and M4-Edges To Determine the Uranium Valence State on

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Use of HERFD−XANES at the U L3- and M4‑Edges To Determine the Uranium Valence State on [Ni(H2O)4]3[U(OH,H2O)(UO2)8O12(OH)3] René Bès,*,†,‡ Murielle Rivenet,§ Pier-Lorenzo Solari,‡ Kristina O. Kvashnina,∥,⊥ Andreas C. Scheinost,∥,⊥ and Philippe M. Martin# †

Synchrotron SOLEIL, Ligne de lumière MARS, L’Orme des Merisiers, Saint Aubin, BP 48, F-91192 Gif-sur-Yvette Cedex, France CEA, DEN, DEC, Centre d’études nucléaires de Cadarache, F-13108 Saint Paul Lez Durance, France § Université Lille, CNRS, Centrale Lille, ENSCL, Université Artois, UMR 8181, UCCS, Unité de Catalyse et Chimie du Solide, F-59000 Lille, France ∥ Rossendorf Beamline (ROBL) at the European Synchrotron Radiation Facility (ESRF), CS40220, F38043 Grenoble, France ⊥ Helmholtz Zentrum Dresden-Rossendorf (HZDR), Institute of Resource Ecology, P.O. Box 510119, 01314 Dresden, Germany # CEA, DEN, DTEC, Centre d’études nucléaires de Marcoule, Bagnols-sur-Cèze F-30207, France ‡

ABSTRACT: We report and discuss here the unambiguous uranium valence state determination on the complex compound [Ni(H 2 O) 4 ] 3 [U(OH,H 2 O)(UO2)8O12(OH)3] by using high-energy-resolution fluorescence detection−Xray absorption near-edge structure spectroscopy (HERFD−XANES). The spectra at both U L3- and M4-edges confirm that all five nonequivalent U atoms are solely in the hexavalent form in this compound, as previously suggested by bondvalence-sum analysis and X-ray diffraction pattern refinement. Moreover, the presence of the preedge feature, due to the 2p3/2−5f quadrupole transition, has been observed in the U L3-edge HERFD−XANES spectrum, in agreement with theoretical and experimental observations of other uranium-based compounds. Recently, this feature has been proposed as a possible tool to determine the uranium oxidation state in a manner similar to that of 3d and 4d metals. Nevertheless, this feature is also very sensitive to the uranium local environment, as revealed by our theoretical calculations, and consequently could not be used to attribute without ambiguity the uranium valence state. In contrast, U M4-edge HERFD−XANES appears to be the most straightforward and reliable way to assess the uranium valence state in very complex materials such as [Ni(H2O)4]3[U(OH,H2O)(UO2)8O12(OH)3] or a mixture of compounds.

1. INTRODUCTION During the last decades, the chemistry and physics of actinides have been at the heart of a worldwide research effort because of their significance in nuclear applications. Because of the longterm storage of UO2 spent nuclear fuel, the behavior of mining waste in the environment, as well as catalysis applications,1,2 uranium is the most studied actinide. Moreover, it is also one of the more fundamentally complex elements because of its valence orbital configuration involving 5f electrons. Such complexity requires the development of experimental and theoretical approaches to increase our understanding of the structure and reactivity of such elements at an atomic scale. The structural and valence state evaluations are of prime importance because of their relevance to this aim. X-ray absorption spectroscopy (XAS) and, in particular, X-ray absorption nearedge structure (XANES) is a widely established method to determine the oxidation states of the elements. The strength of this technique is that no special sample preparation is required, which could alter the oxidation state, and that it is a bulk and nondestructive technique. Moreover, because of its sensitivity to the very local order, it can be used not only for crystalline © XXXX American Chemical Society

materials but also for liquids, gases, or amorphous matter. Nevertheless, in the case of actinide compound materials, one major drawback is the relatively long core−hole lifetime, which strongly affects the resolution of the XANES spectra and thus the valence state determination. In particular, in the case of samples with mixed valence states, the induced spectral broadening is then larger than the expected chemical shift due to a change in the valence state. This problem can be solved at least in part by using high-energy-resolution fluorescence detection−X-ray absorption near-edge structure spectroscopy (HERFD−XANES) because of its ability to collect better resolved spectra.3 In this paper, we aim to demonstrate and discuss the ability of this approach in the case of potential mixed-valence states in complex uraniumcontaining compounds. Uranium oxohydroxide hydrates, with the general formula [Uu(UO2)v(CO3)wOx(OH)y(H2O)z], appear to be excellent candidates for this evaluation because they meet all of the Received: January 4, 2016

A

DOI: 10.1021/acs.inorgchem.6b00014 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 1. Triclinic unit cell as previously solved by XRD and published in ref 10, which shows the layered structure of [Ni(H20)4]3[U(OH,H2O)(UO2)8O12(OH)3]. The five nonequivalent U atoms are also displayed. The figure was made using VESTA 3 software.11

Figure 2. Local environment and geometry around the five nonequivalent U atoms in the [Ni(H2O)4]3[U(OH,H2O)(UO2)8O12(OH)3] crystal structure.9. The figure was made using VESTA 3 software.11

easily complex with carbonate, sulfate, or phosphate ions and subsequently be readily transported in solution to the environment, whereas for reducing conditions, uranium is mainly tetravalent. Because of the low solubility of U4+, uranium under such a form precipitates and is far less mobile than U6+. Consequently, because of the differences in the chemical reactivity controlled by the valence state in a wide range of geochemical processes, perfect knowledge of the uranium valence states in uranium oxohydroxide hydrate compounds is of prime importance to enhance the safety of long-term disposal of spent nuclear fuel. To our knowledge, such an understanding remains an open question in many oxohydroxide hydrate compounds. In this paper, we will focus on only one of such compounds as a test case to demonstrate

foreseen conditions: high relevance, structural complexity with several nonequivalent U atoms in terms of local environment and structure, and a small amount of uranium content. Such compounds are produced during the oxidative alteration of the UO2 matrix or during its dissolution.4−8 Therefore, they play a predominant role in the mechanisms leading to the release of radionuclides from spent fuel into the environment. These types of minerals have mostly a layered structure, and the valence state of uranium can vary from U4+ to U6+. While the chemical toxicity of uranium depends on several environmental parameters, the formation of U6+ complexes highly soluble in groundwater is the most important one.9 Indeed, under oxidizing conditions, the U6+ uranyl ion, which is the most prevalent and stable form of uranium in the environment, can B


