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All sourcesNASA NTRSDOE OSTI
DOE OSTI2023

Density Functional Theory Guided Investigation of Ligand‐Induced Neptunyl‐Neptunyl Interactions

Abstract Actinyl‐actinyl interactions are particularly prevalent for the pentavalent neptunyl cation (Np(V)O 2 ) + where these interactions appear either as a T ‐ or D ‐shape (diamond‐shape). T ‐shaped interactions have been previously identified in high concentration Np(V) solutions containing simple anions (NO 3 − , ClO 4 − , Cl − ) whereas D ‐shaped have only been isolated in the solid‐state in the presence of carboxylate ligands. In this study, Density Functional Theory (DFT) calculations were paired with Raman spectroscopy to evaluate the formation of D ‐shaped interactions in the presence of aliphatic (R=H (formate), CH 3 (acetate), CH 2 CH 3 (propionate)) and aromatic (R=C 6 H 5 (benzoate), C 6 H 4 OH (4‐hydroxybenzoate), C 5 H 4 N (isonicotinate)) carboxylate ligands. DFT studies indicate that the Δ G to form hydrated T ‐ and D ‐shaped forms are not spontaneous but become so with the addition of the carboxylate ligands. Raman spectra of the Np(V) carboxylate solutions contained vibrational modes associated with the D ‐shaped interactions, but spectral changes observed over time indicate a dynamic system. Crystallization experiments from the Np(V) carboxylate systems confirmed the presence of D‐ shaped dimers for the aromatic carboxylates, suggesting that the choice of the anion in solution favors actinyl‐actinyl interactions even at low concentrations (≤20 mM) of Np(V).

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2020

Materials Data on BeH6(NF2)2 by Materials Project

BeH6(NF2)2 is beta Np structured and crystallizes in the monoclinic P2_1/c space group. The structure is zero-dimensional and consists of four BeH6(NF2)2 clusters. Be2+ is bonded in a tetrahedral geometry to four F1- atoms. There are a spread of Be–F bond distances ranging from 1.56–1.58 Å. There are two inequivalent N2- sites. In the first N2- site, N2- is bonded in a trigonal non-coplanar geometry to three H1+ atoms. There is one shorter (1.05 Å) and two longer (1.06 Å) N–H bond length. In the second N2- site, N2- is bonded in a trigonal non-coplanar geometry to three H1+ atoms. There are a spread of N–H bond distances ranging from 1.03–1.07 Å. There are six inequivalent H1+ sites. In the first H1+ site, H1+ is bonded in a single-bond geometry to one N2- atom. In the second H1+ site, H1+ is bonded in a single-bond geometry to one N2- and one F1- atom. The H–F bond length is 1.61 Å. In the third H1+ site, H1+ is bonded in a single-bond geometry to one N2- atom. In the fourth H1+ site, H1+ is bonded in a distorted single-bond geometry to one N2- and one F1- atom. The H–F bond length is 1.58 Å. In the fifth H1+ site, H1+ is bonded in a single-bond geometry to one N2- atom. In the sixth H1+ site, H1+ is bonded in a single-bond geometry to one N2- atom. There are four inequivalent F1- sites. In the first F1- site, F1- is bonded in a distorted bent 120 degrees geometry to one Be2+ and one H1+ atom. In the second F1- site, F1- is bonded in a distorted single-bond geometry to one Be2+ atom. In the third F1- site, F1- is bonded in a distorted single-bond geometry to one Be2+ atom. In the fourth F1- site, F1- is bonded in a water-like geometry to one Be2+ and one H1+ atom.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on TiSi4(NCl7)2 by Materials Project

