Nuclear power is essential in meeting energy demands while tackling climate change crisis. However, one of the main challenges in safely utilizing nuclear energy lies in preventing the environmental pollution from uranium mining to nuclear waste disposal [1-3]. It is thus of great importance to develop targeted waste forms that can efficiently immobilize various types of radionuclides due to their radioactivity and toxicity [4].

Uranyl silicates are one of the most abundant minerals found in the oxidized zone of U deposits worldwide and are typical alteration products formed during weathering of natural uraninite [5-8]. When interacting with spent nuclear waste in deep ground environments, various structures can be formed due to the flexibility of silicate connectivity [9]. Hence uranium silicates have been explored in its application in nuclear waste immobilization [10-13]. Some uranium silicate compounds have been reported to be capable of immobilizing multiple components of spent nuclear fuel, such as cesium, which is one of the most abundant fission products [9]. Uranium(VI) compounds are dominantly presented in the crust environment, but more mobile than uranium(IV) ones, making it less suitable for immobilizing radionuclides [14]. For instance, coffinite is the only known natural U⁴⁺ silicate that form as a primary mineral [15, 16].

Most synthetic explorations have been made towards uranium(VI) compounds in the last few decades [17-20]. Nonetheless, efforts have led to a few novel uranium silicates or germanates (due to their similarities in chemical properties) with oxidation states of uranium other than 6+. For example, Liu et al. [21] obtained Cs₂U^IVSi₆O₁₅ by high-pressure, high-temperature hydrothermal synthesis. On the other hand, two uranium(V) silicates, K(U^VO)Si₂O₆ [22] and K₃(U₃^VO₆)(Si₂O₇) [23], and two uranium(V) germanates, Cs₃U^VGe₇O₁₈ [24] and Rb₃(U₃^VO₆)(Ge₂O₇) [23]. Some mixed-valence uranium silicates and germanates have been reported as well, including [Na₉F₂][(U^VO₂)(U^VIO₂)₂(Si₂O₇)₂] [25], Na₇U^IVO₂(U^VO)₂(U^V/VIO₂)₂SiO₆ [26], and Cs₈U^IV(U^VIO₂)₃(Ge₃O₉)₃·3H₂O [27]. The great contrast of the scarcity of these compounds and the abundance of their uranyl counterparts calls for more discovery through synthetic efforts to enrich its crystal chemistry for immobilizing nuclear waste.

Uranium silicates and germanates both exhibit rich connectivity capabilities. Due to the flexibility of orthosilicate ion SiO₄⁴⁻, various silicon units can be formed, such as Si₂O₇ dimers, Si₄O₁₂ squares, chains, sheets, and 3D frameworks [28]. Furthermore, various 0D [29, 30], 1D [31, 32] and 2D [33] silicate units can bond with typical UO₆ polyhedra to form diverse uranium silicate compounds. Recently, G. Morrison et al. [9] has successfully synthesized four uranium silicate crystals with salt-inclusion, [A_mB_nX][(UO₂)_p(Si_qO_r)_t], by CsCl/CsF flux method. In stacking of uranyl and silicon units, there are diverse U-Si structure with different 10-(ABBBBABBBB), 12-(ABBABBABBABB) and 14-(ABBBBABABBBBAB) membered uranyl silicate rings, indicating the flexibility of this structure. Compared with uranium silicate structure, however, the reports of uranium germanates are relatively limited. The most common GeO₄ tetrahedra bond with UO₆ polyhedra like SiO₄ units, and there are various other modes of germanium coordination with oxygen, such as square pyramids, trigonal bipyramids and octahedra [34]. Moreover, Li et al. [34] synthetized a unique series of open-framework uranyl germanates from different mixed molten fluxes, consisting of 3D 8-, 10-ring channels built upon [UGe₄] pentamers and/or [UGe₆] heptamers. A pentavalent uranium germanate containing 4- and 6-coordinate germanium was reported by Quang et al. by high-temperature, high-pressure hydrothermal method [24].

Most of the reported uranium(IV) and uranium(V) silicates and germanates are synthesized by high-temperature, high-pressure hydrothermal method [35], and there are a few crystals synthesized by molten salt method. Molten salt electrolysis is a promising technology for reprocessing spent nuclear fuels [36, 37]. Most actinides are recovered and decontaminated from the fission products in the molten salt electrorefining process [38]. Molten salt method has also been proposed to synthesize unique host materials for waste salt after the refining process [9]. On the other hand, molten salt method can provide benefits in novel compound synthesis allowing for the preservation of unusual valence states and ionic species, which would be destabilized in conventional solvents [39]. Moreover, the reaction time can be significantly reduced by using the molten salt method [39, 40]. So far, no uranium(IV) silicate and germanate has been reported using molten salt method. Herein, the synthesis of uranium silicates and germanates with less common valence states (IV, V) by molten salt method is worthy of experimental exploration.

