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C: Physical Properties of Materials and Interfaces

DFT + U Simulation of the X-ray Absorption Near-Edge Structure of Bulk UO2 and PuO2
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The Journal of Physical Chemistry C

Cite this: J. Phys. Chem. C 2023, 127, 36, 17994–18000
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https://doi.org/10.1021/acs.jpcc.3c03143
Published September 5, 2023

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Abstract

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Hubbard U-corrected density functional theory within the periodic boundary condition model in the WIEN2k code is used to simulate the actinide LIII and O K edge X-ray absorption near-edge structure (XANES) for UO2 and PuO2. Spin-orbit coupling effects are included, as are possible excitonic effects using supercells with a core hole on one of the atoms. Our calculations yield spectra in excellent agreement with previous experiments and superior to previous simulations. Density of states analysis reveals the mechanism behind the XANES peaks: the main contribution to the U/Pu LIII edges comes from the U/Pu d states hybridized with O p states, while as expected, the O p states primarily determine the O K edges of both UO2 and PuO2. The O K edges also feature O p hybridizing with U/Pu d and f states in the low-energy region and with U/Pu s and p states for the higher-energy peaks.

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Introduction

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The actinide dioxides UO2 and PuO2 are of fundamental interest due in no small part to the fascinating properties of the actinides’ valence 5f electrons. (1) UO2 and PuO2 are also of major industrial importance because of their central role in the nuclear fuel cycle. The majority of the world’s nuclear reactors use UO2 as their fuel, and the recycling of spent Magnox and AGR fuels has, over many decades, separated out significant quantities of Pu, which is currently stored as PuO2 powder in the UK. Within this material, the radioactive decay of the Pu has led to the build-up of both U and Am. Good understanding of the evolution of the PuO2 on a multidecade timescale is important for its safe storage, and so the study of the fundamental properties of PuO2 and Pu/U/Am mixed oxides is essential.
A variety of experimental techniques has been used to explore the electronic structures of UO2 and PuO2, including X-ray absorption spectroscopy (XAS), electron energy loss spectroscopy (EELS), and photoelectron spectroscopy (PES). (2−7) Theoretical methods have also been developed to give better predictions of strongly correlated actinide-containing systems, (8) especially density functional theory (DFT) with Hubbard U corrections, which combines reasonable accuracy with computational efficiency, especially in periodic simulations. (9−11) We are currently part of a joint experiment–theory collaboration, in which one of the principal experimental techniques being employed is X-ray absorption near-edge structure (XANES) spectroscopy, which yields information on both electronic structures and geometries. Theoretical simulation of XANES spectra─the focus of this paper─is important not only to support experimental interpretations but also to provide insight as to the origins of the XANES peaks.
The study of UO2 with XANES spectroscopy began decades ago, focusing mainly on the U L and M edges. (12−14) These data have recently been enhanced by high-energy-resolution fluorescence detection for X-ray absorption spectroscopy (HERFD-XAS) at the U LIII edge. (15,16) The O K edge of UO2 has also attracted significant interest. (17) The Pu LIII and O K edges of PuO2 are spectroscopically similar to their UO2 counterparts. (18−20) A number of simulations of UO2 and PuO2 XANES has also been reported. (21−25) Multiple-scattering calculations within a cluster model reproduce all of the peaks in U LIII XANES but do not give good predictions of the peak positions and intensities. (21) The same method does not fare well at the O K edge, missing key peaks and giving incorrect positions and intensities for others. (25) The Pu LIII edge of PuO2 was calculated in the framework of the Anderson impurity model; (26) the simulation reproduced all of the experimentally reported peaks but did not give good peak positions. It is likely that the failings of the abovementioned simulations result from their being performed on finite clusters, with severe restrictions due to the form of the potential in which the electronic states are calculated. The O K edge of PuO2 has been studied using a periodic model, but it did not consider core-hole (CH) effects and did not reproduce all of the experimental peaks. (27)
In this work, we present the first systematic simulation of the actinide LIII and oxygen K edges of UO2 and PuO2 using Hubbard U-corrected density functional theory within the periodic boundary condition framework. We show that this approach yields excellent agreement with experiment, significantly superior to previously reported simulations. Furthermore, analysis of the electronic structure of the dioxides allows us to provide detailed interpretations of the origins of the peaks in the edge spectra. Our paper is structured as follows. We first provide computational background and details, before setting out our results for the actinide LIII edges. We then discuss the oxygen K edges, including extended analysis of the effects of the choice of Hubbard U, before summarizing our findings in the Conclusions section.

Computational Background and Details

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All XANES simulations have been performed using the periodic WIEN2k code, (28) and a short introduction to WIEN2k is given in the Supporting Information (SI). The experimental lattice parameters for face centered cubic fluorite UO2 and PuO2 are used, 5.47 and 5.40 Å, respectively, (29−31) and the corresponding tetragonal unit cell for the antiferromagnetic (AFM) structure is given in Figure S1a. The 2 × 2 × 1 supercell used for the CH calculations is shown in Figure S1b. A supercell approach is needed since only very few atoms in a solid are actually excited; most remain in their ground state but may contribute to screen the excitonic effects due to the excited core electron on one of the atoms. The generalized gradient approximation of Perdew, Burke, and Ernzerhof (PBE) was chosen, (32) and a Hubbard U correction for the U/Pu 5f electrons with a self-interaction correction for the double counting is adopted. (33,34) RMT was set to 2.35 au for both U and Pu and 1.96 au for O. The cutoff parameter RMTKmax for the basis set is set as 8.5, where Kmax is the cutoff for the basis function wave vector. A Brillouin zone sampling with a 9 × 9 × 6 and 4 × 4 × 7 mesh in the full Brillouin zone is used for the unit cell and supercell, respectively. After testing a wide range of U (0.2–0.6 Ry) and J (U/7–U/4) values for ground-state properties such as magnetic moment and the band gap (Figures S2 and S3), a U value of 0.3 Ry (4.1 eV) and a J value of 0.04 Ry (0.5 eV) were chosen for both UO2 and PuO2, with the inclusion of spin-orbit coupling (SOC), as these parameters give the best prediction for UO2 and PuO2 ground-state properties. They also lie in the range of values used in our previous simulations of these materials using the VASP code. (9) The same value of the Hubbard U is added to the An for both An L edge and O K edge XANES simulations (see below).
Both experiment and previous simulation have found a noncollinear antiferromagnetic (AFM) ground state for UO2. (9,35) By contrast, a nonmagnetic (NM) ground state was found for PuO2 from a previous experiment (36) and our previous simulation, albeit the NM state was obtainable in the latter only by using the occupation matrix control (OMC) approach. (9) We here use the DFT + U method and model both UO2 and PuO2 with collinear AFM ground states. Comparing the current density of states (DOS, a broadening factor of 0.003 mRy is used for all DOS presented in this work) of AFM PuO2 with previous NM simulation, we find that AFM PuO2 yields almost the same DOS for the unoccupied states (Figure S3) and, as we shall show, our PuO2 XANES simulations also suggest that AFM PuO2 is a good approximation for our present purposes. The spin direction of the U/Pu electrons alters between layers to yield the AFM arrangement.
An all-electron method like that implemented in WIEN2k is ideally suited to simulate X-ray absorption spectroscopy. The intensity of the spectrum is given by Fermi’s golden rule according to dipole transitions between initial and final states. (37−39) For XANES, the final state, which has a CH and an excited electron, determines the spectrum. The CH and the excited electron interact with each other and may lead to strong excitonic effects. The excited electron can be treated as either an additional valence electron (added into the lowest conduction bands) or as a constant background charge to keep the whole system neutral. In some systems, exciting only half a core electron may give better simulations because of incomplete screening effects, and in others, one can obtain good XANES without CHs because of strong screening effects. Although not completely ab initio anymore, those parameters should be tested to find the most suitable settings. Therefore, we calculated XANES spectra using a ground-state simulation and a variety of CH calculations: full CH (1CH) and half CH (0.5 CH) are created by removing 1 or 0.5 electrons from U/Pu 2p orbitals for the LIII edge or O 1s orbitals for the O K edge on one of the atoms in the supercells and adding 1 or 0.5 electrons to the valence band (VB) or background (BG). As our systems are spin-polarized, spin up (up) and spin down (dn) 2p or 1s electron holes are also compared for 1CH simulations.

Results and Discussion

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Various CH approximations with different spins and excited electron locations are considered for U the LIII edge XANES of UO2, and the resulting spectra are compared in Figure S4a; the spectra are plotted with very low broadening factors S = 0.2 eV (spectrometer broadening) and G = 1.0 eV (CH lifetime) to yield more detailed XANES. The spectra with different CHs but also those without CH are very similar (Figure S4a). We therefore conclude that the screening of the U 2p CH due to the additional occupied 5f electron is very strong in bulk UO2.
Next, we compare our simulated U LIII edge of ground-state UO2 with two experimental spectra, one XANES and the other HERFD-XAS, (14,16) and with one simulated spectrum using a cluster approach (the only previous simulation we found). (21) There are three clear peaks for U LIII edge XANES, marked as A, B, and C in Figure 1a. expt.2 is the HERFD-XAS spectrum and features clearer peaks with smaller peak widths than expt.1 (XANES) and slightly higher energies for peaks B and C. For comparison with experiment, a broadening factor S = 2.4 eV is chosen according to the energy resolution of the previous HERFD-XAS experiment. (16) A wide range of CH lifetimes G (5.0–10.0 eV) are tested, and a value of 9.0 eV gives the best results (Figure S4); it is also close to the 8.67 eV core-hole lifetime of the 2p3/2 orbital. Our simulation gives almost perfect agreement with experiment for both peak positions and relative intensities (Figure 1a), much better, especially concerning the positions of peaks B and C, than previous cluster modeling results which used the multiple-scattering method. (21) Overall, for U LIII edge XANES of UO2, we conclude that periodic boundary condition simulation without CH gives accurate results with high computational efficiency.

Figure 1

Figure 1. (a) Simulated U LIII edge XANES of UO2 (S = 2.4 eV, G = 9.0 eV) and previous experimental and simulated spectra: expt.1, expt.2, and simu1 are from refs (14,16) and (21), respectively; (b) dipole matrix elements of U s and d states; (c) density of states of U s and d states and U LIII edge XANES of UO2 (S = 2.4 eV and G = 9.0 eV); (d) density of states of U d states and amplified O s (4-fold), p (2-fold), and d (4-fold) states, and U LIII edge XANES of UO2 (S = 2.4 eV and G = 9.0 eV). All calculations are performed without CH. XANES peak A in panels (a) and (d) are aligned and shifted to 10 eV, and the DOS in panels (c) and (d) and dipole matrix elements in panel (b) are also shifted to match the XANES. Spectra and DOS in panels (a), (c), and (d) are shifted vertically for better visibility.

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We further analyzed our simulated spectra to uncover the contributions to the XANES peaks. Motivated by Fermi’s golden rule, we checked the radial transition probability, i.e., the dipole matrix element squared (⟨Ψcore|r|Ψfinal⟩2), and DOS. Together with the angular factors W1,2 = 2/5 and W1,0 = 1, the result gives the energy-dependent transition probability by multiplying the dipole matrix element squared with the corresponding partial DOS. We focus on the unoccupied U s and d states due to the dipole selection rules. The contribution due to U s states is much smaller than U d states because the U s states have a much smaller dipole matrix (Figure 1b) and much lower DOS (Figure 1c). The DOS of U d states are compared with the simulated spectrum in Figure 1d, and the DOS of the O s, p, and d states are also shown. The DOS of the U d states are found in the energy range of all three LIII edge peaks as is hybridization between U d states and O states. Similar DOS shapes are found for U d states and O p and d states around 10 eV. In the energy range 20–30 eV (peak B), clear hybridization between U d states and O p states can be seen, while from 50 to 55 eV (in the energy range of peak C), we find small double peaks for U d states and similar double peaks for O s and p states. Overall, it is mainly the U d states which contribute to the DOS in the region of the spectral peaks, with hybridization contributions from O states, mainly O p.
CH effects at the Pu LIII edge of PuO2 are studied and compared in Figure S5; as with UO2, the choice of CH has negligible effect on the spectra, so the screening effects on the Pu 2p CH are also very strong. A previous simulation of the Np and Am LIII edges of NpO2 and AmO2 also suggested that the CH size does not have much influence on the spectra, (40) and hence, overall, we conclude that the screening effects to the An 2p CH are very strong for AnO2 (An = U–Am).
In Figure 2, we compare our ground-state (no CH) simulation with previous HERFD experiment and simulation. (26) As the energy resolution used is not given in the previous experimental report, the same S and G values (Figure S5) as used for UO2 are employed. As with the U LIII edge, there are three peaks at the Pu LIII edge identified by previous experimental XANES, marked as A, B, and C in Figure 2. Our periodic data give almost perfect simulation, for both peak positions and relative intensities, superior to previous cluster modeling results, especially for the positions of peaks B and C; similar improvements were also found for U LIII above. Overall, our results at the LIII edge of UO2 and PuO2 suggest that the best approach to XANES simulation is periodic boundary conditions without CH.

Figure 2

Figure 2. Simulated Pu LIII edge XANES of PuO2 (no CH, S = 2.4 eV, G = 9.0 eV) and previous experimental and simulated spectra: expt. and simul. are from ref (26). Spectra are shifted vertically for better visibility.

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The mechanism underlying the Pu LIII edge XANES peaks was also studied by analyzing the dipole matrix elements and DOS of the Pu s and d states (Figure S6); as for the U LIII edge, the main contribution is from the d states. The DOS of the Pu d states and those of the O s, p, and d states are compared with Pu LIII XANES in Figure S6. Pu d state DOS are found in the energy range of all three LIII edge peaks, as well as hybridization contributions from O states, so we conclude that the origin of the LIII edge XANES peaks for U and Pu in their dioxides is the same.
Previous experimental works show that the O K edge of UO2 features two regions. (17,22) The first, the up to edge region (0 – 10 eV in Figure 3), has peaks directly connected to the electronic structure, labeled a, b, c, and d. The second, beyond the edge region (>10 eV in Figure 3), features five peaks labeled A, B, C, D, and E─these are related to the crystallographic structure. Similar to the LIII edges, we first look at the CH effects on the O K edge XANES of UO2. Ground-state (no CH) and CH simulations of O K edge spectra are compared in Figure S7 with S = 0.2 eV and G = 0.5 eV; we use smaller G here than for the LIII edge, as the lifetime of O 1s holes should be larger than that of U 2p holes. The CH does not have much influence beyond the edge region, while it does influence the peaks in the lower energy edge region (Figure S7a), so the following discussion focuses on peaks a, b, c, and d. First, we find that the CH has a clear influence on these peaks─the spectrum without a CH has a structure different from the other lines in Figure S7a. Second, the CH size influences the peak intensities and positions (compare 0.5 CH: up-VB with 1CH: up-VB). Finally, the position of the excited electron has a slight influence on the lower energy (<10 eV) peaks; see 1CH: up-VB and 1CH: up-BG in Figure S7a─they are very similar, but the intensities and positions of the first three peaks are slightly different. The initial spin direction (up or down) does not influence the spectra (Figure S7a). Hence, overall, CH effects have clear influence on the O K edge XANES of UO2, i.e., the screening of the O 1s CH is weak in UO2 bulk.