TiSi4(NCl7)2 is beta Np structured and crystallizes in the orthorhombic P2_12_12_1 space group. The structure is zero-dimensional and consists of four TiSi4(NCl7)2 clusters. Ti4+ is bonded in a 4-coordinate geometry to two N3- and two Cl1- atoms. There are one shorter (2.02 Å) and one longer (2.03 Å) Ti–N bond lengths. There are one shorter (2.19 Å) and one longer (2.20 Å) Ti–Cl bond lengths. There are four inequivalent Si4+ sites. In the first Si4+ site, Si4+ is bonded to one N3- and three Cl1- atoms to form corner-sharing SiNCl3 tetrahedra. The Si–N bond length is 1.73 Å. All Si–Cl bond lengths are 2.05 Å. In the second Si4+ site, Si4+ is bonded to one N3- and three Cl1- atoms to form corner-sharing SiNCl3 tetrahedra. The Si–N bond length is 1.71 Å. There are two shorter (2.04 Å) and one longer (2.08 Å) Si–Cl bond lengths. In the third Si4+ site, Si4+ is bonded to one N3- and three Cl1- atoms to form corner-sharing SiNCl3 tetrahedra. The Si–N bond length is 1.73 Å. There are one shorter (2.04 Å) and two longer (2.05 Å) Si–Cl bond lengths. In the fourth Si4+ site, Si4+ is bonded to one N3- and three Cl1- atoms to form corner-sharing SiNCl3 tetrahedra. The Si–N bond length is 1.72 Å. There are two shorter (2.04 Å) and one longer (2.07 Å) Si–Cl bond lengths. There are two inequivalent N3- sites. In the first N3- site, N3- is bonded in a trigonal planar geometry to one Ti4+ and two Si4+ atoms. In the second N3- site, N3- is bonded in a trigonal planar geometry to one Ti4+ and two Si4+ atoms. There are fourteen inequivalent Cl1- sites. In the first Cl1- site, Cl1- is bonded in a distorted single-bond geometry to one Si4+ atom. In the second Cl1- site, Cl1- is bonded in a single-bond geometry to one Si4+ atom. In the third Cl1- site, Cl1- is bonded in a single-bond geometry to one Si4+ atom. In the fourth Cl1- site, Cl1- is bonded in a single-bond geometry to one Si4+ atom. In the fifth Cl1- site, Cl1- is bonded in a single-bond geometry to one Si4+ atom. In the sixth Cl1- site, Cl1- is bonded in a single-bond geometry to one Si4+ atom. In the seventh Cl1- site, Cl1- is bonded in a single-bond geometry to one Si4+ atom. In the eighth Cl1- site, Cl1- is bonded in a single-bond geometry to one Ti4+ atom. In the ninth Cl1- site, Cl1- is bonded in a single-bond geometry to one Si4+ atom. In the tenth Cl1- site, Cl1- is bonded in a single-bond geometry to one Ti4+ atom. In the eleventh Cl1- site, Cl1- is bonded in a single-bond geometry to one Si4+ atom. In the twelfth Cl1- site, Cl1- is bonded in a single-bond geometry to one Si4+ atom. In the thirteenth Cl1- site, Cl1- is bonded in a single-bond geometry to one Si4+ atom. In the fourteenth Cl1- site, Cl1- is bonded in a single-bond geometry to one Si4+ atom.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Mixed Nanosphere Assemblies at a Liquid–Liquid Interface

The in-plane packing of gold (Au), polystyrene (PS), and silica (SiO 2 ) spherical nanoparticle (NP) mixtures at a water–oil interface is investigated in situ by UV–vis reflection spectroscopy. All NPs are functionalized with carboxylic acid such that they strongly interact with amine-functionalized ligands dissolved in an immiscible oil phase at the fluid interface. This interaction markedly increases the binding energy of these nanoparticle surfactants (NPSs). The separation distance between the Au NPSs and Au surface coverage are measured by the maximum plasmonic wavelength (λ max ) and integrated intensities as the assemblies saturate for different concentrations of non-plasmonic (PS/SiO 2 ) NPs. Further, as the PS/SiO 2 content increases, the time to reach intimate Au NP contact also increases, resulting from their hindered mobility. λ max changes within the first few minutes of adsorption due to weak attractive inter-NP forces. Additionally, a sharper peak in the reflection spectrum at NP saturation reveals tighter Au NP packing for assemblies with intermediate non-plasmonic NP content. Grazing incidence small angle X-ray scattering (GISAXS) and scanning electron microscopy (SEM) measurements confirm a decrease in Au NP domain size for mixtures with larger non-plasmonic NP content. The results demonstrate a simple means to probe interfacial phase separation behavior using in situ spectroscopy as interfacial structures densify into jammed, phase-separated NP films.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2020