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Inorganic Chemistry

discriminate between the U5+ and U6+ valence states leads to no direct conclusion. By a comparison of the U(5) local environment with well-known pentavalent and hexavalent uranium containing oxides such as β-U3O812 and NaUO3,13 the main conclusion has been that [Ni(H2O)4]3[U(OH,H2O)(UO2)8O12(OH)3] should contain only hexavalent uranium U6+ and no U5+ or U4+. Up to now, no direct evidence has been reported in the literature, so such a conclusion remained an open question, especially for the U(5) atom. In this paper, we present the direct uranium valence state determination in [Ni(H2O)4]3[U(OH,H2O)(UO2)8O12(OH)3] obtained by XAS at the U L3- (17166 eV) and M4-edges (3728 eV). The standard XANES results as well as the HERFD− XANES ones are presented and discussed in the framework of their abilities to assess unambiguously the uranium valence state in such a complex model compound. Theoretical calculation results are also given to support the experimental results.

the ability of XAS, and especially HERFD−XAS, to provide these key data in complex uranium compounds. The crystal structure of the uranium and nickel oxohydroxide hydrate [Ni(H2O)4]3[U(OH,H2O)(UO2)8O12(OH)3] has been determined by using single-crystal X-ray diffraction (XRD).10 The reported triclinic unit cell is displayed in Figure 1. Its dimensions are a = 8.627(2) Å, b = 10.566(2) Å, c = 12.091(4) Å, α = 110.59(1)°, β = 102.96(2)°, and γ = 105.50(1)°. The [Ni(H2O)4]3[U(OH,H2O)(UO2)8O12(OH)3] layered framework is built from negatively charged sheets of [U(OH,H2O)(UO2)8O12(OH)3]6− and a positively charged interlayer of [Ni(H2O)4]2+. It contains five nonequivalent U atoms, as illustrated by Figure 2. One can also note that one unit cell consists of two U(1), U(2), U(3), and U(4) atoms and one U(5) atom for a total of nine U atoms. The U(1), U(2), and U(3) polyhedra consist in six O atoms and 1 OH group. They form a pentagonal bipyramid [UO2(OH)O4] around uranium. The U(4) atom is composed by six O atoms forming a square bipyramid [(UO2)O4]. In all of these four local environments, one can note the presence of uranyl short oxygen bonds. No uranyl bond is evidenced, however, for the U(5) polyhedron. Its local environment and geometry consist of four shortly bonded O atoms on the equatorial plane and two water or hydroxyl groups at longer distance. They form a distorted octahedron [UO4(OH,H2O)]. The mean U−O distances as well as the corresponding coordination numbers are reported in Table 1. In this table are also given the mean U−U distances and coordination number of the first metallic shell. The presence of uranyl-type bonding around U(1), U(2), U(3), and U(4) indicates unambiguously that these sites are occupied by hexavalent uranium, U6+. The bond-valence-sum analysis, as reported in ref 10, led to the same conclusion. Nevertheless, in the case of U(5), the absence of uranyl-type bonding and the fact that the bond valence sum could not

2. EXPERIMENTAL SECTION 2.1. Sample Synthesis and Preparation. The [Ni(H2O)4]3[U(OH,H2O)(UO2)8O12(OH)3] sample has been synthesized according to the method previously described.10 It mainly consists of a mild hydrothermal reaction of uranyl nitrate and nickel nitrate in a mixture of distilled water and oxalic acid, heated at 453 K for 7 days. Single crystals were collected and crushed into powder. The methodology developed here to determine the uranium valence states in [Ni(H2O)4]3[U(OH,H2O)(UO2)8O12(OH)3] is based on a comparison to well-known reference compounds: UO2 (pure U4+), U4O9 (mixed 50% U4+/50% U5+), U3O8 (mixed 33% U5+/67% U6+), UO2(NO3)2(H2O)6 (pure U6+), and β-UO3 (pure U6+). The UO2 powder was thermally treated for 1 day at 1973 K under argon and 5% H2 atmosphere in order to reach the correct stoichiometry. The U3O8 powder was obtained via isothermal annealing at 1443 K of a sintered UO2 pellet in dry air. The U4O9 powder was prepared by thermal treatment of a mixture of UO2 and U3O8 powders. In this case, the mass fractions of UO2 and U3O8 were chosen to obtain an average UO2.23 composition. The mixture was put in an airtight closed quartz tube and annealed at 1323 K for 30 days and then slowly cooled to room temperature over 12 h. Assuming that the U4O9 phase had an oxygen composition very close to the phase stability limit in the phase diagram, less than 1% U3O8 should remain in the U4O9 sample. The uranyl nitrate UO2(NO3)2(H2O)6 has been kindly supplied by AREVA NC, and the β-UO3 sample was obtained by calcinating uranyl nitrate in air at 723−773 K over several weeks as described in ref 14. The phase identity and purity of all references were confirmed by powder XRD. The obtained powders were individually mixed with boron nitride and pressed into 8-mm-diameter and 2-mm-width pellets. In order to limit the oxidation/hydration process during sample storage, the pellets were individually enclosed in a 8 μm kapton foil. 2.2. XAS. 2.2.1. U L3-Edge X-ray Absorption Fine Structure (EXAFS) Measurement. The U L3-edge EXAFS measurements were carried out at the ROBL, dedicated to actinide elements and located at BM20 of the ESRF in Grenoble, France. The storage-ring operating conditions were 6.0 GeV and 170−200 mA. The photon energy was scanned from 17000 to 18266 eV, using the Si(111) monochromator coupled to rhodium-coated mirrors for the collimation and reduction of higher harmonics. The sample was mounted in a closed-cycle helium cryostat running at 15 K in order to eliminate the thermal vibration contributing to the Debye−Waller factors of EXAFS spectra, which can then be considered as only influenced by structural disorder. EXAFS spectra were collected in transmission mode. The detected intensity was normalized to the incident photon flux, detected by using 30-cm-long ionization chambers running at 1200 V. Energy calibration was done using the K-edge excitation energy of a yttrium (17038 eV) metallic foil. The total energy resolution (incident energy and core− hole lifetime broadening) has been evaluated and is about 8.8 eV.