Figure 3

Figure 3. (a) Simulated O K edge XANES of UO2 for a supercell with 1CH: up-VB (S = 0.2 eV, G = 0.8 eV) and previous experimental and simulated O K edge XANES; expt.1, expt.2, and simul. are from refs (17,22) and (25), respectively. (b) DOS of amplified O p (10-fold) and U s (10-fold), p (10-fold), and d and f (0.5-fold) states and O K edge of UO2 (S = 0.2 eV and G = 0.8 eV); all simulations are performed with a supercell with 1CH generated by moving one 1s electron of O and adding it to the VB. The highest peaks of the XANES are in all figures aligned and shifted to 10 eV. The energy of the DOS is also shifted in panel (b) to match the XANES. Spectra and DOS are shifted vertically for better visibility.

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Figure 4

Figure 4. (a) Simulated O K edge of PuO2 for a supercell with 1CH: up-BG (S = 0.5 eV, G = 0.6 eV) and previous experimental and simulated O K edge data; expt.-SC, expt.-PC, and simul. are from refs (27,29) to (27), respectively. (b) DOS of amplified O p (10-fold) and Pu s (10-fold), p (10-fold), and d and f (0.5-fold) states and O K edge of PuO2 (S = 0.5 eV and G = 0.6 eV); all simulations are performed with a supercell with 1CH generated by removing one 1s electron of O and adding it to the background. The highest peaks of the XANES are in all figures aligned and shifted to 10 eV. The energy of the DOS is also shifted in panel (b) to match the XANES. Spectra and DOS are shifted vertically for better visibility.

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We choose 1CH: up-VB simulation to compare with previous experiments and simulation in Figure 3a; (17,22,25) although 1CH: up-BG also gives a good prediction when compared with experiments, 1CH: up-VB predicts slightly better relative intensities for the first three peaks. The O K edge of UO2 was studied with energy resolution S of 0.2–0.3 eV in previous experiments, so we use S = 0.2 eV for our simulated spectrum. The G value of 0.8 eV is chosen in the energy range 0.4–5.0 eV (Figure S7b). Our simulation gives a much better spectrum than the previous simulation, (25) which predicts only three peaks up to the edge region and 4 peaks beyond the edge region and does not do well with the relative intensities of these peaks. Our simulation reproduces all of the experimental peaks (Figure 3a), matching fairly well the peak positions (Table 1) and relative intensities (Figure 3a). The most notable differences are for peak d (a single broad feature in experiment vs. a sharp peak with a shoulder on the left in theory) and for the position of peak a, which is, however, only a weak shoulder in the experiment.
Table 1. Simulated Energies of O K edge Peaks for UO2 (S = 0.2 eV, G = 0.8 eV) and PuO2 (S = 0.5 eV, G = 0.6 eV), Experimental Data is from References (17,22) and (27). 
 peak (eV)a/a′bcdABCDEaMADb
UO2expt.11.553.655.210.013.916.119.8524.8537.050.319
expt.21.63.45.210.014.016.119.824.536.90.309
this work2.363.204.9610.0013.7615.8819.9024.6237.78 
PuO2expt.24.49.31012.715.519.725.1370.507
this work1.78/3.414.928.9610.0013.1416.0619.4224.8238.92 
a

The position of high-energy peak E is obtained from a simulated spectrum with a lifetime broadening factor G = 4.0 eV.

b

The mean absolute deviation (MAD) of our simulations from the available experimental data are given in the last column.

We further studied the O K edge XANES by analyzing the DOS of the unoccupied states. Due to the dipole selection rule, only O p states contribute directly to the spectrum, but to emphasize possible hybridizations of O p with U s, p, d, and f states, their partial DOS are also shown in Figure 3b. In the low-energy region, there is clear hybridization between O p and U f states up to 6 eV (highlighted with a yellow background in Figure 3b) that contributes to peaks a, b, and c. Hybridization between O p and U d states (gray background) shows U d state contribution to peak c and in particular d. Beyond the edge region, peak A arises from the hybridization between O p and U s states (blue background), peaks B and C come from the hybridization between O p and U p states (orange background), while peaks D and E mainly come from O p states, perhaps with some hybridization from U s and p states. Overall, it is the intensities and positions of the O p states which determine the O K edge peaks, modified by hybridization with U d and f states in the up to edge region and having contributions from U s and p states in the region beyond the edge.
Similar to the O K edge of UO2, previous experiments show that the O K edge of PuO2 can be divided into two parts: up to the edge region (0–10 eV in Figure 4) with four main peaks a, b, c, and d and beyond the edge region (>10 eV in Figure 4) with five peaks A, B, C, D, and E. (19,27,41) We again begin by examining the CH effects. Ground-state (no CH) and CH simulations of O K edge spectra are compared in Figure S8a; as with UO2, CH effects have a clear influence on the O K edge XANES of PuO2, so the screening effect of the valence electrons on the O 1s CH is not very strong.
A full CH generated by removing one O 1s electron and adding it to background gives the best simulation results, so we focus on the 1CH: up-BG spectrum. In Figure 4, the two experimental spectra are from single-crystal 239PuO2 (line expt.-SC) and polycrystalline 242PuO2 (line expt.-PC), and their energy resolution is not larger than 0.6 eV. For better comparison with the experiments, we use an S value of 0.5 eV and a G value of 0.6 eV in the range of 0.4–5.0 eV (Figure S8b). We have discovered one previous simulation, also performed in WIEN2k, though using only the ground-state electronic structure and not considering CH effects. (27)
Our simulation reproduces all of the experimental peaks. Beyond the edge region, we obtain very good positions and relative intensities for peaks A, B, C, D, and E compared with the experiment (Table 1 and Figure 4a). A previous calculation without CH (27) (line simul. in Figure 4a) also yields all of the peaks beyond the edge region with a reasonable position and intensity. Thus, the CH effect has only a slight influence on the beyond edge peaks, as is the case for the O K edge of UO2 (Figure S7). In the up to the edge region, CH simulation does not predict a sufficient number of peaks (line simul. in Figure 4b), while our simulation reproduces all of the experimental peaks, once again proving the importance of CH on the simulation of the O K edge. Our simulation predicts a peak a′ (around 3–4 eV) between peaks a and b, which seems to be also present in the experimental results, especially the single-crystal PuO2 experiment but was not discussed in the experimental work. (27)
The DOS of PuO2 are studied to gain further insight into the XANES peaks, especially for the up to the edge region peaks. O p states (dipole selection rule) and Pu s, p, d, and f states (hybridization) are compared with our simulated XANES in Figure 4b. The O p states clearly determine the XANES peaks. In the low-energy region, the Pu f states hybridize with O p states and contribute to peaks a and a′ (yellow background in Figure 4b), while the Pu d states hybridize with O p states and contribute to peaks b, c, and d (gray background in Figure 4b). Beyond the edge region, hybridization contributions from Pu s and p states are highlighted with blue and orange backgrounds, respectively, in Figure 4b.
There are some small differences between our simulations and previous experiments for the O K edges. For example, in Figure 3a, experiment finds a round peak d for UO2, while our simulation predicts a sharper peak, with a left shoulder peak around 9 eV. Also, the relative intensities and positions of peaks a, b, and c at the UO2 O K edge are slightly different from experiment (Table 1). In Figure 4a, there is a clear peak a′ at about 3 eV in our simulated O K edge of PuO2, though this feature is observed only weakly in the experimental results, and our simulation predicts much lower intensity for peak a at the PuO2 O K edge. We wondered if these small differences could arise from our simulation method, specifically the choice of U in the GGA + U approach or possibly choosing GGA + U over hybrid DFT. However, as we show in Figures S9 and S10, the U value has negligible influence on the peaks’ position and intensity, and DFT + U simulations predict almost the same spectra as hybrid DFT simulations.

Conclusions

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In this contribution, we report periodic DFT + U + SOC simulations of the actinide LIII and O K edges of bulk UO2 and PuO2. Detailed assessment of computational parameters such as the choice of U and J and the treatment of core-hole effects allow us to generate simulated spectra that are in excellent agreement with experimental data. Core-hole effects have little influence on the actinide LIII edges due to the strong screening effect of the valence electrons, and hence using a unit cell without a core hole works well. By contrast, O 1s core holes, which are not fully screened by the valence electrons, have clear influence on the O K edge XANES, mainly on the low-energy, electronic structure-related peaks, and we recommend the use of a full core hole with a supercell for accurate O K edge simulation.
The excellent performance of the methodology employed here within the WIEN2k code gives us much confidence in our simulations, and we are now moving to apply the same approach to study other edges, such as the An M and N, and more complex actinide oxides, e.g., the actinide LIII and O K edges of Pu/U/Am mixed dioxides.

Data Availability

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The data supporting the findings reported in this paper are openly available at DOI: 10.17632/v2x87s8sy9.1.

Supporting Information

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The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.jpcc.3c03143.

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Author Information

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Acknowledgments

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The authors acknowledge financial support from the EPSRC (EP/T013842/1).

References

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This article references 41 other publications.