Materials Data on Zr2Np by Materials Project

NpZr2 crystallizes in the hexagonal P6/mmm space group. The structure is three-dimensional. Np is bonded in a 2-coordinate geometry to two equivalent Np and twelve equivalent Zr atoms. Both Np–Np bond lengths are 2.81 Å. All Np–Zr bond lengths are 3.33 Å. Zr is bonded in a 11-coordinate geometry to six equivalent Np and five equivalent Zr atoms. There are two shorter (2.81 Å) and three longer (3.02 Å) Zr–Zr bond lengths.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Diabatic Valence-Hole Concept

A global diabatization scheme, based on the “valence-hole” concept, has been previously applied to model webs of avoided crossings that exist in four electronic-state symmetry manifolds of C 2 ( 1 Π g , 3 Π g , 1 Σ u + , and 3 Σ u + ). Here, this model is extended to the electronically excited states of four more molecules: CN ( 2 Σ + ), N 2 ( 3 Π u ), SiC ( 3 Π), and Si 2 ( 3 Π g ). Many strangenesses in the spectroscopic observations (e.g., energy level structure, predissociation linewidths, and radiative lifetimes) for all four electronic state systems discussed here are accounted for by this unified model. The key concept of the model is valence-hole electron configurations: 3σ 2 4σ 1 1π 4 5σ 2 in CN ( 2 Σ + ), 2σ g 2 2σ u 1 1π u 4 3σ g 2 1π g 1 in N 2 ( 3 Π u ), 5σ 2 6σ 1 7σ 2 2π 3 in SiC ( 3 Π), and 4σ g 2 4σ u 1 5σ g 2 2π u 3 in Si 2 ( 3 Π g ), all of which have a triply occupied “valence-core” (i.e., 2σ g 2 2σ u 1 or the equivalent). These valence-hole configurations have a nominal bond order of three or higher and correlate with high-energy separated-atom limits with an np ← ns (n = 2, 3) promotion in one of the atomic constituents. On its way to dissociation, the strongly bound diabatic valence-hole state crosses multiple weakly bound or repulsive states, which belong to electron configurations with a completely filled valence-core. These curve crossings between diabatic potentials result in a network of many avoided crossings among multiple electronic states, analogous to the well-studied electronic structure landscape of ionic-covalent crossings in strongly ionic molecules. In conclusion, considering the unique role of valence-hole states in shaping the global electronic structure, the valence-hole concept should be added to our intuitive framework of chemical bonding.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2022

Comparative uptake, translocation, and plant mediated transport of Tc-99, Cs-133, Np-237, and U-238 in Savannah River Site soil columns for the grass species Andropogon virginicus

Here, this study examines the ability of the grass species Andropogon virginicus to alter the subsurface transport and redistribution of a suite of radionuclides ( 99 Tc, 133 Cs (stable analog for 135 Cs and 137 Cs), 237 Np, 238 U) with varying chemical behaviors in a Savannah River Site soil via the use of vegetated and unvegetated soil columns. After an acclimation period, a small volume of solution containing all radionuclides was introduced into the columns via Rhizon© pore water sampling tubes. Plants were grown for an additional 4 weeks before shoots were harvested, and columns were prepared for sampling. Plant presence led to decreased radionuclide release from the columns, mainly due to radionuclide specific combinations of system hydrology differences resulting from plant transpiration as well as plant uptake. For the most mobile radionuclides, 99 Tc followed by 237 Np, plant presence resulted in significantly different soil concentration profiles between vegetated and unvegetated columns, including notable upward migration for 237 Np in columns with plants. Additionally, plant uptake of 99 Tc was the greatest of all the radionuclides, with plant tissues containing an average of 44 % of the 99 Tc, while plant uptake only accounted for <2 % of 237 Np and <0.5 % of 133 Cs and 238 U in the system. Although overall plant uptake of 133 Cs and 238 U were similar, the majority of 133 Cs taken up by plants was associated with 133 Cs already available in the aqueous phase while 238 U uptake was mainly associated with the solid phase, meaning that plant activity resulted in a fraction of the native 238 U being mobilized and thus, made available for plant uptake. Overall, this study quantified the influence of several plant-mediated physical and biogeochemical factors that have significant influence on radionuclide mobility and transport in this complex system which can be further utilized in future system or site-specific environmental transport and risk assessment models.

