Article pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Speciation Change of Uranyl in Lithium Borate Glasses Myrtille O. J. Y. Hunault,*,† Gérald Lelong,‡ Laurent Cormier,‡ Laurence Galoisy,‡ Pier-Lorenzo Solari,† and Georges Calas‡ †
Synchrotron SOLEIL, L’Orme des Merisiers, Saint Aubin BP 48, 91192 Gif-sur-Yvette, France Sorbonne Université, Muséum National d’Histoire Naturelle, UMR CNRS 7590, IRD, Institut de Minéralogie, de Physique des Matériaux et Cosmochimie, IMPMC, 75005 Paris, France
Inorg. Chem. Downloaded from pubs.acs.org by UNIV OF LOUISIANA AT LAFAYETTE on 04/26/19. For personal use only.
‡
S Supporting Information *
ABSTRACT: Determining the uranyl(VI) UO22+ reactivity in crystalline and amorphous oxides is necessary to control its mobility. The intrinsic versatility of borate structural units containing both triangular BO3 and tetrahedral BO4 makes them original and rich hosts for uranyl. As part of the effort to determine the uranium stability in borate oxides, we have determined the speciation of uranium(VI) in two lithium borate glasses containing, respectively, 10 mol % and 30 mol % Li2O using a combined structural and spectroscopic approach based on X-ray absorption spectroscopy (XAS). M4- and L3-edge high-resolution XAS demonstrates the speciation of U(VI) as uranyl in both glasses. Comparison of uranyl bond distances obtained by EXAFS with distances found in borate crystals reveals that in the low alkali borate glass, uranyl is present as hexagonal bipyramids with six equatorial oxygen ligands. This local environment was never observed in any other oxide glass. We show that the increase of the lithium content induces the decrease of the equatorial coordination number. The associated uranyl bond elongation suggests the influence of the alkali cations in relation with drastic changes in the structure of the borate network. The spectroscopic evidence of this speciation change is discussed in terms of change in the uranyl electronic structure and covalency.
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INTRODUCTION
Hexavalent actinides can form highly covalent AnO22+ transdioxo linear species called actinyl. The chemical reactivity of these species is crucial in a number of applications including nuclear and medical domains, geochemistry, and environmental sciences.1,2 The covalency of the actinyl is governed by the localization of the 5f electrons and strongly depends on the nature of the equatorial ligands.3−7 As a model element for the actinide family, in addition to its considerable technological and environmental importance, uranium has received continuous attention for many decades. Under ambient atmosphere, hexavalent uranium, U(VI), dominates in both natural and synthetic systems, mostly as uranyl, UO22+. The linear uranyl cation has two short axial oxygen bonds and forms bipyramids by bonding with 4, 5, or 6 equatorial oxygens Oeq. The reactivity in the equatorial plane explains the association of uranyl groups with other oxoligands, leading to a large number of crystalline structures.8 Crystalline alkali borates show a unique structural versatility based on the coordination change of boron: BO3 and BO4 units, present in various ratios. Earlier work on crystalline uranium borates has revealed only layered structures.9−11 Uranium borates have recently received a renewed interest because of the possibility to build original 3D structures,12 including zeolite-like frameworks,13 for potential application in immobilization of trans-uranium elements. Figure 1 presents the histogram of bond distance occurrences in 21 uranium © XXXX American Chemical Society
Figure 1. U−O bond distance distribution in alkali borate uranyl crystalline compounds depending on the number of Oeq. Vertical dashed lines show the distances obtained by EXAFS fitting for BUL10 and BUL30.