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    Zhu, J.-X.; McMahan, A.; Jones, M.; Durakiewicz, T.; Joyce, J.; Wills, J.; Albers, R. Spectral properties of δ-plutonium: Sensitivity to 5 f occupancy. Phys. Rev. B 2007, 76, 245118  DOI: 10.1103/PhysRevB.76.245118
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    Spectral properties of δ-plutonium: Sensitivity to 5f occupancy
    Zhu, Jian-Xin; McMahan, A. K.; Jones, M. D.; Durakiewicz, T.; Joyce, J. J.; Wills, J. M.; Albers, R. C.
    Physical Review B: Condensed Matter and Materials Physics (2007), 76 (24), 245118/1-245118/6CODEN: PRBMDO; ISSN:1098-0121. (American Physical Society)
    By combining the local d. approxn. (LDA) with dynamical mean field theory (DMFT), we report a systematic anal. of the spectral properties of δ-plutonium with varying 5f occupancy. The LDA Hamiltonian is extd. from a tight-binding fit to full-potential linearized augmented plane-wave calcns. The DMFT equations are solved by the exact quantum Monte Carlo (QMC) method and by the Hubbard-I approxn. We demonstrate strong sensitivity of the spectral properties to the 5f occupancy, which suggests using this occupancy as a fitting parameter in addn. to the Hubbard U. By comparing with photoemission data, we conclude that the "open shell" 5f5 configuration gives the best agreement, resolving the controversy over 5f "open shell" vs. "close shell" at. configurations in δ-Pu.
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  4. 4
    Tobin, J. G.; Ramanantoanina, H.; Daul, C.; Roussel, P.; Yu, S. W.; Nowak, S.; Alonso-Mori, R.; Kroll, T.; Nordlund, D.; Weng, T. C.; Sokaras, D. Unoccupied electronic structure of actinide dioxides. Phys. Rev. B 2022, 105, 125129  DOI: 10.1103/PhysRevB.105.125129
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    Unoccupied electronic structure of actinide dioxides
    Tobin, J. G.; Ramanantoanina, H.; Daul, C.; Roussel, P.; Yu, S.-W.; Nowak, S.; Alonso-Mori, R.; Kroll, T.; Nordlund, D.; Weng, T.-C.; Sokaras, D.
    Physical Review B (2022), 105 (12), 125129CODEN: PRBHB7; ISSN:2469-9969. (American Physical Society)
    Ligand field d. functional theory calcns. of the dioxides of thorium, uranium, and plutonium have been combined with high-energy-resoln. fluorescence detection (HERFD) in x-ray absorption spectroscopy and inverse photoelectron spectroscopy (IPES) measurements to provide powerful insight into the underlying compn. of the unoccupied 5f electronic structure in these 5f localized systems. Fine structure in the 5f5/2 transitions in HERFD can be directly correlated with the fine structure in the leading edge of the IPES. The shapes, intensities, and systematics in HERFD and IPES are explained in a consistent and rigorous fashion in terms of the j-specific 5f electronic structure.
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  5. 5
    Tobin, J. The apparent absence of chemical sensitivity in the 4d and 5d X-ray absorption spectroscopy of uranium compounds. J. Electron Spectrosc. Relat. Phenom. 2014, 194, 14– 22,  DOI: 10.1016/j.elspec.2014.01.020
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    The apparent absence of chemical sensitivity in the 4d and 5d X-ray absorption spectroscopy of uranium compounds
    Tobin, J. G.
    Journal of Electron Spectroscopy and Related Phenomena (2014), 194 (), 14-22CODEN: JESRAW; ISSN:0368-2048. (Elsevier B.V.)
    Recently, X-ray absorption spectroscopy (XAS) and related deriv. measurements have been used to demonstrate that the Pu 5f states are strongly relativistic and have a 5f occupation no. near 5. Owing to the success in this regime, it has been argued that the XAS measurements should be a powerful tool to probe 5f occupation variation, both as a function of elemental nature (actinide at. no.) and as a function of phys. and chem. perturbation, e.g., oxidn. state. It will be shown here that XAS and its related measurements fail in this latter aspect for a wide variety of uranium compds. and materials. Possible causes will be discussed.
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    Tobin, J. G.; Yu, S.-W.; Chung, B. Splittings, satellites and fine structure in the soft X-ray spectroscopy of the actinides. Top. Catal. 2013, 56, 1104– 1111,  DOI: 10.1007/s11244-013-0076-4
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    Splittings, satellites and fine structure in the soft x-ray spectroscopy of the actinides
    Tobin, J. G.; Yu, S.-W.; Chung, B. W.
    Topics in Catalysis (2013), 56 (12), 1104-1111CODEN: TOCAFI; ISSN:1022-5528. (Springer)
    A review. Perhaps the most demanding and powerful actinide spectroscopy is that using soft x-ray and VUV photons. Because of the relatively low energy and fairly small sampling depths of these photons and the corresponding electrons, it is necessary to use un-encapsulated samples with highly cleaned and well-prepd. surfaces. This causes a myriad of sample containment problems for these radioactive materials. Despite these hindrances and difficulties, the soft-x-ray and ultra-violet spectroscopy of the actinides can provide an amazing level of detailed information, particularly having to do with 5f electronic structure. In this paper, the splittings, satellites and fine structure of the following actinide soft x-ray spectroscopies are discussed: XPS; x-ray absorption spectroscopy; and inverse photoelectron spectroscopy, including Bremsstrahlung isochromat spectroscopy and resonant inverse photoelectron spectroscopy.
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    Zhou, F.; Ozoliņš, V. Crystal field and magnetic structure of UO2. Phys. Rev. B 2011, 83, 085106  DOI: 10.1103/PhysRevB.83.085106
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    Crystal field and magnetic structure of UO2
    Zhou, Fei; Ozolins, Vidvuds
    Physical Review B: Condensed Matter and Materials Physics (2011), 83 (8), 085106/1-085106/6CODEN: PRBMDO; ISSN:1098-0121. (American Physical Society)
    The properties of UO2 result from rich f-electron physics, including electronic Coulomb interactions, spin-orbit and crystal-field effects, as well as interionic multipolar coupling. We present a comprehensive theor. study of the electronic structure of UO2 using a combined application of self-consistent DFT+U calcns. and a model Hamiltonian. The Γ5 ground state of U4+ and the energies of crystal-field excitations Γ5 → Γ3,4,1 are reproduced in very good agreement with expt. We also investigate competing noncollinear magnetic structures and confirm 3k as the T = 0 K ground-state magnetic structure of UO2.
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    Wen, X.-D.; Martin, R. L.; Henderson, T. M.; Scuseria, G. E. Density functional theory studies of the electronic structure of solid state actinide oxides. Chem. Rev. 2013, 113, 1063– 1096,  DOI: 10.1021/cr300374y
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    Density Functional Theory Studies of the Electronic Structure of Solid State Actinide Oxides
    Wen, Xiao-Dong; Martin, Richard L.; Henderson, Thomas M.; Scuseria, Gustavo E.
    Chemical Reviews (Washington, DC, United States) (2013), 113 (2), 1063-1096CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)
    A review. We have surveyed a series of methods, including LDA/GGA, DFT+U, SIC, DFT+DMFT, and HSE, assessing theirpredictive capability for the Mott insulating actinide dioxides. Our focus has been on lattice consts., electronic densities-of-states, band gaps, and magnetic and optical properties. Of particular interest to us is the nature and magnitude of the gap. We conclude that the screened hybrid HSE functional gives areasonable reprodn. of the electronic and optical propertiesfor these actinide dioxides when compared with available exptl. data. Unlike DFT+U, HSE does not require the introduction of any material-specific parameters. Problems remaining to be addressed include multiplet effects and the proper treatment of complex magnetic properties. We also find that with a judiciously chosen U, the DFT+U approach can predict reasonable band gaps, generally at the expense of other important properties such as lattice consts. and densities of states. The most dramatic difference between HSE and SIC-DFTor DMFT is in the qual. nature of the gap as a function of the at. no. Z, where SIC and DMFT predict even the early members of the series to be charge-transfer insulators, while HSE correctly predicts the early actinide dioxides to be Mott insulators. The development of HSE has provided new and exciting insights into the fundamental chem. and physics of these strongly correlated insulators. However, there is still work to be done with the primary need for further improvement being assocd. with the ability to describe strongly correlated metals.
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  9. 9
    Chen, J.-L.; Kaltsoyannis, N. DFT+ U study of uranium dioxide and plutonium dioxide with occupation matrix control. J. Phys. Chem. C 2022, 126, 11426– 11435,  DOI: 10.1021/acs.jpcc.2c03804
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    DFT + U Study of Uranium Dioxide and Plutonium Dioxide with Occupation Matrix Control
    Chen, Jia-Li; Kaltsoyannis, Nikolas
    Journal of Physical Chemistry C (2022), 126 (27), 11426-11435CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
    DFT+U with occupation matrix control (OMC) is applied to study computationally bulk UO2 and PuO2, the latter for the first time. Using the PBESol functional in conjunction with OMC locates AFM and NM ground states for UO2 and PuO2, resp., in agreement with exptl. findings. By simulating the lattice parameter, magnetic moment, band gap, and densities of states, U = 4.0 eV is recommended for AFM UO2, yielding data close to expts. for all considered properties. U = 4.5 and 4.0 eV are recommended for NM and AFM PuO2, resp., though much larger U values (c. 10 eV) are required to yield the most recently reported PuO2 band gap. For both oxides, several excited states have similar properties to the ground state, reinforcing the need to employ OMC wherever possible.
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  10. 10
    Chen, J.-L.; Kaltsoyannis, N. Computational study of the bulk and surface properties of minor actinide dioxides MAnO2 (MAn= Np, Am, and Cm); Water adsorption on stoichiometric and reduced {111},{110}, and {100} surfaces. J. Phys. Chem. C 2019, 123, 15540– 15550,  DOI: 10.1021/acs.jpcc.9b02324
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    10
    Computational Study of the Bulk and Surface Properties of Minor Actinide Dioxides MAnO2 (MAn = Np, Am, and Cm); Water Adsorption on Stoichiometric and Reduced {111}, {110}, and {100} Surfaces
    Chen, Jia-Li; Kaltsoyannis, Nikolas
    Journal of Physical Chemistry C (2019), 123 (25), 15540-15550CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
    Hubbard U-cor. generalized gradient approxn. d. functional theory is used to study the bulk and surface properties of minor actinide dioxides NpO2, AmO2, and CmO2. Comparison of lattice parameters, band gaps, and d. of states with exptl. data indicates that PBE + U (U = 4.50 eV, J = 0.50 eV) is the best approach of those considered. Stoichiometric and oxygen vacancy defect {111}, {110}, and {100} surfaces are investigated, including the mol. and dissociative adsorptions of water. Comparison with previous work by our group on UO2 and PuO2 leads to the conclusion that the oxygen defect generation energy is essentially linearly correlated with the An(IV)/(III) redox potential across the AnO2 series from U to Cm. The geometries of water adsorption are similar across the series, whereas adsorption energies generally decrease, with the exception of certain PuO2 surfaces, in agreement with the very hygroscopic nature of PuO2. Surface defects promote dissociative water adsorption.
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    Petit, L.; Svane, A.; Szotek, Z.; Temmerman, W. M.; Stocks, G. M. Electronic structure and ionicity of actinide oxides from first principles. Phys. Rev. B 2010, 81, 045108  DOI: 10.1103/PhysRevB.81.045108
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    Electronic structure and ionicity of actinide oxides from first principles
    Petit, L.; Svane, A.; Szotek, Z.; Temmerman, W. M.; Stocks, G. M.
    Physical Review B: Condensed Matter and Materials Physics (2010), 81 (4), 045108/1-045108/12CODEN: PRBMDO; ISSN:1098-0121. (American Physical Society)
    The ground-state electronic structures of the actinide oxides AO, A2O3, and AO2 (A=U, Np, Pu, Am, Cm, Bk, and Cf) are detd. from first-principles calcns., using the self-interaction cor. local spin-d. approxn. Emphasis is put on the degree of f-electron localization, which for AO2 and A2O3 is found to follow the stoichiometry, namely, corresponding to A4+ ions in the dioxide and A3+ ions in the sesquioxides. In contrast, the A2+ ionic configuration is not favorable in the monoxides, which therefore become metallic. The energetics of the oxidn. and redn. in the actinide dioxides is discussed, and it is found that the dioxide is the most stable oxide for the actinides from Np onward. Our study reveals a strong link between preferred oxidn. no. and degree of localization which is confirmed by comparing to the ground-state configurations of the corresponding lanthanide oxides. The ionic nature of the actinide oxides emerges from the fact that only those compds. will form where the calcd. ground-state valency agrees with the nominal valency expected from a simple charge counting.
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  12. 12
    Chollet, M.; Martin, P.; Degueldre, C.; Poonoosamy, J.; Belin, R.; Hennig, C. Neptunium characterization in uranium dioxide fuel: Combining a XAFS and a thermodynamic approach. J. Alloys Compd. 2016, 662, 448– 454,  DOI: 10.1016/j.jallcom.2015.12.005
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    Neptunium characterization in uranium dioxide fuel: Combining a XAFS and a thermodynamic approach
    Chollet, M.; Martin, P.; Degueldre, C.; Poonoosamy, J.; Belin, R. C.; Hennig, C.
    Journal of Alloys and Compounds (2016), 662 (), 448-454CODEN: JALCEU; ISSN:0925-8388. (Elsevier B.V.)
    The effect of Np incorporation in a UO2 matrix on redox state of U and Np cations has been investigated by X-ray absorption spectroscopy (XAS) on three samples: (U0.9Np0.1)O2, (U0.9Np0.1)O2 + x and pure NpO2, the latter of which was used as a ref. for Np(IV) oxidn. state. XANES at the Uranium LIII edge and XANES and EXAFS at the Neptunium LIII edge show that Uranium only is oxidized to higher (V) and (VI) oxidn. states in the hyperstoichiometric (U0.9Np0.1)O2 + x sample in comparison to the stoichiometric (U0.9Np0.1)O2 where U appears at the (IV) oxidn. state. This contrasts to Neptunium cation that always remains at the (IV) oxidn. state whatever the oxygen stoichiometry. Thermodn. calcns. were performed to complete the exptl. study and lead to the same conclusion. Sep. UO2-NpO2 phases and homogeneous solid soln. were considered. The latter case shows that the energy of mixing is insignificant in this system. The combined exptl. and theor. approaches demonstrate that any excess of oxygen in the system is carried by Uranium.
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    Ditter, A. S.; Pacold, J. I.; Dai, Z.; Lee Davisson, M.; Vine, D.; Donald, S. B.; Chung, B. W.; Shuh, D. K. Submicrometer spectromicroscopy of UO2 aged under high humidity conditions. J. Vac. Sci. Technol., A 2022, 40, 043202  DOI: 10.1116/6.0001880
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    Submicrometer spectromicroscopy of UO2 aged under high humidity conditions
    Ditter, Alex S.; Pacold, Joseph I.; Dai, Zurong; Lee Davisson, M.; Vine, David; Donald, Scott B.; Chung, Brandon W.; Shuh, David K.
    Journal of Vacuum Science & Technology, A: Vacuum, Surfaces, and Films (2022), 40 (4), 043202CODEN: JVTAD6; ISSN:0734-2101. (American Institute of Physics)
    The oxidn. of uranium dioxide is a complicated process, depending on factors including humidity, temp., and microstructure. To further det. the characteristics of this process, UO2 particles were allowed to age and agglomerate under 98relative humidity at room temp. for 378 days. A focused ion beam (FIB) section of this agglomeration was then measured at the O K-edge, U N5-edge, and C K-edge using the scanning transmission x-ray microscope (STXM) at the Advanced Light Source. O K-edge and U N5-edge x-ray absorption measurements allowed for the elemental and chem. species mapping of the agglomerates and indicated the formation of schoepite at the submicrometer scale in specific locations. Non-neg. matrix factorization was employed to elucidate the main components at the O K-edge, which were uranyl (schoepite) formed primarily at the interface of the sample with controlled atm., a UO2-like bulk component present in the majority of the sample, and an oxygen species present at the surface of the FIB section, which is likely adsorbed water. STXM spectromicroscopy measurements at the U N5-edge measurements also confirmed the location of oxidized uranium. This anal. is a valuable insight into the formation of schoepite on UO2 and shows the sensitivity to and utility of STXM spectromicroscopy for uranium speciation. (c) 2022 American Institute of Physics.
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    Kalkowski, G.; Kaindl, G.; Brewer, W.; Krone, W. Near-edge x-ray-absorption fine structure in uranium compounds. Phys. Rev. B 1987, 35, 2667,  DOI: 10.1103/PhysRevB.35.2667
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    Near-edge x-ray-absorption fine structure in uranium compounds
    Kalkowski, G.; Kaindl, G.; Brewer, W. D.; Krone, W.
    Physical Review B: Condensed Matter and Materials Physics (1987), 35 (6), 2667-77CODEN: PRBMDO; ISSN:0163-1829.
    For a no. of metallic and nonmetallic U compds. and a few Th compds., the near-edge EXAFS spectra, taken by transmission and total-electron-yield techniques, are reported at the L3, M3, M4,5, N4,5, and O4,5, thresholds. At L3 and M3, similar spectra with analogous information were obsd. While in the metallic U systems, the white lines at L3 and M3 show only minor variations in position and relative peak height, a shift of ∼4 eV was obsd. between UCl3 and UF4, representing mainly the difference in Coulomb interaction between a 2p3/2 core hole and the 5f configurations. The spectra at the M4,5 and N4,5 thresholds are dominated by intense absorption peaks due to 3d-5f and 4d-5f transitions probing directly the unoccupied 5f states. No multiplet structure due to 3d-5f or 4d-5f exchange interaction was resolved, but the linewidths of the M4,5 peaks in the metallic U systems decrease by ≃20% with increasing U-U distance, reflecting a narrowing of the unoccupied 5f bands. The spectra at the O4,5 thresholds, on the other hand, show resolved structures, which are atomiclike for ThF4 and UF4 and may be described by spin-orbit and exchange splitting of the 5d95f1 and 5d95f3 final states, resp. In metallic U systems, the O4,5 structures are broadened and partly lost, presumably due to the more itinerant nature of the 5f states. Addnl. relatively weak peaks were obsd. in the M4,5 and O4,5 spectra of Th metal, ThO2, UO2, and UF4 at ∼15-38 eV above threshold, which are assigned to multiple-scattering resonances.
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    Butorin, S. M.; Modin, A.; Vegelius, J. R.; Kvashnina, K. O.; Shuh, D. K. Probing chemical bonding in uranium dioxide by means of high-resolution X-ray absorption spectroscopy. J. Phys. Chem. C 2016, 120, 29397– 29404,  DOI: 10.1021/acs.jpcc.6b09335
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    Probing Chemical Bonding in Uranium Dioxide by Means of High-Resolution X-ray Absorption Spectroscopy
    Butorin, Sergei M.; Modin, Anders; Vegelius, Johan R.; Kvashnina, Kristina O.; Shuh, David K.
    Journal of Physical Chemistry C (2016), 120 (51), 29397-29404CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
    A systematic x-ray absorption study at the U 3d, 4d, and 4f edges of UO2 was performed, and the data were analyzed within framework of the Anderson impurity model. By applying the high-energy-resoln. fluorescence-detection (HERFD) mode of x-ray absorption spectroscopy (XAS) at the U 3d3/2 edge and conducting the XAS measurements at the shallower U 4f levels, fine details of the XAS spectra were resolved resulting from reduced core-hole lifetime broadening. This multiedge study enabled a far more effective anal. of the electronic structure at the U sites and characterization of the chem. bonding and degree of the 5f localization in UO2. The results support the covalent character of UO2 and do not agree with the suggestions of rather ionic bonding in this compd. as expressed in some publications.
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    Duan, P.-Q.; Bao, H.-L.; Li, J.; Cao, H.-J.; Huang, Y.-Y. In-situ high-energy-resolution X-ray absorption spectroscopy for UO2 oxidation at SSRF. Nucl. Sci. Tech. 2017, 28, 2,  DOI: 10.1007/s41365-016-0155-x
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    In-situ high-energy-resolution X-ray absorption spectroscopy for UO2 oxidation at SSRF
    Duan, Pei-quan; Bao, Hong-liang; Li, Jiong; Cao, Han-jie; Huang, Yu-ying
    Nuclear Science and Techniques (2017), 28 (1), 2/1-2/4CODEN: NSETEC; ISSN:1001-8042. (Science Press)
    Based on the high-energy-resoln. fluorescence spectrometer on the BL14W1 beamline at Shanghai Synchrotron Radiation Facility, an in-situ high-energyresoln. X-ray absorption spectroscopy technique, with an in-situ heating cell, was developed. The high-energyresoln. fluorescence detection for X-ray absorption near-edge spectroscopy (HERFD-XANES) was tested in a UO2 oxidn. expt. to measure the UL3-edge, with higher signal-to-noise ratio and higher-energy-resoln. than conventional XANES. The technique has potential application for in-situ study of uranium-based materials.
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    Martin, P.; Ripert, M.; Carlot, G.; Parent, P.; Laffon, C. A study of molybdenum behaviour in UO2 by X-ray absorption spectroscopy. J. Nucl. Mater. 2004, 326, 132– 143,  DOI: 10.1016/j.jnucmat.2004.01.006
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    A study of molybdenum behavior in UO2 by X-ray absorption spectroscopy
    Martin, Philippe; Ripert, Michel; Carlot, Gaeelle; Parent, Philippe; Laffon, Carine
    Journal of Nuclear Materials (2004), 326 (2-3), 132-143CODEN: JNUMAM; ISSN:0022-3115. (Elsevier Science B.V.)
    Mo is one of the most abundant fission products created in UO2 fuel. Post-irradn. examns. show that Mo is mainly assocd. with Rh, Ru, Pd and Tc in metallic ppts. However, for low burn-ups, these 5 metal particles cannot be detected. To det. the chem. state of Mo in UO2, XANES expts. were carried out on fresh UO2 pellets implanted with Mo ions. The measurements performed at the O K and Mo LIII edges showed a correlation between the chem. state of Mo and the O/U ratio of the UO2 matrix. Indeed, the Mo located at the grain boundaries is present in both metallic and +IV oxidn. states. This study is the 1st exptl. evidence of the soln. annealing of Mo+IV in the cationic lattice of UO2. Thus, when Mo is oxidized at the +IV state, it remains in soln. in the UO2 matrix.
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    Gerber, E.; Romanchuk, A. Y.; Pidchenko, I.; Amidani, L.; Rossberg, A.; Hennig, C.; Vaughan, G. B. M.; Trigub, A.; Egorova, T.; Bauters, S. The missing pieces of the PuO2 nanoparticle puzzle. Nanoscale 2020, 12, 18039– 18048,  DOI: 10.1039/D0NR03767B
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    The missing pieces of the PuO2 nanoparticle puzzle
