E. Colineau

Plutonium-based superconductivity with a transition temperature above 18 K

Plutonium is a metal of both technological relevance and fundamental scientific interest. Nevertheless, the electronic structure of plutonium, which directly influences its metallurgical properties, is poorly understood. For example, plutoniums 5f electrons are poised on the border between localized and itinerant, and their theoretical treatment pushes the limits of current electronic structure calculations. Here we extend the range of complexity exhibited by plutonium with the discovery of superconductivity in PuCoGa5. We argue that the observed superconductivity results directly from plutoniums anomalous electronic properties and as such serves as a bridge between two classes of spin-fluctuation-mediated superconductors: the known heavy-fermion superconductors and the high-Tc copper oxides. We suggest that the mechanism of superconductivity is unconventional; seen in that context, the fact that the transition temperature, Tc ≈ 18.5 K, is an order of magnitude greater than the maximum seen in the U- and Ce-based heavy-fermion systems may be natural. The large critical current displayed by PuCoGa5, which comes from radiation-induced self damage that creates pinning centres, would be of technological importance for applied superconductivity if the hazardous material plutonium were not a constituent.

Magnetic Memory Effect in a Transuranic Mononuclear Complex

Molecular nanomagnets that display magnetic bistability are the subject of intensive investigation due to their unique potential in ultrahigh-density memory components and spintronic devices. So far, the best practical realization of such single-molecule magnets (SMMs) are polymetallic transition-metal complexes with strong intramolecular exchange coupling, giving rise to a high-spin ground state and negligible intercluster interactions. However, 3d metals are restricted by their comparatively low anisotropy, and SMMs with better performance could be produced by exploiting the higher single-ion anisotropy typical of felectron ions. This possibility has been practically demonstrated by Ishikawa et al., who discovered that mononuclear rare earth metal bis-phthalocyanine compounds (Pc2RE) display magnetic hysteresis under favorable conditions. On these grounds, the use of actinides in molecular magnetism appears timely, and indeed slow relaxation effects have recently been reported in a mononuclear uranium-based molecule. Future SMMs displaying magnetic hysteresis could benefit from the fact that, whilst the 5f electron shell can remain relatively well localized, its larger radial extension with respect to the 4f shell can result both in an increased ligand-field potential (and therefore a higher anisotropy energy barrier) and in the possibility to trigger a sizeable exchange coupling in polynuclear complexes, usually precluded to trivalent rare earth metal ions. Moreover, discrete molecules based on 5f ions should allow much greater understanding of the peculiar behavior observed in actinide materials, including multipolar superexchange coupling. Recently, we obtained evidence that a neptunium trimetallic compound displays slow magnetic relaxation and superexchange interaction; nevertheless, we were unable to find any signs of hysteresis in the measured magnetization curves. Here we report the first observation of such low-temperature magnetic memory effects in another transuranic molecular complex, namely, bis(cyclooctatetraenyl)neptunium(IV), commonly known as neptunocene [Np(COT)2] (COT= C8H8 2 ), which was first described in 1970 and belongs to the whole actinocene row. The molecule has a single Np ion between two planar COT rings in a sandwich structure (Figure 1) with D8h symmetry. [9] The degeneracy of the lowest

Advances in the preparation and characterization of transuranium systems

The most remarkable advance that one can report as regards transuranium systems is certainly the discovery of superconductivity above 18 K in PuCoGa5. Motivated by this discovery, we have investigated bulk samples of both PuCoGa5 and its Rh analogue. Characterizations of polycrystalline Co and Rh compounds are reported. The new PuRhGa5 compound is also found to become superconducting above 8 K.

A Uranium-Based UO2+-Mn2+ Single-Chain Magnet Assembled trough Cation-Cation Interactions

Single-chain magnets (SCMs) are materials composed of magnetically isolated one-dimensional (1D) units exhibiting slow relaxation of magnetization. The occurrence of SCM behavior requires the fulfillment of stringent conditions for exchange and anisotropy interactions. Herein, we report the synthesis, the structure, and the magnetic characterization of the first actinide-containing SCM. The 5f-3d heterometallic 1D chains [{[UO2(salen)(py)][M(py)4](NO3)}]n, (M=Cd (1) and M=Mn (2); py=pyridine) are assembled trough cation-cation interaction from the reaction of the uranyl(V) complex [UO2(salen)py][Cp*2Co] (Cp*=pentamethylcyclopentadienyl) with Cd(NO3)2 or Mn(NO3)2 in pyridine. The infinite UMn chain displays a high relaxation barrier of 134±0.8 K (93±0.5 cm(-1)), probably as a result of strong intra-chain magnetic interactions combined with the high Ising anisotropy of the uranyl(V) dioxo group. It also exhibits an open magnetic hysteresis loop at T<6 K, with an impressive coercive field of 3.4 T at 2 K.