borate and alkali borate crystal structures reported in the literature (see the Supporting Information, Table S1) according to the type of uranium polyhedron found: UO6, UO7, or UO8. We observe that for both UO7 and UO8 Received: January 31, 2019
A
DOI: 10.1021/acs.inorgchem.9b00305 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry
GeV. X-ray absorption near edge structure (XANES) and extended Xray absorption fine structure (EXAFS) were measured at the U L3edge using a double-crystal monochromator (DCM) with a pair of Si(220) crystals. Rejection of higher harmonics as well as vertical focusing was achieved using two platinum-coated mirrors under total reflection at 3.1 mrad, inserted before and after the DCM. The incident energy was calibrated using a metallic yttrium foil (K-edge at 17.038 keV). The incident beam flux was 1.7 × 1011 ph/s. The beam size on sample was 350 μm × 150 μm fwhm (H × V). Transmission XANES and EXAFS were measured in normal incidence using photodiodes. The high-energy resolution fluorescence-detection (HERFD) mode of XAS (also alternatively referred to as highresolution XANES, HR-XANES22) was measured using a crystalanalyzer X-ray spectrometer in the Rowland geometry and a KETEK single element silicium solid state detector. The samples were oriented at 45° with respect to the incident beam. HERFD-XANES at the U L3-edge, was measured using the 880 reflection of four bent Si(220) crystal analyzers with a curvature radius of 0.5 m to analyze the U Lα1 emission line (2p3/23d5/2, 13.615 keV). The total energy resolution for the HERFD-XANES measurement at the U L3-edge was 5 eV as determined from the fwhm of the elastic scattering peak at 17.038 keV. The spectra were normalized to a edge-jump of 1. HERFD-XANES was measured at the U M4-edge (3.728 keV) using the DCM with a pair of Si(111) crystals. Higher harmonics rejection and vertical focusing was achieved using the Si strip of the each mirror inserted before and after the DCM with a 4 mrad incidence angle. The incident energy was calibrated using the absorption K-edge of potassium in a KBr pellet (3.6 keV). The incident X-ray flux on the sample position was 1.9 × 109 ph/s at 3.5 keV. The Mβ emission line (3.339 keV) was analyzed using the 220 reflection of a Si(220) bent diced crystal analyzer with a curvature radius of 1 m. A He-filled chamber was used to reduce the scattering of the emitted X-rays by the air between the sample and the crystal analyzer and the detector. The overall energy resolution of the emission spectrometer was 1.6 eV as determined from the fwhm of the elastic scattering peak at the double energy of 6.676 keV. The spectra were normalized to the maximum of the white-line. Uranyl(VI) nitrate was used as a reference for HERFD-XANES and EXAFS analysis. It was prepared as a pellet and diluted in cellulose. XANES/EXAFS spectra normalization, data extraction, and EXAFS fitting were performed using the LARCH software.23 Radial structure functions were obtained by Fourier transforming k2-weighted EXAFS χ(k) functions between 3.9 and 20 Å−1 using a Kaiser−Bessel window function with a parameter of 3. A multishell approach was employed for data fitting based on the theoretical single and multiple scattering paths (SS and MS paths, respectively) obtained using FEFF8.5 and the structure of LiUO2BO3.24,25 We used SS paths for U−Oax, for U− Oeq, and U−B. One MS path was used for the U−O1−U−O2 (four legged axial path). The S02 factor was determined by fitting the EXAFS signal of the crystalline reference compound uranyl nitrate (see the Supporting Information, Figure S2 and Table S2) and fixed to 0.7 for the fitting of the glass EXAFS data. The shift in the threshold energy, ΔE0, was allowed to vary as a global parameter for all atoms in one fit. All measurements were performed at room temperature. Because the static disorder in glass is significantly higher than the thermal disorder, the use of low-temperature measurements is not expected to provide much improvement on the signal-to-noise ratio at high k-values in the EXAFS.26
polyhedra, the bond distances are separated in two groups: bonds shorter than 1.9 Å and bonds longer than 2.1 Å, which correspond, respectively, to the U−Oax axial bond and U−Oeq equatorial bond, hence confirming the uranyl speciation in all compounds. It is worth noting that while uranyl compounds are in general dominated by UO7 pentagonal bipyramids,8 uranyl borates show a large fraction of UO8 hexagonal bipyramids.14,15 This is assigned to the small size of the BO3 or BO4 units as also observed in nitrate and carbonate compounds. The intrinsic versatility of borate structural units containing both triangular BO3 and tetrahedral BO4 makes them original and rich hosts for uranyl. Alkali borate glasses offer a unique opportunity to determine the reactivity of uranyl in borate oxide structures: the BO3/BO4 ratio depends on the alkali concentration, which changes the structure from a bidimensional network dominated by boroxol ring planes to a tridimensional network dominated by superstructural units.16 The present study aims at using the flexibility of the glass network to explore the reactivity of the uranium (uranyl) with the borate network depending on the alkali cation field strength and the BO3/BO4 ratio. Transition elements (d- and f-) form localized electronic states in the gap of oxide glasses, giving rise to the peculiar optical and magnetic properties17 of most colored glasses. The composition dependent structural changes in borate glasses induce subsequent drastic changes in the speciation of divalent transition metal ions.18 Earlier optical absorption study suggested the modification of the uranyl from regular to distorted in the borate glass upon the increase of the sodium content.19 Such an extreme modification of U(VI) speciation as a function of glass chemistry is unknown in the other types of oxide glasses. In this study, we compare the speciation of uranium in two binary lithium borate glasses, 10Li2O−90B2O3 and 30Li2O− 70B2O3 denoted hereinafter BUL10 and BUL30, respectively, and containing 2 wt % uranium, using X-ray absorption spectroscopy (XAS) and information from the known crystal structures. This study provides the first quantitative structural information about the uranium local environment in borate glasses.