Table 1. Individual and Average Coordination Numbers and Distances (Å) of O and U First Neighbors on [Ni(H2O)4]3[U(OH,H2O)(UO2)8O12(OH)3] from Ref 10a XRD data U atom U(1)

U(2)

U(3)

U(4) U(5) average (per U atom)

O first-shell main characteristics

U first-shell main characteristics

2 O at 1.83(1) 2 O at 2.27(2) 3 O at 2.37(5) 2 O at 1.80(1) 2 O at 2.25(4) 3 O at 2.36(4) 2 O at 1.82(2) 2 O at 2.21(1) 3 O at 2.38(5) 2 O at 1.87(2) 4 O at 2.23(2) 4 O at 2.01(1) 2 O at 2.31(1) 1.8 O at 1.83(3)

2 U at 3.67(1) 1 U at 3.87(0)

2.7 O at 2.2(6) 2.2 O at 2.36(5)

2 U at 3.67(1) 1 U at 3.86(0) 1 U at 3.58(0) 2 U at 3.86(1) 2 U at 3.67(1) 2 U at 3.58(0) 1.8 U at 3.65(4) 0.9 U at 3.86(1)

a

The errors given here represent the standard deviation of the averaged distances. C


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Inorganic Chemistry

Figure 3. Simplified and schematic overview of the uranium theoretical valence configuration evolution during XAS (standard XAS and HERFD− XAS) for U4+, U5+, and U6+ valence states at the U L3- and M4-edges. Note that only the shells involved in the measured transitions are reported. The core shells are in brackets to distinguish them from the valence shells. ET is the transferred energy (final state energy), which corresponds to the difference between the incident and emitted energies. 2.2.2. U L3-Edge HERFD−XANES and Resonant Inelastic X-ray Scattering (RIXS) Measurement. The U L3-edge XANES measurements were carried out at the MARS beamline, dedicated to radioactive samples, located at the French synchrotron radiation facility SOLEIL in Saint Aubin, France,15,16 under top-up 430 mA storage ring mode (2.75 GeV). The photon energy was scanned from 17100 to 17300 eV, using the Si(220) sagittal focusing double-crystal monochromator (DCM). Rejection of higher harmonics as well as vertical collimation/focusing was achieved by two platinum-coated mirrors, placed before and after the DCM, working under total reflection at 3.1 mrad. Energy calibration was done using the K-edge excitation energy of a yttrium (17038 eV) metallic foil. The monochromatic beam was focused to 350 × 350 μm2. The XANES spectra have been simultaneously collected in transmission, total fluorescence yield (TFY), and HERFD modes at room temperature at the U L3-edge. The transmission signal was collected by in-house intensity monitors, as described in refs 15 and 16. TFY was recorded using a Vortex-90-EX silicon drift detector (SDD). The HERFD spectrum was collected by using an emission spectrometer16 equipped with two spherically bended Si(110) crystal and coupled to a Ketek AXAS-M SDD, all arranged in a vertical Rowland geometry. The spectrometer was tuned to the maximum of the U Lα1 (2p3/2−3d5/2, 13614 eV) X-ray emission line using the (880) reflection (analyzer crystals at a Bragg angle of ∼71.5°). The detected intensity was normalized to the incident photon flux. The total energy resolutions (incident convoluted with emitted energy and core−hole lifetime broadening) have been evaluated to 8.3 and 3.9 eV for XANES and HERFD−XANES, respectively.

2.2.3. U M4-edge HERFD−XANES Measurement. The U M4-edge HERFD−XANES measurement was performed at the ID26 beamline17 located at the ESRF. The U M4-edge (3725 eV) incident energy was selected using the Si(111) DCM. Rejection of higher harmonics was achieved by three silicon mirrors at 3.5 mrad working under total reflection. The beam size was estimated to be ∼0.2 mm vertically and 0.5 mm horizontally. HERFD−XANES spectra were measured using a X-ray emission spectrometer equipped with five Si(110) crystal analyzers and a silicon drift diode in a vertical Rowland geometry. The spectrometer was tuned to the maximum of the U Mβ (3d3/2−4f5/2, 3337 eV) X-ray emission line using the (220) reflection (analyzer crystals at a Bragg angle of ∼75.4°). The detected intensity was normalized to the incident flux. The total energy resolution (incident convoluted with emitted energy and core−hole lifetime broadening) has been evaluated to 0.7 eV. 2.2.4. Uranium Valence States As Seen by XAS. To help the reader with the different XAS approaches described along this paper, the uranium valence states as seen by XAS are discussed in the following lines. Figure 3 is a schematic representation of the main process that occurs during XAS in the case of U4+, U5+, and U6+ pure valence state compounds. One must note that only the shells involved in the measured transitions are reported in such a figure. The core shells are displayed in brackets to distinguish them from the valence shells. The X-ray absorption process, as in standard XAS, begins when an incident photon is absorbed by a given core electron (2p3/2 and 3d5/2 in the case of L3- and M4-edges, respectively) promoted to a previously empty or partially empty valence shell (6d and 5f for U L3- and M4edges, respectively). Such a process is represented by a vertical arrow in Figure 3 from the initial state to the intermediate state, which is the D


