5f Bimodal Emission in Europium-Incorporated Uranyl

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

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Tunable 4f/5f Bimodal Emission in Europium-Incorporated Uranyl Coordination Polymers Jian Xie,†,⊥ Yaxing Wang,†,§,⊥ Mark A. Silver,† Wei Liu,† Tao Duan,‡ Xuemiao Yin,† Lanhua Chen,† Juan Diwu,† Zhifang Chai,† and Shuao Wang*,† †

School for Radiological and Interdisciplinary Sciences (RAD-X) and Collaborative Innovation Center of Radiation Medicine of Jiangsu Higher Education Institutions, Soochow University, Suzhou 215123, China ‡ School of National Defence Science & Technology, Southwest University of Science and Technology, Mianyang 621010, China § Key Laboratory of Radiation Physics and Technology, Ministry of Education, Institute of Nuclear Science and Technology, Sichuan University, Chengdu 610064, China S Supporting Information *

ABSTRACT: There have been numerous studies on emission-color regulation by the adjustment of molar amounts of multiple trivalent lanthanide cations, such as Eu3+, Tb3+, Dy3+, and others, in many types of solid host materials. Although uranyl emission originating from charge-transfer transitions has been well-recognized and investigated for many decades, as of now there is no report on tunable 4f/5f bimodal emission based on heterobimetallic lanthanide(III) and uranyl(VI) compounds. In most cases, complete energy transfer between uranyl(VI) and lanthanide(III) centers was observed. In this work, a series of isotypic-europiumincorporated uranyl coordination polymers, Eu@UO2L(DMF) (L2− = 3,5-pyridinedicarboxylate, denoted as 1−10, which represent the different Eu contents in UO2L(DMF); DMF = N,N-dimethylformamide), has been synthesized by solvothermal reactions. Crystallographic evidence of this series unveiled one-dimensional chains of UO22+ as pentagonal-bipyramidal units bridged by pyridinedicarboxylate with no defined, crystallographically unique site containing Eu, even for the products with high concentrations of Eu in this series. However, emission bands characteristic of Eu3+ were clearly observed in every product along with the characteristic uranyl-emission feature when observed with UV−vis fluorescence spectroscopy. Laser-ablation inductively coupled plasma mass spectrometry indicated that europium was concomitant with uranium, corroborating the incorporation of europium into crystals of UO2L(DMF). Systematic control of the solvent ratio (VH2O/VDMF) in each reaction gives rise to an enrichment of Eu3+ in the interior of UO2L(DMF). In addition, the color of emission of these compounds changed significantly from bright red to bright green with decreasing Eu content. This phenomenon occurs from the highly efficient energy transfer between the UO22+ and Eu3+ centers within each sample, providing the first case of a tunable 4f/5f bimodal emission in a mixed 4f/5f-elements-bearing metal−organic-hybrid material.

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INTRODUCTION Interest in investigating actinide-based hybrid materials, such as coordination polymers (CPs), has grown exponentially in the past few decades.1 Radioactive wastes produced in the mining of uranium as well as the long-term management of used nuclear fuel make investigations into the interactions between uranium and naturally occurring functional groups necessary.2 The most plentiful functional groups in environmental systems are carboxylates (e.g., organic aliphatic or aromatic carboxylates), which have been proven to be successful in constructing uranyl coordination polymers.3 The most stable oxidation state of uranium, the hexavalent state, features a linear dioxo unit (OUO), and coordination from mono- or multidentate ligands is restricted to forming bonds around the equatorial plane of this species. The ability of UO22+ to possess different coordination © XXXX American Chemical Society

environments (e.g., 6-, 7-, or 8-fold) permits the formation of a variety of unique and novel frameworks possessing different dimensionalities and structural topologies.1a,4 Exploring the chemical interactions of lanthanides with uranium is imperative to discovering new methods or validating current procedures for recycling nuclear waste, where these elements cohabitate after thorough use of reactor material. Previous work investigating the synthesis and characterization of uranyl CPs have sought to unite 4f and 5f metals; however, the success of preparing mixed uranium− lanthanide compounds is rare because of the large difference in geometry between the linear uranyl and roughly spherical 4f cations. Three strategies have been reported to overcome this Received: September 12, 2017

