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Spin-Orbit Coupling in Actinide Cations
The limiting case of Russell-Saunders coupling, which leads to a maximum spin alignment for the open shell electrons, usually explains the properties of high spin ionic crystals with transition metals. For actinide compounds, the spin-orbit splitting is large enough to cause a significantly reduced spin alignment. Novel concepts are used to explain the dependence of the spin alignment on the 5f shell occupation. We present evidence that the XPS of ionic actinide materials may provide direct information about the angular momentum coupling within the 5f shell.
Probing Orbital Bonding and Surface Interactions of Hydrolyzed Actinide Oxides using Synchrotron X-ray, Vibrational Spectroscopy, and Electron Microscopy
Actinide dioxides (MO 2 , M = Th, U, Np, Pu, Am, Cm) are the simplest actinide compounds and often used as long term storage materials. They can provide fundamental insights into 5f/6d orbital bonding and electronic structure. Scientists have been trying to understand the role of 4f, 5f, and 6d electrons in covalent bonding for decades. Most hydrolysis studies of actinide materials involve uranium and uranium oxides due to their relevance for nuclear fuel and waste. Very few hydrolysis studies, if any, involve thorium, neptunium, and trans-plutonium dioxides or mixed actinides. Actinide surface speciation can be investigated in great detail with spectroscopic techniques such as EXAFS, vibrational spectroscopy, or electron microscopy. Actinide mineral-water interface reactions have been thoroughly summarized for non-oxide actinides; however, the reaction mechanisms provide useful ideas into the incorporation of water into the actinide lattice as a result of hydrolysis. Hydrolysis will have an incremental effect on the crystalline lattice allowing for models to predict the next changes to the electronic states.
Phase Transitions in Tetramethylammonium Hexachlorometalate Compounds (TMA) 2 MCl 6 (M = U, Np, Pt, Sn, Hf, Zr)
A study of the phase transitions occurring in tetramethylammonium hexachlorometalate compounds with M = U IV , Np IV , Zr IV , Sn IV , Hf IV and Pt IV were performed using single-crystal X-ray diffraction across the temperature range 120 - 400K. When the crystals were cooled, movement of the octahedral [MCl 6 ] 2- anions induces a phase transition from Fm3m to Fd3c with a doubling of the unit cell. For the actinide compounds, no correlation between the f-electron configuration and the transition temperature was observed, instead, a correlation between the transition temperatures and both the [MCl 6 ] 2- anion and the TMA cation size is highlighted. Two phase transitions were observed and characterized. The first phase transition occurs with the ordering of the TMA cation and the second from a rotation of the [MCl 6 ] 2- octahedra. A third phase transition was observed at lower temperatures and was ascribed to a tetragonal distortion of the [MCl 6 ] 2- anions. Synthesis and study of their deuterated compounds did not show a significant isotope effect. As a result, Raman spectra performed on the protonated and deuterated compounds indicate only weak hydrogen bonding interactions between the TMA cations and the [MCl 6 ] 2- octahedra.
Final Report on Small Particle Speciation for Forensics Analysis by Soft X-ray Scanning Transmission X-ray Microscopy
Materials of interest for nuclear forensic science are often highly heterogeneous, containing complex mixtures of actinide compounds in a wide variety of matrices. Scanning transmission X-ray microscopy (STXM) is ideally suited to study such materials, as it can be used to chemically image specimens by acquiring X-ray absorption near-edge spectroscopy (XANES) data with 25 nm spatial resolution. In particular, STXM in the soft X-ray synchrotron radiation regime (approximately 120 – 2000 eV) can collect spectroscopic information from the actinides and light elements in a single experiment. Thus, STXM combines the chemical sensitivity of X-ray absorption spectroscopy with high spatial resolution in a single non-destructive characterization method. This report describes the application of STXM to a broad range of nuclear materials. Where possible, the spectroscopic images obtained by STXM are compared with information derived from other analytical methods, and used to make inferences about the process history of each material. STXM measurements can yield information including the morphology of a sample, “elemental maps” showing the spatial distribution of major chemical constituents, and XANES spectra from localized regions of a sample, which may show spatial variations in chemical composition.
