AVS 67 Session AC-Contributed On Demand: Actinides and Rare Earths Contributed On Demand Session

Study of UO₂ corrosion is important in understanding the spent nuclear fuel (SNF) disposal. To address the challenges in investigating large amount of SNF materials, we developed a new approach to incorporate UO₂ powder onto the working electrode of a microfluidic electrochemical cell, aka the system for analysis at the vacuum liquid interface (SALVI) E-cell, to facilitate UO₂corrosion study at the microscale. Instead of using bulk spent fuel pieces as working electrode at the macroscale, UO₂ powder was mixed withpolyvinylidene fluoride (PVDF) and carbon black to form the working electrode with mm dimension and included into the SALVI E-cell. The UO₂ powder electrode went through electrochemical corrosion in 0.1 M NaClO₄ (pH=9.5) aqueous electrolyte using the electrochemical station. Multimodal imaging analysis, including in situ scanning electron microscope (SEM) coupled withEnergy-dispersive X-ray spectroscopy (EDS) as well as ex situ transmission electron microscopy (TEM) and atomic force microscopy (AFM), was applied to reveal the morphological change, oxidation layer distribution, and topographical information of UO₂ before and after corrosion.In addition, X-ray photoelectron spectroscopy (XPS) was used to determine the oxidation state of the UO₂ electrode surface disassembled from the microfluidic E-cell after anodic oxidation. Our results demonstrate a promising approach to characterizing UO₂ corrosion at the microscale using multimodal imaging. Particularly, in situ SEM imaging and EDS mapping allow direct observation of corrosion in liquid. This new approach is useful in studying the interaction between geological repository environments (e.g., groundwater) and SNF to validate and improve the Fuel Matrix Dissolution Model.

Laser ablation (LA) mass spectrometric and static time-of-flight secondary ion mass spectrometry (ToF-SIMS) of metals produces similar ion distributions of large M_xO_y+ ions due to gas phase recombination reactions of small M_xO_y+ ions in the gaseous cloud formed during LA, or selvedge formed in static ToF-SIMS during ion sputtering [1].Specifically in the case of depleted uranium (DU), an expanded understanding of these recombination reactions due to ion sputtering under vacuum and in the presence of oxygen or in atmosphere can further our understanding of nuclear forensics and debris analysis [2, 3].We describe here precise control of large U_x+ and U_xO_y+ ion formation as a function of static ToF-SIMS Ga+ primary ion bombardment parameters.

We report the observation of large U_x+, U_xO_y+, and U_xO_yH_z+ ions (x = 1-14) during 25 keV Ga+ analysis of the DU surface oxide and sputter cleaned metal surface during primary ion bombardment under static conditions.These ions are large enough to suggest that intact cleavage from the DU surface is energetically prohibitive.Rather, collision cascade theory suggests that these large species are the product of post-ionization reactions in the selvedge of DU [4].Here, we report variation in the power law fits of secondary ion counts of U_x+, U_xO_y+ and U_xO_yH_z+ ions as a function of x (x=1-7) due to sputtering 2 nm into the DU surface oxide at ion current densities varying from 20 pA to 20 nA.We also report the variation of power law fits to secondary ion counts of U_x+ ions as a function of x (x = 1-14) due to tuning of instrument cycle time.Finally, we observe different rates of conversion of U_x+ ions to U_xO_y+ as a function of x during in-chamber surface oxidation under UHV conditions from residual moisture during static spectrum collection over a period of 3000 s.

Future applications of this work include monitoring of ion formation and variation in recombination reactions in the presence of oxygen, water vapor, and temperature.This work was also performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under contract DE-AC52-07NA27344.

[1] J. Mass Spectrom. 41, 527–542 (2006)

[2] J. Phys. D: Appl. Phys. 50, 485201 (2017)

[3] Eur. Phys. J. D. 6, 83-97 (1999)

[4] J. Mass Spectrom. 42, 11–19 (2007)

Studying the local moment and 5f-electron occupations sheds insight into the electronic behavior in actinide materials. X-ray absorption spectroscopy (XAS) has been a powerful tool to reveal the valence electronic structure when assisted with theoretical calculations. In this work, we employed the DFT+Gutzwiller rotationally-invariant slave boson method to obtain the local Hamiltonian of the single-impurity Anderson model (SIAM), and used exact diagonalization (ED) method to calculate the XAS spectra from the model. An in-house built computational code was developed for the ED method. By applying this technique to the recently discovered 5f-electron topological Kondo insulator plutonium tetraboride (PuB₄), we were able to determine the signature of 5f-electronic correlation effects in the theoretical X-ray spectra. We found that the Pu 5f-6dhybridization effect provides an extra channel to mix the j=5/2 and 7/2 orbitals in the 5f valence. As a consequence, the resultant electron occupation number and spin-orbit coupling strength deviate from the intermediate coupling regime. We have also applied this approach to the δ-phase of Pu.