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Inorganic Chemistry

Figure 4. U L3-edge transmission XANES spectra and derivatives of the [Ni(H2O)4]3[U(OH,H2O)(UO2)8O12(OH)3] sample and references of UO2 (U4+), U4O9 (U4+/U5+), U3O8 (U5+/U6+), UO2(NO3)2(H2O)6 (U6+), and UO3 (U6+). All of these spectra have been recorded in transmission mode at ROBL (full symbols) and/or at MARS (open symbols). final state of the standard absorption process. Then, one possible relaxation path is the decay of a less bonded electron to fill the hole, leaving behind a new hole and emitting a characteristic X-ray photon. This is the X-ray emission line (Lα1 and Mβ for U L3- and M4-edges, respectively), which is detected with high-energy resolution in the case of HERFD−XAS. The energy difference between the final and initial states corresponds to the transferred energy to the system that remains an excited state. By an increase in the oxidation state of uranium from U4+ to U6+ (from left to right in Figure 3), a chemical shift of some electronvolts to higher energy of the empty or partially empty shells is expected because of a change in the effective nuclei charge, influencing the valence electron configuration. Such an effect is well observed at the U M4-edge, but it is contradicted at the U L3-edge, where the U5+ chemical shift is to lower energy relatively to the U4+ position. This phenomenon is usually explained by the fact that a shorter U−O bonding distance consists of higher mixing of the U 6d and O 2p orbitals in U5+/U6+ than in U4+, leading to a more covalent bond character.18 As a result, the U5+/U6+ 6d states are pulled to lower energy compared to the U4+ 6d states. In the case of a mixed-valence-states compound, all of these signals are superimposed on the collected spectra, and the difficulty is to well determine their relative contributions. This is usually performed by comparison to and linear combination of pure valence spectra from well-known compounds. However, the core−hole broadening remains a limiting factor to the sensibility of this approach, and one must find ways to reduce its effect. 2.2.5. Data Evaluation. The ATHENA software19,20 was used for normalizing XANES and EXAFS spectra from the raw absorption data. Preedge removal and normalization were achieved using linear functions. The energy threshold (E0) values and the white-line maximum energy values of each spectrum were chosen respectively as

the first inflection point and the first knot of the first derivative relatively to the incident energy. Self-absorption correction of the fluorescence spectra has been performed using the tools available on the ATHENA software. 2.2.6. Theoretical Calculations. The XANES theoretical calculations were performed using the finite difference method for nearedge structure (FDMNES) code.21 The crystal data used as input data were the ones previously proposed10 and listed in the Crystallographic Open Database (COD).22,23 Self-consistent-field calculations using the Dirac−Slater approach have been performed for all five nonequivalent U atoms in the [Ni(H2O)4]3[U(OH,H2O)(UO2)8O12(OH)3] structure. XANES simulations were performed using an atomic cluster of 6 Å. From the superposed self-consistent atomic densities in the considered cluster, the Poisson equation was solved to obtain the Coulomb potential. The energy-dependent exchange-correlation potential was evaluated using the local density approximation. The exchange-correlation potential was constructed using both the real Hedin−Lundquist and Von Barth formulations. These calculations were based on static atom supercells of hundreds of atoms, and thermally induced disorder was not considered. Because of the presence of heavy nuclei (U), spin−orbit effects were taken into account in the calculations. Nevertheless, no spin-polarization effect has been noticed from the comparison of spin-polarized spectra. Finally, calculations were performed with and without the quadrupole 2p3/2−5f transition probability in order to assess the 5f contribution on the final spectra at the U L3-edge. The EXAFS data-fitting process has been performed by using the ARTEMIS software in k3 space. The experimental EXAFS spectrum was Fourier-transformed using a Hanning window over the full k range available, i.e., 3.5−14.6 Å−1. The phases and amplitudes of the interatomic scattering paths were calculated with the ab initio code FEFF8.40. The previously published crystal structure has been used to E


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Inorganic Chemistry build the spherical 8 Å clusters of atoms. Three U−O single scattering paths at short, medium, and long radii and two U−U single scattering paths at short and long radii have been used in combination with one U−O−O multiple scattering path for the fitting process. The coordination numbers were fitted for each coordination shell. The shift in the threshold energy (ΔE0) was varied as a global parameter. The scattering factor S02 has been taken as equal to 0.9.

This tends to prove that the main uranium valence state of this compound is U6+, as previously mentioned. Linear combination fitting (LCF) using UO2, U4O9, and β-UO3 reference spectra on a −20 to +25 eV energy range around the edge energy point E0 gives relative weights of 0(7), 0(10), and 100(12)% of UO2, U4O9, and β-UO3 respectively. The deduced uranium valence state weights are 0(7), 0(5), and 100(17) for U4+, U5+, and U6+, respectively. However, because of the core−hole width at U L3-edge (8.2 eV), which leads to spectral broadening, a small amount of U5+ cannot be totally exclude at this point, especially because of the fact that the signal coming from the U(5) atoms represents only 1/9 of the total signal, which is the same order of magnitude as the errors of the LCF approach. In order to ensure that the local environment of the sample corresponds to the one deduced by XRD,10 the EXAFS part of the absorption spectrum has also been collected in transmission mode and is displayed in Figure 5.