59 BASIC BIOLOGICAL SCIENCES↗
DOE OSTI2020

Interaction of hydrogen with actinide dioxide (011) surfaces

The corrosion and oxidation of actinide metals, leading to the formation of metal-oxide surface layers with the catalytic evolution of hydrogen, impacts the management of nuclear materials. In this work, the interaction of hydrogen with actinide dioxide (AnO 2 , An = U, Np, or Pu) (011) surfaces by Hubbard corrected density functional theory (PBEsol+U) has been studied, including spin–orbit interactions and non-collinear 3k anti-ferromagnetic behavior. The actinide dioxides crystalize in the fluorite-type structure, and although the (111) surface dominates the crystal morphology, the (011) surface energetics may lead to more significant interaction with hydrogen. The dissociative adsorption of hydrogen on the UO 2 (0.44 eV), NpO 2 (-0.47 eV), and PuO 2 (-1.71 eV) (011) surfaces has been calculated. It is found that hydrogen dissociates on the PuO 2 (011) surface; however, UO 2 (011) and NpO 2 (011) surfaces are relatively inert. Recombination of hydrogen ions is likely to occur on the UO 2 (011) and NpO 2 (011) surfaces, whereas hydroxide formation is shown to occur on the PuO 2 (011) surface, which distorts the surface structure.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Reactions of Np(VI) and Pu(VI) with Phenanthroline Result in Bending the [NpVIO2]2+ Unit and a Reduced Pu(V) Species

Despite decades of actinyl chemistry, genuinely bent actinyl structures remain rare beyond uranium. We report the first crystallographic characterization of a bent neptunyl(VI) complex, NpO 2 Cl 2 (phen) 2, along with a linear plutonyl species that, unexpectedly, adopts the +V oxidation state. Coordination of two 1,10-phenanthroline ligands to NpO 2 2+ enforces pronounced bending of the O yl –Np–O yl unit to 162° through steric clashes with the axial phenathroline ligand, representing the sharpest angle reported for any neptunyl(VI) complex. In contrast, synthesis with PuO 2 2+ produces a reduced linear Pu(V) species, PuO 2 Cl(phen) 2, underscoring the redox lability and the reluctance of plutonium to distort. Raman and IR spectra yield the first experimental stretching and interaction force constants for plutonyl(V). Comparison with values for plutonyl(VI) and bent and linear uranyl(VI) and neptunyl(VI) compounds shows that reduction perturbs the actinyl bond more than bending within one oxidation state. Electrochemical and spectroscopic studies clarify phenanthroline binding to Pu(VI) and the system’s reactivity. Quantum chemical calculations indicate that bending is energetically favored in the U–Np–Pu series, but the stabilization energy decreases as the actinide atomic number increases. NBO and QTAIM analyses reveal systematic trends: increasing 5f occupancy and decreasing An–O yl bond covalency from U to Pu. These results demonstrate that ligand-induced bending in neptunyl(VI) species is not only electronically feasible but also synthetically achievable, though plutonyl(VI) is constrained by redox reactivity. This work expands the frontier of nonlinear actinyl chemistry and illuminates how structure, oxidation state, and electronic configuration interrelate across the actinyl series.