*J. G. Tobin,S. Nowak, S.-W. Yu, R. Alonso-Mori, T. Kroll, D. Nordlung, T.-C. Weng, D. Sokaras, "EXAFS as a Probe of Actinide Oxide Formation in the Tender X-Ray Regime," Surface Science 698, 121607 (2020) https://doi.org/10.1016/j.susc.2020.121607.

Plutonium has a complex electronic structure, where the role of the 5f orbitals in chemical bonding and the level of covalency has not been understood in detail and is a very active field of research. Resonant inelastic X‑ray scattering (RIXS) is a valuable tool and can lead to a deeper understanding of the electronic structure of plutonium materials.^1,2 Also high-energy resolution X-ray absorption near edge structure (HR-XANES) has proven powerful in plutonium speciation studies.^3,4

Relativistic quantum chemical computations for the Pu⁴⁺ ion and the PuO₂ compound were performed. We will discuss four computational measures of covalency of the Pu 5f orbitals. The PuM_4,5-edge HR-XANES and Pu M_4,5-edge core-to-core 3d4f RIXS on PuO₂ and other Pu⁴⁺ compounds under various experimental conditions were conducted at the INE and ACT beamlines at the Karlsruhe research accelerator (KARA) at KIT in Karlsruhe Germany.^5,6

We found that for PuO₂ the Pu 5f covalent mixing with O valence orbitals overall is relatively small. It is the largest in the 5f(7/2) a_2u orbital with the highest orbital energy. The analysis of the Pu M_4,5-edge HR-XANES in combination with the calculated results, showed that the spectra can not be described with a simple one electron transition between individual 3d and 5f orbitals applying the dipole selection rule. There is considerable amount of redistribution of 5f electrons involved in both the Pu M₄-edge and the Pu M₅-edge absorption processes, i.e. shake-up excitations (J = 5f(5/2) to 5f(7/2)) take place. From the comparison of the computational results with the HR-XANES spectrum, it was found that the second peak in the Pu M₄-edge and the shoulder feature in the Pu M₅-edge spectrum are probing the 5f(7/2) a_2u orbital and are therefore expected to be sensitive to bond variations. It will be discussed how the different interatomic interactions affect the Pu M_4,5‑edge core-to-core 3d4f RIXS map of PuO₂. The here presented spectroscopic and theoretical tools will help to advance the understanding of the electronic structure of Pu materials.

Acknowledgement: We acknowledge funding from the European Research Council (ERC) ( grant agreement n° 101003292). We thank the Institute for Beam Physics and Technology (IBPT), KIT for the operation of the storage ring, the KARA.

(1) Vitova, T. et al. Nat. Commun. 2017, 8 (May), 1–9.

(2) Butorin, S. M. Inorg. Chem. 2020.

(3) Bahl, S. et al. Inorg. Chem. 2017, 56 (22), 13982–13990.

(4) Gerber, E. et al. Nanoscale 2020, 12 (35), 18039–18048.

(5) Rothe, J. et al. Rev. Sci. Instrum. 2012, 83 (4), 043105.

(6) Zimina, A. et al. Rev. Sci. Instrum. 2017, 88 (113113), 1–12.

A thorough exploration of uranium oxide surface chemistry is critical to understanding UO₂ corrosion in the nuclear fuel cycle and potential for catalytic reactivity. Nanoscale uranium oxides provide a template through which to study the surface chemistry of actinide materials. Synthesis of actinide nanoparticles, however, has not been empirically well-developed, making investigations into their size- and surface-dependent properties relatively unexplored. Synthesis of nanoparticle constructs in the range between 2 and 100 nm utilizing colloidal and template-based approaches will be discussed, along with their structural attributes on the atomic and nanometer length scales. Characterization of such particles, in a manner which resolves surface chemistry, becomes challenging using conventional characterization techniques. In particular, the high curvature of nanoparticle surfaces and complex solution interface chemistry makes surface techniques appropriate to thin film systems insufficient structural probes.

We overcome the presented challenge through focusing on the study of ultrasmall nanoparticles, which present particularly high surface area to volume ratios. The high percentage of surface atoms makes bulk characterization techniques such as X-ray absorption spectroscopy (XAFS) become surface-sensitive probes, enabling us to resolve surface structure in atomistic detail. New models will be presented that provide an indication of elemental speciation of surface atoms and sites for surface ligand binding, through combining geometric models with structural parameters extracted from XAFS. Results obtained through this work are broadly applicable towards resolving nanoparticle surface chemistry and provide foundational methodology towards exploring the nanoscale properties of actinide oxides.

