Observation of an Unusual Uranyl Cation–Cation Interaction in the

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

Observation of an Unusual Uranyl Cation−Cation Interaction in the Strongly Fluorescent Layered Uranyl Phosphates Rb6[(UO2)7O4(PO4)4] and Cs6[(UO2)7O4(PO4)4] Christian A. Juillerat, Emily E. Moore, Theodore Besmann, and Hans-Conrad zur Loye* Department of Chemistry and Biochemistry and the Center for Hierarchical Wasteform Materials (CHWM), University of South Carolina, Columbia, South Carolina 29208, United States S Supporting Information *

der, ACS grade), AlPO4 (Alfa Aesar, powder, 99.99%), CsCl (Alfa Aesar, powder, 99%), and RbCl (Alfa Aesar, powder, 99.8%) were used as received. Caution! Although the uranium precursor used contained depleted uranium, standard safety measures for handling radioactive substances must be followed. For each reaction, 0.5 mmol of UF4, 0.2 mmol of AlPO4, and 10 mmol of RbCl or 20 mmol of CsCl were loaded into alumina crucibles with alumina caps and heated to 875 °C in 1.5 h, held for 12 h, and cooled to 450 °C at 6 °C/h. The products were then sonicated in water and isolated via vacuum filtration in good yield (∼70%; Figure 1). The orange plate single crystals were hand

ABSTRACT: Single crystals of two new uranyl phosphates, A6[(UO2)7O4(PO4)4] (A = Cs, Rb), featuring cation−cation interactions (CCIs) rarely observed in uranium(VI) compounds were synthesized by molten flux methods. This structure crystallizes in the triclinic space group P1̅ with lattice parameters, a = 9.2092(4) Å, b = 9.8405(4) Å, c = 10.1856(5) Å, α = 92.876(2)°, β = 95.675(2)°, and γ = 93.139(2)° for A = Cs and a = 9.2166(9) Å, b = 9.3771(10) Å, c = 10.1210(11) Å, α = 89.981(4)°, β = 96.136(4)°, and γ = 92.790(4)° for A = Rb. The optical properties are reported for both compounds and compared to a layered uranyl phosphate, K4[(UO2)3O2(PO4)2], having a similar phosphuranylitebased structure but no CCIs. Partial ion exchange of Cs and Rb cations into the Rb6[(UO2)7O4(PO4)4] and Cs 6 [(UO 2 ) 7 O 4 (PO 4 ) 4 ] structures, respectively, was achieved.

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ranium chemistry has been studied since the mid-20th century for predominantly nuclear weapons and nuclear energy applications and, more recently, for applications related to environmental protection and nuclear waste storage.1−4 We are interested in studying uranium(VI) chemistry to primarily develop the basic understanding necessary to design the next nuclear wasteforms.5 Uranium(VI) ubiquitously appears in the form of UO22+, uranyl, an ion that features strong axial oxygen bonds with average distances of ∼1.8 Å.6 Because of the stronger axial U−O bond, the “yl” oxygen atoms are typically inert and do not participate in additional bonding. Cation−cation interactions (CCIs) are an exception where the “yl” oxygen bonds with another uranium center. CCIs are known in other penta- and hexavalent actinides that form the AnO2+ ion (An = U−Am);7 however, they are much less frequent among uranyl materials. In a recent review, only 50 out of 2500 uranium(VI) compounds were found to contain CCIs;8 noticeably, of those 50 CCIcontaining compounds, only one was observed in a layered structure. Several layered uranyl oxychlorides have also been reported, where, unlike in the phosphates described herein, the CCIs are sterically mediated by the larger chlorine atoms.9 The observed layered uranyl phosphate compounds feature a uranyl CCI that is, to date, unreported in the literature for uranyl systems. The title compounds were synthesized by molten flux methods.10 UF4 (International Bio-Analytical Industries, pow© XXXX American Chemical Society

Figure 1. Orange plate crystals of Rb6[(UO2)7O4(PO4)4] (a) and Cs6[(UO2)7O4(PO4)4] (b).

