Uranium-Induced Changes in Crystal-Field and Covalency Effects of

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

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Uranium-Induced Changes in Crystal-Field and Covalency Effects of Th4+ in Th1−xUxO2 Mixed Oxides Probed by High-Resolution X‑ray Absorption Spectroscopy Hongliang Bao,† Peiquan Duan,†,‡ Jing Zhou,† Hanjie Cao,†,‡ Jiong Li,† Haisheng Yu,† Zheng Jiang,† Hongtao Liu,† Linjuan Zhang,† Jian Lin,† Ning Chen,§ Xiao Lin,†,‡ Yancheng Liu,† Yuying Huang,*,† and Jian-Qiang Wang*,† Downloaded via UNIV OF SOUTH DAKOTA on August 27, 2018 at 21:09:16 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

†

Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai 201800, PR China University of Chinese Academy of Sciences, Beijing 100049, PR China § Canadian Light Source, University of Saskatchewan, Saskatoon, Saskatchewan S7N 2 V3, Canada ‡

S Supporting Information *

ABSTRACT: Knowledge of the local Th structure is a prerequisite for a better understanding of the physicochemical properties of the thoriumbased mixed oxides (Th-MOX) involved in the Th-based nuclear fuel cycle. The crystalline electric field (CEF) splitting of the 6d shell in Th1−xUxO2 (x = 0.25, 0.5, 0.75) solid solution was probed by the Th L3 edge high-energy-resolution fluorescence-detected (HERFD) X-ray absorption near-edge spectroscopy (XANES) collected at the Lβ5 emission line, which cannot be obtained by conventional X-ray absorption methods. The detected CEF split between the 6d eg and t2g orbitals in ThO2 consisting of ordered Th−O8 cubes with cubic symmetry is ∼3.5 eV for the Th4+ ion. Because the split peaks of the white line corresponding to the crystal-field splitting of the unoccupied 6d states were resolved in the HERFD-XANES spectra, the analysis of these split peaks combined with first-principles calculations revealed that an increase of the U content involves the distortion of the Th−O8 cubes in the Th1−xUxO2 mixed oxides. The lower symmetry of the Th−O8 cube induced by the incorporated U tends to reduce the local crystal-field around Th4+ as well as the hybridization of Th 6t2g−O 2p which is mainly responsible for the covalent property of the Th−O bond. The phenomenon is noticeable in Th0.25U0.75O2, whose CEF splitting is decreased by approximately 10%, and covalent mixing between Th 6d t2g and O 2p orbitals is substantially reduced compared to that of pure ThO2.

1. INTRODUCTION

To understand how the structure of Th-MOX impacts physical and chemical properties, such as melting point, solubility, and thermal conductivity, it is necessary to study the structure at the atomic scale. Furthermore, the experimental work on the microstructures can be closely interfaced with first-principles computational approaches to establish validated models for new insights and predictive understanding of the chemical and physical properties of Th-based MOX under extreme environments as well as their ability to accommodate fission products and highly radioactive wastes. The local structure of Th is closely related to the physicochemical properties of Th-MOX involved in the thorium-based nuclear fuel cycle.5,6 Therefore, it is important to study the microstructural responses of ThO2 to actinide doping or mixing. Pure ThO2 has a fluorite structure in which each Th atom is surrounded by eight O atoms forming a Th−O8 cube.

Thorium-based mixed oxides (Th-MOX) as potential advanced fuel materials have received increased interest due to greater thorium abundance and their some beneficial properties.1,2 Although 232Th is not a fissile isotope, it can change into 233U via neutron capture. In addition, due to its high capability to resist aqueous corrosion, as well as isomorphism with other tetravalent actinide dioxides (AnO2), thorium dioxide has been proposed for use as a matrix material for geological disposal of long-life radioactive waste such as minor actinides.3,4 Thorium−uranium mixed oxides (Th1−xUxO2) exist in different stages of the thorium fuel cycle. ThO2 as a breeder material can gradually change into Th1−xUxO2 after neutron radiation; moreover, thorium is also generally mixed with 235U oxide to maintain the core critical until sufficient 233U is generated. In addition, Th1−xUxO2 components exist in spent Th-based nuclear fuel; additionally, radionuclides, such as 234U, can be embedded into the ThO2 matrix for geological storage. © XXXX American Chemical Society

Received: April 28, 2018

A

DOI: 10.1021/acs.inorgchem.8b01142 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