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DOI: 10.1021/acs.inorgchem.7b02304 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

measured. Here, boric acid is a mineralizing agent that can promote the growth of high-quality single crystals. Much smaller crystals can also be obtained in the absence of boric acid but are not suitable for crystallography analysis. Elemental analysis: calcd C, 23.61%; N, 5.51%; H, 1.97% and found C, 23.72%; N, 5.94%; H, 2.02%. Single-Crystal X-ray Diffraction. The data were collected at 223 K using a Turbo X-ray Source (Mo Kα radiation, λ = 0.71073 Å) adopting the direct-drive rotating-anode technique and a CMOS detector. The crystals were mounted on Cryoloops with Paratone oil and optically aligned on a Bruker D8-Venture single-crystal X-ray diffractometer equipped with a digital camera. The structures were solved by direct methods and refined on F2 by full-matrix least-squares methods using SHELXTL.13 Powder patterns (Figure S1) show that 1−10 crystallized in the same structure, so only the structural data of 1 was collected in this work. All nonhydrogen atoms were refined with anisotropic-displacement parameters, and the carbon-bound hydrogen atoms were introduced at calculated positions. The crystal data and structure-refinement parameters are given in Table 1.

concern. Thuéry and co-workers used ligands that had strong coordination sites with differing functional groups (i.e., heterofunctional) to synthesize heterobimetallic frameworks.5 Loiseau et al. synthesized a pair of uranyl−lanthanide mellitates that possessed cation−cation interactions and two different uranyl coordination environments to facilitate crystallization.4b Finally, Lii and co-workers synthesized a uranyl−europium germanate under high-temperature, highpressure hydrothermal and flux-growth reactions.6 In addition, energy transfer between UO22+ and Ln3+ has attracted much attention over the last few decades, having been observed in various media, including solutions,7 glasses,8 polymers,9 inorganic phosphor materials,10 and phosphates.11 The photoluminescence properties of these uranyl−lanthanide compounds have been thoroughly investigated, revealing the role that unique physical factors within these materials have in stirring interesting electronic behavior. Cahill12 and Lii6 have reported a few uranyl−lanthanide heterobimetallic materials in which the uranyl cation sensitizes lanthanide luminescence while emission from uranyl is absent from the photoluminescence spectrum. In contrast to this, Loiseau at al. reported several uranyl−lanthanide organic-coordinated polymers whose photoluminescence spectra only show the characteristic emission bands of UO22+. In these experiments, highly efficient energy transfer occurs between the UO22+ and Ln3+ centers. To our knowledge, there has been no investigation of a uranyl−lanthanide heterobimetallic CP that displays the emission bands of UO22+ and Ln3+ simultaneously. Herein, we report the first uranyl−europium CP that exhibits the emission features of both 4f and 5f species. Although photoluminescence spectroscopy supports the coexistence of Eu3+ in this uranyl CP, crystallographic data does not provide evidence of the presence of Eu3+. However, because the crystallographic technique and refinement process removes insufficient signals lying beneath the noise and because the emission intensity relative to the quantity of Eu3+ is large, we conjecture that the content of europium is too low to be observed in this structure with SC-XRD. Therefore, ICP-AES, LA-ICP-MS, and SEM experiments were conducted to confirm the existence of europium, quantify the amount of the europium content, and provide discernible depth distribution in these crystals. Furthermore, photoluminescence-lifetime measurements were gathered to define the energy-transfer mechanism in this system.

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Table 1. Crystallographic Data for 1

a

formula

Eu@UO2L(DMF)

MM (g mol−1) cryst syst. space group a (Å) b (Å) c (Å) β (°) V (Å3) Z Dc (g cm−3) μ (mm−1) F (000) T (K) R1,a wR2b (I > 2σ(I)) R1,a wR2b (all data)

508.23 monoclinic P21/n 9.758(2) 13.437(4) 10.968(3) 115.572(7) 1297.2(6) 4 2.602 12.546 928.0 223 0.0275, 0.1051 0.0488, 0.0791