actinides↗
DOE OSTI2020

Materials Data on Np2Co17 by Materials Project

Np2Co17 crystallizes in the orthorhombic Cmcm space group. The structure is three-dimensional. there are two inequivalent Np sites. In the first Np site, Np is bonded in a 12-coordinate geometry to eighteen Co atoms. There are a spread of Np–Co bond distances ranging from 2.80–3.16 Å. In the second Np site, Np is bonded in a 2-coordinate geometry to twenty Co atoms. There are a spread of Np–Co bond distances ranging from 2.75–3.05 Å. There are eight inequivalent Co sites. In the first Co site, Co is bonded in a 2-coordinate geometry to one Np and thirteen Co atoms. There are a spread of Co–Co bond distances ranging from 2.35–2.67 Å. In the second Co site, Co is bonded to two equivalent Np and ten Co atoms to form CoNp2Co10 cuboctahedra that share corners with fourteen CoNp2Co10 cuboctahedra, edges with six CoNp3Co9 cuboctahedra, and faces with ten CoNp2Co10 cuboctahedra. There are four shorter (2.35 Å) and four longer (2.36 Å) Co–Co bond lengths. In the third Co site, Co is bonded to two equivalent Np and ten Co atoms to form CoNp2Co10 cuboctahedra that share corners with fourteen CoNp2Co10 cuboctahedra, edges with six CoNp3Co9 cuboctahedra, and faces with ten CoNp2Co10 cuboctahedra. There are four shorter (2.35 Å) and four longer (2.36 Å) Co–Co bond lengths. In the fourth Co site, Co is bonded in a 12-coordinate geometry to two Np and ten Co atoms. There are a spread of Co–Co bond distances ranging from 2.28–2.51 Å. In the fifth Co site, Co is bonded in a 12-coordinate geometry to two Np and ten Co atoms. There are a spread of Co–Co bond distances ranging from 2.28–2.51 Å. In the sixth Co site, Co is bonded in a 12-coordinate geometry to two Np and ten Co atoms. There are two shorter (2.41 Å) and two longer (2.46 Å) Co–Co bond lengths. In the seventh Co site, Co is bonded to three Np and nine Co atoms to form a mixture of distorted face, edge, and corner-sharing CoNp3Co9 cuboctahedra. Both Co–Co bond lengths are 2.34 Å. In the eighth Co site, Co is bonded to three Np and nine Co atoms to form a mixture of distorted face, edge, and corner-sharing CoNp3Co9 cuboctahedra.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Characterizing the effect of incrementally increasing dry bulb temperature on linear and nonlinear measures of heart rate variability in nonpregnant, mid-gestation, and late-gestation sows

Abstract Characterizing the sow physiological response to an increased heat load is essential for effective heat stress mitigation. The study objective was to characterize the effects of a 400-min heating episode on sow heart rate variability (HRV) at different reproductive stages. HRV is a commonly used noninvasive proxy measure of autonomic function. Twenty-seven sows were enrolled in the study according to their gestation stage at time of selection: 1) nonpregnant (NP; n = 7), 2) mid-gestation (MID; 57.3 ± 11.8 d gestation; n = 11), and 3) late-gestation (LATE; 98.8 ± 4.9 d gestation; n = 8). The HRV data utilized in the study were collected from each pig as the dry bulb temperature in the room increased incrementally from 19.84 ± 2.15 °C to 35.54 ± 0.43 °C (range: 17.1–37.5 °C) over a 400-min period. After data collection, one 5-min set of continuous heart rate data were identified per pig for each of nine temperature intervals (19–20.99, 21–22.99, 23–24.99, 25–26.99, 27–28.99, 29–30.99, 31–32.99, 33–34.99, and 35–36.99 °C). Mean inter-beat interval length (RR), standard deviation of r-r intervals (SDNN), root mean square of successive differences (RMSSD), high frequency spectral power (HF), sample entropy (SampEn), short-term detrended fluctuation analysis (DFAα1), and three measures (%REC, DET, LMEAN) derived from recurrence quantification analysis were calculated for each data set. All data were analyzed using the PROC GLIMMIX procedure in SAS 9.4. Overall, LATE sows exhibited lower RR than NP sows (P < 0.01). The standard deviation of r–r intervals and RMSSD differed between each group (P < 0.01), with LATE sows exhibiting the lowest SDNN and RMSSD and NP sows exhibiting the greatest SDNN and RMSSD. Late-gestation sows exhibited lower HF than both MID and NP sows (P < 0.0001), greater DFA values than NP sows (P = 0.05), and greater DET compared to MID sows (P = 0.001). Late-gestation also sows exhibited greater %REC and LMEAN compared to MID (P < 0.01) and NP sows (all P < 0.01). In conclusion, LATE sows exhibited indicators of greater autonomic stress throughout the heating period compared to MID and NP sows. However, temperature by treatment interactions were not detected as dry bulb increased. Future studies are needed to fully elucidate the effect of gestational stage and increasing dry bulb temperature on sow HRV.