The HERFD-XAS spectra at different edges, such as the U L3 and M edges, have different widths of the spectral features.37 The HERFD-XAS techniques were applied at the U L3 and M edges to study the U compounds and determine the chemical state of uranium oxides.38−40 The HERFD-XANES spectra at the Th or U L3 edge are generally recorded by monitoring the maximum of the Lα1 (3d5/2−2p3/2) emission line, which has the highest transition probability compared with other emission lines. The spectral broadening can be further reduced if the L3 edge XANES spectrum is collected at the Lβ5 (5d5/2− 2p3/2) emission line.41 The U L3 edge HERFD-XANES spectrum recorded using the Lβ5 emission line was first used to detect the crystal-field splitting of the U 6d states in UO2.42 This approach makes it possible to detect the crystal-field effects on the Th 6d states in Th-MOX, which cannot be obtained by conventional X-ray absorption methods. In this work, by adjusting the amount of the metal precursors of Th(NO3)4·6H2O and UO2(NO3)2·6H2O, we synthesized thorium−uranium MOX with nominal Th/U atomic ratios of 3:1, 1:1, and 1:3, which are denoted as Th0.75U0.25O2, Th0.5U0.5O2, and Th0.25U0.75O2, respectively. The local structures of Th in the Th1−xUxO2 (x = 0.25, 0.5, 0.75) mixed oxides, including the crystal-field splitting and distribution of the Th 6d states, are probed by using the Th L3 edge HERFD-XANES spectra collected at the Lβ5 emission line combined with the first-principles calculations based on the local-density approximation (LDA) taking into account Coulomb interaction U (LDA + U).

Uranium dioxide shares the same crystal structure as thorium dioxide with similar lattice parameters. Significantly, ThO2 and UO2 have very different electronic structures.7 Thorium dioxide is a charge-transfer insulator with a band gap of 6−7 eV,8 while uranium dioxide is a Mott insulator with an electronic band gap of 2−2.5 eV.9 The 6d orbitals of thorium are lower in energy than its 5f orbitals, which is distinct from other actinides. Furthermore, thorium has the smallest difference in energy between the 6d and 5f orbitals among actinides, suggesting the most favorable 6d−5f admixture from an energy viewpoint.10 If oxygen is regarded as divalent, then the stoichiometry implies 5f0 and 5f2 configurations for Th and U atoms in AnO2 with An4+, respectively. However, ThO2 has been considered to have partial 5f occupation due to the hybridization of Th 6d−Th 5f−O 2p.8 Recently, ThO2 was first experimentally demonstrated to have a significant covalency component originating from the Th 6d−Th 5f−O 2p hybridization, for which the Th 6d occupancy with 0.20 electrons is nearly twice that of the Th 5f states.11 However, the Th 6d orbitals are expected to split into two groups due to the crystalline electric field (CEF) interaction in ThO2. These features of thorium strongly suggest that exploring the 6d orbitals of Th-based oxides is also of great significance. The addition of U into the ThO2 matrix will lead to changes in the local geometric and electronic structure of Th.12,13 It is well-established that the Th1−xUxO2 mixed oxides tend to form solid solutions over the whole composition range with lattice parameters following Vegard’s law.14,15 Further studies16,17 showed that the Th−O and U−O bond lengths in Th1−xUxO2 vary slightly from those of their parents and differ substantially from the weighted average cation−anion distance, implying the distortion of the Th−O8 cubes around U atoms. The Th−O interaction in ThO2 can be qualitatively regarded as a dominant ionic bond with partial covalent properties. Because the 6d orbitals of Th4+ are more extended than the 5f orbitals, the 6d orbitals are more sensitive to the crystal field and more likely to form the framework of covalent bonds with specific orientations. The changes in the local structure of Th induced by U in Th1−xUxO2 can be reflected in the Th 6d states. Of particular interest is the change of the Th−O interaction when U is included in the ThO2 matrix. X-ray absorption spectroscopy (XAS), containing the X-ray absorption nearedge spectroscopy (XANES) region and extended X-ray absorption fine structure (EXAFS) region, is a useful technique with the characteristic of element selection.18 XAS can provide complementary information about the local geometric and electronic structure of the central absorption atoms.19 XAS was already used to study the local structure of Th−Am,20 Th− Ce,21 U−Pu,22 U−Am,23,24 U−La,25 and Pu−Am26 mixed oxides. The EXAFS data at the L3 edge have been used to explore the local geometric structure of Th and U in Th1−xUxO2 mixed oxides.27,28 The XANES spectra at the L3 and M edges have also been employed to probe the local electronic structure of Th and U, including the chemical valence and unoccupied 5f and 6d states.8,29−31 Nevertheless, both thorium and uranium have a large lifetime broadening at the L3 and M edges,32,33 rendering the conventional XAS technique less sensitive to the fine distribution of the unoccupied electronic states. Owing to recent advances in spectroscopy, high-energyresolution fluorescence-detected X-ray absorption spectroscopy (HERFD-XAS) with reduced spectral broadening can be obtained by employing an X-ray emission spectrometer.34−36