It is common to attempt to improve the energy resolution of XPS in order to obtain more information about the electronic structure of the system studied. However, it may not be possible to improve the resolution because unresolved final states features are present which lead to broad features. These unresolved features may arise from closely spaced multiplets for the angular momentum coupling of the core and valence open shell electrons. They may also arise from excitations to higher lying vibrational levels for the final ionic states which may be especially important when bond distances for the core-ionized states are very different from those for the initial state; see, for example, Ref. [1]. When the energy separation of the final states are less than or comparable to the lifetime of the core-hole, it will not be possible to resolve the states and there will only be a broadening, often quite significant, for the observed peak composed of these unresolved features. Thus, for example, the U(5f_7/2) peak of UO₂ has a FWHM of 1.4 eV although the instrumental resolution was 0.3 eV. [2] Similar large FWHM have been observed for U in different oxidation states. [3] In order to be able to relate the widths of these broadened features to the chemical and physical interactions in the system, it is necessary to understand the separate contributions of the multiplet splittings and the vibrational excitations. It has been shown that, for U(IV) 4f XPS in UO₂, the contributions of the multiplet splitting and the vibrational excitations are comparable, each contributing ~0.5 eV to the FWHM. [2] In the present work, the contributions of these mechanisms are examined for U(V) and U(VI) oxidation states. In addition, the broadening is examined for the XPS of different core levels where the relative importance of multiplet and vibrational broadening is different from that for the U(4f) XPS. The theoretical predictions for these different parameters can be validated paving the way to extract chemical information from the measured FWHM. [4] The theoretical framework for these predictions is based on wavefunctions for embedded cluster models of the oxide.

1. C. J. Nelin et al. Angew. Chem. Int. Ed. 50, 10174 (2011).
2. P. S. Bagus, C. J. Nelin, S. Rennie, G. H. Lander, and R. Springel, (In preparation).
3. T. Gouder, R. Eloirdi, and R. Caciuffo, Scientific Reports 8, 8306 (2018).
4. P. S. Bagus, E. S. Ilton, and C. J. Nelin, Catal. Lett. 148, 1785 (2018).

Besides the fundamental importance, uranium hydride has considerable relevance for nuclear energy and devices, which motivates continuous effort to understand its formation and properties [1], in particular its electronic structure.

Photoelectron Spectroscopy is a method which directly probes the electronic structure. However, surface oxidation is an issue, which we overcome using the strategy of thin film synthesis. Surprisingly, it became possible to stabilize the UH₂ phase (non-existent as a bulk) in a thin film form [2].

XPS was first used for analysis of the samples quality. One of the difficulties is that the H-1s line is a part of the U valence band, hence it cannot be used to quantify the H concentration. We followed an empirical approach based on gradual increase of H₂ partial pressure in the working gas (Ar) while monitoring variations of the U-4f core-level spectra until saturation is reached [3].

Details of electronic states in the vicinity of the Fermi level are explored by UPS, surpassing XPS both by intensity and energy resolution (about 70 meV). Comparison of UPS spectra of U and UH₃ revealed that the spectra are similar just at the Fermi level (if properly normalized), the maximum for UH₃ is slightly displaced from the Fermi level to ≈140 meV binding energy. The U metal has DOS increasing up to the Fermi level and the Fermi-Dirac cutoff forms the maximum at 90 meV. Another feature of UH₃ (UH₂ is very similar) is the broad shoulder at 0.5 eV (ascribed to 5f multiplet from GGA+U calculations). Details of the DOS shape resist to a quantitative description using DFT calculations [1]. However, spectral density obtained from DMFT calculations captures both the maximum just below E_F and the 0.5 eV shoulder. This stresses the importance of electron correlations for the description of U hydrides.

As the XPS spectra of UH₃ and UH₂ are so similar, it is hard to distinguish which species was deposited. Magnetization measurements turned particularly useful. While both species are ferromagnets, their T_C values are different (165 K for UH₃ and 120 K for UH₂) [2]. Using the M(T) dependence, we can assess the phase composition (pure UH₂, UH₃ or mixed-phase) of the samples.

This work was supported by the Czech Science Foundation under the grants No. 18-02344S and 21-09766S and b y the Grant Agency of the Charles University under Project No. 814120.

References

[1]L. Havela et al., J.El.Spectroscopy Rel.Phenomena239, 146904 (2020).

[2]L. Havela et al., Inorg.Chem.57, 14727-14732 (2018).

[3]T. Gouder et al., Phys.Rev.B70, 235108 (2004).

We employ material-specific dynamical mean-field theory (LDA+DMFT) to investigate the electronic structure of UH3. We use the computed electronic structure to model valence-band photoemission and x-ray absorption spectra. We investigate the sensitivity of our results to the Coulomb U parameter that enters the LDA+DMFT method, since there is a large spread of recommended values reported in the literature, ranging from 0.5 eV [1] to more than 5 eV [2].

To understand the origin of certain spectral features, we analyze how the many-body eigenstates of the DMFT auxiliary impurity model evolve when the hybridization between the uranium 5f states and the hydrogen 1s states is ramped up from zero (ionic 5f3 model) to the realistic value. This theoretical experiment enables us to illustrate how some of the photoemission features relate to the final-state multiplets of the 5f3 to 5f2 transition. The LDA+DMFT method thus provides a solid ground for the earlier empirical interpretation of the photoemission spectra in terms of the 5f atomic multiplets [3].

This work was supported by the Czech Science Foundation under the grant No. 21-09766S.

[1] L. Havela at al., J. Electron Spectrosc. Relat. Phenom. 239, 146904 (2020).

[2] R.-s. Li et al., Chem. Phys. 538, 110876 (2020).

[3] T. Gouder et al., MRS Proceedings 986,0986-OO01-02 (2006).