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EXPERIMENTAL SECTION
Synthesis. Lithium borate glasses with the composition xLi2O· (100 − x)B2O3 were synthesized with x equal to 10 or 30 mol % of alkali and will be referred to as BUL10 and BUL30, respectively. Glass samples were prepared from appropriate amounts of dried Li2CO3 and H3BO3 and U3O8 as a source of purified uranium 238. First the boric acid and alkali carbonate reagents were finely ground and melted in a Pt crucible at 1200 °C for 30 min, then quenched, and the resulting glass was again finely ground. The obtained glass was melted again together with 2 wt % of U3O8 at 1000 °C during 30 min. The glass was quenched, finely ground, and melted again for 30 min at 1000 °C in order to improve glass homogeneity. The melt was poured and quenched between two copper plates forming glass pellets of ∼3 mm thickness and annealed for 2 h at 50 °C below the glass transition temperature. BUL10 glass was bright light green while BUL30 glass was dark yellow (see Figure S1). The bulk samples were used to avoid the hydration of the glass with the air humidity. Samples were confined using polyimide film in a specific sample holder for X-ray absorption analyses. X-ray Absorption Spectroscopy. X-ray absorption spectra (XAS) were measured at the MARS beamline at the SOLEIL synchrotron (Saint-Aubin, France).20,21 The storage ring was operating in top-up mode at an electron current of 500 mA, 2.5
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RESULTS AND DISCUSSION Oxidation State and Electronic Delocalization. Since uranium is a multivalent ion, it is first important to determine its oxidation state in the glass. This can be done by probing the empty 5f shell using U M4-edge XAS corresponding to the dipole absorption transition from the core−shell 3d3/2. It has been demonstrated that uranium M4-edge HERFD-XANES, obtained by measuring the Mβ emission (4f5/2 to 3d3/2 decay) and by doing so reducing the effect of the core-hole B
DOI: 10.1021/acs.inorgchem.9b00305 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry broadening, allows unambiguously determining the oxidation state of U ions as compared to U L3-edge XANES that probes the 6d empty states.27−30 Comparison of the energy position of the first peak of the U M4-edge HERFD-XANES (Figure 2a) of the two glasses with
Furthermore, we observe the shift of the 5fδ/ϕu peak to higher energy and the 5fσu* peak to lower energy from BUL10 to BUL30. Similar variations were reported for UO 3 polymorphs and CaU2O7 in comparison with metaschoepite.38 The variation of uranyl 5f orbitals suggested by U M4-edge HERFD-XANES is consistent with the observed different colors of BUL10 and BUL30 (see Figure S1).19,39 While the former is similar to the color of uranyl nitrate, supporting a similar uranyl bonding and hence local structure, the color of the latter suggests structural changes in the uranyl polyhedrons. Uranium Local Environment. The U L3-edge HERFDXANES probes the empty 6d final states. Figure 3 shows the U
Figure 3. U L3-edge HERFD-XANES of BUL10 (green) and BUL30 (red) compared to uranyl nitrate (black dotted line). The shift of the energy position of features (A) and (B) is discussed in the main text.