2. EXPERIMENTAL AND CALCULATION METHODS 2.1. Preparation of Th1−xUxO2 Mixed Oxides. Caution! Natural uranium and thorium were used in the synthetic methods given below. Although the corresponding activity is low (1 × 103−3 × 103 Bq), the usual precautions for working with radioactive elements should be followed. The thorium−uranium MOX samples were synthesized by the coprecipitation method. Appropriate quantities of Th(NO3)4· 6H2O and UO2(NO3)2·6H2O were mixed in deionized water and stirred for 10 min. Then, an excess of ammonia−water was added until a light yellow precipitate formed. This mixture was stirred for 30 min and then allowed to settle. The clear solution was decanted, and the precipitate was washed with deionized water until neutral pH. The precipitate was dried at 80 °C for 2 days followed by calcination at 800 °C in air for 6 h, and then, a black massive solid was generated. The black solid was ground into powder in an agate mortar and was then thermally treated for 14 h at 1300 °C under a 95% Ar and 5% H2 atmosphere. By adjusting the amount of the metal precursors of Th(NO3)4·6H2O and UO2(NO3)2·6H2O, we synthesized Th1−xUxO2 (x = 0.25, 0.5, 0.75) mixed oxides with nominal Th/U atomic ratios of 3:1, 1:1, and 1:3, which are denoted as Th0.75U0.25O2, Th0.5U0.5O2, and Th0.25U0.75O2, respectively. Pure ThO2 and UO2 samples were also prepared by a similar process for comparison. 2.2. XRD and XAS Characterizations. The powder X-ray diffraction (XRD) patterns of the samples were recorded on a Bruker D8 Advance X-ray diffractometer using a Ni-filtered Cu Kα radiation source operated at 40 kV and 40 mA. A 2θ interval between 10 and 90° was used with a step size of 0.02° and a step time of 0.15 s. The conventional XAS data at the Th and U L3 edges were collected on beamline BL14W1 at the Shanghai Synchrotron Radiation Facility (SSRF). The electron storage ring was operated at 3.5 GeV with a beam current of approximately 250 mA in top-up operation. A double Si ⟨111⟩ crystal monochromator was employed for the incident energy selection. The high-order harmonic component was rejected using a harmonic rejection mirror. The XAS measurements were carried out in the transmission mode by employing two gas-filled ionization chambers as a photon detector. Standard procedures were followed to analyze the XAS data using the software package B

DOI: 10.1021/acs.inorgchem.8b01142 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Demeter.43 The backscattering amplitude and phase shift were calculated with the program FEFF 9.0.44 Fourier transform (FT) was performed on the k3-weighted EXAFS oscillations from 3.0 to 12.0 Å−1. A window of 1.0−4.5 Å in r-space was used for the curve fitting of FT-EXAFS data. Amplitude reduction factor S02 values of 0.75 and 0.76 were determined by fitting the reference ThO2 and UO2, respectively. Coordination numbers (N) were set to the ideal values of the crystalline structure. Other structural parameters, such as the bond distance (R), Debye−Waller factor (σ2), and inner potential shift (ΔE0), were obtained from the fitting. 2.3. HERFD-XANES Measurements. The HERFD-XANES experiments were also performed on beamline BL14W1 at the SSRF. A double Si ⟨311⟩ crystal monochromator was employed for the incident energy selection. The incident beam size on the samples was ∼0.3 mm vertically and 0.3 mm horizontally in the Si ⟨311⟩ focus mode. The HERFD-XANES spectra were measured in the highenergy-resolution fluorescence detection (HERFD) mode using an Xray emission spectrometer.45 The sample, analyzer crystal, and photon detector (a silicon drift detector) were arranged in a vertical Rowland circle geometry with 1000 mm diameter, equal to the bending radius of the analyzer crystal. The analyzer crystals composed by three spherically curved Si crystals in the ⟨10 10 0⟩ reflection with a Bragg angle at 84.9° were used to diffract and focus the Lβ5 (5d5/2 → 2p3/2) fluorescence onto the photon detector. The HERFD-XANES data were obtained by directly collecting the Lβ5 (16.2075 keV) emission line intensity as a function of the incident energy rather than cutting on the resonant inelastic X-ray scattering (RIXS) maps. A combined (incident convoluted with emitted) energy resolution of ∼1.7 eV was achieved in the experiments. Prior to the HERFD-XANES measurements, each sample was prepared as a pressed pellet of ∼200 mg and coated with an 8 μm Kapton film. Three scans were performed for each sample, without any radiation damage was observed during the experiment. 2.4. Computational Details. The first-principles calculations for the electronic structures of ThO2, UO2, and Th1−xUxO2 were performed using the WIEN2K package46 based on the full-potential linearized augmented plane wave (FPLAPW) plus local orbital (LO) method.47,48 For ThO2 and UO2, the face-centered cubic structures were used as the initial models with lattice parameters of 0.558 and 0.545 nm, respectively, extracted from the experimental XRD data. The Th0.75U0.25O2, Th0.5U0.5O2, and Th0.25U0.75O2 samples were modeled by a supercell containing four unit-cells of ThO2 with one, two, and three thorium atoms being substituted by uranium, respectively. The expansion of the basis function was up to RMT × Kmax = 7 (RMT is the muffin−tin sphere radii and Kmax the maximum modulus for the reciprocal lattice vector) to ensure the accuracy of the basis set. Correlation effects for the 5f states of uranium and thorium were taken into account by applying the LDA + U approximation with U and J set to 4.5 and 0.54 eV,49 respectively. All atoms were fully relaxed until the magnitude of the force on each atom fell below 0.05 eV/Å, and self-consistency was reached when the energy converged to 10−4 eV. The optimized bulk structures of ThO2, Th1−xUxO2, and UO2 for the computational work are summarized in the Supporting Information (section S6). The average Th−O and Th−Th(U) bond lengths in the optimized structures are in accordance with the EXAFS results.

