11a) contain six-and eight-membered rings. The graph topology for 1 are rather close to those reported for the structure of zeolite merlinoite, K6Ca2[Al10Si22O64]·20H2O [55,56] (Figure 11d). Considering these related structures, we conclude that this framework tolerates both the replacement of K + by Rb + and the variation of the water content in the channels. This The Considering these related structures, we conclude that this framework tolerates both the replacement of K + by Rb + and the variation of the water content in the channels. This suggests that some exchange properties may be expected. These features are even more pronounced for the frameworks in 1-4. In 1, the channels contain Rb + cations and water molecules, while in Cs 2 [(UO 2 ) 2 (Si 8 O 19 )] [24], only Cs + cations. The framework in 2 is also rather "elastic": its channels can be filled by Cs + , K + /Rb + and water molecules. The framework in 3 and 4 remains stable upon replacement of Cs + by Rb + and F − by Cl − . Considering that the size difference between F − and Cl − is essentially larger than that between Cl − and Br − , existence of the corresponding bromide analogs, at least with smalland medium-size alkali metal cations and maybe even Ag + and Tl + , does not seem unlikely; at least some of these species can likely be prepared via cation/anion exchange using lowtemperature eutectic halide melts, which is of certain interest considering immobilization of 137 Cs + or 36 Cl − .
The compounds 1-4 were prepared at harsh conditions (900 • C). Despite the gross differences in preparation conditions, they share some common structural features: for instance, their silicate architectures contain [Si 6 O 18 ] 12− building blocks. It needs to be noted that while the silicon source in the hydrothermal syntheses is very likely the very reactive form of dissolved silica, in high-temperature syntheses it is the glassy form of SiO 2 which is also rather reactive compared to its crystalline forms, particularly quartz. Yet, the presence of some activators, such as PbO or HF or alkali fluorides, which react with silica at much lower temperatures, compared to uranium oxides, is also important. The activation is either due to the attack of the initially smooth and less reactive silica surface, or via the formation of volatile and reactive species, such as SiF 4 or UO 2 F 2 ; note, however, that the exact sequence of reactions is obscure (neither activator is incorporated into the uranyl silicates reported here). The role of PbO is also in oxidation of U 3 O 8 into U VI compounds. This suggests that certain silicate species are either most easily formed during synthesis or exhibit exceptional stability to occur under totally dissimilar synthesis conditions. The possible templating role of alkali metal and uranyl cations in formation of complex silicate architectures under hydrous and (nearly) anhydrous conditions is also an open and appealing question. The structural similarities and dissimilarities between silicates, germanates, and some more distant relatives, such as alumo-or gallophosphates are also of essential interest. Investigations aimed at finding at least primary and partial answers to these questions are currently underway.

Conclusions
Four novel uranyl-alkali metal silicates have been synthesized via high-temperature synthesis in evacuated "activated" silica tubes using PbO as an oxidizer and fluxing agent. Their crystal structures can be described as frameworks containing channels with effective radii of up to 11.62 × 10.54 Å; these are filled by alkali metal cations and water molecules. These frameworks are relatively stable against substitution in the cation sublattice and variation of water content; this indicates the possibility of exchange reactions. The frameworks are comprised of edge-sharing UO n and SiO 4 polyhedra. Despite essential differences, these structures share some common features, i.e., the polysilicate anions in 1 and 2 are comprised of hexameric [Si 6 O 18 ] 12− rings, while in 3 and 4, of tetrameric [Si 4 O 12 ] 8− rings. Topological analysis of other known uranyl silicate structures shows that these hexa-and tetrameric rings are found very commonly as secondary building blocks.   Figure S3. IR absorption spectra of (K,Rb) 2 [(UO 2 )(Si 10 [57][58][59][60][61][62][63]