Actinide Compounds Research Articles

Spectroscopic characterization of actinide materials

Advanced spectroscopic techniques provide new and unique tools for unraveling the nature of the electronic structure of actinide materials. Inelastic neutron scattering experiments, which address temporal aspects of lattice and magnetic fluctuations, probe electromagnetic multipole interactions and the coupling between electronic and vibrational degrees of freedom. Nuclear magnetic resonance clearly demonstrates different magnetic ground states at low temperature. Photoemission spectroscopy provides information on the occupied part of the electronic density of states and has been used to investigate the momentum-resolved electronic structure and the topology of the Fermi surface in a variety of actinide compounds. Furthermore, x-ray absorption and electron energy-loss spectroscopy have been used to probe the relativistic nature, occupation number, and degree of localization of 5f electrons across the actinide series. More recently, element- and edge-specific resonant and non-resonant inelastic x-ray scattering experiments have provided the opportunity of measuring elementary electronic excitations with higher resolution than traditional absorption techniques. Here, we will discuss results from these spectroscopic techniques and what they tell us of the electronic and magnetic properties of selected actinide materials.

Reassessing the melting temperature of PuO2

The melting behavior is a fundamental property of a material, closely related to its structure and thermodynamic stability, and has therefore been a crucial subject of research for ages. The melting point is also an important engineering parameter, as it defines the operational limits of a material in its application environment. This point becomes critical in nuclear engineering where the thermo-mechanical stability of a nuclear fuel element is a key factor determining fuel performance and safety. However, experimental difficulties stemming from the extreme temperatures, complex pressure-temperature-composition relations, and the high radioactivity make the study of melting of refractory actinide compounds particularly challenging. As a consequence, experimental data are rare and subject to large uncertainties, and more reliable experimental techniques are badly needed. A novel experimental approach is presented here, yielding new data and allowing a re-assessment of the PuO 2 melting behaviour.

ChemInform Abstract: Syntheses and Characterization of Some Solid‐State Actinide (Th, U, Np) Compounds

AbstractReview: 86 refs.

Fabrication and characterization of minor actinides bearing fuels obtained by conventional powder metallurgy process

The CEA is currently assessing minor actinides (MA) recycling in nuclear fuels in fast neutrons reactors (FNR). Two routes are investigated: homogeneous recycling, where MA are added up to several percents in UPu-type fuel to be used in the whole core, and heterogeneous recycling, which consists in higher amounts of MA in uranium oxide fuel used in the periphery of the core. The studies include various subject areas: neutronics, thermo-physical properties, coolant type (sodium, lead, bismuth, helium), etc…, and experimental work concerning the fabrication of minor actinides compounds. The first part of this work consisted in the structural study by XRD of AmO 2. We studied the lattice parameter change versus time due to α self-irradiation. In this context, AmO 2 powder was prepared in the ATALANTE facility in the CEA Marcoule. It is a first approach of swelling phenomenon at the structural scale which has been extended at the macroscopic scale in the second part of the work with the study of the geometrical stability in time of UO 2 sintered pellets containing 30% Am. Pellets were fabricated by conventional powder metallurgy process in hot cell at the laboratory scale. Swelling behaviour has been studied by accurate diameter measurements performed along the pellets and correlated to the cumulative α decay dose. Up to 2.1 × 10 17 α decay in the sample, no evidence of pellet macroscopic swelling was observed.

Cation o' nine tails

The field of actinide chemistry is still young, not least because the radioactivity of these elements makes them difficult to work with. A study now reveals details of how actinide compounds might behave in water.

Syntheses and structures of three disulfoxide uranyl complexes

Three disulfoxide uranyl complexes [UO2(DBSOB)(NO3)2] n (1), [UO2(DBM)2]2(DBSOB) (2), and [UO2(PMBP)2]2(DBSOB) (3) (DBSOB = 1,4-di(butylsulfinyl)butane, HDBM = dibenzoylmethane, HPMBP = 1-phenyl-3-methyl-4-benzoyl-5-pyrazolone) were synthesized and characterized. The [UO2(NO3)2] groups are connected by bridging disulfoxide ligands DBSOB to form a 1-D zigzag chain in 1. Two [UO2(DBM)2] or [UO2(PMBP)2] groups are connected by a bridging DBSOB to form the dimeric structures of 2 or 3, respectively. Complexes 1, 2, and 3 are the first structurally characterized disulfoxide–actinide compounds. Thermal stabilities of 1, 2, and 3 were investigated.