Figure 1. XRD patterns of ThO2 (black), Th0.75U0.25O2 (red), Th0.5U0.5O2 (blue), Th0.5U0.5O2 (magenta), and UO2 (olive).

S1), which can be attributed to contraction of the lattice constant. The amount that the diffraction peaks shift increases with the U content in Th1−xUxO2 relative to ThO2 (Figure S1). It should be also noted that some splitting of the ⟨220⟩, ⟨222⟩, ⟨311⟩, ⟨400⟩, ⟨420⟩, and ⟨422⟩ index peaks of Th0.25U0.75O2 can be observed, which can be attributed to some degree of UO2-rich segregation. The reduction of U6+ to U4+ needs to go through an intermediate process of U3O8 generating when UO2(NO3)2·6H2O is used as precursor. Because the U3O8 has much different crystalline structure from ThO2, phase segregation between U3O8 and ThO2 is easy to occur in the intermediate process, resulting in a certain degree of UO2-rich segregation in finally Th1−xUxO2 samples. This phenomenon becomes more obvious when the content of U is higher, so the index peaks in the XRD pattern of Th0.25U0.75O2 shows some splitting. The values of the crystallographic axis a in the crystal cells of ThO2, Th1−xUxO2, and UO2 calculated from the XRD data are compiled in Table 1. It can be concluded that the lattice constant in Th1−xUxO2 approximately follows Vegard’s law. To investigate the local structure of Th and U in Th1−xUxO2, we performed XAS measurements at room temperature. The Th and U L3 edge XANES data are shown in Figure 2. The Th L3 edge XANES spectra of Th1−xUxO2 (Figure 2a) have the same edge positions and similar features to that of ThO2, corresponding to the dominant Th4+ ion in Th1−xUxO2. Similarly, the U L3 edge XANES spectra of Th1−xUxO2 (Figure 2b) are similar to that of UO2 with the same edge position, corresponding to the dominant U4+ ion in Th1−xUxO2. The XRD and XAS results reveal that the Th1−xUxO2 samples are the expected solid solutions with lattice constants approximately following Vegard’s law. The structural parameters obtained from the L3 edge EXAFS data of ThO2, Th1−xUxO2, and UO2 are also compiled in Table 1. The EXAFS analysis only gives average information and has difficulty in distinguishing between the backscattered atoms Th and U. For Th1−xUxO2, the average Th−O bond is longer than the U−O bond, and the average Th−Th(U) distance is longer than the U−U(Th) distance. These results suggest that the dominant local structures of Th and U in Th1−xUxO2 maintain those of the parent ThO2 and UO2, respectively. The impact of existence of U tends to mainly influence the first shell Th−O cubic coordination (i.e., the adjacent Th−O8 cubes).

3. RESULTS AND DISCUSSION 3.1. XRD and XAS Data. The crystalline structure of the prepared Th1−xUxO2 samples was determined by XRD. Powder XRD profiles of ThO2, Th1−xUxO2, and UO2 are shown in Figure 1. The diffraction peaks at approximately 27.7, 32, 45.9, 54.5, 57, 66.9, 73.8, 76, and 84.9° can be assigned to the ⟨111⟩, ⟨200⟩, ⟨220⟩, ⟨311⟩, ⟨222⟩, ⟨400⟩, ⟨331⟩, ⟨420⟩, and ⟨422⟩ diffraction indexes, respectively, revealing a fluorite structure for ThO2, Th1−xUxO2, and UO2. It should be noted that the corresponding diffraction peaks in Th1−xUxO2 shift to higher angles to various extents compared with ThO2 (Table C

DOI: 10.1021/acs.inorgchem.8b01142 Inorg. Chem. XXXX, XXX, XXX−XXX

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Table 1. Structural Parameters Obtained from the Analysis of the XRD Data and the EXAFS Spectra at the Th and U L3 Edge for ThO2, Th1−xUxO2, and UO2 sample

a (Å)

ThO2

5.58

Th0.75U0.25O2

5.56

Th0.5U0.5O2

5.52

Th0.25U0.75O2

5.47

UO2

5.45

paths

N

R (Å)

Th−O Th−Th Th−O Th−Th(U) U−O U−U(Th) Th−O Th−Th(U) U−O U−U(Th) Th−O Th−Th(U) U−O U−U(Th) U−O U−U