Figure 2. U M4-edge HERFD-XANES (a) in BUL10 (green), BUL30 (red), and uranyl nitrate (black dotted line) showing the three features assigned according to the molecular orbitals of the uranyl cation (b).
L3-edge HERFD-XANES spectra of BUL10 and BUL30 compared to uranyl nitrate. The spectra are better resolved than standard transmission XANES spectra (see the Supporting Information, Figure S3) as expected from the use of the emission spectrometer that reduces the effect of the core-hole broadening. The energy position of the white line and of the inflection point of the HERFD-XANES spectrum of BUL10 at 17176.2 and 17173 eV, respectively, are the same as observed for the uranyl nitrate. These values are consistent with HERFD-XANES data recorded on the same beamline for U(VI) coordination complexes.28 The position of the white line and of the inflection point of the HERFD-XANES spectrum of BUL30 at 17176.8 and 17173 eV, respectively, are close to those measured for BUL10 confirming similar speciation of uranium than in BUL10 as U(VI). The shoulder (A) at 17190 eV in the XANES is assigned to multiplescattering (MS) from the uranyl linear structure.34,36,40 Altogether, U L3-edge HERFD-XANES confirms the presence of uranium as uranyl in both BUL10 and BUL30 in agreement with U M4-edge HERFD-XANES. The U L3-edge HERFD-XANES spectrum of BUL10 (Figure 3) almost perfectly overlaps with the spectrum of uranyl nitrate, suggesting similar U 6d final state electronic structure and thus similar uranyl local structure. The spectrum of BUL30 shows the shift of the feature (A) toward lower energies. According to the inverse relation between the position of the MS resonances (A) and bond lengths,40 this suggests the increase of the uranyl U−Oax bond distance in BUL30 compared to BUL10 and uranyl nitrate. The second feature (B) observed around 17210 eV is consistent with
the reference uranyl(VI) nitrate reveals that the synthesis in air conditions leads to the dominant hexavalent oxidation state. Reduced species such as U(V) and U(IV) are nondetectable and their contribution is neglected in the following. The obtained U M4-edge HERFD-XANES spectra show three main features assigned to the splitting of the 5f orbitals. In the isolated uranyl linear cation, described in the D∞h point group, the 5f orbitals interact with the oxygen 2p orbitals and split into the nonbonding 5fδ/ϕu, and the antibonding molecular orbitals (MOs) with 5fπu* and 5fσu* character. Although the ordering of the antibonding orbitals is still debated,5 several authors have used DFT calculations and obtained the order 5fπ* < 5fσ*.4,31−33 A simplified picture of the 5f MOs scheme based on these results is given in Figure 2b. From this scheme, the three features observed in the M4edge HERFD-XANES spectra have been assigned to transitions to the nonbonding 5fδ/ϕu and the antibonding 5fπu* and 5fσu* states in that order.22,32,33 In particular, the high energy feature 5fσ* at 3.734 keV is assigned to the oxygen−uranium charge transfer induced by the strong σ overlap along the trans-dioxo bond.22,27,34 As a consequence, the M4-edge HERFD-XANES spectra reveal that uranium(VI) is present as uranyl in both glasses. Uranium speciation in BUL10 and BUL30 as uranyl agrees with the former UV−vis spectroscopy results obtained on sodium borate glasses.19 Uranyl was previously reported in other silicate glasses from optical or X-ray spectroscopy studies.35−37 C