    Gerber, Evgeny; Romanchuk, Anna Yu.; Pidchenko, Ivan; Amidani, Lucia; Rossberg, Andre; Hennig, Christoph; Vaughan, Gavin B. M.; Trigub, Alexander; Egorova, Tolganay; Bauters, Stephen; Plakhova, Tatiana; Hunault, Myrtille O. J. Y.; Weiss, Stephan; Butorin, Sergei M.; Scheinost, Andreas C.; Kalmykov, Stepan N.; Kvashnina, Kristina O.
    Nanoscale (2020), 12 (35), 18039-18048CODEN: NANOHL; ISSN:2040-3372. (Royal Society of Chemistry)
    The nanoscience field often produces results more mystifying than any other discipline. It has been argued that changes in the plutonium dioxide (PuO2) particle size from bulk to nano can have a drastic effect on PuO2 properties. Here we report a full characterization of PuO2 nanoparticles (NPs) at the at. level and probe their local and electronic structures by a variety of methods available at the synchrotron, including extended X-ray absorption fine structure (EXAFS) at the Pu L3 edge, X-ray absorption near edge structure (XANES) in high energy resoln. fluorescence detection (HERFD) mode at the Pu L3 and M4 edges, high energy X-ray scattering (HEXS) and X-ray diffraction (XRD). The particles were synthesized from precursors with different oxidn. states of plutonium (III, IV, and V) under various environmentally and waste storage relevant conditions (pH 8 and pH > 10). Our exptl. results analyzed with state-of-the-art theor. approaches demonstrate that well dispersed, cryst. NPs with a size of ∼2.5 nm in diam. are always formed in spite of diverse chem. conditions. Identical crystal structures and the presence of only the Pu(IV) oxidn. state in all NPs, reported here for the first time, indicate that the structure of PuO2 NPs is very similar to that of the bulk PuO2. All methods give complementary information and show that investigated fundamental properties of PuO2 NPs, rather than being exotic, are very similar to those of the bulk PuO2.
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  19. 19
    Dalodière, E.; Virot, M.; Morosini, V.; Chave, T.; Dumas, T.; Hennig, C.; Wiss, T.; Dieste Blanco, O.; Shuh, D. K.; Tyliszcak, T. Insights into the sonochemical synthesis and properties of salt-free intrinsic plutonium colloids. Sci. Rep. 2017, 7, 43514  DOI: 10.1038/srep43514
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    Insights into the sonochemical synthesis and properties of salt-free intrinsic plutonium colloids
    Dalodiere Elodie; Virot Matthieu; Morosini Vincent; Chave Tony; Nikitenko Sergey I; Dumas Thomas; Venault Laurent; Moisy Philippe; Hennig Christoph; Wiss Thierry; Dieste Blanco Oliver; Shuh David K; Tyliszcak Tolek
    Scientific reports (2017), 7 (), 43514 ISSN:.
    Fundamental knowledge on intrinsic plutonium colloids is important for the prediction of plutonium behaviour in the geosphere and in engineered systems. The first synthetic route to obtain salt-free intrinsic plutonium colloids by ultrasonic treatment of PuO2 suspensions in pure water is reported. Kinetics showed that both chemical and mechanical effects of ultrasound contribute to the mechanism of Pu colloid formation. In the first stage, fragmentation of initial PuO2 particles provides larger surface contact between cavitation bubbles and solids. Furthermore, hydrogen formed during sonochemical water splitting enables reduction of Pu(IV) to more soluble Pu(III), which then re-oxidizes yielding Pu(IV) colloid. A comparative study of nanostructured PuO2 and Pu colloids produced by sonochemical and hydrolytic methods, has been conducted using HRTEM, Pu LIII-edge XAS, and O K-edge NEXAFS/STXM. Characterization of Pu colloids revealed a correlation between the number of Pu-O and Pu-Pu contacts and the atomic surface-to-volume ratio of the PuO2 nanoparticles. NEXAFS indicated that oxygen state in hydrolytic Pu colloid is influenced by hydrolysed Pu(IV) species to a greater extent than in sonochemical PuO2 nanoparticles. In general, hydrolytic and sonochemical Pu colloids can be described as core-shell nanoparticles composed of quasi-stoichiometric PuO2 cores and hydrolyzed Pu(IV) moieties at the surface shell.
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    Martin, P.; Grandjean, S.; Ripert, M.; Freyss, M.; Blanc, P.; Petit, T. Oxidation of plutonium dioxide: an X-ray absorption spectroscopy study. J. Nucl. Mater. 2003, 320, 138– 141,  DOI: 10.1016/S0022-3115(03)00180-6
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    Oxidation of plutonium dioxide: an X-ray absorption spectroscopy study
    Martin, Philippe; Grandjean, Stephane; Ripert, Michel; Freyss, Michel; Blanc, Pierre; Petit, Thierry
    Journal of Nuclear Materials (2003), 320 (1-2), 138-141CODEN: JNUMAM; ISSN:0022-3115. (Elsevier Science B.V.)
    X-ray absorption spectroscopy expts. were conducted to characterize Pu dioxide oxidn. The sample prepn. adopted does not enable elaboration of hyper-stoichiometric Pu dioxide. These results could mean that Pu dioxide oxidn. only occurs under very specific conditions.
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    Hudson, E. A.; Rehr, J.; Bucher, J. Multiple-scattering calculations of the uranium L3-edge X-ray-absorption near-edge structure. Phys. Rev. B 1995, 52, 13815,  DOI: 10.1103/PhysRevB.52.13815
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    Multiple-scattering calculations of the uranium L3-edge x-ray-absorption near-edge structure
    Hudson, E. A.; Rehr, J. J.; Bucher, J. J.
    Physical Review B: Condensed Matter (1995), 52 (19), 13815-26CODEN: PRBMDO; ISSN:0163-1829. (American Physical Society)
    A theor. study of the uranium L3-edge x-ray absorption near-edge structure (XANES) is presented for several uranium compds., including oxides, intermetallics, uranyl fluoride, and α-uranium. Calcns. were performed using FEFF6, an ab initio multiple-scattering (MS) code that includes the most important features of current theories. The results, which account for both the fine structure χ and the at. like background μ0 of the absorption coeff. μ, are compared to new and previously measured exptl. spectra, revealing very good agreement for most systems. For several compds., a more detailed theor. anal. detd. the influence of cluster size and scattering order upon the calcd. spectra. Results indicate that MS paths and scattering paths that include rather distant atoms make significant contributions for UO2, whereas XANES for crystals with lower symmetry and d. can be modeled using only shorter single-scattering paths. In most cases, assumption of a screened final state in the calcn. gives better agreement with expt. than use of an unscreened final state. The successful modeling of spectra for a variety of different uranium compds., with differing spectral features, indicates that the semirelativistic treatment of XANES used here is adequate even for heavy elements. The well-known resonance, obsd. exptl. for uranyl (UO22+) compds. ≈15 eV above the white line, is successfully modeled here for the first time, using multiple-scattering paths within the O-U-O axial bonds. Overlapping muffin-tin spheres were required in the calcn., probably as a result of the short uranyl axial bonds.
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    Jollet, F.; Petit, T.; Gota, S.; Thromat, N.; Gautier-Soyer, M.; Pasturel, A. The electronic structure of uranium dioxide: an oxygen K-edge X-ray absorption study. J. Phys.: Condens. Matter 1997, 9, 9393,  DOI: 10.1088/0953-8984/9/43/022
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    The electronic structure of uranium dioxide: an oxygen K-edge x-ray absorption study
    Jollet, F.; Petit, T.; Gota, S.; Thromat, N.; Gautier-Soyer, M.; Pasturel, A.
    Journal of Physics: Condensed Matter (1997), 9 (43), 9393-9401CODEN: JCOMEL; ISSN:0953-8984. (Institute of Physics Publishing)
    The O K-edge x-ray absorption near edge structure (XANES) spectrum of UO2 is presented and interpreted. A comparison with that corresponding to CeO2 is made. First-principles-based calcns. using the LSDA+U approach allows us to link each feature present in the spectra to the specific at. arrangement and electronic structure of the compd. The structures at the edge originate from oxygen 2p states hybridized with U 5f and 6d orbitals and the 6d eg-t2g splitting is found to be 4.8 eV. The structures due to O 2p-U 5f hybridization are found to be lower in energy than the structures due to the O 2p-U 6d hybridization. On this basis, UO2 can be considered as an f-f Mott-Hubbard insulator.
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    Modin, A.; Suzuki, M.; Vegelius, J.; Yun, Y.; Shuh, D.; Werme, L.; Nordgren, J.; Oppeneer, P. M.; Butorin, S. M. 5 f-Shell correlation effects in dioxides of light actinides studied by O 1s x-ray absorption and emission spectroscopies and first-principles calculations. J. Phys.: Condens. Matter 2015, 27, 315503  DOI: 10.1088/0953-8984/27/31/315503
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    5 f -shell correlation effects in dioxides of light actinides studied by O 1s x-ray absorption and emission spectroscopies and first-principles calculations
    Modin A; Suzuki M-T; Vegelius J; Yun Y; Shuh D K; Werme L; Nordgren J; Oppeneer P M; Butorin S M
    Journal of physics. Condensed matter : an Institute of Physics journal (2015), 27 (31), 315503 ISSN:.
    Soft x-ray emission and absorption spectroscopic data are reported for the O 1s region of a single crystal of UO2, a polycrystalline NpO2 sample, and a single crystal of PuO2. The experimental data are interpreted using first-principles correlated-electron calculations within the framework of the density functional theory with added Coulomb U interaction (DFT+U). A detailed analysis regarding the origin of different structures in the x-ray emission and x-ray absorption spectra is given and the effect of varying the intra-atomic Coulomb interaction-U for the 5 f electrons is investigated. Our data indicate that O 1s x-ray absorption and emission spectroscopies can, in combination with DFT+U calculations, successfully be used to study 5 f -shell Coulomb correlation effects in dioxides of light actinides. The values for the Coulomb U parameter in these dioxides are derived to be in the range of 4-5 eV.
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    Ramanantoanina, H.; Kuri, G.; Martin, M.; Bertsch, J. Study of electronic structure in the L-edge spectroscopy of actinide materials: UO2 as an example. Phys. Chem. Chem. Phys. 2019, 21, 7789– 7801,  DOI: 10.1039/C9CP01021A
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    Study of electronic structure in the L-edge spectroscopy of actinide materials: UO2 as an example
    Ramanantoanina, Harry; Kuri, Goutam; Martin, Matthias; Bertsch, Johannes
    Physical Chemistry Chemical Physics (2019), 21 (15), 7789-7801CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
    While the electronic structure calcn. for actinide materials, using ligand-field phenomenol. in conjunction with d. functional theory (LFDFT) treating configurations with single or two open-shells 5f and 6d electrons, is well established and currently practiced, the consideration of the three open-shells electron configurations for LFDFT treatment is a challenging task addressed in the present work. Herein, we report the first-principles method, developed for the first time on the basis of LFDFT, to evaluate the uranium L3-edge X-ray absorption near-edge structure (XANES), which requires non-equiv. active electrons within the 2p, 5f and 6d orbitals of the uranium ion. The theor. results, when compared with the exptl. XANES data measured from uranium dioxide fresh fuel pellets and rector-exposed spent fuel materials, show good agreement with the exptl. findings elucidating the local oxidn. in the spent fuel materials. This report is relevant for the commonly used L-edge spectroscopy of actinide isotopes and important for understanding the structural, optical and electronic properties of actinide-based materials.
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    Wu, Z. Y.; Jollet, F.; Gota, S.; Thromat, N.; Gautier-Soyer, M.; Petit, T. X-ray absorption at the oxygen K edge in cubic f oxides examined using a full multiple-scattering approach. J. Phys.: Condens. Matter 1999, 11, 7185,  DOI: 10.1088/0953-8984/11/37/314
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    X-ray absorption at the oxygen K edge in cubic f oxides examined using a full multiple-scattering approach
    Wu, Z. Y.; Jollet, F.; Gota, S.; Thromat, N.; Gautier-Soyer, M.; Petit, T.
    Journal of Physics: Condensed Matter (1999), 11 (37), 7185-7194CODEN: JCOMEL; ISSN:0953-8984. (Institute of Physics Publishing)
    The O K-edge x-ray absorption near-edge-structure (XANES) spectra of UO2 and CeO2 are presented and interpreted. Using different-size clusters around the excited atom in the full multiple-scattering (MS) simulation, we are able to link the features present in the spectra of each oxide to its specific at. arrangement and electronic structure. The structures at the edge originate from O 2p states hybridized with f and d orbitals of the cation split by the cubic crystal field. All of the other features come from MS with the neighboring shells of the central O atoms.
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    Kvashnina, K. O.; Romanchuk, A. Y.; Pidchenko, I.; Amidani, L.; Gerber, E.; Trigub, A.; Rossberg, A.; Weiss, S.; Popa, K.; Walter, O. A Novel Metastable Pentavalent Plutonium Solid Phase on the Pathway from Aqueous Plutonium(VI) to PuO2 Nanoparticles. Angew. Chem., Int. Ed. 2019, 58, 17558– 17562,  DOI: 10.1002/anie.201911637
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    A Novel Metastable Pentavalent Plutonium Solid Phase on the Pathway from Aqueous Plutonium(VI) to PuO2 Nanoparticles
    Kvashnina, Kristina O.; Romanchuk, Anna Yu.; Pidchenko, Ivan; Amidani, Lucia; Gerber, Evgeny; Trigub, Alexander; Rossberg, Andre; Weiss, Stephan; Popa, Karin; Walter, Olaf; Caciuffo, Roberto; Scheinost, Andreas C.; Butorin, Sergei M.; Kalmykov, Stepan N.
    Angewandte Chemie, International Edition (2019), 58 (49), 17558-17562CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Here we provide evidence that the formation of PuO2 nanoparticles from oxidized PuVI under alk. conditions proceeds through the formation of an intermediate PuV solid phase, similar to NH4PuO2CO3, which is stable over a period of several months. For the first time, state-of-the-art expts. at Pu M4 and at L3 absorption edges combined with theor. calcns. unambiguously allow to det. the oxidn. state and the local structure of this intermediate phase.
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    Modin, A.; Yun, Y.; Suzuki, M.-T.; Vegelius, J.; Werme, L.; Nordgren, J.; Oppeneer, P.; Butorin, S. Indication of single-crystal PuO2 oxidation from O 1 s x-ray absorption spectra. Phys. Rev. B 2011, 83, 075113  DOI: 10.1103/PhysRevB.83.075113
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    Indication of single-crystal PuO2 oxidation from O 1s x-ray absorption spectra
    Modin, A.; Yun, Y.; Suzuki, M.-T.; Vegelius, J.; Werme, L.; Nordgren, J.; Oppeneer, P. M.; Butorin, S. M.
    Physical Review B: Condensed Matter and Materials Physics (2011), 83 (7), 075113/1-075113/7CODEN: PRBMDO; ISSN:1098-0121. (American Physical Society)
    The electronic structure of single-crystal PuO2 is studied using O 1s x-ray absorption (XA) and x-ray emission. Interpretation of the exptl. data is supported by extensive first-principles calcns. on the basis of the d. functional theory+U approach. The measured XA spectra show a significant difference in intensity for the first two peaks between different spots or areas on the single crystal. Our theor. simulations show that the first peak, at ∼531 eV, can be attributed to O 2p-Pu 5f hybridization, while the second peak, at ∼533.4 eV, is due to hybridization of O 2p with Pu d states. The reasons for the obsd. differences in the O 1s XA spectra are explored by calcg. a no. of defect structures PuO2±x as well as by simulating the existence of Pu(V) sites. Our results indicate the presence of oxidn. states higher than Pu(IV) in some areas of the single crystal. The findings also suggest that plutonium oxide with a Pu fraction in an oxidn. state higher than Pu(IV) consists of inequivalent Pu sites with Pu(IV)O2 and Pu(V)O2 rather than representing a system where the Pu oxidn. state is constantly fluctuating between Pu(IV) and Pu(V).
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    Blaha, P.; Schwarz, K.; Tran, F.; Laskowski, R.; Madsen, G. K.; Marks, L. D. WIEN2k: An APW+ lo program for calculating the properties of solids. J. Chem. Phys. 2020, 152, 074101  DOI: 10.1063/1.5143061
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    WIEN2k: An APW+lo program for calculating the properties of solids
    Blaha, Peter; Schwarz, Karlheinz; Tran, Fabien; Laskowski, Robert; Madsen, Georg K. H.; Marks, Laurence D.
    Journal of Chemical Physics (2020), 152 (7), 074101CODEN: JCPSA6; ISSN:0021-9606. (American Institute of Physics)
    The WIEN2k program is based on the APW plus local orbitals (APW + lo) method to solve the Kohn-Sham equations of d. functional theory. The APW + lo method, which considers all electrons (core and valence) self-consistently in a full-potential treatment, is implemented very efficiently in WIEN2k, since various types of parallelization are available and many optimized numerical libraries can be used. Many properties can be calcd., ranging from the basic ones, such as the electronic band structure or the optimized at. structure, to more specialized ones such as the NMR shielding tensor or the elec. polarization. After a brief presentation of the APW + lo method, we review the usage, capabilities, and features of WIEN2k (version 19) in detail. The various options, properties, and available approxns. for the exchange-correlation functional, as well as the external libraries or programs that can be used with WIEN2k, are mentioned. Refs. to relevant applications and some examples are also given. (c) 2020 American Institute of Physics.
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    Idiri, M.; Le Bihan, T.; Heathman, S.; Rebizant, J. Behavior of actinide dioxides under pressure: UO2 and ThO2. Phys. Rev. B 2004, 70, 014113  DOI: 10.1103/PhysRevB.70.014113
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    Behavior of actinide dioxides under pressure: UO2 and ThO2
    Idiri, M.; Le Bihan, T.; Heathman, S.; Rebizant, J.
    Physical Review B: Condensed Matter and Materials Physics (2004), 70 (1), 014113/1-014113/8CODEN: PRBMDO; ISSN:0163-1829. (American Physical Society)
    The structural high-pressure properties of ThO2 and UO2 were studied in diamond-anvil cells up to max. pressures of 80 and 69 GPa, resp. Both Th and U dioxides exhibit an identical sequence of structural transitions under pressure; both transform rather sluggishly to the cotunnite-type (orthorhombic Pnma) high-pressure structure. This study which was performed under better hydrostatic conditions than previous expts. has enabled the authors to det. reliable compressibility parameters for the two dioxides: ThO2: B0 = 198(2) GPa and B'0=4.6(3); UO2: B0 = 207(2) GPa and B'0=4.5(4). In the case of UO2 the ambient pressure cubic phase was still found to be present at 69 GPa, which is in contradiction with earlier measurements. With regards to these results and re-calcns. performed on other actinide dioxides PuO2 and AmO2, the authors obtain a different evolution of the bulk moduli through the actinide dioxide series.
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    Yamashita, T.; Nitani, N.; Tsuji, T.; Kato, T. Thermal expansion of neptunium-uranium mixed oxides. J. Nucl. Mater. 1997, 247, 90– 93,  DOI: 10.1016/S0022-3115(97)00031-7
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    Thermal expansion of neptunium-uranium mixed oxides
    Yamashita, Toshiyuki; Nitani, Noriko; Tsuji, Toshihide; Kato, Tetsuya
    Journal of Nuclear Materials (1997), 247 (), 90-93CODEN: JNUMAM; ISSN:0022-3115. (Elsevier)
    Thermal expansions of NpyU1-yO2 solid solns. were investigated between room temp. and 1273 K by a high temp. x-ray diffraction technique. The lattice parameters of NpyU1-yO2 solid solns. at high temps. were given in polynomial expressions of temp. High temp. heat capacities, Cp, of NpyU1-yO2 solid solns. were estd. from the thermodn. relation using the measured thermal expansions and literature data. The estd. errors in the calcd. Cp of UO2 were less than ±5%.
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    Haschke, J. M.; Allen, T. H.; Morales, L. A. Reaction of plutonium dioxide with Water: formation and properties of PuO2. Science 2000, 287, 285– 287,  DOI: 10.1126/science.287.5451.285
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    Reaction of plutonium dioxide with water: formation and properties of PuO2+x
    Haschke, John M.; Allen, Thomas H.; Morales, Luis A.
    Science (Washington, D. C.) (2000), 287 (5451), 285-287CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
    PuO2+x, a high-compn. (x ≤ 0.27) phase contg. Pu(VI), is the stable binary oxide in air. This nonstoichiometric oxide forms by reaction of dioxide with water and by water-catalyzed reaction of dioxide with oxygen. The PuOz + H2O reaction rate is 0.27 nmol per m squared per h at 250C; the activation energy at 250 to 3500C is 39 kJ per mol. Slow kinetics and a low lattice parameter-compn. dependence for fluorite-related PuO2+x are consistent with a failure to observe the phase in earlier studies. Perplexing aspects of plutonium oxide chem. can now be explained.
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    Physical Review Letters (1996), 77 (18), 3865-3868CODEN: PRLTAO; ISSN:0031-9007. (American Physical Society)
    Generalized gradient approxns. (GGA's) for the exchange-correlation energy improve upon the local spin d. (LSD) description of atoms, mols., and solids. We present a simple derivation of a simple GGA, in which all parameters (other than those in LSD) are fundamental consts. Only general features of the detailed construction underlying the Perdew-Wang 1991 (PW91) GGA are invoked. Improvements over PW91 include an accurate description of the linear response of the uniform electron gas, correct behavior under uniform scaling, and a smoother potential.
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    Anisimov, V. I.; Solovyev, I.; Korotin, M.; Czyżyk, M.; Sawatzky, G. Density-functional theory and NiO photoemission spectra. Phys. Rev. B 1993, 48, 16929,  DOI: 10.1103/PhysRevB.48.16929
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    Density-functional theory and NiO photoemission spectra
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    Physical Review B: Condensed Matter and Materials Physics (1993), 48 (23), 16929-34CODEN: PRBMDO; ISSN:0163-1829.
    The generalization of the local-d.-approxn. method for the systems with strong Coulomb correlations is proposed, which restores the discontinuity in the one-electron potential as in the exact d. functional. The method is based on the model-Hamiltonian approach and allows the authors to take into account the nonsphericity of the Coulomb and exchange interactions. The calcn. scheme could be regarded as a first-principle method due to the absence of adjustable parameters. The method was applied to the calcn. of the photoemission (x-ray photoemission spectroscopy) and bremsstrahlung isochromat spectra of NiO.
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    Laskowski, R.; Madsen, G. K.; Blaha, P.; Schwarz, K. Magnetic structure and electric-field gradients of uranium dioxide: An ab initio study. Phys. Rev. B 2004, 69, 140408  DOI: 10.1103/PhysRevB.69.140408
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    Magnetic structure and electric-field gradients of uranium dioxide: An ab initio study
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Cite this: J. Phys. Chem. C 2023, 127, 36, 17994–18000
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Published September 5, 2023