8 12 8 12 8 12 8 12 8 12 8 12 8 12 8 12

2.41 3.96 2.40 3.93 2.36 3.92 2.40 3.92 2.35 3.89 2.38 3.91 2.35 3.88 2.34 3.87

σ2 (× 10−3 Å2) 4.1 3.7 4.0 4.2 3.2 4.9 2.8 4.1 3.5 4.0 4.9 8.3 6.3 4.8 5.2 3.8

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

1.2 0.7 1.0 0.7 0.9 1.1 1.2 1.2 1.1 1.2 1.0 1.8 0.9 0.4 1.0 0.4

ΔE0 (eV) 3.7 4.8 3.0 4.2 4.7 5.1 3.0 3.8 2.5 3.3 1.0 3.5 4.5 4.0 2.5 4.0

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

1.1 1.2 1.0 1.2 1.0 1.1 1.0 1.2 1.0 1.3 0.8 1.3 1.2 1.0 1.0 1.1

about the unoccupied Th 6d states related to the local symmetry of Th in Th1−xUxO2. Uranium dioxide is susceptible to oxidation. In contrast, thorium dioxide is chemically inert and displays a stronger Th−O8 cube structure. For the Th1−xUxO2 mixed oxides, the white lines of the Th L3 edge are more resistant to oxygen defects compared to the U L3 edge. A close observation of the XANES data (Figure 2) reveals that there are progressive changes in the intensity of the white line of Th L3 edge XANES. In contrast, the U L3 edge XANES data show not only resolved changing in white line intensity but also progressive changing in the resolution of the first postwhite line feature. The significant change in intensity of the white line of the Th L3 edge can be attributed to the change of the Th−O8 cube structure. However, the conventional XANES at the Th L3 edge has large spectral broadening and is insensitive to the changes in the local structure of Th. 3.2. HERFD-XANES Data of ThO2. To further investigate the local structure of Th in Th1−xUxO2, XANES spectra at the Th L3 edge were obtained with higher energy resolution by reducing spectral broadening. The HERFD-XANES spectrum at the Th L3 edge of ThO2 collected by monitoring the Lβ5 emission line and the corresponding XANES spectrum collected in the conventional transmission mode are compared in Figure 3. The HERFD-XANES spectra of ThO2 was obtained by directly collecting the Lβ5 (16.2075 keV) fluorescence rather than directly cutting (Figure S9) along the white dash line in the RIXS maps (Figure S7 and S8). The white line at approximately 16306 eV in the conventional XANES spectrum mainly originates from the electron transition from Th 2p3/2 to unoccupied 6d states. The HERFD-XANES spectrum displays more subtle features, and the split of the white line (peaks A and B) can be observed due to the higher energy resolution. ThO2 has a fluorite-type crystal structure in which each Th atom is located at the body center of a cube composed of eight oxygen atoms. The Th 6d states in ThO2 containing dz2, dx2−y2, dxy, dyz, and dxz orbitals are expected to split into two groups, the so-called eg band (dz2 and dx2−y2) and t2g band (dxy, dyz, and dxz), due to the crystal-field effect. The principal origins of splitting peaks A and B in ThO2 were further confirmed by first-principles calculations. The densities of electronic states (DOS) of ThO2 were extracted

Figure 2. (a) Th L3 edge XANES spectra for Th0.75U0.25O2 (red), Th0.5U0.5O2 (blue), Th0.25U0.75O2 (magenta), and ThO2 (black). (b) U L3 edge XANES spectra for Th0.75U0.25O2 (red), Th0.5U0.5O2 (magenta), Th0.25U0.75O2 (blue), and UO2 (black).

Compared to the EXAFS, which supplies radial structural information, XANES is more sensitive to the spatial structure and can provide the local electronic structure associated with the spatial configuration as well. According to the dipole selection rules, the white line of the XANES spectra at the Th L3 edge corresponds to the electronic transitions from the 2p to 6d orbit. The intensity of the white line carries information D

DOI: 10.1021/acs.inorgchem.8b01142 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

5) were also obtained to study the effects on the local structure of Th when thorium atoms in ThO2 are partly substituted by

Figure 3. Th L3 edge HERFD-XANES spectrum of ThO2 recorded using the X-ray emission spectrometer set to the Lβ5 emission energy is compared to the conventional XANES spectrum collected in transmission mode.

Figure 5. Experimental Th L3 edge HERFD-XANES spectra (collecting the Lβ5 fluorescence) of Th0.75U0.25O2 (red), Th0.5U0.5O2 (blue), Th0.25U0.75O2 (magenta), and ThO2 reference (black).

from the calculations. The unoccupied Th 6d DOS described in the forms of eg and t2g are shown in Figure 4. The theoretical