DOI: 10.1021/acs.inorgchem.9b00305 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry previous results obtained on uranyl compounds.40 Although Farges et al. have related the energy position of the feature (B) to the U−Oeq bond distance (inverse relation), Allen et al. have shown based on FEFF calculations that the U−Oeq distance has little effect on the energy position of feature (B) and that the scattering from the axial oxygen is the dominating effect.40 However, the shift of the energy position of feature (B) observed is opposite to the shift of the energy position of feature (A). Assuming both shifts are inversely correlated to bond distances, both cannot be related to the change of the U−Oax bond distance alone. Unfortunately, only a few studies report the cross-analysis of L3-edge XANES and EXAFS. As will be shown in the EXAFS analysis reported below, the present study supports the interpretation of Farges et al. that the shift of feature (B) correlates with the decrease of the U−Oeq bond distance in BUL30. EXAFS was used to determine quantitative structural information on the local environment of uranyl in these two borate glasses. U L3-edge EXAFS oscillations were measured in BUL10 and BUL30 up to k = 22 Å−1 as presented in Figure 4a
and show significant differences. This is further enhanced by the Fourier transforms (FT) in Figure 4b showing two clearly distinct nearest neighbors for BUL10, while in the case of BUL30 we observed several superimposed contributions. We note that the EXAFS signal contains a small contribution from the double-excitation around k = 11 Å−1 assigned to L3N6,7 shake up effect (star in Figure 4a), which results in contributions at R < 1.2 Å in the FT signal.41 Being aware of this contribution and since this distance is lower than any expected contributions from uranium neighbors, we chose not to modify the EXAFS data and to keep the double excitation. Comparison of the EXAFS signal and FT data of BUL10 with uranyl nitrate reveals high similarities and supports the similar speciation of U(VI) as uranyl in agreement with the M4-edge HERFD-XANES results. The short distance in the FT-EXAFS signal is assigned to the U−Oax shell and the second distance is assigned to the U−Oeq shell. These two contributions give rise to typical interferences in the EXAFS signal around k = 7 Å−1 (arrow in Figure 4a). In contrast, in the case of BUL30, similar interferences are not observed in the EXAFS and confirms that the local environment of the uranyl is different than in BUL10. This is clearly evidenced by the filtered EXAFS data in Figure 4c. In order to obtain quantitative information on the local structure of uranyl species in the glasses, further analyses based on the EXAFS fitting are required. Uranyl Structure in BUL10. We used a bottom-up strategy for a shell by shell fitting of the EXAFS data starting from the first oxygen shells to obtain bond distances. In a first step we have floated coordination numbers. For BUL10, we obtained a first distance to the axial oxygens at d(U−Oax) = 1.76 Å and a second distance for the equatorial oxygens at d(U−Oeq) = 2.50 Å. These distances are typical of uranyl polyhedron and thus confirm the speciation as uranyl. The linear bonds formed by the axial oxygens give rise to a multiple scattering contribution at twice d(U−Oax) = 3.6 Å.42,43 Comparison of these distances with those observed in uranyl borate crystals (see vertical dashed green lines on Figure 1) confirms the uranyl speciation and agrees with bond distances typical for UO8. This is the first evidence of an equatorial coordination number as high as 6 for uranyl in glasses. The obtained coordination number for the equatorial shell was close to 6 (see Table S3). Because coordination numbers derived from EXAFS suffer from large errors and are further correlated to the disorder described by the Debye−Waller factor, we decided to fix the coordination numbers in agreement with the bond distances: the number of the uranyl axial oxygen is fixed to 2 and the number of equatorial oxygens is fixed to 6. Further variations of the EXAFS amplitude is only captured by the fitting of the Debye−Waller