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  • Abstract

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    Figure 1

    Figure 1. (a) Simulated U LIII edge XANES of UO2 (S = 2.4 eV, G = 9.0 eV) and previous experimental and simulated spectra: expt.1, expt.2, and simu1 are from refs (14,16) and (21), respectively; (b) dipole matrix elements of U s and d states; (c) density of states of U s and d states and U LIII edge XANES of UO2 (S = 2.4 eV and G = 9.0 eV); (d) density of states of U d states and amplified O s (4-fold), p (2-fold), and d (4-fold) states, and U LIII edge XANES of UO2 (S = 2.4 eV and G = 9.0 eV). All calculations are performed without CH. XANES peak A in panels (a) and (d) are aligned and shifted to 10 eV, and the DOS in panels (c) and (d) and dipole matrix elements in panel (b) are also shifted to match the XANES. Spectra and DOS in panels (a), (c), and (d) are shifted vertically for better visibility.

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    Figure 2

    Figure 2. Simulated Pu LIII edge XANES of PuO2 (no CH, S = 2.4 eV, G = 9.0 eV) and previous experimental and simulated spectra: expt. and simul. are from ref (26). Spectra are shifted vertically for better visibility.

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    Figure 3

    Figure 3. (a) Simulated O K edge XANES of UO2 for a supercell with 1CH: up-VB (S = 0.2 eV, G = 0.8 eV) and previous experimental and simulated O K edge XANES; expt.1, expt.2, and simul. are from refs (17,22) and (25), respectively. (b) DOS of amplified O p (10-fold) and U s (10-fold), p (10-fold), and d and f (0.5-fold) states and O K edge of UO2 (S = 0.2 eV and G = 0.8 eV); all simulations are performed with a supercell with 1CH generated by moving one 1s electron of O and adding it to the VB. The highest peaks of the XANES are in all figures aligned and shifted to 10 eV. The energy of the DOS is also shifted in panel (b) to match the XANES. Spectra and DOS are shifted vertically for better visibility.

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    Figure 4

    Figure 4. (a) Simulated O K edge of PuO2 for a supercell with 1CH: up-BG (S = 0.5 eV, G = 0.6 eV) and previous experimental and simulated O K edge data; expt.-SC, expt.-PC, and simul. are from refs (27,29) to (27), respectively. (b) DOS of amplified O p (10-fold) and Pu s (10-fold), p (10-fold), and d and f (0.5-fold) states and O K edge of PuO2 (S = 0.5 eV and G = 0.6 eV); all simulations are performed with a supercell with 1CH generated by removing one 1s electron of O and adding it to the background. The highest peaks of the XANES are in all figures aligned and shifted to 10 eV. The energy of the DOS is also shifted in panel (b) to match the XANES. Spectra and DOS are shifted vertically for better visibility.