uranium. The Th1−xUxO2 samples show spectral features similar to those of ThO2, with the same position of the absorption edge. Significantly, no obvious changes in the shape and the center position of peak B were observed for the Th1−xUxO2 samples compared with ThO2. In contrast, peak A in Th1−xUxO2 exhibits observable changes in shape, and its center position displays a shift toward high energy relative to ThO2 (Figure S4). To determine the positions of the centers of peaks A and B, peak fitting of line shapes to the HERFDXANES spectra by Gaussian functions was performed (Figure S5). The fit parameters estimated from fitting the HERFDXANES data at the Th L3 edge of Th1−xUxO2 and ThO2 are summarized in Table S2. There is a peak drifting trend revealed for peak A in step with the increasing of U concentration. In addition, the relative intensities of peaks A and B in Th1−xUxO2 are apparently higher than those in ThO2. It should be noted that the XAS data collected in the fluorescence mode strongly depend on the element concentration, sample thickness, and fluorescence mode detection geometry due to the self-absorption effect.50 Because HERFDXAS is a fluorescence-based technique, the shape of HERFDXANES spectra may be impacted when concentrated or thick samples are measured.51 Self-absorption occurs when the induced fluorescence photons are partially absorbed in the sample. The predominant phenomenon of self-absorption is the intensity reduction of the white line. Th1−xUxO2 samples with the same thickness in the thick limit and the same detection geometry were used in the HERFD-XANES scans, so the self-absorption effect is determined primarily by the concentration. The Th1−xUxO2 samples have nearly the same concentrations because U and Th have nearly equal atomic numbers. Therefore, the absorptions of the Th1−xUxO2 samples to Lβ5 fluorescence are approximately the same throughout the experiments. If the decrease in the peak B intensity in ThO2 caused by self-absorption is assumed to be Δ, then the corresponding value for Th1−xUxO2 can be evaluated to be approximately Δ. Similarly, the same phenomena should occur for peak A when only self-absorption is considered. The heights of peaks A and B for Th1−xUxO2 and their increment percentages in height (Phi) relative to ThO2 are summarized in Table S3. The

Figure 4. Comparison of the experimental Th L3 edge HERFDXANES spectrum of ThO2 (black), the calculated HERFD-XANES spectrum (red), and the unoccupied Th 6d DOS described in the forms of eg (blue) and t2g (magenta).

HERFD-XANES curve was obtained by Lorentzian broadening of the unoccupied 6d DOS and by adding a background with the help of the arctangent function. The arctangent background represents the transitions of the excited electrons to the continuum and gives a contribution in the form of the absorption edge jump in addition to the calculated DOS. The comparison of the experimental and calculated HERFDXANES spectra of ThO2 at the Th L3 edge is presented in Figure 4. The experimental HERFD-XANES spectrum can be fairly described by the calculated spectrum. Therefore, the peaks A and B in the measured HERFD-XANES spectrum of ThO2 at the Th L3 edge come mainly from the contributions of the unoccupied eg and t2g bands of the Th 6d states, respectively. The distance between the two peaks is approximately 3.5 eV, revealing that the energy separation between the 6d eg and t2g bands in ThO2 is ∼3.5 eV. The main features of the experimental Th L3 edge HERFD-XANES data of ThO2 can be also reproduced by the calculated spectrum obtained by using the FEFF 9.6 code (Figure S10). 3.3. HERFD-XANES Data of Th1−xUxO2. The HERFDXANES data of Th1−xUxO2 samples at the Th L3 edge (Figure E

DOI: 10.1021/acs.inorgchem.8b01142 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 6. Total and partial densities of states of ThO2 (left panel), Th0.25U0.75O2 (middle panel), and UO2 (right panel) extracted from the LDA + U calculations.

DOS above and below the Fermi level in comparison with ThO2, there is a narrow band at approximately −0.5 eV originating from both O 2p and U 5f contributions. The distribution of the Th 6d states is important here because these states mainly contribute to the Th L3 edge HERFD-XANES spectra due to the dominating dipole transitions from the 2p3/2 core level to unoccupied 6d states. The calculated Th 6d DOS of Th1−xUxO2 and ThO2 described in the forms of eg and t2g are shown in Figure 7. For the unoccupied part of the Th 6d DOS, the position of the t2g band is higher than that of the eg band at the energy scale, although there is some overlap. In contrast, for the occupied part of the Th 6d DOS, the area of the t2g band is obviously larger than that of the eg band. The influence on the distribution of the Th 6d states after U is added into the ThO2 lattice is analyzed according to the calculated Th 6d DOS. Figure 7 shows that the entire Th 6d DOS moves toward lower energy once U is added into ThO2. It should be noted that the curve of the eg band changes with the U content, implying that the eg band is susceptible to the addition of U. Upon closer inspection of the unoccupied Th 6d DOS, the distance between the centroids of the eg and t2g bands in

increment percentage of the height of peak B (PhB) for the Th0.5U0.5O2 sample (14.4%) is approximately 2.1 times larger than that of Th0.75U0.25O2 (6.8%). Moreover, the PhB of the Th0.25U07.5O2 sample (29.8%) is approximately 4.4 times larger than that of Th0.75U0.25O2 (6.8%). Significantly, the ratio (PBA) of PhB to PhA in the Th1−xUxO2 samples increases with U content. These results suggest that there are other factors influencing the heights of both peaks A and B in addition to the self-absorption effect. 3.4. Calculated DOS of Th1−xUxO2. The densities of electronic states (DOS) of ThO2 and UO2 obtained from the first-principles calculations are shown in Figure 6. The calculated values of the band gap in ThO2 and UO2 are ∼5.0 eV and ∼2.3 eV, respectively. For ThO2 (Figure 6, left panel), the main contribution to the DOS above the Fermi level comes from the Th 5f states, with some admixture of both the Th 6d and O 2p states. The 5f DOS is localized at approximately 7.5 eV, while the 6d DOS is fairly delocalized in the range of 5−12.5 eV. In contrast, the O 2p DOS dominates in the occupied part of the valence band, with some admixture of both the Th 6d and 5f states. For UO2 (Figure 6, right panel), in addition to the similar main contributions to the F

