Investigation of ammonium diuranate calcination with high temperature X-ray diffraction

The thermal decomposition of ammonium diuranate (ADU) in air is investigated using in-situ high-temperature x-ray diffraction (HT-XRD), thermogravimetry, and differential thermal analysis (TG/DTA). Data have been collected in the temperature range from 30 to 1000 °C, allowing to observe the sequence of phase transformations and to assess the energy changes involved in the calcination of ADU. The starting material 2UO3•NH3•3H2O undergoes a process involving several endothermic and exothermic reactions. In situ HT-XRD shows that amorphous UO3 is obtained after achieving complete dehydration at 300 °C, and denitration at about 450 °C. After cooling from heat treatment at 600 °C, a crystalline UO3 phase appears, as displayed by ex-situ XRD. The self-reduction of UO3 into orthorhombic U3O8 takes place at about 600 °C, but a long heat treatment or higher temperature is required to stabilize the structure of U3O8 at room temperature. U3O8 remains stable in air up to 850 °C. Above this temperature, oxygen losses lead to the formation of U3O8-x, as demonstrated by subtle changes in the diffraction pattern and by a mass loss recorded by TGA.JRC.E.6-Actinide researc

Probing magnetism in the vortex phase of PuCoGa5 by X-ray magnetic circular dichroism

We have measured X-ray magnetic circular dichroism (XMCD) spectra at the Pu M4;5 absorption edges from a newly-prepared high-quality single crystal of the heavy fermion superconductor 242PuCoGa5, exhibiting a critical temperature Tc = 18.7 K. The experiment probes the vortex phase below Tc and shows that an external magnetic field induces a Pu 5f magnetic moment at 2 K equal to the temperature-independent moment measured in the normal phase up to 300 K by a SQUID device. This observation is in agreement with theoretical models claiming that the Pu atoms in PuCoGa5 have a nonmagnetic singlet ground state resulting from the hybridization of the conduction electrons with the intermediate-valence 5f electronic shell. Unexpectedly, XMCD spectra show that the orbital component of the 5f magnetic moment increases significantly between 30 and 2 K; the antiparallel spin component increases as well, leaving the total moment practically constant. We suggest that this indicates a low-temperature breakdown of the complete Kondo-like screening of the local 5f moment.JRC.G.I.5-Advanced Nuclear Knowledg

Single-Electron Uranyl Reduction by a Rare-Earth Cation

Reducing the irreducible. Incorporation of a lanthanide cation into the bottom coordination pocket of a uranyl Pacman complex results in single electron reduction to form stable pentavalent uranyl-lanthanide complexes with uranyl-oxo-lanthanide bonds.JRC.E.6-Actinides researc

Tris‐{hydridotris(1‐pyrazolyl)borato}actinide Complexes: Synthesis, Spectroscopy, Crystal Structure, Bonding Properties and Magnetic Behaviour

The isostructural compounds of the trivalent actinides uranium, neptunium, plutonium, americium, and curium with the hydridotris(1-pyrazolyl)borato (Tp) ligand An[η$_{3}$-HB(N$_{2}$C$_{3}$H$_{3}$)$_{3}$]$_{3}$ (AnTp$_{3}$) have been obtained through several synthetic routes. Structural, spectroscopic (absorption, infrared, laser fluorescence) and magnetic characterisation of the compounds were performed in combination with crystal field, density functional theory (DFT) and relativistic multiconfigurational calculations. The covalent bonding interactions were analysed in terms of the natural bond orbital (NBO) and quantum theory of atoms in molecules (QTAIM) models

Tris‐{hydridotris(1‐pyrazolyl)borato}actinide Complexes: Synthesis, Spectroscopy, Crystal Structure, Bonding Properties and Magnetic Behaviour

The isostructural compounds of the trivalent actinides uranium, neptunium, plutonium, americium, and curium with the hydridotris(1-pyrazolyl)borato (Tp) ligand An[η$_{3}$-HB(N$_{2}$C$_{3}$H$_{3}$)$_{3}$]$_{3}$ (AnTp$_{3}$) have been obtained through several synthetic routes. Structural, spectroscopic (absorption, infrared, laser fluorescence) and magnetic characterisation of the compounds were performed in combination with crystal field, density functional theory (DFT) and relativistic multiconfigurational calculations. The covalent bonding interactions were analysed in terms of the natural bond orbital (NBO) and quantum theory of atoms in molecules (QTAIM) models

Oxo-Functionalization and Reduction of the Uranyl Ion through Lanthanide-Element Bond Homolysis:Synthetic, Structural, and Bonding Analysis of a Series of Singly Reduced Uranyl-Rare Earth 5f¹-4f^<em>n</em> Complexes

The heterobimetallic complexes [{UO2Ln-(py)2(L)}2], combining a singly reduced uranyl cation and a rare-earth trication in a binucleating polypyrrole Schiff-base macrocycle (Pacman) and bridged through a uranyl oxo-group, have been prepared for Ln = Sc, Y, Ce, Sm, Eu, Gd, Dy, Er, Yb, and Lu. These compounds are formed by the single-electron reduction of the Pacman uranyl complex [UO2(py)(H2L)] by the rare-earth complexes LnIII(A)3 (A = N(SiMe3)2, OC6H3But 2-2,6) via homolysis of a Ln−A bond. The complexes are dimeric through mutual uranyl exo-oxo coordination but can be cleaved to form the trimetallic, monouranyl “ate” complexes [(py)3LiOUO(μ-X)Ln(py)(L)] by the addition of lithium halides. X-ray crystallographic structural characterization of many examples reveals very similar features for monomeric and dimeric series, the dimers containing an asymmetric U2O2 diamond core with shorter uranyl U=O distances than in the monomeric complexes. The synthesis by LnIII−A homolysis allows [5f1-4fn]2 and Li[5f1-4fn] complexes with oxobridged metal cations to be made for all possible 4fn configurations. Variable-temperature SQUID magnetometry and IR, NIR, and EPR spectroscopies on the complexes are utilized to provide a basis for the better understanding of the electronic structure of f-block complexes and their f-electron exchange interactions. Furthermore, the structures, calculated by restricted-core or allelectron methods, are compared along with the proposed mechanism of formation of the complexes. A strong antiferromagnetic coupling between the metal centers, mediated by the oxo groups, exists in the UVSmIII monomer, whereas the dimeric UVDyIII complex was found to show magnetic bistability at 3 K, a property required for the development of single-molecule magnets.JRC.E.6-Actinide researc