3. RESULTS AND DISCUSSION 3.1. Standard U L3-Edge Experiments. The XANES spectra collected in transmission mode are reported in Figure 4. The XANES spectrum is composed of one intense peak, the so-called white line situated at 17177.8(5) eV, and an additional resonance at higher energy around 17189.5(5) eV. The inflection point E0 has been found at 17173.2(5) eV. On the left side of the white line, one can also observe a slight shoulder, responsible for the asymmetry of the white line. Such asymmetry has been reported by Soldatov et al.24 to a pure U5+ valence state in systematic sets of ternary uranium oxides NaUO3, KUO3, and RbUO3. Indeed, by a comparison with other ternary compounds such as BaUO3 (pure U4+), BaUO4 (pure U6+), and Ba2U2O7 (pure U5+), they have not observed such a shoulder on U4+ and U6+ compounds. They also stressed that, in order to get a clear understanding of the influence of the uranium oxidation state on the XANES spectral features, one must compare XANES from isostructural materials of very similar crystal structures. In our case, such similar compounds were not available, so a comparison has been performed, as displayed in Figure 4, to well-known compounds such as UO3 and UO2, as well as intermediate oxidized materials. In such a figure, one can observe that the shoulder feature is also present on U6+ compounds such as β-UO3. This confirms the fact that the presence of this feature is not sufficient to accurately attribute the uranium valence state in complex materials. By using standard methods to analyze the oxidation states, i.e., by studying the chemical shift of both the white-line and edge energy position point E0 given in Table 2, one can deduce that the [Ni(H2O)4]3[U(OH,H2O)(UO2)8O12(OH)3] compound is very close to the U6+ valence state rather than the lower oxidation state.

Figure 5. U L3-edge k3-weighted EXAFS data of [Ni(H2O)4]3[U(OH,H2O)(UO2)8O12(OH)3] (upper part) and the corresponding Fourier transform (lower part). The best-fit result is presented as a full red line. The main paths, calculated using the ab initio FEFF8.40 code from the XRD deduced structure, are also displayed.

Table 2. Uranium L3-Edge E0 and White-Line Position As Measured in Transmission Mode in ROBL and MARS Beamlines

This spectrum is characterized by one main peak localized around 1.5 Å. It corresponds to the contribution of the different O shells around each U atom. This peak is strongly asymmetric, in good agreement with the presence of short (about 1.9 Å), medium (about 2.2 Å), and long (about 2.4 Å) U−O distances. A good agreement between the experiment and fitted data confirms the validity of the structural model used in the present analysis. The crystallographic parameters derived from the EXAFS fitting are reported in Table 3. The experimental average U−O and U−U distances are similar to the theoretical distances. Moreover, the experimental averaged coordination numbers are also in line with the theoretical ones. The Debye−Waller values are consistent with the distance dispersions on the longer distances compared to the shorter ones. 3.2. High-Resolution U L3-Edge Experiments. RIXS data for the U L3-edge (2p3/2 → 6d dipole transition) are shown in Figure 6. It has been collected by scanning the incident energy (also called the excitation energy) across the U

U L3-Edge XANES (Transmission) compound

E0 position (eV)

white-line position (eV)

17173.3(5)

17177.8(5)

[Ni(H2O)4]3[U(OH,H2O) (UO2)8O12(OH)3] [Ni(H2O)4]3[U(OH,H2O) (UO2)8O12(OH)3] UO2 (pure U4+)

17173.5(5)

17177.5(5)

17170.2(5)

17175.5(5)

U4O9 (50% U4+ + 50% U5+)

17171.4(5)

17176.4(5)

U3O8 (33% U5+ + 67% U6+)

17171.9(5)

17179.5(5)

U3O8 (33% U5+ + 67% U6+)

17172.2(5)

17179.1(5)

UO3 (pure U6+)

17172.3(5)

17177.5(5)

UO2(NO3)2(H2O)6 (pure U6+)

17172.9(5)

17176.8(5)

ref this work (ROBL) this work (MARS) this work (ROBL) this work (ROBL) this work (ROBL) this work (MARS) this work (MARS) this work (MARS) F


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Inorganic Chemistry

Table 3. Structural Parameters Obtained by Fitting the EXAFS Spectrum at the U L3-Edge Compared to the Theoretical Values Derived from the Crystal Structure Previously Publisheda Theory

a

Experiment

Shell

Distance (Å)

Coordination number

Distance (Å)

Coordination number

Debye−Waller factor σ2 (Å2)

Correlation factor R

Short U−O Medium U−O Long U−O Short U−U Long U−U

1.83(3) 2.2(6) 2.36(5) 3.65(4) 3.86(1)

1.8 2.7 2.2 1.8 0.9

1.832(5) 2.241(5) 2.42(1) 3.65(2) 3.86(2)

1.6(4) 3.4(4) 2.1(4) 1.6(4) 0.9(4)

0.004(1) 0.006(1) 0.008(2) 0.007(2) 0.006(2)

0.015 between 1.2 - 4 Å

The theoretical distance errors are the standard deviation of the averaged distances.

Figure 6. Experimental RIXS map plotted as a contour map over the U L3-edge near the Lα1 emission line for [Ni(H2O)4]3[U(OH,H2O)(UO2)8O12(OH)3]. The HERFD−XANES spectrum corresponds to the diagonal cut (dotted line) through the RIXS plane at the maximum of the emission line, far from the edge.