In this work, we report two tetravalent uranium silicate and germanate compounds containing T₃O₉ trimer by flux method, M₂U^IVT₃O₉ (M = K, Cs; T = Si, Ge).

Single crystals M₂U^IVT₃O₉ were grown by a mixture of 0.5 mmol U₃O₈ and 4 mmol SiO₂ or GeO₂ under a mixture of 20 mmol KBr and 20 mmol KF, and 9 mmol CsF and 11 mmol CsCl, respectively. The starting materials were placed in a silver crucible that was 2.5 cm (top), 1.5 cm (bottom) in diameter and 5.7 cm in height. The silver crucibles were loosely covered with sliver lids and placed in a muffle furnace inside the glove box under inert gas atmosphere and were gradually heated to 800 ℃ in 1.5 h, isothermed for 12 h, and cooled slowly to 400 ℃ or 500 ℃ at a rate of 6 ℃/h, at which temperature the muffle furnace was shut off. After removing redundant salt by washing using deionized water, the crystals were obtained via crystal picking and shown in Fig. S1 (Supporting information). However, the resulting crystals have a low yield (< 10%) and not enough amount can be harvested for further investigation such as powder X-ray diffraction for phase purity determination.

The crystal structure confirmations were executed via intensity data collection on a Bruker D8 CCD diffractometer equipped with a Mo K_α radiation (λ = 0.7107 Å) at 270 K and a CMOS PHOTON 100 detector. An original structure was acquired using Olex2 directly. Further refined structure to determine the final structural model (R₁ = 0.0698, 0.0144). A summary of the basic crystallographic data for the two crystals is provided in Table 1, with specific details provided in the Supporting information (Table S1). Moreover, Scanning Electron Microscopy (SEM) equipped with Energy Dispersive Spectroscopy (EDS) confirmed the presence of Cs, K, Ge, Si, U and O in M₂U^IVT₃O₉, respectively, shown in Fig. S2 (Supporting information).

K₂USi₃O₉ (1), crystallizes in the monoclinic space group P1₂₁/n1 with lattice parameters a = 7.1076 Å, b = 10.4776 Å, c = 12.2957 Å, γ = 120° and V = 915.67 ų. The asymmetric unit consist of four U sites, three Si sites, two K sites, and nine O sites. All UO₆ octahedra have six long U–O bonds (2.239–2.256 Å), which agrees with the other U(IV) inorganic compound [27]. Using the bond valences parameters R = 2.1 Å and b = 0.373 Å for U(IV)-O the valence of U is calculated to be 4.04, which agrees with the occurrence of U⁴⁺ in these sites [41]. The Si atoms coordinate with four O atoms to form a SiO₄ tetrahedra. The Si-O bond distances in Si₃O₉ trimers range between 1.594 and 1.645 Å. These values are in good agreement with values reported in previous studies describing the family of uranium silicate compounds [22, 29]. The specific bond distances are listed in Table S2 (Supporting information). The structure of K₂USi₃O₉ is shown in Figs. 1a and b. Each SiO₄ tetrahedron shares two corners with two other tetrahedra to form a silicon oxide trimer (Si₃O₉⁶⁻). Each silicon oxide trimer shares its corners with six UO₆ octahedra along c direction, forming a 3D uranium silicate framework [USi₃O₉]^2- (Figs. 1b and d). This framework is charge-balanced by K⁺ cations located in voids formed by the sequence of UO₆−SiO₄ connectivity. Another channel along the a and b axis of [USi₃O₉]⁻ framework is ~ 6.79 × 4.29 Å in dimensions, as shown in Fig. 1c.

Cs₂UGe₃O₉ (2) crystallizes in a hexagonal space group P-6 with lattice constants of a = 7.5138 Å, b = 7.5138 Å, c = 11.0114 Å, γ = 120° and V = 538.38 ų. The structure of Cs₂UGe₃O₉ consists of a unique U site, one Ge site, two unique Cs sites and two unique O sites. In this structure, U(1)O₆ octahedron contains six long regular U-O bonds (2.237 Å × 6), and the valence of U(1) is calculated to be 4.15 of according to its bond distance, using parameters R = 2.1 Å and b = 0.373 Å [41]. The Ge atoms are tetrahedrally coordinated by O atoms with short Ge-O bond distances, 1.711–1.767 Å. Its specific bond distances are listed in Table S2 (Supporting information). The structure of Cs₂UGe₃O₉ is shown in Figs. 1a and b, which is isostructural to K₂USi₃O₉ described above. Germanium oxide trimer (Ge₃O₉⁶⁻) coordinate with UO₆ octahedra to form a 3D uranium germanate framework [UGe₃O₉]²⁻ which is charge-balanced by Cs⁺ cations. Its channel along the a and b axis of [UGe₃O₉]⁻ framework is ~6.82 × 5.32 Å in dimensions, as shown in Fig. 1c.