ÄÅ É 2 2 Ñ1/2 R1 = ∑ F0 − Fc /∑ F0 bwR 2 = ÅÅÅÅ∑ w(F0 2 − Fc 2) /∑ w(F0 2) ÑÑÑÑ ÅÇ ÑÖ

Powder X-ray Diffraction. Powder patterns were collected from 5 to 50° with a step of 0.02° on a Bruker D8 advance X-ray diffractometer with Cu Kα radiation (λ = 1.54056 Å) equipped with a Lynxeye one-dimensional detector. UV−Vis-Absorption and Photoluminescence Spectroscopy. UV−Vis-absorption spectra were recorded from single crystals using a Craic Technologies microspectrophotometer. The crystals were placed on quartz slides, and the data were collected after optimization of the microspectrophotometer. Photoluminescence spectra were recorded from single crystals of 1−10 using a Craic Technologies microspectrophotometer. The crystals were placed on quartz slides, and the data were collected after optimization of the microspectrophotometer. Inductively Coupled Plasma Atomic-Emission Spectrometry (ICP-AES). Ten milligrams of the crystalline products of 1, 3, 5, 7, and 9 were each dissolved in 5 mL of 5% HNO3. After complete dissolution, each solution was filtered through 0.22 μm filtration membranes, and the concentrations of UO22+ and Eu3+ in the resulting filtrates were determined. Scanning Electron Microscopy and Energy-Dispersive X-ray Spectroscopy. Scanning-electron-microscopy energy-dispersivespectroscopy (SEM/EDS) images and data were collected using an FEI Quanta 200FEG instrument. The energy of the electron beam was maintained at 30 kV, and the spectrum acquisition time was set at 100 s. Samples were mounted directly onto carbon-conductive tape with a gold coating.

EXPERIMENTAL SECTION

Caution! All the uranium compounds used in these studies contained depleted uranium; standard precautions were performed for handling radioactive materials, and all the studies were conducted in a laboratory dedicated to studies involving actinide elements. Reagents. UO2(NO3)2·6H2O, Eu(NO3)3·6H2O, H2L (H2L = 3,5pyridinedicarboxylic acid), H3BO3, DMF, and distilled water were used as received from commercial suppliers without further purification. Synthesis of Eu@UO2L(DMF) (1−10). UO2(NO3)2·6H2O (0.0502 g, 0.1 mmol), Eu(NO3)3·6H2O (0.0133 g, 0.03 mmol), H2L (0.0334 g, 0.2 mmol), H3BO3 (0.0120 g, 0.2 mmol; molar ratio = 1:0.3:2:2), and 5 mL of the mixed solvent (VH2O:VDMF = 0.8:2.2, 1.0:2.0, 1.2:1.8, 1.4:1.6, 1.6:1.4, 1.8:1.2, 2.0:1.0, 2.2:0.8, 2.4:0.6, or 2.6:0.4 mL) were loaded into 10 mL vials. The vials were then sealed and heated to 100 °C for 12 h and cooled to room temperature under ambient condition. Isotypic, yellowish, needlelike crystals of Eu@ UO2L(DMF) (1−10) were isolated as pure products (Figure S1). Crystals suitable for X-ray structural analysis were collected and B


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Figure 1. Structural features of crystalline product 1, including the seven-coordinate environment of UO22+ (a), coordination by DMF (b), propagation of 1D chains of UO2L2(DMF) (c), and the resulting staggered formation of 1D chains into flat sheets from parallel π-stacking (d). Infrared Spectroscopy. Infrared spectra were recorded on powdered samples using a Thermo Scientific Nicolet iS50 instrument in the range of 400−4000 cm−1 at room temperature. Lifetime Measurements. Photoluminescence-lifetime measurements were recorded using an Edinburgh FLS920 steady-state fluorimeter with a time-correlated-single-photon-counting (TCSPC) spectrometer and an LED lamp as the excitation source. Laser-Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS). Laser-ablation analysis was conducted using a Thermo Fisher Element2 ICP-MS instrument coupled to a UP213 Nd:YAG laser-ablation system (New Wave Research). Elementalincorporation analyses consisted of a 50 s measurement of background ion signals prior to a 40 s measurement of the ion signals (238U and 152Eu). Analyses were conducted in medium-massresolution mode (resolution = mass/peak width ≈ 4000) to eliminate possible spectral interferences. The ablated particles were transported from the ablation cell to the ICP-MS instrument using He as the carrier gas at a flow rate of 1 L/min. The crystals were ablated using a range of spot sizes between 15 and 30 μm, a repetition rate of 2 Hz, and 45−50% power output corresponding to an energy density of 12−15 J/cm2. Using these ablation conditions, the depth of penetration of the laser passed 5 μm.14 Thermogravimetric Analysis (TGA). TGA was carried out on a NETZSCH STA 449 F3 Jupiter instrument in the range of 30−900 °C under a nitrogen flow at a heating rate of 10 °C/min.