picked from other minor unidentified phases to obtain a phasepure sample, and the phase purity was confirmed by grinding the crystals into a powder and collecting powder X-ray diffraction (PXRD) data using a Bruker D2 Phaser equipped with an LYNXEYE silicon strip detector and a Cu Kα source (Figure S1). The intensities of the calculated PXRD patterns of both compounds differ from the experimentally observed intensities due to the presence of severe preferred orientation of the plate crystals in the (0 1 −1) direction. The structure of each material was determined by singlecrystal X-ray diffraction using a Bruker D8 Quest single-crystal Xray diffractometer equipped with a Mo Kα microfocus source (λ = 0.71073 Å). The absorption correction was performed using the SAINT+ and SADABS programs within the APEX 3 software.11 After reduction and absorption correction, the structure was solved by SHELXT, an intrinsic phasing solution method, and refined using SHELXL, both of which were used within the Olex 2 GUI.12−14 Full crystallographic data (Table S1) and a description of the crystallographic structure refinement are Received: February 2, 2018

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DOI: 10.1021/acs.inorgchem.8b00302 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry given in the Supporting Information (CSD#s 434015−434016). Elemental compositions of the compounds were confirmed qualitatively by energy-dispersive spectroscopy (EDS) using a TESCAN Vega-3 SBU scanning electron microscope equipped with an EDS detector. A6[(UO2)7O4(PO4)4] is constructed of uranyl phosphate layers of the composition [(UO2)7O4(PO4)4]6−, which are charge-balanced by Cs+ or Rb+ cations located between the layers. The layers are made up of phosphate tetrahedra, uranyl pentagonal bipyramids, and uranyl hexagonal bipyramids. The layer can be deconstructed into units of the phosphuranylite (PU) topology that are connected into chains via edge-sharing between two pentagonal bipyramids (Figure 2). These chains are

Figure 3. CCI observed in A6[(UO2)7O4(PO4)4]. Bond distances of the cesium analogue are provided. Uranyl bonds are in boldface for emphasis.

La5Re3MnO16 and Ti−Ti bonds of 2.760 Å in Nd3Ti4O12.16 While the U−O bond distances involved in the CCIs in this structure are longer than average, the U−U bond is shorter at 3.563 Å. Both U−O10 bonds are longer than typical U−O bond distances, which is characteristic of CCIs. For A = Cs, the U3− O10 bond length is 1.843 Å compared to the other uranyl bond distances in the structure, which are in the range 1.785−1.799 Å. The equatorial U3−O10 bond is 2.523 Å and is longer than other U−O bond distances in the same polyhedron. Bond distances and bond valence sums for the uranyl and phosphate polyhedra for both structures are listed in Tables S2 and S3.17,18 Optical measurements on both compounds were performed using a PerkinElmer Lambda 35 UV−vis spectrometer equipped with an integrating sphere and a PerkinElmer LS55 luminescence spectrometer. The UV−vis diffuse-reflectance data were internally converted to absorbance using the Kubelka−Munk equation.19 The UV−vis data show absorption edges of 570 and 550 nm with estimated band gaps of 2.2 and 2.3 eV for the cesium and rubidium compounds, respectively (Figure S2). UV−vis and fluorescence data for each compound show similar features, as expected for isostructural materials. It is generally accepted that uranyl phases containing CCIs luminesce more intensely than those not containing CCIs. To test this phenomenon, the fluorescence spectra of the rubidium and cesium analogues and that of a structurally related phosphate compound recently prepared by our group, K4[(UO2)3O2(PO4) 2],20 were collected. For the cesium analogue, a 10% attenuator was used to decrease the intensity to measurable levels. The fluorescence emission spectra feature one large peak at 548 nm (Figure 4). Visually, the luminescence

Figure 2. Construction of A6[(UO2)7O4(PO4)4] from the PU units and dimers. The uranyl polyhedra are shown in yellow, phosphate tetrahedra in magenta, cesium atoms in dark blue, and oxygen atoms in red. CCIs are outlined in black.