factor, which in this case will describe mainly the structural disorder around uranium. All results are compiled in Table 1. Figure 5 shows the fitted EXAFS signal compared to the experimental data. The main FT-EXAFS contributions (Figure 5a) and the EXAFS oscillations (Figure 5b) are reproduced. In BUL10, the Debye−Waller factor of the axial oxygens was σ2(Oax) = 1.6 × 10−3 Å2. This relatively low value agrees with the regular and rigid linear bonds of the uranyl species.44 The Debye−Waller factor of the equatorial oxygens in BUL10 is σ2(Oeq) = 7.9 × 10−3 Å2. This value is larger than σ2(Oax) and reflects the disorder of the equatorial coordination shell of the uranyl in the glass. In crystals, as revealed in the histogram in Figure 1, the distribution of equatorial oxygen distances for UO8 polyhedron is spread into two groups: a short and a long
Figure 4. (a) EXAFS signal obtained at the U L3-edge for BUL10 (green), BUL30 (red), and uranyl nitrate (dotted black). The doubleexcitation occurring around k = 11 Å−1 is indicated by a star.41 The Fourier transform window is shown. (b) Fourier transforms of the EXAFS signals for BUL10, BUL30, and uranyl nitrate. (c) Filtered EXAFS signal corresponding to the first shells of O as selected with the Fourier transform window shown in part b. D
DOI: 10.1021/acs.inorgchem.9b00305 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry Table 1. Structural Parameters Obtained by Fitting the EXAFS Spectra of BUL10 and BUL30a sample
shell
BUL10
Oax Oeq B Oax MS Oax Oeq Oeq B Oax MS
BUL30
N 2 6 2 2 2 5 1 2 2
(fixed) (fixed) (fixed) (fixed) (fixed) (fixed) (fixed) (fixed) (fixed)
R (Å)
σ2 (10−3 Å2)
ΔE0 (eV)
R-factor
± ± ± ± ± ± ± ± ±
1.3 ± 0.2 8.2 ± 0.9 10.5 ± 12.5 b 2.3 ± 0.2 11.5 ± 0.9 4.3 ± 1.8 6.7 ± 3.5 b
3.0 ± 1.3 c c c 1.0 ± 1.6 c c c c
0.05
1.76 2.5 3.08 3.61 1.81 2.28 2.53 2.91 3.61
0.004 0.01 0.10 0.02 0.002 0.008 0.01 0.03 0.004
0.03
a
MS: Multiple-Scattering bFixed to the same as for the single scattering U−Oax path. cFixed value for all paths.
Figure 5. (a) Magnitude and imaginary part of Fourier transforms of the fitted and experimental EXAFS signals for BUL10 (top) and BUL30 (bottom). The contributions of the various shells are assigned with arrows and discussed in the main text. (b) EXAFS signal fit compared to experimental data for BUL10 (top) and BUL30 (bottom). The stars indicate the double-excitation.
UO7 species. Fixing the coordination number of 5 for the first U−Oeq shell, the obtained Debye−Waller factor σ2(Oeq) = 11.8 × 10−3 Å2 reveals a higher static disorder of the equatorial coordination shell than in BUL10. This suggests the increase of the disorder in BUL30 and agrees again with the trend observed in crystals where, contrary to the two narrow d(U− Oeq) distributions observed for UO8 units in crystals (Figure 1), the distribution observed for UO7 units is broader. According to Figure 1, the second U−Oeq distance is closer to typical long bonds found in UO8 species. The fact that we can observe this clear contribution suggests that it arises from a well-defined uranyl equatorial shell and not from a distorted uranyl polyhedron. Thus, this suggests the presence of UO8 species together with UO7 in the BUL30 glass. The corresponding U−Oax contribution could not be disentangled from the U−Oax of the UO7 dominant contribution. The distances obtained for BUL30, as well as the sample color, are similar to the distances found in albite silicate glasses.36 However, the equatorial coordination number was found to be as low as four in albite. We believe that this value might have been underestimated from intrinsic errors in the coordination number fit. Indeed, uranyl pentagonal bipyramids are more often observed in uranyl silicate crystals, such as soddyite and weeksite for instance,8 where UO7 forms chains.