(2.313(5) Å, 2.318(6) Å; 2.480(5) Å, 2.438(5) Å), and one oxygen atom from DMF (2.413(5) Å). The L2− ligand coordinates three uranium atoms, chelating one metal center using one bidentate carboxylate group and bonding to two other centers using the second carboxylate group. This feature bridges pentagonal bipyramids and extends infinite chargebalanced UO2-ligand chains with coordinating DMF alternating on either side of the chain (Figure 1c). The distance between two adjacent chains is approximately 2.808 Å, suggesting significant parallel π-stacking between aromatic rings of the L2− ligands from nearby chains (Figure 1d).15 Therefore, flat sheets of staggered UO2-ligand chains result from the presence of π···π interactions (the red box shown in Figure 1d highlights this structural feature). Although significant amounts of europium were used in preparing these crystals, no independent crystallographic sites containing Eu were observed in this structure. Inductively Coupled Plasma Atomic-Emission-Spectrometry (ICP-AES) Analysis. ICP-AES was chosen as a technique to meticulously determine the contents of uranium and europium in this series of compounds. In order to create a more linear series of these samples, products 1, 3, 5, 7, and 9 were selected for comparison. As shown in Table 2, there is no significant change in the content of UO22+ between these samples, the average concentration being approximately 1.2880

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RESULTS AND DISCUSSION Structural Description. Single-crystal-X-ray-diffraction studies revealed that 1 crystallizes in the monoclinic and centrosymmetric space group P21/n. As shown in Figure 1d, the overall structure is based on charge-balanced 1D chains with π···π interactions. The asymmetric unit contains one crystallographically independent UO22+ ion, one L2− ligand, and one DMF molecule (Figure 1b). Uranium centers are enveloped in seven-coordinate pentagonal-bipyramidal environments with bonds to the two axial oxo atoms (1.770(5) Å, 1.773(5) Å), four oxygen atoms in two bidentate L2−

Table 2. Content of U, Eu, and U/Eu in Selected Eu@ UO2L(DMF)

C

sample

mmol U/g solid

mmol Eu/g solid

Eu/U (%)

U/Eu (%)

1 3 5 7 9

1.3035 1.2982 1.2421 1.2844 1.3126

0.0393 0.0227 0.0093 0.0050 0.0050

3.0 1.7 0.75 0.39 0.38

33.1 57.2 133.5 256.7 261.4


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Inorganic Chemistry mmol of UO22+ per gram of sample. The presence of europium in these samples is also exposed, the incorporation of which exists anywhere from 0.0393−0.0050 mmol per gram of sample. This amount corresponds to 3.0−0.38% of the uranium content across the series, so the inability to determine a crystallographically independent site for europium from the SC-XRD experiments is reasonable. When reviewing the inverse content ratio of this series of samples (i.e., the ratio of uranium compared with europium), these values increase from 33.1 to 261.4%. ICP-AES analysis shows that the enrichment of Eu into the interior of UO2L(DMF) relates to the solvent ratio (VH2O/ VDMF), in that DMF provides more Eu to the structure, but water does the opposite. Given that DMF acts as both a solvent and a ligand source in the formation of UO2L(DMF), which may originally bind to Eu3+, it is reasonable that more DMF would aid in bringing more Eu3+ into the structure during the in situ formation process. However, this observation leaves the question: where is europium in these crystals? Three potential possibilities are proposed: first, Eu is deposited on the surface of the crystals; second, Eu resides in the interstitial open space between adjacent uranium−organic chains as hydrated Eu3+ cations; or third, Eu dopes onto U sites. Elemental Mapping and LA-ICP-MS Analysis. The results above confirm that Eu3+ is incorporated into these crystal structures in varying concentrations; however, the distribution of Eu3+ within these crystals also deserves further study. Two crystals of 1 were chosen for analysis with LA-ICPMS on the basis of their high crystallinity as well as their high Eu content. As shown in Figure 2, elemental mapping analysis