Agriculture↗
DOE OSTI2023

Divergent Stabilities of Tetravalent Cerium, Uranium, and Neptunium Imidophosphorane Complexes**

Abstract The study of the redox chemistry of mid‐actinides (U−Pu) has historically relied on cerium as a model, due to the accessibility of trivalent and tetravalent oxidation states for these ions. Recently, dramatic shifts of lanthanide 4+/3+ non‐aqueous redox couples have been established within a homoleptic imidophosphorane ligand framework. Herein we extend the chemistry of the imidophosphorane ligand (NPC=[N=P t Bu(pyrr) 2 ] − ; pyrr=pyrrolidinyl) to tetrahomoleptic NPC complexes of neptunium and cerium ( 1‐M , 2‐M , M=Np, Ce) and present comparative structural, electrochemical, and theoretical studies of these complexes. Large cathodic shifts in the M 4+/3+ (M=Ce, U, Np) couples underpin the stabilization of higher metal oxidation states owing to the strongly donating nature of the NPC ligands, providing access to the U 5+/4+ , U 6+/5+ , and to an unprecedented, well‐behaved Np 5+/4+ redox couple. The differences in the chemical redox properties of the U vs . Ce and Np complexes are rationalized based on their redox potentials, degree of structural rearrangement upon reduction/oxidation, relative molecular orbital energies, and orbital composition analyses employing density functional theory.

Otte, Kaitlyn S.↗
DOE OSTI2023

Divergent Stabilities of Tetravalent Cerium, Uranium, and Neptunium Imidophosphorane Complexes

The study of the redox chemistry of mid-actinides (U–Pu) has historically relied on cerium as a model, due to the accessibility of trivalent and tetravalent oxidation states for these ions. Recently, dramatic shifts of lanthanide 4+/3+ non-aqueous redox couples have been established within a homoleptic imidophosphorane ligand framework. Herein we extend the chemistry of the imidophosphorane ligand (NPC=[N=P t Bu(pyrr) 2 ] – ; pyrr=pyrrolidinyl) to tetrahomoleptic NPC complexes of neptunium and cerium (1-M, 2-M, M=Np, Ce) and present comparative structural, electrochemical, and theoretical studies of these complexes. Large cathodic shifts in the M 4+/3+ (M=Ce, U, Np) couples underpin the stabilization of higher metal oxidation states owing to the strongly donating nature of the NPC ligands, providing access to the U 5+/4+ , U 6+/5+ , and to an unprecedented, well-behaved Np 5+/4+ redox couple. Furthermore, the differences in the chemical redox properties of the U vs. Ce and Np complexes are rationalized based on their redox potentials, degree of structural rearrangement upon reduction/oxidation, relative molecular orbital energies, and orbital composition analyses employing density functional theory.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2023

Multiconfiguration Pair-Density Functional Theory for Vertical Excitation Energies in Actinide Molecules

Modeling actinides with electronic structure theories is challenging because these systems present a strong ligand field and metal–ligand covalency. Here, we systematically investigate the effectiveness of pair-density functional theory (PDFT) for the calculation of vertical excitation energies in An(III), [An III Cl 6 ] 3– , and [An VI O 2 ] 2+ (An = U, Np, Pu, and Am). We compare the performance of PDFT, hybrid PDFT, and multistate PDFT with traditional active-space methods followed by perturbation theory, like multistate CASPT2, and with experimental data. Overall, multistate PDFT gives quantitative agreement with multistate CASPT2 at a significantly reduced computational cost.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2020