Figure 7. Top: U L 3 -edge of [Ni(H 2 O) 4 ] 3 [U(OH,H 2 O)(UO2)8O12(OH)3] recorded in transmission, TFY, and partial fluorescence yield or HERFD mode. The latter spectrum has been collected by using the X-ray emission spectrometer of the MARS beamline (SOLEIL) tuned to the Lα1 emission line (13618 eV). The results of the FDMNES theoretical calculation with and without the quadrupole component are also given. Note that the theoretical spectrum has not been convoluted with the core−hole broadening Lorentzian and the continuum relative step function. Bottom: Calculated occupied and unoccupied projected U d and f and O p DOS of [Ni(H2O)4]3[U(OH,H2O)(UO2)8O12(OH)3] around the U L3-edge showing the contribution of the dipole (2p3/2 → 6d) and quadrupole (2p3/2 → 5f) transitions on the above experimental spectra.

L3-edge at different emission energies around the U Lα1 emission line (3d5/2 → 2p3/2 decay channel) of [Ni(H2O)4]3[U(OH,H2O)(UO2)8O12(OH)3]. These data are displayed as a contour map in a plane of incident and finalstate energies, which is the difference between the incident and emission energies, i.e., the energy transferred to the system. This energy roughly corresponds to the binding energy of the U 3d5/2 shell. It has been found at 3557(2) eV, which is higher than the reported value of 3551.7(3) eV.25 This could be a consequence of the oxidation state of the U atoms in our compound compared to the reported one, for which no information is given. A scan at the maximum of the U Lα1 emission line (HERFD−XANES) corresponds to a diagonal cut through the RIXS plane. In Figure 6, except HERFD− XANES, no additional feature is revealed in the RIXS map. Consequently, a detailed analysis of the spectral features can be performed on the basis of the HERFD−XANES spectrum. The HERFD−XANES spectrum is compared with standard XANES spectra collected in both transmission and TFY modes in Figure 7 (top). The HERFD−XANES spectrum shows much sharper features compared to those collected in the standard modes. This is coherent with the fact that the spectral broadening in HERFD−XANES is mainly determined by the lifetime broadening of the 3d5/2 core−hole (3.54 eV) and no longer by the 2p3/2 core−hole (8.16 eV), as demonstrated by the total energy resolutions of 8.3 and 3.9 eV for XANES and HERFD−XANES, respectively.

The HERFD spectrum is composed of an intense peak, the white line, situated at 17176.9(5) eV, followed by a second broad peak at 17187.0(5) eV. The inflection point E0 is located at 17174.5(5) eV. On the left side of the white line, one can also observe a shoulder at 17168.4(5) eV. At 17186.9(5) eV, a second broad resonance is also observed. All of these features are well in agreement with the calculated spectrum given in Figure 7 (top). Nevertheless, the intensity of the shoulder on the left side of the white line is better reproduced by taking into account the quadrupole component of the electronic transitions from the 2p3/2 to the partially empty U f shells. According to the calculated density of states (DOS) given in Figure 7 (bottom), such a quadrupole transition from the 2p3/2 to 5f orbitals is probably a consequence of U 5f−O 2p orbital mixing. This is in good agreement with the results published by Vitova et al.26 on UO2, [UO2Py5][KI2Py2], and [UO2(NO3)2(H2O)6] uranium compounds. G


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Inorganic Chemistry

Figure 8. U L3-edge HERFD−XANES spectra of [Ni(H2O)4]3[U(OH,H2O)(UO2)8O12(OH)3], UO2 (U4+), U3O8 (U5+/U6+), UO2(NO3)2(H2O)6 (U6+), and β-UO3 (U6+) recorded at MARS (SOLEIL).

Table 4. Energy of the U L3-Edge E0 and Main Resonances Determined in HERFD Mode in Comparison to Previously Published Dataa U L3-Edge HERFD−XANES (Lα1 Emission Line)

a

compound

E0 position

first resonance (White-line)

second resonance

[Ni(H2O)4]3[U(OH,H2O)(UO2)8O12(OH)3] UO2 (pure U4+) UO2 (pure U4+) UO2 (pure U4+) U3O8 (33% U5+ + 67% U6+) [UO2Py5][KI2Py2] (pure U5+) UO3 (pure U6+) UO2(NO3)2(H2O)6 (pure U6+) UO2(NO3)2(H2O)6 (pure U6+) UO2(NO3)2(H2O)6 (pure U6+)

17174.6(5) 17169.2(5) m.d. m.d. 17173.2(5) m.d. 17174.6(5) 17174.5(5) m.d. m.d.

17176.9(5) 17174.6(5) 17175.0(5) 17174.0(5) 17176.6(5) 17173.0(5) 17176.9(5) 17176.5(5) 17176.2(5) 17176.5(5)

17186.9(5) 17190.5(5) m.d. m.d.

17185.1(5) 17189.5(5) m.d. m.d.

ref this this 26 27 this 26 this this 27 26

work (MARS) work (MARS)

work (MARS) work (MARS) work (MARS)

m.d. represents missing data.

respectively. However, because of the fact that the spectral broadening as well as the quadrupole transition feature can partially occult the expected chemical shift per oxidation state, a small amount of U5+ can still not be totally excluded in our case by using HERFD−XANES at the U L3-edge despite the increase in the valence state sensitivity. Nevertheless, one can attempt to assess the presence of a unique valence state by taking a deeper look at the individual DOS and more specifically the position of the quadrupole electronic transition. Indeed, as mentioned by Vitova et al.,26 the position and intensity of this preedge feature can be used for uranium valence state determination. However, it can also be sensitive to changes in the symmetry, type of axial or