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      Wen, Xiao-Dong; Martin, Richard L.; Henderson, Thomas M.; Scuseria, Gustavo E.
      Chemical Reviews (Washington, DC, United States) (2013), 113 (2), 1063-1096CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)
      A review. We have surveyed a series of methods, including LDA/GGA, DFT+U, SIC, DFT+DMFT, and HSE, assessing theirpredictive capability for the Mott insulating actinide dioxides. Our focus has been on lattice consts., electronic densities-of-states, band gaps, and magnetic and optical properties. Of particular interest to us is the nature and magnitude of the gap. We conclude that the screened hybrid HSE functional gives areasonable reprodn. of the electronic and optical propertiesfor these actinide dioxides when compared with available exptl. data. Unlike DFT+U, HSE does not require the introduction of any material-specific parameters. Problems remaining to be addressed include multiplet effects and the proper treatment of complex magnetic properties. We also find that with a judiciously chosen U, the DFT+U approach can predict reasonable band gaps, generally at the expense of other important properties such as lattice consts. and densities of states. The most dramatic difference between HSE and SIC-DFTor DMFT is in the qual. nature of the gap as a function of the at. no. Z, where SIC and DMFT predict even the early members of the series to be charge-transfer insulators, while HSE correctly predicts the early actinide dioxides to be Mott insulators. The development of HSE has provided new and exciting insights into the fundamental chem. and physics of these strongly correlated insulators. However, there is still work to be done with the primary need for further improvement being assocd. with the ability to describe strongly correlated metals.
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    9. 9
      Chen, J.-L.; Kaltsoyannis, N. DFT+ U study of uranium dioxide and plutonium dioxide with occupation matrix control. J. Phys. Chem. C 2022, 126, 11426– 11435,  DOI: 10.1021/acs.jpcc.2c03804
      9
      DFT + U Study of Uranium Dioxide and Plutonium Dioxide with Occupation Matrix Control
      Chen, Jia-Li; Kaltsoyannis, Nikolas
      Journal of Physical Chemistry C (2022), 126 (27), 11426-11435CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
      DFT+U with occupation matrix control (OMC) is applied to study computationally bulk UO2 and PuO2, the latter for the first time. Using the PBESol functional in conjunction with OMC locates AFM and NM ground states for UO2 and PuO2, resp., in agreement with exptl. findings. By simulating the lattice parameter, magnetic moment, band gap, and densities of states, U = 4.0 eV is recommended for AFM UO2, yielding data close to expts. for all considered properties. U = 4.5 and 4.0 eV are recommended for NM and AFM PuO2, resp., though much larger U values (c. 10 eV) are required to yield the most recently reported PuO2 band gap. For both oxides, several excited states have similar properties to the ground state, reinforcing the need to employ OMC wherever possible.
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    10. 10
      Chen, J.-L.; Kaltsoyannis, N. Computational study of the bulk and surface properties of minor actinide dioxides MAnO2 (MAn= Np, Am, and Cm); Water adsorption on stoichiometric and reduced {111},{110}, and {100} surfaces. J. Phys. Chem. C 2019, 123, 15540– 15550,  DOI: 10.1021/acs.jpcc.9b02324
      10
      Computational Study of the Bulk and Surface Properties of Minor Actinide Dioxides MAnO2 (MAn = Np, Am, and Cm); Water Adsorption on Stoichiometric and Reduced {111}, {110}, and {100} Surfaces
      Chen, Jia-Li; Kaltsoyannis, Nikolas
      Journal of Physical Chemistry C (2019), 123 (25), 15540-15550CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
      Hubbard U-cor. generalized gradient approxn. d. functional theory is used to study the bulk and surface properties of minor actinide dioxides NpO2, AmO2, and CmO2. Comparison of lattice parameters, band gaps, and d. of states with exptl. data indicates that PBE + U (U = 4.50 eV, J = 0.50 eV) is the best approach of those considered. Stoichiometric and oxygen vacancy defect {111}, {110}, and {100} surfaces are investigated, including the mol. and dissociative adsorptions of water. Comparison with previous work by our group on UO2 and PuO2 leads to the conclusion that the oxygen defect generation energy is essentially linearly correlated with the An(IV)/(III) redox potential across the AnO2 series from U to Cm. The geometries of water adsorption are similar across the series, whereas adsorption energies generally decrease, with the exception of certain PuO2 surfaces, in agreement with the very hygroscopic nature of PuO2. Surface defects promote dissociative water adsorption.
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    11. 11
      Petit, L.; Svane, A.; Szotek, Z.; Temmerman, W. M.; Stocks, G. M. Electronic structure and ionicity of actinide oxides from first principles. Phys. Rev. B 2010, 81, 045108  DOI: 10.1103/PhysRevB.81.045108
      11
      Electronic structure and ionicity of actinide oxides from first principles
      Petit, L.; Svane, A.; Szotek, Z.; Temmerman, W. M.; Stocks, G. M.
      Physical Review B: Condensed Matter and Materials Physics (2010), 81 (4), 045108/1-045108/12CODEN: PRBMDO; ISSN:1098-0121. (American Physical Society)
      The ground-state electronic structures of the actinide oxides AO, A2O3, and AO2 (A=U, Np, Pu, Am, Cm, Bk, and Cf) are detd. from first-principles calcns., using the self-interaction cor. local spin-d. approxn. Emphasis is put on the degree of f-electron localization, which for AO2 and A2O3 is found to follow the stoichiometry, namely, corresponding to A4+ ions in the dioxide and A3+ ions in the sesquioxides. In contrast, the A2+ ionic configuration is not favorable in the monoxides, which therefore become metallic. The energetics of the oxidn. and redn. in the actinide dioxides is discussed, and it is found that the dioxide is the most stable oxide for the actinides from Np onward. Our study reveals a strong link between preferred oxidn. no. and degree of localization which is confirmed by comparing to the ground-state configurations of the corresponding lanthanide oxides. The ionic nature of the actinide oxides emerges from the fact that only those compds. will form where the calcd. ground-state valency agrees with the nominal valency expected from a simple charge counting.
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    12. 12
      Chollet, M.; Martin, P.; Degueldre, C.; Poonoosamy, J.; Belin, R.; Hennig, C. Neptunium characterization in uranium dioxide fuel: Combining a XAFS and a thermodynamic approach. J. Alloys Compd. 2016, 662, 448– 454,  DOI: 10.1016/j.jallcom.2015.12.005
      12
      Neptunium characterization in uranium dioxide fuel: Combining a XAFS and a thermodynamic approach
      Chollet, M.; Martin, P.; Degueldre, C.; Poonoosamy, J.; Belin, R. C.; Hennig, C.
      Journal of Alloys and Compounds (2016), 662 (), 448-454CODEN: JALCEU; ISSN:0925-8388. (Elsevier B.V.)
      The effect of Np incorporation in a UO2 matrix on redox state of U and Np cations has been investigated by X-ray absorption spectroscopy (XAS) on three samples: (U0.9Np0.1)O2, (U0.9Np0.1)O2 + x and pure NpO2, the latter of which was used as a ref. for Np(IV) oxidn. state. XANES at the Uranium LIII edge and XANES and EXAFS at the Neptunium LIII edge show that Uranium only is oxidized to higher (V) and (VI) oxidn. states in the hyperstoichiometric (U0.9Np0.1)O2 + x sample in comparison to the stoichiometric (U0.9Np0.1)O2 where U appears at the (IV) oxidn. state. This contrasts to Neptunium cation that always remains at the (IV) oxidn. state whatever the oxygen stoichiometry. Thermodn. calcns. were performed to complete the exptl. study and lead to the same conclusion. Sep. UO2-NpO2 phases and homogeneous solid soln. were considered. The latter case shows that the energy of mixing is insignificant in this system. The combined exptl. and theor. approaches demonstrate that any excess of oxygen in the system is carried by Uranium.
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    13. 13
      Ditter, A. S.; Pacold, J. I.; Dai, Z.; Lee Davisson, M.; Vine, D.; Donald, S. B.; Chung, B. W.; Shuh, D. K. Submicrometer spectromicroscopy of UO2 aged under high humidity conditions. J. Vac. Sci. Technol., A 2022, 40, 043202  DOI: 10.1116/6.0001880
      13
      Submicrometer spectromicroscopy of UO2 aged under high humidity conditions
      Ditter, Alex S.; Pacold, Joseph I.; Dai, Zurong; Lee Davisson, M.; Vine, David; Donald, Scott B.; Chung, Brandon W.; Shuh, David K.
      Journal of Vacuum Science & Technology, A: Vacuum, Surfaces, and Films (2022), 40 (4), 043202CODEN: JVTAD6; ISSN:0734-2101. (American Institute of Physics)
      The oxidn. of uranium dioxide is a complicated process, depending on factors including humidity, temp., and microstructure. To further det. the characteristics of this process, UO2 particles were allowed to age and agglomerate under 98relative humidity at room temp. for 378 days. A focused ion beam (FIB) section of this agglomeration was then measured at the O K-edge, U N5-edge, and C K-edge using the scanning transmission x-ray microscope (STXM) at the Advanced Light Source. O K-edge and U N5-edge x-ray absorption measurements allowed for the elemental and chem. species mapping of the agglomerates and indicated the formation of schoepite at the submicrometer scale in specific locations. Non-neg. matrix factorization was employed to elucidate the main components at the O K-edge, which were uranyl (schoepite) formed primarily at the interface of the sample with controlled atm., a UO2-like bulk component present in the majority of the sample, and an oxygen species present at the surface of the FIB section, which is likely adsorbed water. STXM spectromicroscopy measurements at the U N5-edge measurements also confirmed the location of oxidized uranium. This anal. is a valuable insight into the formation of schoepite on UO2 and shows the sensitivity to and utility of STXM spectromicroscopy for uranium speciation. (c) 2022 American Institute of Physics.
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    14. 14
      Kalkowski, G.; Kaindl, G.; Brewer, W.; Krone, W. Near-edge x-ray-absorption fine structure in uranium compounds. Phys. Rev. B 1987, 35, 2667,  DOI: 10.1103/PhysRevB.35.2667
      14
      Near-edge x-ray-absorption fine structure in uranium compounds
      Kalkowski, G.; Kaindl, G.; Brewer, W. D.; Krone, W.
      Physical Review B: Condensed Matter and Materials Physics (1987), 35 (6), 2667-77CODEN: PRBMDO; ISSN:0163-1829.
      For a no. of metallic and nonmetallic U compds. and a few Th compds., the near-edge EXAFS spectra, taken by transmission and total-electron-yield techniques, are reported at the L3, M3, M4,5, N4,5, and O4,5, thresholds. At L3 and M3, similar spectra with analogous information were obsd. While in the metallic U systems, the white lines at L3 and M3 show only minor variations in position and relative peak height, a shift of ∼4 eV was obsd. between UCl3 and UF4, representing mainly the difference in Coulomb interaction between a 2p3/2 core hole and the 5f configurations. The spectra at the M4,5 and N4,5 thresholds are dominated by intense absorption peaks due to 3d-5f and 4d-5f transitions probing directly the unoccupied 5f states. No multiplet structure due to 3d-5f or 4d-5f exchange interaction was resolved, but the linewidths of the M4,5 peaks in the metallic U systems decrease by ≃20% with increasing U-U distance, reflecting a narrowing of the unoccupied 5f bands. The spectra at the O4,5 thresholds, on the other hand, show resolved structures, which are atomiclike for ThF4 and UF4 and may be described by spin-orbit and exchange splitting of the 5d95f1 and 5d95f3 final states, resp. In metallic U systems, the O4,5 structures are broadened and partly lost, presumably due to the more itinerant nature of the 5f states. Addnl. relatively weak peaks were obsd. in the M4,5 and O4,5 spectra of Th metal, ThO2, UO2, and UF4 at ∼15-38 eV above threshold, which are assigned to multiple-scattering resonances.
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    15. 15
      Butorin, S. M.; Modin, A.; Vegelius, J. R.; Kvashnina, K. O.; Shuh, D. K. Probing chemical bonding in uranium dioxide by means of high-resolution X-ray absorption spectroscopy. J. Phys. Chem. C 2016, 120, 29397– 29404,  DOI: 10.1021/acs.jpcc.6b09335
      15
      Probing Chemical Bonding in Uranium Dioxide by Means of High-Resolution X-ray Absorption Spectroscopy
      Butorin, Sergei M.; Modin, Anders; Vegelius, Johan R.; Kvashnina, Kristina O.; Shuh, David K.
      Journal of Physical Chemistry C (2016), 120 (51), 29397-29404CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
      A systematic x-ray absorption study at the U 3d, 4d, and 4f edges of UO2 was performed, and the data were analyzed within framework of the Anderson impurity model. By applying the high-energy-resoln. fluorescence-detection (HERFD) mode of x-ray absorption spectroscopy (XAS) at the U 3d3/2 edge and conducting the XAS measurements at the shallower U 4f levels, fine details of the XAS spectra were resolved resulting from reduced core-hole lifetime broadening. This multiedge study enabled a far more effective anal. of the electronic structure at the U sites and characterization of the chem. bonding and degree of the 5f localization in UO2. The results support the covalent character of UO2 and do not agree with the suggestions of rather ionic bonding in this compd. as expressed in some publications.
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    16. 16
      Duan, P.-Q.; Bao, H.-L.; Li, J.; Cao, H.-J.; Huang, Y.-Y. In-situ high-energy-resolution X-ray absorption spectroscopy for UO2 oxidation at SSRF. Nucl. Sci. Tech. 2017, 28, 2,  DOI: 10.1007/s41365-016-0155-x
      16
      In-situ high-energy-resolution X-ray absorption spectroscopy for UO2 oxidation at SSRF
      Duan, Pei-quan; Bao, Hong-liang; Li, Jiong; Cao, Han-jie; Huang, Yu-ying
      Nuclear Science and Techniques (2017), 28 (1), 2/1-2/4CODEN: NSETEC; ISSN:1001-8042. (Science Press)
      Based on the high-energy-resoln. fluorescence spectrometer on the BL14W1 beamline at Shanghai Synchrotron Radiation Facility, an in-situ high-energyresoln. X-ray absorption spectroscopy technique, with an in-situ heating cell, was developed. The high-energyresoln. fluorescence detection for X-ray absorption near-edge spectroscopy (HERFD-XANES) was tested in a UO2 oxidn. expt. to measure the UL3-edge, with higher signal-to-noise ratio and higher-energy-resoln. than conventional XANES. The technique has potential application for in-situ study of uranium-based materials.
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    17. 17
      Martin, P.; Ripert, M.; Carlot, G.; Parent, P.; Laffon, C. A study of molybdenum behaviour in UO2 by X-ray absorption spectroscopy. J. Nucl. Mater. 2004, 326, 132– 143,  DOI: 10.1016/j.jnucmat.2004.01.006
      17
      A study of molybdenum behavior in UO2 by X-ray absorption spectroscopy
      Martin, Philippe; Ripert, Michel; Carlot, Gaeelle; Parent, Philippe; Laffon, Carine
      Journal of Nuclear Materials (2004), 326 (2-3), 132-143CODEN: JNUMAM; ISSN:0022-3115. (Elsevier Science B.V.)
      Mo is one of the most abundant fission products created in UO2 fuel. Post-irradn. examns. show that Mo is mainly assocd. with Rh, Ru, Pd and Tc in metallic ppts. However, for low burn-ups, these 5 metal particles cannot be detected. To det. the chem. state of Mo in UO2, XANES expts. were carried out on fresh UO2 pellets implanted with Mo ions. The measurements performed at the O K and Mo LIII edges showed a correlation between the chem. state of Mo and the O/U ratio of the UO2 matrix. Indeed, the Mo located at the grain boundaries is present in both metallic and +IV oxidn. states. This study is the 1st exptl. evidence of the soln. annealing of Mo+IV in the cationic lattice of UO2. Thus, when Mo is oxidized at the +IV state, it remains in soln. in the UO2 matrix.
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    18. 18
      Gerber, E.; Romanchuk, A. Y.; Pidchenko, I.; Amidani, L.; Rossberg, A.; Hennig, C.; Vaughan, G. B. M.; Trigub, A.; Egorova, T.; Bauters, S. The missing pieces of the PuO2 nanoparticle puzzle. Nanoscale 2020, 12, 18039– 18048,  DOI: 10.1039/D0NR03767B
      18
      The missing pieces of the PuO2 nanoparticle puzzle
      Gerber, Evgeny; Romanchuk, Anna Yu.; Pidchenko, Ivan; Amidani, Lucia; Rossberg, Andre; Hennig, Christoph; Vaughan, Gavin B. M.; Trigub, Alexander; Egorova, Tolganay; Bauters, Stephen; Plakhova, Tatiana; Hunault, Myrtille O. J. Y.; Weiss, Stephan; Butorin, Sergei M.; Scheinost, Andreas C.; Kalmykov, Stepan N.; Kvashnina, Kristina O.