further connected into sheets by corner and edge sharing through phosphate tetrahedra, similar to the PU topology, and by uranyl CCIs that are formed by the edge sharing of two pentagonal-bipyramidal uranyl units. The CCIs in the structure are created by uranyl pentagonal bipyramids that share a unique edge that includes one axial (O10) and one equatorial oxygen (O10), thereby forming two simultaneous CCIs between them (Figure 3). This CCI has only been observed in a neptunium(V) compound, Na4[(NpO2)2C6(COO)6]·8H2O, and is rare even among these compounds.7 These CCIs would be classified as type 7 uranyl interactions, based on the recently published classification scheme by Read et al.9 This type of CCI in uranium(VI) chemistry was first hypothesized by Fortier et al., but no example compounds of this type were given, and this class of CCI was not reported in the 2014 review of CCIs in uranium(V) by Serezhkin et al.8,15 This CCI is reminiscent of edge-sharing octahedra in metal oxide materials such as pillared perovskites, Nd3Ti4O12 and La5Mo4O16, where the edge-sharing octahedra have exceptionally short M−M bond distances, i.e., Re−Re bonds of 2.407 Å in

Figure 4. Emission and excitation spectra for Cs6[(UO2)7O4(PO4)4] and Rb6[(UO2)7O4(PO4)4].

excited at 365 nm using a hand-held UV lamp is very intense and significantly more so than that of K4[(UO2)3O2(PO4)2] (Figure 5). The structure of K4[(UO2)3O2(PO4)2] is also based on the PU topology, supporting the theory that it is the presence of CCIs between the PU units in the title structures that results in a much more intense luminescence. B

DOI: 10.1021/acs.inorgchem.8b00302 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry

Tables of crystallographic information, select bond distances, bond valence sums, PXRD patterns, UV−vis spectra, and description of VBT methods (PDF) Accession Codes

CCDC 1814994−1814995 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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Figure 5. Powders of Cs6[(UO2)7O4(PO4)4] (a), Rb6[(UO2)7O4(PO4)4] (b), and K4[(UO2)3O2(PO4)2] (c) shown under artificial light (top) and under long-wave (365 nm) UV light (bottom).

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Volume-based thermodynamics has been used to calculate the Gibbs energy of the title compounds using methods described in an earlier work19 with minor adaptations explained in the Supporting Information. Values of −9306 and −9062 kJ/mol, for the rubidium and cesium phases, respectively, were obtained, indicating that the structures are very stable. This supports the experimental observation that the title compounds can be synthesized over a wide range of temperatures (775−925 °C) and reagent-to-flux ratios. The results were also compared to the Gibbs energy values for the A4[(UO2)3O2(PO4)2] family (average energy of −6665 kJ/mol)19 and showed that the CCI structure is significantly more favorable. Ion-exchange experiments were performed on ground samples of both the cesium and rubidium analogues to determine if the cesium and rubidium cations could be replaced by rubidium and cesium, respectively. The cesium and rubidium analogues were soaked in aqueous 7 M RbCl and 11 M CsCl, respectively, for 8 days without stirring in an oil bath set to 90 °C. Partial ion exchange was successful and confirmed by examining the PXRD and EDS results. PXRD (Figure S4) confirmed that the layers remain intact during the ion-exchange process, and EDS confirmed the presence of a second cation in the structure. The ion exchange was more extensive in the cesium phase than in the rubidium phase, resulting in approximate ratios of 2:1 Rb/Cs for Rb6[(UO2)7O4(PO4)4] soaked in CsCl and 1:1 for Cs6[(UO2)7O4(PO4)4] soaked in RbCl. These results suggest that the rubidium phase is able to diffuse more quickly into the cesium phase, which has a larger interlayer gallery, as compared to the reverse process, which would require the cesium cations to push the layers apart in order to interdiffuse. In summary, we have synthesized and characterized two unique layered uranyl phosphates, A6[(UO2)7O4(PO4)4] (A = Cs, Rb), which feature a CCI not previously observed in uranyl chemistry. The two phosphates were synthesized via molten flux growth methods and feature strong luminescence. The luminescence of these CCI-containing compounds is noticeably more intense than that of the structurally related layered uranyl phosphate, K4[(UO2)3O2(PO4)2], which is also based on the PU topology but has no CCIs. Basic ion-exchange experiments demonstrated that the alkali-metal cations can be partially exchanged by other monovalent cations.

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Hans-Conrad zur Loye: 0000-0001-7351-9098 Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Research was conducted by the CHWM, an Energy Frontier Research Center, and supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering, under Award DE-SC0016574. C. Juillerat was additionally supported by an NSF IGERT Graduate Fellowship under Grant 1250052.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b00302. C

DOI: 10.1021/acs.inorgchem.8b00302 Inorg. Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.inorgchem.8b00302 Inorg. Chem. XXXX, XXX, XXX−XXX