Table S2 lists selected specific interatomic bond distances of the two M₂U^IVT₃O₉ (M = K, Cs; T = Si, Ge) structures. Figs. 1e and f showed the U-O bond distances of each unique U atom in both structures. The channel of [USi₃O₉]²⁻ framework (~6.79 × 4.29 Å) is smaller than that of [UGe₃O₉]²⁻ one (~6.82 × 5.32 Å). This is consistent with the result that the smaller cation K⁺ is immobilized in the [USi₃O₉]²⁻ framework while the [UGe₃O₉]²⁻ framework traps Cs⁺ cation.

The structures of K₂USi₃O₉ and Cs₂UGe₃O₉ consist of three-membered single-ring Si₃O₉⁶⁻ and Ge₃O₉⁶⁻ trimers. These T₃O₉⁶⁻ trimers strike resemblance with previously reported structures [24]. Nguyen et al. [27] reported mixed-valence uranium germanate Cs₈U^IV(U^VIO₂)₃(Ge₃O₉)₃·3H₂O, in which Ge₃O₉⁶⁻ trimers connect U^IVO₆ and U^VIO₆ to form a three-dimensional framework with 9-ring channels occupied by Cs cations and H₂O molecules (Fig. 2a); whereas in Cs₂UGe₃O₉, the channels are only occupied by Cs cations. On the other hand, they also [24] reported a Cs₃UGe₇O₁₈ structure, where Ge₃O₉⁶⁻ trimers coordinate with U^VO₆ and GeO₆ to form a 3D framework, with Ge−O−Ge bond angles of 130. 39 (17)°, 124. 55 (16)° and 127. 69 (17)°, respectively (Fig. 2b). Both previously reported structures (Cs₈U^IV(U^VIO₂)₃(Ge₃O₉)₃·3H₂O and Cs₂UGe₇O₁₈) were, however, obtained by high-temperature, high-pressure hydrothermal reactions, in which the reactions were heated at 585 ℃ for 2 and 4 days with a slow cooling rate of 2 ℃/h. Furthermore, there were no previous reports on Si₃O₉⁶⁻ related frameworks.

The two M₂U^IVT₃O₉ (M = K, Cs; T = Si, Ge) structures were optimized by density functional theory (DFT) calculations. More details are given in Supporting information. Compared to the experimentally measured bond distance, the calculated bond distances of U-O are between 2.27 and 2.29 Å. The similar bond distances indicate that both structures are stable. The optimized CIF file can be found in the Supporting Information. The calculated formation enthalpy is −6.30 and −5.36 and eV/atom for K₂USi₃O₉ and Cs₂UGe₃O₉, respectively. Their relatively large absolute values demonstrate the high stability of these two kinds of compounds. The total and projected density of states (DOS) of each element for these three compounds are illustrated in Fig. 3. For the structures of K₂USi₃O₉ and Cs₂UGe₃O₉, their orbitals of U and O atoms are hybridized in the range of about −21 to −16, and −10 to 0 eV, respectively, indicating the strong U-O bonds. Similarly, there are overlaps between the Si and O orbitals of K₂USi₃O₉ and between the Ge and O orbitals of Cs₂UGe₃O₉ in a wide range, indicating the strong Si-O and Ge-O bonds. In a word, the strong interaction between metal atoms and oxygen atoms leads to the high stability of these two compounds.

In summary, we have successfully synthesized two tetravalent uranium silicate and germanate crystals containing a three-membered single-ring T₃O₉⁶⁻ under flux growth method under inert gas atmosphere. Bond valence sum calculations have confirmed the valence states of uranium to be 4+. K₂USi₃O₉ is the first uranium silicate that contains the Si₃O₉⁶⁻ trimers. Molten salt method under inactive gas have demonstrated its potential in synthesizing a variety of uranium compounds with less common valence states, and immobilizing different types of radionuclides such as uranium and cesium in one crystal structure. More work is being conducted in exploring novel uranium structures by this group.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This work was supported by the National Science Fund for Young Scholars (No. 22106165), the National Science Fund for Distinguished Young Scholars (No. 21925603), the Major Program of the National Natural Science Foundation of China (No. 21790373) and the National Natural Science Foundation of China (No. U20B2020).

Supplementary materials

Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2022.03.026.