distance, as also observed for uranium nitrate (see Table S2 in the Supporting Information). In the case of the glass BUL10, it was not possible to distinguish between the two bond distances and the obtained value is thus an average. This supports the higher static disorder in glass as compared to crystals.3,44 Uranyl Structure in BUL30. In the case of BUL30, the EXAFS fitting reveals a complex first shell of oxygens: a first short distance at d(U−Oax) = 1.81 Å assigned to axial oxygens and associated to a multiple scattering contribution at 3.61 Å and a longer distance at d(U−Oeq) = 2.28 Å assigned to equatorial oxygens. However, this is not all, since we observed another contribution in the FT-EXAFS around 2.3 Å (uncorrected from phase shift). The fitting of this contribution using a simple scattering path gives a d(U−Oeq) = 2.53 Å. Comparison with the bond distances observed in crystals (see vertical dashed red lines on Figure 1) confirms the uranyl speciation and the axial bond length is increased compared to BUL10. With a coordination number set to 2, the Debye− Waller factor increases to σ2(Oax) = 2.3 × 10−3 Å2, suggesting a less rigid trans-dioxo bond in agreement with the longer U− Oax bond length. Although the equatorial oxygen coordination number obtained by fitting is close to 7 (see Table S4), the second bond distance is much shorter than in BUL10 and agrees with the typical equatorial bond length corresponding to E
DOI: 10.1021/acs.inorgchem.9b00305 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry This would support the presence of UO7 species in silicate glasses similarly to BUL30 glass. The evidence of two coexisting uranyl environments in BUL30 is similar to the case of the waste storage SON68 borosilicate glass, where two uranium(VI) species have been suggested from time-resolved photoluminescence spectroscopy.37 Assuming that the UO8 species remaining in BUL30 give a similar M4-edge XAS signature to BUL10, we estimated from the M4-edge data that the uranyl species in BUL30 could contain up to 40% of UO8 (see the Supporting Information, Figure S4). The decrease of the coordination number of uranyl in BUL glasses agrees with the results reported for divalent transition metal ions, in which coordination number decreases from 6 to 5 and 4 upon increasing the alkali content in binary borate glasses.18,19,45 The partial conversion observed for lithium cations agrees with the intermediate speciation reported for divalent nickel and cobalt ions.18,45 Uranyl Electronic Structure Change. At the stage of the interpretations and discussions of the results, it is interesting to highlight what we can further understand from spectroscopy. M4-edge HERFD-XANES, L3-edge HERFD-XANES, and EXAFS altogether demonstrated the speciation of uranium(VI) as uranyl and the change of the equatorial coordination number. Structural results obtained by EXAFS can now help us to further interpret the shifts observed in energy position of the three features in the M4-edge HERFD-XANES spectra, assigned to 5f states. We can now correlate the increase of the uranyl U−Oax bond distance with the shift to lower energy of the 5fσu* peak. As the energy of the nonbonding 5fδ/ϕu states do not change, this suggests the stabilization of the 5fσu* assigned to U−O charge transfer. Stabilization may be the result of the decrease of the σ overlap correlated to the bond elongation. This picture would agree with the one proposed by Vitova et al.,22 who have suggested that the energy difference between the 5fδ/ϕu peak and the 5fσu* peak is a qualitative measurement of the overlap-driven covalency of the uranyl bond. Altogether, it suggests the decrease of the uranyl U−Oax bond covalency with increasing the U−Oax bond distance and decreasing the number of equatorial ligands. The shortening of the U−Oeq bonds could be related to the stronger interaction in the equatorial plane and would support the increase of the electronic delocalization of the uranyl 5f orbitals in the equatorial plane. Further work based on the calculation of the electronic structure and population analysis of the uranyl would help confirming these interpretations. Relation with the Glass Network. In the BUL10 glass, between the U−Oeq contribution and the MS contribution in the FT-EXAFS (Figure 5a), we observe another weak contribution at ∼2.8 Å (uncorrected from phase-shift). According to crystalline uranyl borates (Figure 1), such a large distance could arise from the boron neighbors of borate units sharing an edge with the equatorial shell of the UO8 polyhedron as illustrated in Figure 6. Edge-sharing geometry is a reasonable condition to enable the observation in EXAFS as it creates a rigid connection and minimizes the static radial disorder. Neighboring carbon contributions were similarly observed in the EXAFS of uranyl carbonates.46 The fit of this contribution is difficult because of its weak contribution in the EXAFS signal. We have attempted it with a single scattering path U−B and obtained: d(U−B) = 3.08 Å (see the Supporting Information, Figure S5), which agrees with the typical U−B distances observed in crystals (see Figure 1). A coordination number fixed to 2 leads to a significantly high
Figure 6. Schematic representation of the local environment of uranyl in borate glasses and possible connectivity with neighboring boron atoms and uranyl groups (green, uranium; red, oxygen; gray, boron).