      Nanoscale (2020), 12 (35), 18039-18048CODEN: NANOHL; ISSN:2040-3372. (Royal Society of Chemistry)
      The nanoscience field often produces results more mystifying than any other discipline. It has been argued that changes in the plutonium dioxide (PuO2) particle size from bulk to nano can have a drastic effect on PuO2 properties. Here we report a full characterization of PuO2 nanoparticles (NPs) at the at. level and probe their local and electronic structures by a variety of methods available at the synchrotron, including extended X-ray absorption fine structure (EXAFS) at the Pu L3 edge, X-ray absorption near edge structure (XANES) in high energy resoln. fluorescence detection (HERFD) mode at the Pu L3 and M4 edges, high energy X-ray scattering (HEXS) and X-ray diffraction (XRD). The particles were synthesized from precursors with different oxidn. states of plutonium (III, IV, and V) under various environmentally and waste storage relevant conditions (pH 8 and pH > 10). Our exptl. results analyzed with state-of-the-art theor. approaches demonstrate that well dispersed, cryst. NPs with a size of ∼2.5 nm in diam. are always formed in spite of diverse chem. conditions. Identical crystal structures and the presence of only the Pu(IV) oxidn. state in all NPs, reported here for the first time, indicate that the structure of PuO2 NPs is very similar to that of the bulk PuO2. All methods give complementary information and show that investigated fundamental properties of PuO2 NPs, rather than being exotic, are very similar to those of the bulk PuO2.
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    19. 19
      Dalodière, E.; Virot, M.; Morosini, V.; Chave, T.; Dumas, T.; Hennig, C.; Wiss, T.; Dieste Blanco, O.; Shuh, D. K.; Tyliszcak, T. Insights into the sonochemical synthesis and properties of salt-free intrinsic plutonium colloids. Sci. Rep. 2017, 7, 43514  DOI: 10.1038/srep43514
      19
      Insights into the sonochemical synthesis and properties of salt-free intrinsic plutonium colloids
      Dalodiere Elodie; Virot Matthieu; Morosini Vincent; Chave Tony; Nikitenko Sergey I; Dumas Thomas; Venault Laurent; Moisy Philippe; Hennig Christoph; Wiss Thierry; Dieste Blanco Oliver; Shuh David K; Tyliszcak Tolek
      Scientific reports (2017), 7 (), 43514 ISSN:.
      Fundamental knowledge on intrinsic plutonium colloids is important for the prediction of plutonium behaviour in the geosphere and in engineered systems. The first synthetic route to obtain salt-free intrinsic plutonium colloids by ultrasonic treatment of PuO2 suspensions in pure water is reported. Kinetics showed that both chemical and mechanical effects of ultrasound contribute to the mechanism of Pu colloid formation. In the first stage, fragmentation of initial PuO2 particles provides larger surface contact between cavitation bubbles and solids. Furthermore, hydrogen formed during sonochemical water splitting enables reduction of Pu(IV) to more soluble Pu(III), which then re-oxidizes yielding Pu(IV) colloid. A comparative study of nanostructured PuO2 and Pu colloids produced by sonochemical and hydrolytic methods, has been conducted using HRTEM, Pu LIII-edge XAS, and O K-edge NEXAFS/STXM. Characterization of Pu colloids revealed a correlation between the number of Pu-O and Pu-Pu contacts and the atomic surface-to-volume ratio of the PuO2 nanoparticles. NEXAFS indicated that oxygen state in hydrolytic Pu colloid is influenced by hydrolysed Pu(IV) species to a greater extent than in sonochemical PuO2 nanoparticles. In general, hydrolytic and sonochemical Pu colloids can be described as core-shell nanoparticles composed of quasi-stoichiometric PuO2 cores and hydrolyzed Pu(IV) moieties at the surface shell.
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    20. 20
      Martin, P.; Grandjean, S.; Ripert, M.; Freyss, M.; Blanc, P.; Petit, T. Oxidation of plutonium dioxide: an X-ray absorption spectroscopy study. J. Nucl. Mater. 2003, 320, 138– 141,  DOI: 10.1016/S0022-3115(03)00180-6
      20
      Oxidation of plutonium dioxide: an X-ray absorption spectroscopy study
      Martin, Philippe; Grandjean, Stephane; Ripert, Michel; Freyss, Michel; Blanc, Pierre; Petit, Thierry
      Journal of Nuclear Materials (2003), 320 (1-2), 138-141CODEN: JNUMAM; ISSN:0022-3115. (Elsevier Science B.V.)
      X-ray absorption spectroscopy expts. were conducted to characterize Pu dioxide oxidn. The sample prepn. adopted does not enable elaboration of hyper-stoichiometric Pu dioxide. These results could mean that Pu dioxide oxidn. only occurs under very specific conditions.
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    21. 21
      Hudson, E. A.; Rehr, J.; Bucher, J. Multiple-scattering calculations of the uranium L3-edge X-ray-absorption near-edge structure. Phys. Rev. B 1995, 52, 13815,  DOI: 10.1103/PhysRevB.52.13815
      21
      Multiple-scattering calculations of the uranium L3-edge x-ray-absorption near-edge structure
      Hudson, E. A.; Rehr, J. J.; Bucher, J. J.
      Physical Review B: Condensed Matter (1995), 52 (19), 13815-26CODEN: PRBMDO; ISSN:0163-1829. (American Physical Society)
      A theor. study of the uranium L3-edge x-ray absorption near-edge structure (XANES) is presented for several uranium compds., including oxides, intermetallics, uranyl fluoride, and α-uranium. Calcns. were performed using FEFF6, an ab initio multiple-scattering (MS) code that includes the most important features of current theories. The results, which account for both the fine structure χ and the at. like background μ0 of the absorption coeff. μ, are compared to new and previously measured exptl. spectra, revealing very good agreement for most systems. For several compds., a more detailed theor. anal. detd. the influence of cluster size and scattering order upon the calcd. spectra. Results indicate that MS paths and scattering paths that include rather distant atoms make significant contributions for UO2, whereas XANES for crystals with lower symmetry and d. can be modeled using only shorter single-scattering paths. In most cases, assumption of a screened final state in the calcn. gives better agreement with expt. than use of an unscreened final state. The successful modeling of spectra for a variety of different uranium compds., with differing spectral features, indicates that the semirelativistic treatment of XANES used here is adequate even for heavy elements. The well-known resonance, obsd. exptl. for uranyl (UO22+) compds. ≈15 eV above the white line, is successfully modeled here for the first time, using multiple-scattering paths within the O-U-O axial bonds. Overlapping muffin-tin spheres were required in the calcn., probably as a result of the short uranyl axial bonds.
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    22. 22
      Jollet, F.; Petit, T.; Gota, S.; Thromat, N.; Gautier-Soyer, M.; Pasturel, A. The electronic structure of uranium dioxide: an oxygen K-edge X-ray absorption study. J. Phys.: Condens. Matter 1997, 9, 9393,  DOI: 10.1088/0953-8984/9/43/022
      22
      The electronic structure of uranium dioxide: an oxygen K-edge x-ray absorption study
      Jollet, F.; Petit, T.; Gota, S.; Thromat, N.; Gautier-Soyer, M.; Pasturel, A.
      Journal of Physics: Condensed Matter (1997), 9 (43), 9393-9401CODEN: JCOMEL; ISSN:0953-8984. (Institute of Physics Publishing)
      The O K-edge x-ray absorption near edge structure (XANES) spectrum of UO2 is presented and interpreted. A comparison with that corresponding to CeO2 is made. First-principles-based calcns. using the LSDA+U approach allows us to link each feature present in the spectra to the specific at. arrangement and electronic structure of the compd. The structures at the edge originate from oxygen 2p states hybridized with U 5f and 6d orbitals and the 6d eg-t2g splitting is found to be 4.8 eV. The structures due to O 2p-U 5f hybridization are found to be lower in energy than the structures due to the O 2p-U 6d hybridization. On this basis, UO2 can be considered as an f-f Mott-Hubbard insulator.
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      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADyaK2sXnt1aqsr8%253D&md5=dfa883de9139997b8172fd10d38d28cc
    23. 23
      Modin, A.; Suzuki, M.; Vegelius, J.; Yun, Y.; Shuh, D.; Werme, L.; Nordgren, J.; Oppeneer, P. M.; Butorin, S. M. 5 f-Shell correlation effects in dioxides of light actinides studied by O 1s x-ray absorption and emission spectroscopies and first-principles calculations. J. Phys.: Condens. Matter 2015, 27, 315503  DOI: 10.1088/0953-8984/27/31/315503
      23
      5 f -shell correlation effects in dioxides of light actinides studied by O 1s x-ray absorption and emission spectroscopies and first-principles calculations
      Modin A; Suzuki M-T; Vegelius J; Yun Y; Shuh D K; Werme L; Nordgren J; Oppeneer P M; Butorin S M
      Journal of physics. Condensed matter : an Institute of Physics journal (2015), 27 (31), 315503 ISSN:.
      Soft x-ray emission and absorption spectroscopic data are reported for the O 1s region of a single crystal of UO2, a polycrystalline NpO2 sample, and a single crystal of PuO2. The experimental data are interpreted using first-principles correlated-electron calculations within the framework of the density functional theory with added Coulomb U interaction (DFT+U). A detailed analysis regarding the origin of different structures in the x-ray emission and x-ray absorption spectra is given and the effect of varying the intra-atomic Coulomb interaction-U for the 5 f electrons is investigated. Our data indicate that O 1s x-ray absorption and emission spectroscopies can, in combination with DFT+U calculations, successfully be used to study 5 f -shell Coulomb correlation effects in dioxides of light actinides. The values for the Coulomb U parameter in these dioxides are derived to be in the range of 4-5 eV.
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    24. 24
      Ramanantoanina, H.; Kuri, G.; Martin, M.; Bertsch, J. Study of electronic structure in the L-edge spectroscopy of actinide materials: UO2 as an example. Phys. Chem. Chem. Phys. 2019, 21, 7789– 7801,  DOI: 10.1039/C9CP01021A
      24
      Study of electronic structure in the L-edge spectroscopy of actinide materials: UO2 as an example
      Ramanantoanina, Harry; Kuri, Goutam; Martin, Matthias; Bertsch, Johannes
      Physical Chemistry Chemical Physics (2019), 21 (15), 7789-7801CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
      While the electronic structure calcn. for actinide materials, using ligand-field phenomenol. in conjunction with d. functional theory (LFDFT) treating configurations with single or two open-shells 5f and 6d electrons, is well established and currently practiced, the consideration of the three open-shells electron configurations for LFDFT treatment is a challenging task addressed in the present work. Herein, we report the first-principles method, developed for the first time on the basis of LFDFT, to evaluate the uranium L3-edge X-ray absorption near-edge structure (XANES), which requires non-equiv. active electrons within the 2p, 5f and 6d orbitals of the uranium ion. The theor. results, when compared with the exptl. XANES data measured from uranium dioxide fresh fuel pellets and rector-exposed spent fuel materials, show good agreement with the exptl. findings elucidating the local oxidn. in the spent fuel materials. This report is relevant for the commonly used L-edge spectroscopy of actinide isotopes and important for understanding the structural, optical and electronic properties of actinide-based materials.
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    25. 25
      Wu, Z. Y.; Jollet, F.; Gota, S.; Thromat, N.; Gautier-Soyer, M.; Petit, T. X-ray absorption at the oxygen K edge in cubic f oxides examined using a full multiple-scattering approach. J. Phys.: Condens. Matter 1999, 11, 7185,  DOI: 10.1088/0953-8984/11/37/314
      25
      X-ray absorption at the oxygen K edge in cubic f oxides examined using a full multiple-scattering approach
      Wu, Z. Y.; Jollet, F.; Gota, S.; Thromat, N.; Gautier-Soyer, M.; Petit, T.
      Journal of Physics: Condensed Matter (1999), 11 (37), 7185-7194CODEN: JCOMEL; ISSN:0953-8984. (Institute of Physics Publishing)
      The O K-edge x-ray absorption near-edge-structure (XANES) spectra of UO2 and CeO2 are presented and interpreted. Using different-size clusters around the excited atom in the full multiple-scattering (MS) simulation, we are able to link the features present in the spectra of each oxide to its specific at. arrangement and electronic structure. The structures at the edge originate from O 2p states hybridized with f and d orbitals of the cation split by the cubic crystal field. All of the other features come from MS with the neighboring shells of the central O atoms.
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    26. 26
      Kvashnina, K. O.; Romanchuk, A. Y.; Pidchenko, I.; Amidani, L.; Gerber, E.; Trigub, A.; Rossberg, A.; Weiss, S.; Popa, K.; Walter, O. A Novel Metastable Pentavalent Plutonium Solid Phase on the Pathway from Aqueous Plutonium(VI) to PuO2 Nanoparticles. Angew. Chem., Int. Ed. 2019, 58, 17558– 17562,  DOI: 10.1002/anie.201911637
      26
      A Novel Metastable Pentavalent Plutonium Solid Phase on the Pathway from Aqueous Plutonium(VI) to PuO2 Nanoparticles
      Kvashnina, Kristina O.; Romanchuk, Anna Yu.; Pidchenko, Ivan; Amidani, Lucia; Gerber, Evgeny; Trigub, Alexander; Rossberg, Andre; Weiss, Stephan; Popa, Karin; Walter, Olaf; Caciuffo, Roberto; Scheinost, Andreas C.; Butorin, Sergei M.; Kalmykov, Stepan N.
      Angewandte Chemie, International Edition (2019), 58 (49), 17558-17562CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Here we provide evidence that the formation of PuO2 nanoparticles from oxidized PuVI under alk. conditions proceeds through the formation of an intermediate PuV solid phase, similar to NH4PuO2CO3, which is stable over a period of several months. For the first time, state-of-the-art expts. at Pu M4 and at L3 absorption edges combined with theor. calcns. unambiguously allow to det. the oxidn. state and the local structure of this intermediate phase.
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    27. 27
      Modin, A.; Yun, Y.; Suzuki, M.-T.; Vegelius, J.; Werme, L.; Nordgren, J.; Oppeneer, P.; Butorin, S. Indication of single-crystal PuO2 oxidation from O 1 s x-ray absorption spectra. Phys. Rev. B 2011, 83, 075113  DOI: 10.1103/PhysRevB.83.075113
      27
      Indication of single-crystal PuO2 oxidation from O 1s x-ray absorption spectra
      Modin, A.; Yun, Y.; Suzuki, M.-T.; Vegelius, J.; Werme, L.; Nordgren, J.; Oppeneer, P. M.; Butorin, S. M.
      Physical Review B: Condensed Matter and Materials Physics (2011), 83 (7), 075113/1-075113/7CODEN: PRBMDO; ISSN:1098-0121. (American Physical Society)
      The electronic structure of single-crystal PuO2 is studied using O 1s x-ray absorption (XA) and x-ray emission. Interpretation of the exptl. data is supported by extensive first-principles calcns. on the basis of the d. functional theory+U approach. The measured XA spectra show a significant difference in intensity for the first two peaks between different spots or areas on the single crystal. Our theor. simulations show that the first peak, at ∼531 eV, can be attributed to O 2p-Pu 5f hybridization, while the second peak, at ∼533.4 eV, is due to hybridization of O 2p with Pu d states. The reasons for the obsd. differences in the O 1s XA spectra are explored by calcg. a no. of defect structures PuO2±x as well as by simulating the existence of Pu(V) sites. Our results indicate the presence of oxidn. states higher than Pu(IV) in some areas of the single crystal. The findings also suggest that plutonium oxide with a Pu fraction in an oxidn. state higher than Pu(IV) consists of inequivalent Pu sites with Pu(IV)O2 and Pu(V)O2 rather than representing a system where the Pu oxidn. state is constantly fluctuating between Pu(IV) and Pu(V).
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    28. 28
      Blaha, P.; Schwarz, K.; Tran, F.; Laskowski, R.; Madsen, G. K.; Marks, L. D. WIEN2k: An APW+ lo program for calculating the properties of solids. J. Chem. Phys. 2020, 152, 074101  DOI: 10.1063/1.5143061
      28
      WIEN2k: An APW+lo program for calculating the properties of solids
      Blaha, Peter; Schwarz, Karlheinz; Tran, Fabien; Laskowski, Robert; Madsen, Georg K. H.; Marks, Laurence D.
      Journal of Chemical Physics (2020), 152 (7), 074101CODEN: JCPSA6; ISSN:0021-9606. (American Institute of Physics)