Synthesis and Characterization of Layered Actinide (U, Np, Pu) Oxide and Hydroxide Phases
Systematic structural comparisons across the early actinides remain limited by the scarcity of well-defined transuranic layered oxide and oxyhydroxide phases. Here, we report the synthesis and single-crystal characterization of new layered actinide compounds spanning U, Np, and Pu obtained under mild hydrothermal conditions in concentrated alkali hydroxide media. These include hydrated oxides α-Cs 2 U 2 O 7 •0.5H 2 O and Rb 2 An 2 O 7 •0.5H 2 O (An = Np, Pu), oxy-hydroxides Rb 6 [(AnO 2 ) 6 O 8 (OH) 2 ]•xH 2 O x = 0, 0.5 (An = U, Np) and Rb 4 [(UO 2 ) 5 O 6 (OH) 2 ]•2H 2 O, as well as nitrate-intercalated compounds Cs 6 [(AnO 2 ) 3 O 4 (OH) 2 ](NO 3 ) 2 (An = Np, Pu). Single-crystal X-ray diffraction studies reveal extended two-dimensional architectures constructed from edge- and vertex-sharing actinyl polyhedra, with systematic evolution in equatorial coordination, hydration, and anionic sheet topology across the U–Np–Pu series. Incorporation of nitrate anions within the interlayer region of Cs 6 [(AnO 2 ) 3 O 4 (OH) 2 ](NO 3 ) 2 establishes a previously unobserved structural motif in layered transuranic oxyhydroxides, demonstrating an additional pathway for anion-mediated framework stabilization. Correlation of crystallographic metrics with single-crystal Raman spectroscopy provides new vibrational benchmarks linking differences in An═O yl bond lengths to the equatorial coordination and the interstitial cations. These findings expand the structural hierarchy of layered actinide materials and address clarifying periodic trends governing topology, bonding, and vibrational signatures in high-valent 5ƒ oxide systems.
Isolation and characterization of a covalent Ce IV -Aryl complex with an anomalous 13 C chemical shift
The synthesis of bona fide organometallic Ce IV complexes is a formidable challenge given the typically oxidizing properties of the Ce IV cation and reducing tendencies of carbanions. Herein, we report a pair of compounds comprising a Ce IV – C aryl bond [Li(THF) 4 ][Ce IV (κ 2 - ortho -oxa)(MBP) 2 ] ( 3-THF ) and [Li(DME) 3 ][Ce IV (κ 2 - ortho -oxa)(MBP) 2 ] ( 3-DME ), ortho -oxa = dihydro-dimethyl-2-[4-(trifluoromethyl)phenyl]-oxazolide, MBP 2– = 2,2'-methylenebis(6- tert -butyl-4-methylphenolate), which exhibit Ce IV – C aryl bond lengths of 2.571(7) – 2.5806(19) Å and strongly-deshielded, Ce IV – C ipso 13 C{ 1 H} NMR resonances at 255.6 ppm. Computational analyses reveal the Ce contribution to the Ce IV – C aryl bond of 3-THF is ~12%, indicating appreciable metal-ligand covalency. Computations also reproduce the characteristic 13 C{ 1 H} resonance, and show a strong influence from spin-orbit coupling (SOC) effects on the chemical shift. The results demonstrate that SOC-driven deshielding is present for Ce IV – C ipso 13 C{ 1 H} resonances and not just for diamagnetic actinide compounds.
Interband Transitions and Critical Points of Single-Crystal Thoria Compared with Urania
The interband transitions of UO 2 are validated independently through cathode luminescence. A picture emerges consistent with density functional theory. While theory is generally consistent with experiment, it is evident from the comparison of UO 2 and ThO 2 that the choice of functional can significantly alter the bandgap and some details of the band structure, in particular at the conduction band minimum. Strictly ab initio predictions of the optical properties of the actinide compounds, based on density functional theory alone, continue to be somewhat elusive.
THEORY FOR THE XPS OF ACTINIDES
Two aspects of the electronic structure of actinide oxides that significantly affect the XPS spectra are described; these aspects are also important for the materials properties of the oxides. The two aspects considered are: (1) The spin-orbit coupling of the open 5f shell electrons in actinide cations and how this coupling affects the electronic structure. And, (2) the covalent character of the metal oxygen interaction in actinide compounds. Because of this covalent character, there are strong departures from the nominal oxidation states that are significantly larger in core-hole states than in the ground state. The consequences for the XPS of this covalent character are examined. A proper understanding of the way in which they influence the XPS makes it possible to use the XPS to correctly characterize the electronic structure of the oxides.