Probing electronic correlations in actinide materials using multipolar transitions

We report nonresonant inelastic x-ray scattering from the semi-core 5d levels of several actinide compounds. Dipole-forbidden, high-multipole features form a rich bound-state spectrum dependent on valence electron configuration and spin-orbit and Coulomb interactions. Cross-material comparisons, together with the anomalously high Coulomb screening required for agreement between atomic multiplet theory and experiment, demonstrate sensitivity to the neighboring electronic environment, such as is needed to address long-standing questions of electronic localization and bonding in 5f compounds.

Electronic Structure Analysis of USiO

Uranium is a member of Actinides and plays important role in nuclear science and technology. Electronic and structural investigations of actinide compounds attract major interest in science. The electronic structure and chemical bonding of coffinite USiO4 are investigated by X-ray Absorption Fine Structure spectroscopy (XAFS). U L3- edge absorption spectrum in USiO4 is compared with U L3-edge spectra in UO2 and UTe due to their different electronic and chemical structures. The study presents XANES (x-ray Absorption Near-Edge Structure) and Extended XAFS (EXAFS) calculations of USiO4 thin films. The full multiple scattering approach has been applied to the calculation of U L3 edge XANES spectra of USiO4, UO2 and UTe, based on different choices of one electron potentials according to Uranium coordinations by using the real space multiple scattering method FEFF 8.2 code.

An atomistic simulation of the structural and vibrational properties of A4Fe3Al32 (A = Th, U)

An atomistic simulation of the structural properties of the new ternary A4Fe3Al32 compounds, where A is Th, U, has been carried out using interatomic pair potentials based on the lattice inversion method. A4Fe3Al32 adopts the orthorhombic structure described by space group Cmmm. The unit cell contains 77 atoms which occupy 78 positions. Fe atoms prefer to substitute for Al in 4h sites. The Wyckoff positions 4g and 2d are partially occupied by Fe atoms. Calculated lattice constants are found to agree with a report in the literature. In particular, the phonon densities of states of these actinide compounds were evaluated for the first time. The analysis for the inverted potentials explains qualitatively the contributions of different atoms to the vibrational modes.

Magnetism and superconductivity in the new family of actinide compounds : AnPd5Al2

AnPd5Al2 (An: actinide and rare earths) crystallizing in the tetragonal ZrNi2Al5-type structure are a new family of intermetallic compounds exhibiting exotic magnetic and superconducting properties. The first compound discovered in this family, NpPd5Al2, shows heavy-fermion superconductivity at the rather high temperature of 5 K. The uranium analog UPd5Al2 has a non-magnetic ground state in spite of the Curie-Weiss behavior at high temperatures. CePd5Al2 is an anisotropic antiferromagnet. Application of pressure on this compound leads to the suppression of antiferromagnetic ordering and finally superconductivity appears above 10 GPa.

Di-2-pyridyl ketone in actinide chemistry: Dinuclear uranyl complexes

Treatment of UO 2X 2 (X = OAc, Cl, NO 3) with 1 mol equiv of (py) 2CO in THF afforded the adducts [UO 2X 2{(py) 2CO}] in almost quantitative yields. The same reactions in MeOH, in the presence of NEt 3 for X = Cl and NO 3, gave yellow crystals of [(UO 2X) 2{μ-(py) 2C(OMe)O} 2]·MeOH (X = OAc, 1·MeOH and X = Cl, 2·MeOH) and [{UO 2(NO 3)} 2{μ-(py) 2C(OMe)O} 2] ( 3). Reactions of UO 2X 2 (X = OAc, Cl) with 2 mol equiv of (py) 2CO and NEt 3 in MeOH or further treatment of 1 and 2 with 1 mol equiv of (py) 2CO and NEt 3 afforded the methoxide derivative [{UO 2(OMe)} 2{μ-(py) 2C(OMe)O} 2] ( 4), while UO 2(NO 3) 2 was transformed into [{UO 2(NO 3)}{UO 2(OH)}{μ-(py) 2C(OMe)O} 2] ( 5). In these first structurally characterized actinide compounds with a (py) 2CO-based ligand, the uranium atoms are located at the center of pentagonal (X = Cl and OMe) or hexagonal (X = OAc and NO 3) bipyramids sharing one edge defined by the μ-alkoxo oxygen atoms. Crystals of [{UO 2(OMe)} 2{μ-(py) 2C(OMe)O} 2]·[(UO 2) 4{μ 2-(py) 2C(OMe)O} 2(μ 2-OAc) 2(μ 3-O) 2(MeOH) 2]·H 2O ( 6·H 2O) were serendipitously obtained in one experiment with [UO 2(OAc) 2(H 2O) 2] and (py) 2CO.