      The WIEN2k program is based on the APW plus local orbitals (APW + lo) method to solve the Kohn-Sham equations of d. functional theory. The APW + lo method, which considers all electrons (core and valence) self-consistently in a full-potential treatment, is implemented very efficiently in WIEN2k, since various types of parallelization are available and many optimized numerical libraries can be used. Many properties can be calcd., ranging from the basic ones, such as the electronic band structure or the optimized at. structure, to more specialized ones such as the NMR shielding tensor or the elec. polarization. After a brief presentation of the APW + lo method, we review the usage, capabilities, and features of WIEN2k (version 19) in detail. The various options, properties, and available approxns. for the exchange-correlation functional, as well as the external libraries or programs that can be used with WIEN2k, are mentioned. Refs. to relevant applications and some examples are also given. (c) 2020 American Institute of Physics.
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    29. 29
      Idiri, M.; Le Bihan, T.; Heathman, S.; Rebizant, J. Behavior of actinide dioxides under pressure: UO2 and ThO2. Phys. Rev. B 2004, 70, 014113  DOI: 10.1103/PhysRevB.70.014113
      29
      Behavior of actinide dioxides under pressure: UO2 and ThO2
      Idiri, M.; Le Bihan, T.; Heathman, S.; Rebizant, J.
      Physical Review B: Condensed Matter and Materials Physics (2004), 70 (1), 014113/1-014113/8CODEN: PRBMDO; ISSN:0163-1829. (American Physical Society)
      The structural high-pressure properties of ThO2 and UO2 were studied in diamond-anvil cells up to max. pressures of 80 and 69 GPa, resp. Both Th and U dioxides exhibit an identical sequence of structural transitions under pressure; both transform rather sluggishly to the cotunnite-type (orthorhombic Pnma) high-pressure structure. This study which was performed under better hydrostatic conditions than previous expts. has enabled the authors to det. reliable compressibility parameters for the two dioxides: ThO2: B0 = 198(2) GPa and B'0=4.6(3); UO2: B0 = 207(2) GPa and B'0=4.5(4). In the case of UO2 the ambient pressure cubic phase was still found to be present at 69 GPa, which is in contradiction with earlier measurements. With regards to these results and re-calcns. performed on other actinide dioxides PuO2 and AmO2, the authors obtain a different evolution of the bulk moduli through the actinide dioxide series.
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    30. 30
      Yamashita, T.; Nitani, N.; Tsuji, T.; Kato, T. Thermal expansion of neptunium-uranium mixed oxides. J. Nucl. Mater. 1997, 247, 90– 93,  DOI: 10.1016/S0022-3115(97)00031-7
      30
      Thermal expansion of neptunium-uranium mixed oxides
      Yamashita, Toshiyuki; Nitani, Noriko; Tsuji, Toshihide; Kato, Tetsuya
      Journal of Nuclear Materials (1997), 247 (), 90-93CODEN: JNUMAM; ISSN:0022-3115. (Elsevier)
      Thermal expansions of NpyU1-yO2 solid solns. were investigated between room temp. and 1273 K by a high temp. x-ray diffraction technique. The lattice parameters of NpyU1-yO2 solid solns. at high temps. were given in polynomial expressions of temp. High temp. heat capacities, Cp, of NpyU1-yO2 solid solns. were estd. from the thermodn. relation using the measured thermal expansions and literature data. The estd. errors in the calcd. Cp of UO2 were less than ±5%.
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    31. 31
      Haschke, J. M.; Allen, T. H.; Morales, L. A. Reaction of plutonium dioxide with Water: formation and properties of PuO2. Science 2000, 287, 285– 287,  DOI: 10.1126/science.287.5451.285
      31
      Reaction of plutonium dioxide with water: formation and properties of PuO2+x
      Haschke, John M.; Allen, Thomas H.; Morales, Luis A.
      Science (Washington, D. C.) (2000), 287 (5451), 285-287CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
      PuO2+x, a high-compn. (x ≤ 0.27) phase contg. Pu(VI), is the stable binary oxide in air. This nonstoichiometric oxide forms by reaction of dioxide with water and by water-catalyzed reaction of dioxide with oxygen. The PuOz + H2O reaction rate is 0.27 nmol per m squared per h at 250C; the activation energy at 250 to 3500C is 39 kJ per mol. Slow kinetics and a low lattice parameter-compn. dependence for fluorite-related PuO2+x are consistent with a failure to observe the phase in earlier studies. Perplexing aspects of plutonium oxide chem. can now be explained.
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    32. 32
      Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized gradient approximation made simple. Phys. Rev. Lett. 1996, 77, 3865,  DOI: 10.1103/PhysRevLett.77.3865
      32
      Generalized gradient approximation made simple
      Perdew, John P.; Burke, Kieron; Ernzerhof, Matthias
      Physical Review Letters (1996), 77 (18), 3865-3868CODEN: PRLTAO; ISSN:0031-9007. (American Physical Society)
      Generalized gradient approxns. (GGA's) for the exchange-correlation energy improve upon the local spin d. (LSD) description of atoms, mols., and solids. We present a simple derivation of a simple GGA, in which all parameters (other than those in LSD) are fundamental consts. Only general features of the detailed construction underlying the Perdew-Wang 1991 (PW91) GGA are invoked. Improvements over PW91 include an accurate description of the linear response of the uniform electron gas, correct behavior under uniform scaling, and a smoother potential.
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    33. 33
      Anisimov, V. I.; Solovyev, I.; Korotin, M.; Czyżyk, M.; Sawatzky, G. Density-functional theory and NiO photoemission spectra. Phys. Rev. B 1993, 48, 16929,  DOI: 10.1103/PhysRevB.48.16929
      33
      Density-functional theory and NiO photoemission spectra
      Anisimov, V. I.; Solovyev, I. V.; Korotin, M. A.; Czyzyk, M. T.; Sawatzky, G. A.
      Physical Review B: Condensed Matter and Materials Physics (1993), 48 (23), 16929-34CODEN: PRBMDO; ISSN:0163-1829.
      The generalization of the local-d.-approxn. method for the systems with strong Coulomb correlations is proposed, which restores the discontinuity in the one-electron potential as in the exact d. functional. The method is based on the model-Hamiltonian approach and allows the authors to take into account the nonsphericity of the Coulomb and exchange interactions. The calcn. scheme could be regarded as a first-principle method due to the absence of adjustable parameters. The method was applied to the calcn. of the photoemission (x-ray photoemission spectroscopy) and bremsstrahlung isochromat spectra of NiO.
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    34. 34
      Laskowski, R.; Madsen, G. K.; Blaha, P.; Schwarz, K. Magnetic structure and electric-field gradients of uranium dioxide: An ab initio study. Phys. Rev. B 2004, 69, 140408  DOI: 10.1103/PhysRevB.69.140408
      34
      Magnetic structure and electric-field gradients of uranium dioxide: An ab initio study
      Laskowski, Robert; Madsen, Georg K. H.; Blaha, Peter; Schwarz, Karlheinz
      Physical Review B: Condensed Matter and Materials Physics (2004), 69 (14), 140408/1-140408/4CODEN: PRBMDO; ISSN:0163-1829. (American Physical Society)
      The magnetic structure of uranium dioxide was studied using linear APW ab initio calcns., taking into account spin-orbit coupling, strong Coulomb correlation (using the LDA+U approach) and noncollinear magnetism. The collinear 1-k antiferromagnetic type-I structure and the noncollinear antiferromagnetic 2-k and 3-k orderings were tested. The 1-k and 2-k structures can be excluded by a comparison between the calcd. and exptl. uranium elec. field gradients (EFG). The measured EFG agrees with theory only in the 3-k structure and an addnl. deformation of the oxygen cage with an oxygen displacement of ΔO = 0.16 a u. Also the magnetic moments and the corresponding hyperfine fields agree reasonably well with expt.
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    35. 35
      Kern, S.; Loong, C.-K.; Lander, G. Crystal-field transitions in f-electron oxides. Phys. Rev. B 1985, 32, 3051,  DOI: 10.1103/PhysRevB.32.3051
      35
      Crystal-field transitions of f-electron oxides
      Kern, S.; Loong, C. K.; Lander, G. H.
      Physical Review B: Condensed Matter and Materials Physics (1985), 32 (5), 3051-7CODEN: PRBMDO; ISSN:0163-1829.
      Neutron inelastic scattering was used to measure the (ground state)-to-(excited state) crystal-field transitions in PrO2 (130 meV), BaPrO3 (255 meV), and UO2 (∼160 meV). Details are given of these neutron expts. with the epithermal neutrons from the Argonne National Lab. spallation source. From the obsd. transitions, the values of the crystal-field parameter V4 = A4〈r4〉 deduced are PrO2 (-66 meV), BaPrO3 (119 meV), and UO2 (-385 meV). Comparisons are made with V4 values deduced for metallic systems and those detd. by optical techniques for dil. lanthanides in transparent hosts. In the case of UO2, two peaks were seen, one at 155 meV, and the other at 172 meV. This structure exists both below and above the Neel temp. TN (30.8 K), and is discussed in terms of mechanisms that might exist in UO2. Several further neutron expts. are suggested, since energy transfers above ∼100 meV can now be measured at small (i.e., .ltorsim.5 Å-1) values of the momentum transfer.
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    36. 36
      Kern, S.; Loong, C.-K.; Goodman, G.; Cort, B.; Lander, G. Crystal-field spectroscopy of PuO2: further complications in actinide dioxides. J. Phys.: Condens. Matter 1990, 2, 1933,  DOI: 10.1088/0953-8984/2/7/024
      36
      Crystal-field spectroscopy of plutonium dioxide: further complications in actinide dioxides
      Kern, S.; Loong, C. K.; Goodman, G. L.; Cort, B.; Lander, G. H.
      Journal of Physics: Condensed Matter (1990), 2 (7), 1933-40CODEN: JCOMEL; ISSN:0953-8984.
      Neutron spectroscopy was performed on a sample of PuO2, enriched to 94% with the nonabsorbing isotope 242Pu, to measure the crystal-field potential. Levels were found consistent with the weak-field case crystal-field parameters recently derived for UO2 and NpO2, but inconsistent with a weak-field interpretation of the magnetic susceptibility measurements on PuO2. In the light of these discrepancies, it is believed that the conventional crystal-field model is inadequate. PuO2 is probably the simplest material to test any new ideas because the ground state is a nonmagnetic singlet.
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    37. 37
      Neckel, A.; Rastl, P.; Eibler, R.; Weinberger, P.; Schwarz, K. Results of self-consistent band-structure calculations for ScN, ScO, TiC, TiN, TiO, VC, VN and VO. J. Phys. C: Solid State Phys. 1976, 9, 579,  DOI: 10.1088/0022-3719/9/4/008
      37
      Results of self-consistent band-structure calculations for scandium nitride, scandium monoxide, titanium carbide, titanium nitride, titanium monoxide, vanadium monocarbide, vanadium mononitride and vanadium monoxide
      Neckel, A.; Rastl, P.; Eibler, R.; Weinberger, P.; Schwarz, K.
      Journal of Physics C: Solid State Physics (1976), 9 (4), 579-92CODEN: JPSOAW; ISSN:0022-3719.
      By using the augmented-plane-wave method in connection with the Xα method for exchange, the self-consistent band structures for 8 compds. were calcd. By means of the wavefunctions, a charge anal. is presented. The Slater-Koster LCAO interpolation scheme is applied to yield accurate ds. of states and partial ds. of states.
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    38. 38
      Schwarz, K.; Neckel, A.; Nordgren, J. On the X-ray emission spectra from FeAl. J. Phys. F: Met. Phys. 1979, 9, 2509,  DOI: 10.1088/0305-4608/9/12/023
      38
      On the x-ray emission spectra from iron-aluminum 1:1 compound
      Schwarz, K.; Neckel, A.; Nordgren, J.
      Journal of Physics F: Metal Physics (1979), 9 (12), 2509-21CODEN: JPFMAT; ISSN:0305-4608.
      The x-ray emission spectra of FeAl were interpreted using the results of an APW band structure calcn. (Pechter, K., et al., 1979) for the simple Fe-K, Fe-L3, and Al-K spectra and for the composite Fe-M2,3 Al-L2,3 spectrum. The effects of nonradiative transition rates, ionization probabilities, and absorption corrections on the relative intensities of the latter 2 spectra were considered. The Fe-M2,3 intensity is weakened by a factor of ∼200 with respect to the Al-L2,3 spectrum due to these effects. The theor. intensity ratio of these 2 spectra is ≈ 1/2 the exptl. ratio.
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    39. 39
      Schwarz, K.; Wimmer, E. Electronic structure and X-ray emission spectra of YS in comparison with NbC. J. Phys. F: Met. Phys. 1980, 10, 1001,  DOI: 10.1088/0305-4608/10/5/028
      39
      Electronic structure and x-ray emission spectra of yttrium monosulfide in comparison with niobium carbide
      Schwarz, Karlheinz; Wimmer, Erich
      Journal of Physics F: Metal Physics (1980), 10 (5), 1001-12CODEN: JPFMAT; ISSN:0305-4608.
      Self-consistent calcns. are reported of the energy level band structure of YS. The calcns. were carried out using the APW method and either the Xα exchange potential or the Wigner potential. The nonmetal p/metal d interaction, discussed in terms of l-like charges, is strongly reduced in YS compared with the isostructural and isoelectronic NbC. The S-K, S-LIII, Y-LIII, Y-MIII, and Y-Mv x-ray emission spectra are calcd. from the band structure results. The closely related Y-LIII and Y-MIII spectra are predicted to be significantly different, owing to important matrix element effects.
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    40. 40
      Suzuki, C.; Nishi, T.; Nakada, M.; Akabori, M.; Hirata, M.; Kaji, Y. Core-hole effect on XANES and electronic structure of minor actinide dioxides with fluorite structure. J. Phys. Chem. Solids 2012, 73, 209– 216,  DOI: 10.1016/j.jpcs.2011.10.043
      40
      Core-hole effect on XANES and electronic structure of minor actinide dioxides with fluorite structure
      Suzuki, Chikashi; Nishi, Tsuyoshi; Nakada, Masami; Akabori, Mitsuo; Hirata, Masaru; Kaji, Yoshiyuki
      Journal of Physics and Chemistry of Solids (2012), 73 (2), 209-216CODEN: JPCSAW; ISSN:0022-3697. (Elsevier Ltd.)
      The authors investigated theor. core-hole effects on X-ray absorption near-edge structures (XANES) of Np and Am LIII in neptunium dioxide (NpO2) and americium dioxide (AmO2) with CaF2-type crystal lattices using the all-electron full-potential linearized augmented plane-wave (FP-LAPW) method. The peak creation mechanism of XANES was shown by examg. the electronic structures of these oxides, which indicated that core-hole screening was more marked for AmO2 than for NpO2 because of the difference in the charge transfer between these oxides. Furthermore, the results of charge d. anal. suggested that the white line was assigned to the quasi-bound state composed of the localized Np d or Am d components and O components, and that the tail structure was created as a result of delocalized standing waves between the Np or Am atoms.
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    41. 41
      Dalodière, E.; Virot, M.; Morosini, V.; Chave, T.; Dumas, T.; Hennig, C.; Wiss, T.; Dieste Blanco, O.; Shuh, D. K.; Tyliszcak, T. Insights into the sonochemical synthesis and properties of salt-free intrinsic plutonium colloids. Sci. Rep. 2017, 7, 43514  DOI: 10.1038/srep43514
      41
      Insights into the sonochemical synthesis and properties of salt-free intrinsic plutonium colloids
      Dalodiere Elodie; Virot Matthieu; Morosini Vincent; Chave Tony; Nikitenko Sergey I; Dumas Thomas; Venault Laurent; Moisy Philippe; Hennig Christoph; Wiss Thierry; Dieste Blanco Oliver; Shuh David K; Tyliszcak Tolek
      Scientific reports (2017), 7 (), 43514 ISSN:.
      Fundamental knowledge on intrinsic plutonium colloids is important for the prediction of plutonium behaviour in the geosphere and in engineered systems. The first synthetic route to obtain salt-free intrinsic plutonium colloids by ultrasonic treatment of PuO2 suspensions in pure water is reported. Kinetics showed that both chemical and mechanical effects of ultrasound contribute to the mechanism of Pu colloid formation. In the first stage, fragmentation of initial PuO2 particles provides larger surface contact between cavitation bubbles and solids. Furthermore, hydrogen formed during sonochemical water splitting enables reduction of Pu(IV) to more soluble Pu(III), which then re-oxidizes yielding Pu(IV) colloid. A comparative study of nanostructured PuO2 and Pu colloids produced by sonochemical and hydrolytic methods, has been conducted using HRTEM, Pu LIII-edge XAS, and O K-edge NEXAFS/STXM. Characterization of Pu colloids revealed a correlation between the number of Pu-O and Pu-Pu contacts and the atomic surface-to-volume ratio of the PuO2 nanoparticles. NEXAFS indicated that oxygen state in hydrolytic Pu colloid is influenced by hydrolysed Pu(IV) species to a greater extent than in sonochemical PuO2 nanoparticles. In general, hydrolytic and sonochemical Pu colloids can be described as core-shell nanoparticles composed of quasi-stoichiometric PuO2 cores and hydrolyzed Pu(IV) moieties at the surface shell.
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