Harnessing Solvent Displacement Crystallization for Actinide Synthesis: Insights from Uranyl Oxalate
To address the challenge of actinide crystallization in systems with a low chemical potential, solvent displacement crystallization (SDC) techniques are applied to synthesize uranyl oxalate in a series of alcohols with varying solvent polarity. This work demonstrates the simplicity of applying SDC to actinides and indicates that solvent polarity affects crystallizations. Uranyl oxalate trihydrate was synthesized from methanol, ethanol, 1-propanol, and isopropanol as additive solvents, with characterization indicating an absence of solvent influence on the bulk structure. The choice of solvent did cause changes to the observed morphology and particle size. Additionally, the total yield of uranyl oxalate was found to decrease with increasing solvent polarity. These data support the use of SDC techniques for the crystallization of high-solubility actinide compounds.
A Screened Hybrid DFT Study of Actinide Oxides, Nitrides, and Carbides
A systematic study of the structural, electronic, and magnetic properties of actinide oxides, nitrides, and carbides (AnX1–2 with X = C, N, O) is performed using the Heyd–Scuseria–Ernzerhof (HSE) hybrid functional. Our computed results show that the screened hybrid HSE functional gives a good description of the electronic and structural properties of actinide dioxides (strongly correlated insulators) when compared with available experimental data. However, there are still some problems reproducing the electronic properties of actinide nitrides and carbides (strongly correlated metals). In addition, in order to compare with the results by HSE, the structures, electronic, and magnetic properties of these actinide compounds are also investigated in the PBE and PBE+U approximation. Interestingly, the density of states of UN obtained with PBE compares well with the experimental photoemission spectra, in contrast to the hybrid approximation. This is presumably related to the need of additional screening in the Hartree–Fock exchange term of the metallic phases.
Compression of curium pyrrolidine-dithiocarbamate enhances covalency
Curium is unique in the actinide series because its half-filled 5f 7 shell has lower energy than other 5f n configurations, rendering it both redox-inactive and resistant to forming chemical bonds that engage the 5f shell. This is even more pronounced in gadolinium, curium’s lanthanide analogue, owing to the contraction of the 4f orbitals with respect to the 5f orbitals. However, at high pressures metallic curium undergoes a transition from localized to itinerant 5f electrons. This transition is accompanied by a crystal structure dictated by the magnetic interactions between curium atoms. Therefore, the question arises of whether the frontier metal orbitals in curium(III)–ligand interactions can also be modified by applying pressure, and thus be induced to form metal–ligand bonds with a degree of covalency. Here we report experimental and computational evidence for changes in the relative roles of the 5f/6d orbitals in curium–sulfur bonds in [Cm(pydtc) 4 ] – (pydtc, pyrrolidinedithiocarbamate) at high pressures (up to 11 gigapascals). We compare these results to the spectra of [Nd(pydtc) 4 ] – and of a Cm(III) mellitate that possesses only curium–oxygen bonds. Compared with the changes observed in the [Cm(pydtc) 4 ] – spectra, we observe smaller changes in the f–f transitions in the [Nd(pydtc) 4 ] – absorption spectrum and in the f–f emission spectrum of the Cm(III) mellitate upon pressurization, which are related to the smaller perturbation of the nature of their bonds. These results reveal that the metal orbital contributions to the curium–sulfur bonds are considerably enhanced at high pressures and that the 5f orbital involvement doubles between 0 and 11 gigapascal. Furthermore, our work implies that covalency in actinides is complex even when dealing with the same ion, but it could guide the selection of ligands to study the effect of pressure on actinide compounds.
Open questions in transplutonium coordination chemistry
Over the past decade, momentous progress has been made in the characterization of late actinide compounds. Here the authors highlight how advances in spectroscopic and computational tools have developed our understanding of fundamental transplutonium bonding interactions, and discuss whether covalency and heterogeneity changes in 5f-orbital bonding could be harnessed in environmentally and industrially relevant systems.
X-ray absorption spectra of f-element complexes: insight from relativistic multiconfigurational wavefunction theory
X-ray absorption near edge structure (XANES) spectroscopy, coupled with ab initio calculations, has emerged as the state-of-the-art tool for elucidating the metal-ligand bonding in f-element complexes. Furthermore, this highlight presents recent efforts in calculating XANES spectra of lanthanide and actinide compounds with relativistic multiconfiguration wavefunction approaches that account for differences in donation bonding in the ground state (GS) versus a core-excited state (ES), multiplet effects, and spin-orbit-coupling. With the GS and ES wavefunctions available, including spin-orbit effects, an arsenal of chemical bonding tools that are popular among chemists can be applied to rationalize the observed intensities in terms of covalent bonding.