Crystal structure of complexes of pentavalent neptunium with 1,8-bis[2-(diphenylphosphinylmethyl)]phenoxy-3,6-dioxaoctane (L) and hexavalent actinides (U, Np, Pu) with 1,8-bis[2-(diphenylphosphinyl)]phenoxy-3,6-dioxaoctane (L1)

Complexes of pentaand hexavalent actinides with phosphoryl-containing podands incorporating a triethylene glycol fragment and 2-(diphenylphosphinyl)phenyl (L1) or 2-(diphenylphosphinylmethyl)phenyl (L) end groups were synthesized and structurally characterized. The complexes can be described by the following formulas: [NpO2L(C2H5OH)(NO3)] (I) for pentavalent neptunium and [AnO2(L1)2](OH)2 · nH2O (II), where An is U (IIa), Np (IIb), and Pu (IIc), for the isostructural compounds of hexavalent actinides. In I, L is a bidentate bridging ligand. The Np5+ coordination polyhedron is a pentagonal bipyramid. The bipyramid equatorial plane is formed by the oxygen atoms of two podands L, the bidentate nitrate ion, and the ethanol OH group. The oxygen atoms of the phosphoryl groups of the podand are involved in the coordination environment of two NpO2+ cations where they connect the electrically neutral neptunoyl nitrate fragments to infinite chains along the [100] direction, which are in turn connected into ribbons by strong hydrogen bonds. The crystal of II consists of the complex cations [AnO2(L1)2]2+, hydroxyl ions, and water molecules of crystallization. The environment of AnO22+ is formed by four ligands L1 whose oxygen atoms form a tetragonal-bipyramidal coordination environment. Each of the two crystallographically independent ligands L1 is connected to two AnO2+ cations. This gives positively charged layers of actinyl cations perpendicular to the [010] direction connected by molecular ligands. The layers contain channels accommodating hydroxyl ions and crystallization water molecules.

NDTB‐1: A Supertetrahedral Cationic Framework That Removes TcO4− from Solution

A cubic thorium borate possesses a porous supertetrahedral cationic framework with extraframework borate anions. These anions are readily exchanged with a variety of environmental contaminants, especially those from the nuclear industry, including chromate and pertechnetate.

Syntheses and characterization of some solid-state actinide (Th, U, Np) compounds

In this Perspective we discuss a variety of methods that are broadly applicable to the syntheses of solid-state compounds. To illustrate the application of these methods we use solid-state actinide compounds, an area of chemistry that has been the focus of this laboratory for the last decade. We have synthesized single crystals, primarily of actinide chalcogenide compounds, by a variety of techniques. The benefits and drawbacks of each will be discussed in detail. Because of their propensity towards high coordination numbers and their ability to take on a variety of formal oxidation states, actinide compounds adopt structures illustrated here that are often different from those adopted by transition-metal or even lanthanide compounds. Often, there are corresponding differences in their physical properties, and these are touched upon here.

Synthesis of zirconia sol stabilized by trivalent cations (yttrium and neodymium or americium): a precursor for Am-bearing cubic stabilized zirconia

Recent concepts for nuclear fuel and targets for transmuting long-lived radionuclides (minor actinides) and for the development of innovative Gen-IV nuclear fuel cycles imply fabricating host phases for actinide or mixed actinide compounds. Cubic stabilized zirconia (Zr, Y, Am)O(2-x) is one of the mixed phases tested in transmutation experiments. Wet chemical routes as an alternative to the powder metallurgy are being investigated to obtain the required phases while minimizing the handling of contaminating radioactive powder. Hydrolysis of zirconium, neodymium (a typical surrogate for americium) and yttrium in aqueous media in the presence of acetylacetone was firstly investigated. Progressive hydrolysis of zirconium acetylacetonate and sorption of trivalent cations and acacH on the zirconia particles led to a stable dispersion of nanoparticles (5-7 nm) in the 6-7 pH range. This sol gels with time or with temperature. The application to americium-containing solutions was then successfully tested: a stable sol was synthesized, characterized and used to prepare cubic stabilized zirconia (Zr, Y, Am)O(2-x).

Density functional theory calculations on magnetic properties of actinide compounds

We have performed a detailed analysis of the magnetic (collinear and non-collinear) order and the atomic and electron structures of UO(2), PuO(2) and UN on the basis of density functional theory with the Hubbard electron correlation correction (DFT + U). We have shown that the 3-k magnetic structure of UO(2) is the lowest in energy for the Hubbard parameter value of U = 4.6 eV (and J = 0.5 eV) consistent with experiments when Dudarev's formalism is used. In contrast to UO(2), UN and PuO(2) show no trend for a distortion towards rhombohedral structure and, thus, no complex 3-k magnetic structure is to be anticipated in these materials.