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of bonding and antibonding in the 6d t2g component is larger than that in the 6d eg component, implying a smaller value of EM0−EL0 in the hybridization of Th 6d eg states with O 2p states. However, the O 2p band prefers to hybridize with the Th 6t2g band rather than the Th 6eg band because the dxy, dyz, and dxz orbitals can better match and have more overlap with the O 2p orbitals for the case of cubic 8-coordinated Th−O in ThO2, namely, the hybridization of Th 6t2g−O 2p has larger hopping matrix (HML). Although mismatch of symmetry between both the dz2 and dx2−y2 orbitals of Th 6d and O 2p atomic orbitals is unfavorable for the hybridization of Th 6eg− O 2p, the Th 6eg band can participate in the hybridization with O 2p states by admixture with Th 5f states because the Th 6d eg states have almost the same energy as Th 5f states. These features reveal that the hybridization of Th 6t2g−O 2p is mainly responsible for the covalency of the Th−O bond in ThO2. Th0.25U0.75O2 was chosen as a representative case to study the influence on the Th−O interaction induced by U because its experimental HERFD-XANES spectrum has the most significant difference from that of ThO2 among Th1−xUxO2. The calculated DOS of Th0.25U0.75O2 (Figure 6, middle panel) shows that an introduction of U into the ThO2 lattice seems to cause a reduction in the band gap compared to pure ThO2. The O 2p DOS of Th0.25U0.75O2 has a similar narrow band at approximately −0.5 eV to that of UO2. A closer inspection of the DOS profiles in Th0.25U0.75O2 (Figure 8b) reveals a smaller distance between the major structures in the occupied and unoccupied Th 5f part compared with ThO2, implying the enhancement of Th 5f−O2p hybridization. Moreover, Th0.25U0.75O2 appears to have a larger distance between the dominant structures of the bonding and antibonding 6d t2g components compared with ThO2, implying the reduction of the hybridization of Th 6t2g−O 2p. Further inspection of the DOS profiles of Th0.25U0.75O2 allows one to conclude that there are significant changes in the shapes of both the unoccupied Th 6d eg and Th 5f DOS curves, rather than in the unoccupied Th 6d t2g part, compared to ThO2. 3.6. Crystal Field around Th4+ and Covalency of Th− O in Th1−xU xO 2 . To obtain a clear and reasonable interpretation of the Th L3 edge HERFD-XANES experimental data, the expected changes in the spectral features induced by changes in the local structure of Th were analyzed. More

Figure 7. eg (black) and t2g (red) components in the Th 6d DOS of ThO2 and Th1−xUxO2.

Th1−xUxO2 decreases as the U content increases; moreover, the area ratio of the t2g and eg bands increases with the U content. The results are consistent with the change trends of peaks A and B in the experimental HERFD-XANES data of Th1−xUxO2 at the Th L3 edge. 3.5. Covalent Characteristics of Th−O in ThO2 and Th1−xUxO2. To obtain a clear physical picture, a qualitative model of molecular orbitals (MOs) was adopted to obtain a simple expression of the partial covalent properties of Th−O bonds. Generally, the symmetries of two atomic orbitals need to match for hybridization to occur between the two orbitals, and the extent of the hybridization is further affected by the charge transfer energy (EM0−EL0) and the overlapping between the two orbitals (Figure S6). A cluster of Th−O8 cubes has been adopted as a molecular model to analyze the covalency of Th−O in ThO2.52 The hybridization between the Th 6d−5f states and O 2p states in Th1−xUxO2 and ThO2 can be qualitatively expressed with a similar physical image of the coordination bonding. The Th−O interaction in ThO2 can be qualitatively regarded as a dominant ionic bond with a few covalent components originating mainly from the hybridization of Th 5f−Th 6d−O 2p. The calculated DOS of Th 5f, Th 6d, and O 2p in ThO2 are together exhibited in Figure 8. For ThO2 (Figure 8a), the Th 6d DOS follow the O 2p and U 5f profiles, revealing the hybridization of Th 5f−Th 6d−O 2p. It should be noted that the distance between the major structures

Figure 8. Comparison of the Th 5f (blue), Th 6d (magenta), and O 2p (olive) DOS in ThO2 (a) and Th0.25U0.75O2 (b) obtained from the LDA + U calculations. The intensities of the marked peaks within the range of the braces for the Th 5f DOS of ThO2 and Th0.25U0.75O2 are reduced to 10% of the intensity for clarity. G