Organometallic neptunium(III) complexes

Studies of transuranic organometallic complexes provide a particularly valuable insight into covalent contributions to the metal–ligand bonding, in which the subtle differences between the transuranium actinide ions and their lighter lanthanide counterparts are of fundamental importance for the effective remediation of nuclear waste. Unlike the organometallic chemistry of uranium, which has focused strongly on UIII and has seen some spectacular advances, that of the transuranics is significantly technically more challenging and has remained dormant. In the case of neptunium, it is limited mainly to NpIV. Here we report the synthesis of three new NpIII organometallic compounds and the characterization of their molecular and electronic structures. These studies suggest that NpIII complexes could act as single-molecule magnets, and that the lower oxidation state of NpII is chemically accessible. In comparison with lanthanide analogues, significant d- and f-electron contributions to key NpIII orbitals are observed, which shows that fundamental neptunium organometallic chemistry can provide new insights into the behaviour of f-elements

Actinides, elements like no others

The radioactive chemical elements that follow actinium in the Periodic Table form the actinide series. These elements are the backbone of nuclear fission technologies for electricity supply, with important applications in other strategic fields, from water management to space exploration and human health. As the atomic number increases in the series from 90 to 103, added electrons enter the highly anisotropic 5f shell. This shell is characterized by a radial wavefunction extending relatively far from the nucleus, so that 5f electrons can either form band states or retain a localized behaviour. With 5f-electrons poised at the edge between non-bonding and bonding configurations, actinide elements are prone to lattice instabilities and plutonium, which goes through six allotropic forms in heating to its melting point, is a fascinating example of how complex their structural behaviour can be. Equally intricate are the electronic properties of actinides. From one side, the intra-atomic electron correlation is strong, for the large electric charge of the nucleus pulls the electrons close to each other, favoring the formation of a magnetic moment. On the other hand, quantum fluctuations of electronic and magnetic degrees of freedom can be so strong that the magnetism melts, promoting emergent properties and new classes of materials behaviour around the points of instability. Other sources of complexity are the hybridization between 5f and conduction-electron states, which may give rise to a variety of different phenomena, from unconventional superconductivity to topologically protected states, and the interplay of spin and unquenched orbital degrees of freedom, which can result in exotic phase transitions driven by hidden order parameters. Besides fundamental science interests, achieving a deep understanding of actinides is vital to ensuring a safe deployment of civil nuclear technologies. However, only a few facilities are available worldwide where actinide materials can be safely investigated. Among these, a prominent position is occupied by the laboratories operated by the Joint Research Centre (JRC) of the European Commission. In its establishment located near the charming German town of Karlsruhe (one of the sites of the JRC’s Directorate for Nuclear Safety and Security), the JRC operates state-of-the-art instruments for measuring spectroscopic, thermodynamic, magnetic, and electrical transport properties of radioactive materials, together with specialised facilities for the preparation of high-quality samples, from single crystals to organometallic complexes and epitaxial thin films. Available techniques include, among others, magic-angle-spinning nuclear magnetic resonance, photoemission and Mössbauer spectroscopy, SQUID magnetometry, Seebeck-, and Hall-effect probes. By exploring materials properties in a wide range of temperature, pressure, and magnetic field, studies performed at the JRC are helping in bringing the actinide knowledge to a “material-by-design” level. A considerable effort is also dedicated to the development of radiopharmaceuticals for targeted cancer therapies based on alpha particles emitters, and to the development of radioisotopes power systems for the European space exploration programme. Safety precautions at Universities in Europe have almost excluded the possibility of working beyond uranium in the periodic table; with JRC’s unique facilities being opened to the academic community on the basis of peer- reviewed proposals, Europe’s researchers can still be at the cutting edge of this vital field.JRC.G.I.5-Advanced Nuclear Knowledg

Emergent phenomena in actinides: Multipolar order, correlation effects, and unconventional superconductivity Phénomènes émergents dans les actinides: ordre multipolaire, effets de corrélation et supraconductivité non conventionnelle

This thematic issue of Comptes Rendus Physique is devoted to exotic emergent phenomena exhibited by actinide compounds. We hope that the range of articles presented in this thematic issue of Comptes Rendus Physique will stimulate the interest of the readers in actinide materials, a treasure box of fascinating physical phenomena that challenge and motivate fundamental scientific theory at large. Ce dossier des C. R. Physique est consacré aux nouvelles propriétés exotiques présentées par certains composés d'actinides. Nous espérons que ce dossier de Comptes Rendus Physique puisse stimuler l'intérêt des lecteurs sur les actinides, un coffre à trésors empli de phénomènes physiques fascinants qui défient et encouragent la théorie physique fondamentale dans son ensemble.JRC.E.6-Actinide researc