Superconductivity in the Heusler Family of Intermetallics

Przeprowadzono badania szeregu związkow nadprzewodzących z klasy związkow Heuslera, w szczegolności rodziny (Sc, Y, Lu)Pd2Sn i APd2M (A= Hf, Zr, i M = In, Al). Zwrocono uwage na istotny wplyw sprzezenia elektron - fonon na obserwowane nadprzewodnictwo.

Direct observation of electric-quadrupolar order in UO2

We report direct experimental evidence for long-range antiferro ordering of the electricquadrupole moments on the U ions. Resonant x-ray scattering experiments at the uranium M4 absorption edge show a characteristic dependence in the integrated intensity upon rotation of the crystal around the scattering vector. Although quadrupolar order in uranium dioxide was advocated already in the 1960s no experimental evidence for this phenomenon was provided until now. We conclude with a possible model to explain the phase diagram of the solid solutions of UO2 and NpO2. We suggest that in the region 0.30 < x < 0.75 neither the transverse nor the longitudinal quadrupole ordering can dominate, leading to frustration and only short-range ordering.

Localized 5f electrons in superconducting PuCoIn5: consequences for superconductivity in PuCoGa5

The physical properties of the first In analog of the PuMGa(5) (M = Co, Rh) family of superconductors, PuCoIn(5), are reported. With its unit cell volume being 28% larger than that of PuCoGa(5), the characteristic spin-fluctuation energy scale of PuCoIn(5) is three to four times smaller than that of PuCoGa(5), which suggests that the Pu 5f electrons are in a more localized state relative to PuCoGa(5). This raises the possibility that the high superconducting transition temperature T(c) = 18.5 K of PuCoGa(5) stems from the proximity to a valence instability, while the superconductivity at T(c) = 2.5 K of PuCoIn(5) is mediated by antiferromagnetic spin fluctuations associated with a quantum critical point.

Synthesis of bimetallic uranium and neptunium complexes of a binucleating macrocycle and determination of the solid-state structure by magnetic analysis.

Syntheses of the bimetallic uranium(III) and neptunium(III) complexes [(UI)(2)(L)], [(NpI)(2)(L)], and [{U(BH(4))}(2)(L)] of the Schiff-base pyrrole macrocycles L are described. In the absence of single-crystal structural data, fitting of the variable-temperature solid-state magnetic data allows the prediction of polymeric structures for these compounds in the solid state.

Thorium/uranium mixed oxide nanocrystals: Synthesis, structural characterization and magnetic properties

One of the primary aims of the actinide community within nanoscience is to develop a good understanding similar to what is currently the case for stable elements. As a consequence, efficient, reliable and versatile synthesis techniques dedicated to the formation of new actinide-based nano-objects (e.g. nanocrystals) are necessary. Hence, a “library” dedicated to the preparation of various actinidebased nanoscale building blocks is currently being developed. Nanoscale building blocks with tunable sizes, shapes and compositions are of prime importance. So far, the non-aqueous synthesis method in highly coordinating organic media is the only approach which has demonstrated the capability to provide size and shape control of actinide-based nanocrystals (both for thorium and uranium, and recently extended to neptunium and plutonium). In this paper, we demonstrate that the non-aqueous approach is also well adapted to control the chemical composition of the nanocrystals obtained when mixing two different actinides. Indeed, the controlled hot co-injection of thorium acetylacetonate and uranyl acetate (together with additional capping agents) into benzyl ether can be used to synthesize thorium/uranium mixed oxide nanocrystals covering the full compositional spectrum. Additionally, we found that both size and shape are modified as a function of the thorium:uranium ratio. Finally, the magnetic properties of the different thorium/uranium mixed oxide nanocrystals were investigated. Contrary to several reports, we did not observe any ferromagnetic behavior. As a consequence, ferromagnetism cannot be described as a universal feature of nanocrystals of non-magnetic oxides as recently claimed in the literature.

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 U(III) 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 Np(IV). Here we report the synthesis of three new Np(III) organometallic compounds and the characterization of their molecular and electronic structures. These studies suggest that Np(III) complexes could act as single-molecule magnets, and that the lower oxidation state of Np(II) is chemically accessible. In comparison with lanthanide analogues, significant d- and f-electron contributions to key Np(III) orbitals are observed, which shows that fundamental neptunium organometallic chemistry can provide new insights into the behaviour of f-elements.