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possible to study the local electronic structure of Th−O bonds in Th-MOX, which is affected by the chemical environment of Th atoms. According to the relative positions and relative strengths of peaks A and B in HERFD-XANES spectra at the Th L3 edge, it has demonstrated that the introduction of U into the ThO2 lattice causes local structure distortion in the Th1−xUxO2 solid solution. It seems that the crystal field around Th4+ and the covalency of Th−O in Th1−xUxO2 are sensitive to the local symmetry of Th. The distortion of the Th−O8 cube reduces the crystal-field splitting between the Th 6d eg and t2g orbitals as well as the overlap between the Th 6d t2g orbitals and O 2p orbitals.

detailed information on the local structure of Th in Th1−xUxO2 can be extracted by comparing the experimental data and the deductive phenomenon. According to the above EXAFS results, the substitution of Th in ThO2 by U leads to a reduction of the average Th−O bond length, revealing that the Th−O8 cubes around the substitutional positions are disturbed and tend to contract. A shorter Th−O bond in the Th−O8 cube without considering any distortion should result in an increased interval between peaks A and B in Th L3 edge HERFD-XANES spectra as a result of the crystal-field effect. The deductive phenomenon contradicts the experimental phenomenon. To eliminate the contradiction, the distortion of the Th−O8 cube induced by U must be taken into account. The distortion of the Th−O8 cube will increase the overlap of Th 6eg−O 2p while decreasing the overlap of Th 6t2g−O 2p, leading to peak A moving further toward higher energy while peak B shifts to a lower energy to compensate for the reverse movement caused by the contraction of the Th−O bond. In addition, the hybridization between Th 6d and Th 5f states also affects the shape and position of peaks A and B. Favorable hybridization of Th 6eg−Th 5f changes the shape of peak A and moves peak A to the high energy. In contrast, unfavorable hybridization of Th 6t2g−Th 5f has little effect on both the shape and position of peak B. A shorter Th−O bond should exhibit a stronger covalent property due to the enhancement of the overlap of Th 5f−Th 6d−O 2p without considering both charge transfer and distortion. However, the increase of the overlap of Th 6eg− O 2p without considering the Th 6d−Th 5f hybridization and the decrease of the overlap of Th 6t2g−O 2p caused by the distortion of the Th−O8 cube correspond to the decrease of the unoccupied eg band and the increase of the unoccupied t2g band, respectively. The results agree with the experimental phenomenon that the value of PBA in HERFD-XANES spectra at the Th L3 edge increases with the U content of Th1−xUxO2, revealing that the distortion of the Th−O8 cube caused by the substitution of Th with U can lead to observable modifications in the local electronic structure of the corresponding Th−O interaction. In addition, there are two probable main reasons for that the absolute intensity of peak A in the Th1−xUxO2 is slightly higher than ThO2. Although absolute intensity of white line peaks is mainly determined by corresponding unoccupied DOS, it is also influenced by other factors, especially the selfabsorption effect. The self-absorption effect on ThO2 should tend to be more obvious than Th1−xUxO2 because Th has absorption to the Lβ5 fluorescence higher than U, though the self-absorption effects on Th1−xUxO2 and ThO2 are estimated to be approximately the same. The other probable reason is the hybridization between the Th 6d and Th 5f states. The Th 6eg band can participate in the hybridization with O 2p states by admixture with Th 5f states, which may cause a certain degree of increase in DOS of the unoccupied eg band.

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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.8b01142. Diffraction angle 2θ of the intense diffraction peaks for ThO2, Th1−xUxO2, and UO2; k3-weighted Th and U L3edge experimental χ(k) data and fit in k-space as well as the corresponding Fourier transform for ThO2, UO2, and Th1−xUxO2 samples; optimized bulk structures of ThO2, Th1−xUxO2, and UO2; RIXS map of ThO2 around Th Lβ5 line on the incident energy versus emission energy scale as well as the incident energy versus transfer energy scale (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (Y.H). *E-mail: [email protected] (J.-Q.W). ORCID

Zheng Jiang: 0000-0002-0132-0319 Hongtao Liu: 0000-0001-6450-2585 Jian Lin: 0000-0002-3536-220X Jian-Qiang Wang: 0000-0003-4123-7592 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful for the financial support of National Natural Science Foundation of China (grant nos. U1532259, 21701183, U1732112, and 21573273) and Strategic Priority Research Program of the Chinese Academy of Sciences (grant no. XDA02040600). We are also thankful to the members of the Beamline BL14W1 at SSRF for both XAS and HERFDXANES beam time.

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REFERENCES

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4. CONCLUSION The crystalline electric field interaction in thorium dioxide with a local symmetry of the Th−O8 cube induces the Th 6d orbitals split into the eg band (dz2 and dx2−y2) and t2g band (dxy, dyz, and dxz). The splitting of the white line into peaks A and B, mainly originating from the electron transition from 2p to the unoccupied part of the eg and t2g band, respectively, can be observed in the Th L3 edge HERFD-XANES spectra collected at the Lβ5 emission line. The detected crystal-field splitting between the Th 6d eg and t2g orbitals in ThO2 is ∼3.5 eV. It is H

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