Materials Data on NaNp3FeF15 by Materials Project

NaNp3FeF15 crystallizes in the monoclinic Cc space group. The structure is three-dimensional. there are two inequivalent Na1+ sites. In the first Na1+ site, Na1+ is bonded to twelve F1- atoms to form NaF12 cuboctahedra that share faces with two equivalent FeF6 octahedra. There are a spread of Na–F bond distances ranging from 2.63–2.69 Å. In the second Na1+ site, Na1+ is bonded in a distorted octahedral geometry to six F1- atoms. There are a spread of Na–F bond distances ranging from 2.34–2.41 Å. There are six inequivalent Np+3.67+ sites. In the first Np+3.67+ site, Np+3.67+ is bonded in a 9-coordinate geometry to nine F1- atoms. There are a spread of Np–F bond distances ranging from 2.20–2.53 Å. In the second Np+3.67+ site, Np+3.67+ is bonded in a 9-coordinate geometry to nine F1- atoms. There are a spread of Np–F bond distances ranging from 2.21–2.53 Å. In the third Np+3.67+ site, Np+3.67+ is bonded in a 9-coordinate geometry to nine F1- atoms. There are a spread of Np–F bond distances ranging from 2.20–2.52 Å. In the fourth Np+3.67+ site, Np+3.67+ is bonded in a 9-coordinate geometry to nine F1- atoms. There are a spread of Np–F bond distances ranging from 2.21–2.54 Å. In the fifth Np+3.67+ site, Np+3.67+ is bonded in a 9-coordinate geometry to nine F1- atoms. There are a spread of Np–F bond distances ranging from 2.21–2.53 Å. In the sixth Np+3.67+ site, Np+3.67+ is bonded in a 9-coordinate geometry to nine F1- atoms. There are a spread of Np–F bond distances ranging from 2.20–2.56 Å. There are two inequivalent Fe3+ sites. In the first Fe3+ site, Fe3+ is bonded to six F1- atoms to form FeF6 octahedra that share faces with two equivalent NaF12 cuboctahedra. There are five shorter (2.01 Å) and one longer (2.02 Å) Fe–F bond lengths. In the second Fe3+ site, Fe3+ is bonded in an octahedral geometry to six F1- atoms. There are a spread of Fe–F bond distances ranging from 2.08–2.14 Å. There are thirty inequivalent F1- sites. In the first F1- site, F1- is bonded in a distorted trigonal planar geometry to one Na1+ and two Np+3.67+ atoms. In the second F1- site, F1- is bonded in a distorted trigonal planar geometry to one Na1+ and two Np+3.67+ atoms. In the third F1- site, F1- is bonded in a distorted trigonal planar geometry to one Na1+ and two Np+3.67+ atoms. In the fourth F1- site, F1- is bonded in a 3-coordinate geometry to two Np+3.67+ and one Fe3+ atom. In the fifth F1- site, F1- is bonded in a 3-coordinate geometry to two Np+3.67+ and one Fe3+ atom. In the sixth F1- site, F1- is bonded in a 3-coordinate geometry to two Np+3.67+ and one Fe3+ atom. In the seventh F1- site, F1- is bonded in a 3-coordinate geometry to one Na1+ and two Np+3.67+ atoms. In the eighth F1- site, F1- is bonded in a 3-coordinate geometry to one Na1+ and two Np+3.67+ atoms. In the ninth F1- site, F1- is bonded in a 3-coordinate geometry to one Na1+ and two Np+3.67+ atoms. In the tenth F1- site, F1- is bonded in a 3-coordinate geometry to one Na1+ and two Np+3.67+ atoms. In the eleventh F1- site, F1- is bonded in a 3-coordinate geometry to one Na1+ and two Np+3.67+ atoms. In the twelfth F1- site, F1- is bonded in a 3-coordinate geometry to one Na1+ and two Np+3.67+ atoms. In the thirteenth F1- site, F1- is bonded in a 2-coordinate geometry to one Na1+, one Np+3.67+, and one Fe3+ atom. In the fourteenth F1- site, F1- is bonded in a 2-coordinate geometry to one Na1+, one Np+3.67+, and one Fe3+ atom. In the fifteenth F1- site, F1- is bonded in a 2-coordinate geometry to one Na1+, one Np+3.67+, and one Fe3+ atom. In the sixteenth F1- site, F1- is bonded in a 2-coordinate geometry to one Na1+, one Np+3.67+, and one Fe3+ atom. In the seventeenth F1- site, F1- is bonded in a 2-coordinate geometry to one Na1+, one Np+3.67+, and one Fe3+ atom. In the eighteenth F1- site, F1- is bonded in a 2-coordinate geometry to one Na1+, one Np+3.67+, and one Fe3+ atom. In the nineteenth F1- site, F1- is bonded in a distorted trigonal planar geometry to one Na1+ and two Np+3.67+ atoms. In the twentieth F1- site, F1- is bonded in a distorted trigonal planar geometry to one Na1+ and two Np+3.67+ atoms. In the twenty-first F1- site, F1- is bonded in a distorted trigonal planar geometry to one Na1+ and two Np+3.67+ atoms. In the twenty-second F1- site, F1- is bonded in a 3-coordinate geometry to two Np+3.67+ and one Fe3+ atom. In the twenty-third F1- site, F1- is bonded in a 3-coordinate geometry to two Np+3.67+ and one Fe3+ atom. In the twenty-fourth F1- site, F1- is bonded in a 3-coordinate geometry to two Np+3.67+ and one Fe3+ atom. In the twenty-fifth F1- site, F1- is bonded in a bent 150 degrees geometry to two Np+3.67+ atoms. In the twenty-sixth F1- site, F1- is bonded in a bent 150 degrees geometry to two Np+3.67+ atoms. In the twenty-seventh F1- site, F1- is bonded in a bent 150 degrees geometry to two Np+3.67+ atoms. In the twenty-eighth F1- site, F1- is bonded in a bent 150 degrees geometry to two Np+3.67+ atoms. In the twenty-ninth F1- site, F1- is bonded in a bent 150 degrees geometry to two Np+3.67+ atoms. In the thirtieth F1- site, F1- is bonded in a bent 150 degrees geometry to two Np+3.67+ atoms.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Hydroxypyridinone-based stabilization of Np(IV) enabling efficient U/Np/Pu separations in the Adapted PUREX process