Theoretical prediction of a highly responsive material: Spin fluctuations and superconductivity in FeNiB 2 system
By analyzing the Fe-Ni-B compositional diagram, we predict an energetically and dynamically stable FeNiB 2 compound. This system belongs to the class of highly responsive state of materials, as it is very sensitive to the external perturbations. This state is also characterized by a high level of spin fluctuations, which strongly influence possible magnetic long- and short-range orders. Furthermore, we demonstrate that these antiferromagnetically dominating fluctuations could lead to the appearance of spin mediated superconductivity. The obtained results suggest a promising avenue for the search of strong spin fluctuation systems and related superconductors. Spin fluctuations (SFs) in itinerant electron magnets play an important role in many metallic systems including weak itinerant magnets, heavy fermion compounds, actinides, Invar alloys, and many magnetoresistive materials. SFs also have been identified in high-temperature superconductors and recently discovered Fe-based superconductors with a clear suggestion about their crucial influence on the mechanisms of unconventional superconductivity. In the majority of cases, superconductivity appears near the magnetic quantum critical point, where magnetism has a pure itinerant character with no local moments involved. This is exactly a case when quantum SFs (including spin zero-point motion) are strong and provide a dominating contribution to many observable physical properties. However, as a typical example, in a case of superconductivity, the direct calculations and prediction of critical temperature from first principles are still not possible. In this situation, one can focus on searching for materials with a strong level of SF, leading to the possible development of promising physical properties. In this paper, we will introduce qualitatively the concept of a highly responsive state (HRS) (see Fig. 1). This term is used here to indicate that a physical system is in a state characterized by an order parameter, which is very sensitive to the external perturbations (pressure, magnetic fields, etc.). The HRS is naturally related to closeness to the instability and, as usual in such situations, one can expect that fluctuations are very important.
Ground-state wave function of plutonium in PuSb as determined via x-ray magnetic circular dichroism
Measurements of x-ray magnetic circular dichroism (XMCD) and x-ray absorption near-edge structure (XANES) spectroscopy at the Pu M₄,₅ edges of the ferromagnet PuSb are reported. Using bulk magnetization measurements and a sum rule analysis of the XMCD spectra, we determine the individual orbital [μ L = 2.8(1)μ B /Pu] and spin moments [μ S = –2.0(1)μ B /Pu] of the Pu 5f electrons for the first time. Atomic multiplet calculations of the XMCD and XANES spectra reproduce well the experimental data and are consistent with the experimental value of the spin moment. These measurements of L z and S z are in excellent agreement with the values that have been extracted from neutron magnetic form factor measurements, and confirm the local character of the 5f electrons in PuSb. We demonstrate that a split M₅ as well as a narrow M₄ XMCD signal may serve as a signature of 5f electron localization in actinide compounds.
Magnetocrystalline anisotropy in UMn 2 Ge 2 and related Mn-based actinide ferromagnets
We present magnetization isotherms in pulsed magnetic fields up to 62 Tesla, supported by first principles calculations, demonstrating a huge uniaxial magnetocrystalline anisotropy energy - approximately 20 MJ/m 3 - in UMn 2 Ge 2 . This large anisotropy results from the extremely strong spin-orbit coupling affecting the uranium 5 f electrons, which in the calculations exhibit a substantial orbital moment exceeding 2 μ B . Finally, we also find from theoretical calculations that a number of isostructural Mn-actinide compounds are expected to have similarly large anisotropy.
Examination of Exogenous and Endogenous Irradiation Effects on PuO2 and its Precursors via EPR Spectroscopy
The spectroscopic examination of actinide compounds are of con-siderable interest for elucidating the role of 5f electrons in chemical bonding. For example, this could be directly detected via EPR spectroscopy through the hyperfine interaction of an unpaired Pu electron with a given ligand. However, typically fast spin-lattice re-laxation times [1] require the use of low temperatures, multiplying the difficulty of working with radio-toxic Pu compounds. Instead, we looked for radicals at room temperature produced from endoge-nous α-decay or exogenous γ-irradiation. PuO2 and its precursor PuIII2(C2O4)3∙nH2O were used as examples because of their ubiquity in the literature.