Debye−Waller factor suggesting relatively high disorder. Floating the coordination number during the fit leads to nonrealistic high values and highlights the difficulty of fitting the contribution from the third shell neighbors in glasses.47 This suggests that uranyl species are bound to borate units, which could be part of the glass network. It is not possible to distinguish between BO3 and BO4 units since both can be found in edge-sharing geometry and at similar U−B distances. In the FT-EXAFS (Figure 5a), we further observe contributions above 3.5 Å (uncorrected from phase-shift). From basic geometrical considerations, such distances may correspond to corner-sharing borate units or neighboring uranium from an edge-sharing uranyl polyhedron (see Figure 6). According to the larger back scattering amplitude from uranium compared to boron, the contribution from neighboring uranium should be dominating in the EXAFS signal. Further work is necessary to confirm the fitting and assignment of these contributions. In the case of BUL30 glass, the FT-EXAFS data do not show a clear contribution around 2.5−2.7 Å (uncorrected from phase shift); however, it seems that the three SS and the MS paths used for the fitting do not reproduce the FT signal at these distances well (see the Supporting Information, Figure S6). It is possible that additional contributions lead to partial cancellation of the EXAFS signal. Because this distance range corresponds to the U−B distances, we have attempted to add a fourth U−B contribution. The obtained fitted distance, 2.91 Å, with a coordination number fixed to 2 (floating the coordination number leads to similar values) leads to a better fit of the FT amplitude and imaginary part (Figure 5a) and supports that there are additional contributions from edgesharing neighboring borate units. Contributions to the EXAFS signal from third and fourth coordination shells are weak (see at 4.2 Å uncorrected from phase-shift, Figure 5a), yet they cannot be completely ruled out. A closer look at the uranyl environments in crystalline borate structures shows that there is no clear correlation between the fraction of BO4 units and the equatorial coordination of the uranyl (see Table S1). This suggests that the BO4/BO3 ratio alone does not control the speciation of uranyl via the oxygen electronegativity of the borate unit. This is an important difference with the general conclusions found for divalent transition metal ions.19,45 The comparison between UO2B2O4 and LiUO2BO3, the structure of which is based on chains of edge-sharing UO8 and UO7 units, respectively, while boron is only present as BO3, suggests that the alkalis may play a role in the alteration of the uranyl bond. This is in contrast with the often reported poor reactivity of the uranyl oxygens. If the F
DOI: 10.1021/acs.inorgchem.9b00305 Inorg. Chem. XXXX, XXX, XXX−XXX
Inorganic Chemistry
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BO4/BO3 ratio alone is not the controlling factor of uranyl speciation, the glass structure and the nature of the superstructural units may play a role by the free space available to the uranyl, in particular when changing from a bidimensional boroxol ring-based glass network to a tridimensional network.48 Altogether, this highlights the flexibility of the equatorial coordination shell of the uranyl to adapt to the glass network. Further work is underway to assess the influence of the nature of the alkali ions on uranyl speciation in borate glasses and connect it to the medium range structure of these glasses.
CONCLUSION The presence of uranium as uranyl in lithium borate glasses is demonstrated. The local structure of the uranyl polyhedrons is determined and we demonstrate the speciation as uranyl hexagonal bipyramids UO8 in the 10 mol % lithium borate glass and a combination of UO8 and UO7 pentagonal bipyramid in the 30 mol % lithium borate glass. The alkali content increase in the borate glasses induces the decrease of the equatorial coordination of the uranyl cation similarly to divalent transition metal cations. In both glasses, we found evidence of connection with edge-sharing borate units. These results obtained on borate glasses with simple chemical composition provide basic understanding for more complex glass systems, among which those used for nuclear waste storage. Our combined spectroscopic and crystallographic approach associates structural determination with probing the uranyl 5f and 6d states. This initial study opens the path to develop a fruitful experimental approach of major questions of crystal chemistry, such as the covalency of uranyls. Further work, supported by electronic structure calculations on crystalline compounds, would help assess the relationship between uranyl structure and electronic delocalization of the uranium 5f and 6d states. ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.9b00305. List of the uranyl borate crystalline structures and complementary information on sample characterization and data analysis (PDF)
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REFERENCES
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Myrtille O. J. Y. Hunault: 0000-0002-3754-8630 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank Kristina Kvashnina and Jean-Pascal Rueff for lending crystal analyzers and the SOLEIL synchrotron for providing beamtime. G
DOI: 10.1021/acs.inorgchem.9b00305 Inorg. Chem. XXXX, XXX, XXX−XXX
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DOI: 10.1021/acs.inorgchem.9b00305 Inorg. Chem. XXXX, XXX, XXX−XXX