Figure 3. Demonstration of the detectable ion-count dependence of the laser-ablation time in 1.

differences in formal charge and coordination geometry. We therefore propose that Eu3+ most likely resides in the interstitial space of 1. As shown in Figure 4b, the diameter

Figure 4. (a) Crystal structure of 1. (b) Potential position of Eu3+ in the structure. Figure 2. SEM image and EDX elemental maps of 1. (a) Crystalline SEM image. (b−f) Elemental maps of C (b, red), N (c, green), O (d, yellow), U (e, blue), and Eu (f, dark red).

of the largest open cavity in the structure is about 4.9 Å, perfectly matching a hydrated Eu3+ ion.16 Therefore, this cavity is most likely the position where the Eu3+ ions are located. Optical Analysis. The photoluminescence spectra of the isolated single-crystal products 1−10 measured under λEx = 365 nm at room temperature expose optical behaviors atypical of other uranyl−organic CPs. Interestingly, as shown in Figure 5, we found that the characteristic photoluminescenceemission bands of the uranyl(VI) and europium(III) ions coexist in the spectra measured from 450 to 750 nm. In this first instance of a luminescence-mixed uranyl−europium CP, we continued by characterizing the emission features of this

reveals that Eu3+ is uniformly distributed within 1. Figure 3 illustrates the ion-count dependence of the laser-ablation time in 1, and the signals of 238U and 152Eu are detected at the same time. Furthermore, the LA-ICP-MS analysis reveals that Eu3+ is concomitant with UO 22+ , displaying discernible depth distribution within 1 and excluding the possibility of the presence of Eu3+ on the crystal surface. The substitution of uranyl sites by Eu3+ is also very unlikely given their obvious D


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Inorganic Chemistry

Figure 7. CIE chromaticity diagram of 1, 3, 5, 7, 8, and 10.

of samples. This corresponds well with the ICP-AES data presented in Table 2. As the content of Eu3+ decreases in these samples, the emission bands corresponding to Eu3+ decrease, as would be expected. Concurrent with this phenomenon is the increase in intensity from the emission bands of UO22+ in this system. In general, the emission from europium is easily affected by coordinating species. For instance, coordination by water molecules especially quenches the emission signatures of europium in materials.19 However, according to these characterization and quantification techniques, in our materials, the concentration of europium in the solid matrix should play a key role by affecting the intensity of emission. That is, the ability to tune the color of emission in this series originates from the varied europium content that is incorporated in these uranyl−organic CPs. In addition, with such a small concentration of Eu3+ in this system (at 3.0%), the emission of uranyl is gradually quenched, indicating efficient energy transfer between the two species in this 4f/5f CP. In pursuit of an explanation of the energy-transfer mechanism, photoluminescence-decay analysis is employed. Investigation of the Energy-Transfer Mechanism. The photoluminescence and ICP-AES data reveal that the luminescence of these compounds is highly dependent on the Eu content and can be fine-tuned anywhere from bright red to bright green with decreasing Eu content. Therefore, we theorize that there exists an efficient energy-transfer mechanism via which absorbed energy from UO22+ is nonradiatively transferred to excite nearby Eu3+, thereby reducing the intensity of the radiative emission from UO22+. In order to elucidate the energy-transfer process in this series, photoluminescence-decay profiles were investigated on samples of 1, 4, 7, and 10 using λEx = 365 nm and focusing on emission at 515 nm for the UO22+ species and 616 nm for the Eu3+ species. The lifetime values for the emission from UO22+ decreases from 69.2 to 46.3 μs with increasing Eu content (Figure 8a), whereas the lifetime values of Eu3+ increase from 1.3 to 4.1 ms (Figure 8b), which bolsters the existence of an energy-transfer process between UO22+ and Eu3+ in this 4f/5f CP.