This preedge feature is also observed on other uranium compounds, as shown in Figure 8. It is clearly visible on U6+ compounds as a peak around 17168.5(5) eV on the first derivative relative to incoming energy. The main feature’s positions in energy for all of the shown spectra and some available literature data are given in Table 4. LCF using UO2, U3O8, and β-UO3 references on a −20 to +20 eV energy range around the edge energy point E0 has been performed using the HERFD spectra. LCF confirmed the U6+ nature of the studied compound by giving relative weights of 0(3), 0(5), and 100(5)% of UO2, U3O8, and β-UO3, respectively. The deduced uranium valence state weights are then equal to 0(3), 0(3), and 100(8) for U4+, U5+, and U6+, H


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Inorganic Chemistry

geometries are very close for the three U sites [two long U−U and one short U−U for U(3) instead of two short U−U and one long U−U for U(1) and U(2)]. As demonstrated by the weak overlap between the 5f and 6d DOS from 4 to 9 eV above EFermi, all 6d shells are here slightly mixed with a 5f shell, which consists of five sharp peaks. These U shells are strongly mixed with the O 2p shell as expected. This phenomenon seems to be more localized with only three of five sharp features overlapped for the uranyl bonding (axial O) with a very short bonding length (around 1.8 Å) for the equatorial O atoms. The U(4) site presents less localized empty d and f orbitals. Indeed, for this uranium site, one can observe a splitting into two peaks of the 6d DOS: one, as expected, around 10 eV and the second around 14.5 eV. Such a new peak seems to be mixed with the 2p orbitals of the equatorial O atoms. The mixing of U(4) 5f and 6d shells is more visible than those in the three previous U sites. These DOS differences are probably a consequence of the square-bipyramidal geometry instead of the pentagonal-bipyramidal geometry but must be confirmed by further studies. Because U(4), as well as U(1), U(2), and U(3), has been previously attributed to the U6+ valence state, one can conclude that the preedge feature seems to be sensitive to the local environment for a given uranium valence state. The U(5) site shows strongly localized empty d orbitals, as demonstrated by the two sharp peaks at 8.5 and 18 eV above EFermi. The latter is probably related to the long bonding of the two axial O atoms in the U(5) geometry, as revealed by the O 2p−U 6d mixing around 18 eV. The peak observed for the U(4) atoms around 14.5 eV is also present but is less intense. This peak is slightly mixed with the 2p orbitals of the U(5)’s equatorially bonded O atoms, confirming the relationship between the square-bipyramidal geometry and such a feature in the U 6d shell. The 5f orbitals are 2 times more intense than those for the previous U sites. These shells are also mixed with the O 2p shells, but such an overlap seems to be less important compared to the other nonequivalent U sites. So, no direct evidence of the valence state of U(5) could be extracted from the DOS. However, one can argue that slight changes in the local environment (ligand type and geometry) in this compound could potentially be followed by the intensity and position changes on the preedge (U 5f shell) and satellite (U 6d shell) peaks observed in the experimental spectrum. Indeed, passing from a pentagonal bipyramid to a square bipyramid tends to localize the U 5f orbitals and to split the U 6d orbitals, depending on the ligand types, bonding distances, and eventual distortions. Nevertheless, because of the important core−hole broadening at the U L3-edge, the study of the 6d shell splitting remains limited to significant modifications of the local geometry. A higher sensitivity could be expected at the M4-edge because it allows direct probing of the 5f electronic shells. 3.3. U M4-Edge Experiments. In order to more precisely assess the uranium valence state of U(5), HERFD−XANES at the U M4-edge (3d3/2 → 5f dipole transition) has been performed by employing an emission spectrometer tuned to the maximum of the Mβ emission line (4f5/2 → 3d3/2 decay channel). In that case, the spectral broadening becomes in the range of the expected chemical shift per oxidation state as demonstrated by Kvashnina et al.28 Moreover, in such a spectrum, no quadrupole transition feature that can partially limit the chemical shift determination can occur.

equatorial ligands, bonding character of the given compounds, etc. Additional studies are required to confirm the use of this feature for unknown compounds. In our case, one can consider that the identical position of the 5f DOS for all individual U atoms is a fingerprint of a unique valence state of uranium in [Ni(H2O)4]3[U(OH,H2O)(UO2)8O12(OH)3]. A more detailed study of the individual DOS calculated for all five nonequivalent U atoms is shown in Figure 9.

Figure 9. Comparison of the U d and f and O p DOS for the five nonequivalent U atoms as calculated with FDMNES for the published [Ni(H2O)4]3[U(OH,H2O)(UO2)8O12(OH)3] crystal structure. Each O p DOS is the one calculated for the first neighboring O atoms of each given U atom. These O p DOS are here divided into two groups: the first one refers to the axial bonding (uranyl-like, 2 O), and the second one represents the equatorial bonding (4 or 5 O). For the sake of clarity and comparison, the intensity scale is the same for all individual DOS.

This figure shows that the d DOS of U(4) and U(5) are strongly different from those of the other U atoms, in good agreement with their different local environment (nature and number of the ligands) and geometry. The U(1) and U(2) 6d DOS are mainly characterized by an asymmetric broad peak around 9.5 eV relative to the Fermi level. One can also remark that the d and f orbitals of the U(3) site are slightly shifted to lower energy by 1 eV and present an additional very sharp peak around 7.5 eV. These features could be attributed to the difference in the U−U shell at a longer distance because the O I