Effects of the first hydration sphere and the bulk solvent on the spectra of the f2isoelectronic actinide compounds: U4+, NpO2+, and PuO22+

The electronic spectra of the 5f(2) isoelectronic actinide compounds U(4+), NpO(2)(+), and PuO(2)(2+) have been investigated theoretically both in gas phase and in solution. In the latter case the solvent was modelled by a saturated first hydration sphere, five water molecules for NpO(2)(+), and PuO(2)(2+) and eight for U(4+), and a continuum model describing the remaining solvent. The transition energies and oscillator strengths were obtained at the spin-orbit level using the relativistic wave function based multi-configuration methods CASPT2 (complete active space with second-order perturbation theory) and MRCI + DC (Davidson corrected multi-reference configuration interaction), followed by a spin-orbit CI based on a dressed effective spin-orbit Hamiltonian. This study is an attempt to contribute to an enhanced understanding of the electronic structure of tetravalent actinide ions and actinyl(v) and (vi) ions. The spin-orbit MRCI and spin-orbit CASPT2 transitions energies have been compared for the bare ions, leading us to the conclusion that the spin-orbit CASPT2 approach is reasonably accurate and can be used with confidence for the calculation of the hydrated species. The first hydration sphere and the bulk solvent lift degeneracies, but the effect on the transition energies is fairly small for the two actinyl ions, while it is larger, up to several thousands of wave numbers for U(4+). The calculations allowed us to make assignments of the experimentally observed absorption spectra for all species. The computed transition energies and intensities compared favourably with experiment.

Performance of Relativistic Effective Core Potentials in DFT Calculations on Actinide Compounds

Density functional theory (DFT) calculations using relativistic effective core potentials (RECPs) have emerged as a robust and fast method of calculating the structural parameters and energy changes of the thermochemical reactions of actinide complexes. A comparative investigation of the performance of the Stuttgart small-core and large-core RECPs in DFT calculations has been carried out. The vibrational frequencies and reaction enthalpy changes of several uranium(VI) compounds computed using these RECPs were compared to those obtained using DFT and a four-component one-electron scalar relativistic approximation of the full Dirac equation with large all-electron basis sets (AE). The relativistic AE method is a full solution of the Dirac equation with all spin components separated out. This method gives the "correct" answer (with respect to scalar relativity) which should be closest to experimental values when an adequate density functional is used and in the absence of significant spin-orbit effects. The small-core RECP always show better agreement with the four-component scalar- relativistic AE method than the large-core RECP. We conclude that the 5s, 5p, and 5d orbitals are of great importance in determining the chemistry of actinide complexes. Instances in which large-core RECPs give better agreement with experimental data are attributed to either experimental uncertainties or error cancellations.

Krylov implementation of the hybridization expansion impurity solver and application to 5-orbital models

We present an implementation of the hybridization expansion impurity solver which employs sparse-matrix exact-diagonalization techniques to compute the time evolution of the local Hamiltonian. This method avoids computationally expensive matrix-matrix multiplications and becomes advantageous over the conventional implementation for models with five or more orbitals. In particular, this method will allow the systematic investigation of 7-orbital systems (lanthanide and actinide compounds) within single-site dynamical mean-field theory. We illustrate the power and usefulness of our approach with dynamical mean-field results for a 5-orbital model which captures some aspects of the physics of the iron-based superconductors.

Novel heptavalent actinide compounds: tetrasodium dihydroxidotetraoxidoneptunate(VII) hydroxide dihydrate and its plutonium analogue

The title compounds, Na(4)[NpO(4)(OH)(2)]OH.2H(2)O and Na(4)[PuO(4)(OH)(2)]OH.2H(2)O, are isostructural and isomorphous, and contain complex [AnO(4)(OH)(2)](3-) anions (Ac is an actinide) in the form of distorted tetragonal bipyramids, Na(+) cations, crystallization water molecules and outer-sphere OH groups. The complex [AnO(4)(OH)(2)](3-) anions occupy general positions and the coordinated OH groups deviate significantly from a centrosymmetric relative orientation. The [AnO(4)(OH)(2)](3-) anions exhibit anisotropic actinide contraction; the shortening of the An-O(hydroxide) bonds on going from Np to Pu is greater than that of the AnO(4) groups.