Figure 5. Photoluminescence spectra (λEx = 365 nm) of 1, 3, 5, 7, 8, and 10 from 450 to 750 nm at room temperature normalized at 616 nm (a) and 514 nm (b).

system. The first set of five emission bands observed at 472, 492, 513, 537, and 562 nm belong to the UO22+ species, which are considered to be characteristic of the vibronic progression corresponding to the S11 → S00 and S10 → S0ν (ν = 0−3) electronic transitions, respectively(Figure 5a).17 Additionally, the luminescent bands typical of Eu3+ appear at 592, 616, 653, and 700 nm via a ligand-to-metal energy-transfer mechanism, which correspond to transitions from the 5D0 state to the 7FJ (J = 1−4) levels,18 respectively (Figure 5b). Although we cannot identify the specific position of Eu3+ in 1 using crystallographic data, the presence of Eu3+ can also be supported by the luminescence spectra. Photoluminescence spectra and photographs and a CIE chromaticity diagram of 1, 3, 5, 7, 8, and 10 are shown in Figures 5, 6, and 7, respectively. Notably, the photoluminescence photographs of these crystals show a gradual change in color from bright red to yellow to bright green, which is more easily observed in the CIE chromaticity diagram. Additionally, the intensity of the emission associated with the vibronic progression of UO22+ increases inversely compared with the intensity of the emission from europium in this series

Figure 6. Photoluminescence photographs of 1, 3, 5, 7, 8, and 10. E


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Inorganic Chemistry

first case of tunable 4f/5f bimodal emission in a mixed 4f/5felements-bearing metal−organic-hybrid material, which may find further applications as LED or sensor materials.

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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.7b02304. ICP-AES-sample preparation, PXRD patterns, UV−Vis absorption spectra, SEM-EDS analysis, IR spectra, and TGA data (PDF) Accession Codes

CCDC 1573219 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Yaxing Wang: 0000-0002-1842-339X Shuao Wang: 0000-0002-1526-1102 Author Contributions ⊥ 2+

3+

Figure 8. Photoluminescence-decay-time data of UO2 (a) and Eu (b) at emission signals of 515 and 616 nm, respectively, for 1, 4, 7, and 10 (λEx = 365 nm).

J.X. and Y.W. contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Foundation of China (21422704, 21790370, 21790374, and 21471107), the Science Challenge Project (JCKY2016212A504), a project funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), and the “Young Thousand Talented Program” in China.

The energy-transfer process in this system can be expounded upon,20 in that once excitation with a photon of 365 nm light occurs, the UO22+ moiety goes on to an excited state in which energy transmits from the triplet excited state (3πg) of UO22+ to the 5D1 energy level of Eu3+ through a nonradiative pathway. In the excited state (5D1), Eu3+ relaxes to the 5D0 excited state via a nonradiative pathway. Finally, electrons in this lowest excited state emit photoluminescence characteristic of Eu3+ as they transit to the 7FJ (J = 1−4) levels.

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REFERENCES

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CONCLUSION Uranium and lanthanides coexist in nuclear waste after uranium oxide (UO2) fuel rods are used in fission reactors, highlighting the importance of studies that focus on the chemical interactions of these elements that cohabitate in materials. In this work, a series of CP compounds containing UO22+ and varying quantities of Eu3+ were synthesized via solvothermal conditions. Though crystallographic evidence is insufficient to explain the existence of Eu3+ in this uranyl-based CP, photoluminescence, ICP-AES, and LA-ICP-MS analyses expose the presence of Eu3+ in these materials. Additionally, efficient energy transfer from UO22+ to Eu3+ was observed, in which an inverse correlation exists between the intensities of the emission features of either species based on the content of Eu in this structure. A suitable mechanism has been proposed for this energy-transfer process on the basis of the photoluminescence and lifetime data gathered. We believe this work provides a new strategy for making heterobimetallic lanthanide(III) and uranyl(VI) compounds and provides the F


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5f Bimodal Emission in Europium-Incorporated Uranyl

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