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Inorganic Chemistry

the β-UO3 spectral features clearly demonstrate without ambiguity that all U atoms in [Ni(H2O)4]3[U(OH,H2O)(UO2)8O12(OH)3] are U6+ ones, as proposed in the previous work.10 The local environment of the five nonequivalent U sites in βUO3, as available in ref 14 and listed in COD, are distorted pentagonal-bipyramidal for U(1), U(2), U(4), and U(5) and slightly distorted square-bipyramidal for U(3). Such local environments are not so different from the [Ni(H2O)4]3[U(OH,H2O)(UO2)8O12(OH)3] ones, which are three pentagonal bipyramids and two square bipyramids [assuming that U(5) is also one square bipyramid]. These similar local geometries could explain the comparable spectral features observed in both the L3- and M4-edge spectra of [Ni(H2O)4]3[U(OH,H2O)(UO2)8O12(OH)3] and β-UO3. One can also note that the white line of β-UO3 is broader at low energy than that of [Ni(H2O)4]3[U(OH,H2O)(UO2)8O12(OH)3]. Moreover, the features at higher energy are also broadened. Such differences are here only because of the local environment differences between the U atoms in these two compounds. As partially deduced by the DOS study (Figure 9), such differences can reflect the effect of bipyramid distortions (shift toward lower energy), the type of bonding (O only or with some OH/H2O group), and their distances to the U 5f electronic orbital. A complete study of these effects is out of the scope of this paper but surely represents the next step to a more detailed study of the uranium electronic structure in complex compounds by HERFD−XANES at the M4-edge.

The measured [Ni(H2O)4]3[U(OH,H2O)(UO2)8O12(OH)3] spectrum is compared to the UO2, U4O9, U3O8 and β-UO3 spectra in Figure 10.

Figure 10. U M4-edge HERFD−XANES spectra and derivatives of UO2 (U4+), U4O9 (U4+/U5+), U3O8 (U5+/U6+), β-UO3 (U6+), and [Ni(H2O)4]3[U(OH,H2O)(UO2)8O12(OH)3] recorded using the Xray emission spectrometer of the ID26 beamline (ESRF), tuned to the Mβ emission line (energy 3339.6 eV).

The [Ni(H2O)4]3[U(OH,H2O)(UO2)8O12(OH)3] spectrum is composed by a main peak at 3727.0(3) eV, followed by a resonance at 3728.8(3) eV. No additional peak or any asymmetry is observed. As a consequence, a pure uranium valence state could be expected. The observed spectral features and positions are very similar to those observed in the pure U6+ compound β-UO3. They are also very different compared to the mixed-valence-state compounds U4O9 and U3O8 as well as UO2. The positions of the main resonances given in Table 5 and the similarities with

4. CONCLUSION We report here the direct measurement of the uranium valence state on the five nonequivalent uranium complex compound [Ni(H2O)4]3[U(OH,H2O)(UO2)8O12(OH)3] by means of XANES and HERFD−XANES experiments at both U L3and M4-edges and a subsequent comparison with well-known uranium compounds. The U L3-edge experimental results supported by the FDMNES calculations show a preedge feature, due to the quadrupole 2p3/2−5f transition, as already observed in other uranium compounds.25 This preedge feature seems to be sensitive to the local environment as well as to the valence state, but further study is mandatory to precisely extract insight from such a feature in the case of unknown uranium compounds. The calculated spectra, obtained on the basis of the previously published [Ni(H 2 O) 4 ] 3 [U(OH,H 2 O)(UO2)8O12(OH)3] crystal structure, is in very good agreement with the experimental one. The calculated uranium DOS reveals that, upon passing from a pentagonal-bipyramid site to a square-bipyramidal site, a split the 6d shell occurs into peaks separated by 5−10 eV depending on the bonding distance distortions and/or the nature of the ligand. The U M4-edge experimental results demonstrate that only hexavalent uranium is present in this complex compound. On the basis of our results, the ability of the U L3- and M4-edges to assess the uranium valence state in complex uranium compounds has been discussed. The U L3-edge generally suffers from the influence of the core−hole and the presence of the quadrupole preedge feature. Both affect the detectable amount of one valence state in mixed states. These effects have not been observed in the U M4-edge, which appears to be more sensitive that the U L3-edge. Consequently, such an edge should be favored in the evaluation of low concentrated uranium valence states in mixed-valence compounds.

Table 5. U M4-Edge Main Resonance Position As Measured in This Study and Compared with Data Reported in the Literature U M4-Edge HERFD−XANES (Mβ Emission Line)

compound

first resonance (eV)

second resonance (eV)

3727.0(3)

3728.8(3)

[Ni(H2O)4]3[U(OH,H2O) (UO2)8O12(OH)3] UO2 (pure U4+)

3725.3(3)

UO2 (pure U4+) U4O9 (50% U4+ + 50% U5+)

3725.3(4) 3725.2(3)

U4O9 (50% U4+ + 50% U5+) U4O9 (50% U4+ + 50% U5+) U3O8 (33% U5+ + 67% U6+)

3725.2(3) 3725.1(4) 3726.4(3)

U3O8 (33% U5+ + 67% U6+) U3O8 (33% U5+ + 67% U6+) UO3 (pure U6+)

3726.4(3) 3726.5(4) 3726.9(3)

3728.6(3)

UO3 (pure U6+) UO2(acac)2 (pure U6+)

3726.8(3) 3726.9(4)

3728.6(3) 3728.8(4)

3726.2(3) 3726.3(3) 3726.3(4)

ref this work this work 28 this work 29 28 this work 29 28 this work 29 28 J


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Inorganic Chemistry

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Kvashnina, K. O.; Cheetham, A. K.; Konings, R. J. M. Inorg. Chem. 2015, 54, 3552−3561.

AUTHOR INFORMATION

Corresponding Author

*E-mail: rene.bes@aalto.fi. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The authors thank Philippe Devaux (UCCS) for the synthesis of uranium compounds and the preparation of pellets. The authors also thank the ESRF and SOLEIL for the provision of beamtime.

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REFERENCES

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and M4-Edges To Determine the Uranium Valence State on

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