The classical process to recover uranium (U) and plutonium (Pu) from used nuclear fuel using tributyl phosphate (TBP), namely the Plutonium Uranium Redox EXtraction (PUREX) process, is complicated by the persistent presence of neptunium (Np) and thus requires extra purification steps. The concept of Adapted PUREX seeks to achieve Np recovery by adjusting the valence of the metal more effectively, thereby controlling its behavior more precisely. This study introduces the use of an aqueous hydroxypyridinone chelator, 3,4,3-LI(1,2-HOPO) (abbreviated as HOPO), to dictate the behavior of Np for recovery and meanwhile simplify cumbersome reprocessing steps. The interactions between Np and HOPO were probed mechanistically by way of absorption spectrophotometry, in conjunction with cyclic voltammetry. UV–Vis-NIR spectra illustrated the reduction of NpO 2 2+ to Np 4+ , with a fast reaction rate. Cyclic voltammetry revealed quasi-reversible processes between the oxidized and reduced forms of the ligand and its Np complexes. The corresponding heterogeneous rate constants (k 0 ) were estimated from the peak-to-peak separation potentials (ΔE p ), at ~ 4 – 35 μm/s for both HOPO and NpHOPO, with scan rates of 0.01 – 0.4 V/s. Meanwhile, the electromotive force (E MF ) as well as the change of Gibbs free energy (ΔG) were assessed from the half-wave potential (E 1/2 ), demonstrating the completeness of NpO 2 2+ reduction to Np 4+ by HOPO. The cumulative formation constant of the resulting NpHOPO complex (logβ 101 ) was determined by metal competition titration to be 42.0 ± 0.6, corroborating the extraordinarily high affinity of HOPO to tetravalent metal ions. Here, the prowess of valence control by HOPO and the high stability of the formed complex resulted in enhanced separations of Np from U and of Pu from U, with a maximum separation factor of ~7000 for both, nearly 90- and 10300-fold higher, respectively, than the values obtained using conventional PUREX formulae.

38 RADIATION CHEMISTRY, RADIOCHEMISTRY, AND NUCLEA↗