Mean-field model for magnetic orders in NpTGa5 with T=Co, Ni or Rh

Characteristics of magnetic transitions in NpTGa$_5$ with T=Co, Ni, Rh are explained in a unified way with use of a crystalline electric field (CEF) model of localized 5$f^4$ electrons. The model takes a CEF doublet and a singlet as local states, and includes dipolar and quadrupolar intersite interactions in the mean-field theory. Diverse ordering phenomena are derived depending on the magnitude of interaction parameters, which qualitatively reproduce the experimentally observed magnetic behaviors in NpTGa$_5$. The quadrupole degrees of freedom are essential to the diverse magnetic orders. It is argued that NpRhGa$_5$ is close to a multicritical point where quadrupoles and dipoles with different directions are competing to order.Comment: 17 pages, 10 figure

Structural, electronic, and magnetic characteristics of Np_2Co_(17)

A previously unknown neptunium-transition-metal binary compound Np_2Co_(17) has been synthesized and characterized by means of powder x-ray diffraction, ^(237)Np Mössbauer spectroscopy, superconducting-quantum-interference-device magnetometry, and x-ray magnetic circular dichroism (XMCD). The compound crystallizes in a Th_2Ni_(17)-type hexagonal structure with room-temperature lattice parameters α=8.3107(1) Å and c=8.1058(1) Å. Magnetization curves indicate the occurrence of ferromagnetic order below T_C>350 K. Mössbauer spectra suggest a Np^(3+) oxidation state and give an ordered moment of μ_(Np)=1.57(4) μ_B and μ_(Np)=1.63(4) μ_B for the Np atoms located, respectively, at the 2b and 2d crystallographic positions of the P6_3/mmc space group. Combining these values with a sum-rule analysis of the XMCD spectra measured at the neptunium M_(4,5) absorption edges, one obtains the spin and orbital contributions to the site-averaged Np moment [μ_S=−1.88(9) μ_B, μ_L=3.48(9) μ_B]. The ratio between the expectation value of the magnetic-dipole moment and the spin magnetic moment (m_(md)/μS=+1.36) is positive as predicted for localized 5f electrons and lies between the values calculated in intermediate-coupling (IC) and jj approximations. The expectation value of the angular part of the spin-orbit-interaction operator is in excellent agreement with the IC estimate. The ordered moment averaged over the four inequivalent Co sites, as obtained from the saturation value of the magnetization, is μ_(Co)≃1.6 μ_B. The experimental results are discussed against the predictions of first-principles electronic-structure calculations based on the spin-polarized local-spin-density approximation plus the Hubbard interaction

Orbital-based Scenario for Magnetic Structure of Neptunium Compounds

In order to understand a crucial role of orbital degree of freedom in the magnetic structure of recently synthesized neptunium compounds NpTGa_5 (T=Fe, Co, and Ni), we propose to discuss the magnetic phase of an effective two-orbital model, which has been constructed based on a j-j coupling scheme to explain the magnetic structure of uranium compounds UTGa_5. By analyzing the model with the use of numerical technique such as exact diagonalization, we obtain the phase diagram including several kinds of magnetic states. An orbital-based scenario is discussed to understand the change in the magnetic structure among C-, A-, and G-type antiferromagnetic phases, experimentally observed in NpFeGa_5, NpCoGa_5, and NpNiGa_5.Comment: 18 pages, 8 figures, to appear in New Journal of Physic

Muon spin rotation measurements of the superfluid density in fresh and aged superconducting PuCoGa5_5

We have measured the temperature dependence and magnitude of the superfluid density $\rho_{\rm s}(T)$ via the magnetic field penetration depth $\lambda(T)$ in PuCoGa$_5$ (nominal critical temperature $T_{c0} = 18.5$ K) using the muon spin rotation technique in order to investigate the symmetry of the order parameter, and to study the effects of aging on the superconducting properties of a radioactive material. The same single crystals were measured after 25 days ($T_c = 18.25$ K) and 400 days ($T_c = 15.0$ K) of aging at room temperature. The temperature dependence of the superfluid density is well described in both materials by a model using d-wave gap symmetry. The magnitude of the muon spin relaxation rate $\sigma$ in the aged sample, $\sigma\propto 1/\lambda^2\propto\rho_s/m^*$, where $m^*$ is the effective mass, is reduced by about 70% compared to fresh sample. This indicates that the scattering from self-irradiation induced defects is not in the limit of the conventional Abrikosov-Gor'kov pair-breaking theory, but rather in the limit of short coherence length (about 2 nm in PuCoGa$_5$) superconductivity.Comment: 11 page

Radiation Effects in Americium Ceramic Compounds

Along with uranium dioxide, mixed oxides play an essential role as fuels for present or next generation nuclear reactors. Although their production has been studied extensively, the methods applied currently result in materials that are usually not homogeneous at an atomic scale. To improve the quality of the powders (homogeneity and sintering properties), the hydrothermal decomposition of oxalate method was investigated. Preliminary attempts for the (Th,Pu)O2, (Np,Pu)O2 and (Th,Np,Pu)O2 systems were reported elswhere. The procedure allowed the production of nanometric powders of U1-xPuxO2 solid solution in the full composition range. Partial oxidation of U(IV) to U(V) was observed, while plutonium was assumed to maintain its tetravalent oxidation state. Thanks to the low-temperature of the synthesis, such mixed oxides were used as reagents for the fabrication of more complicated systems. The aliovalent americium, which has a preference for the trivalent oxidation state, can be accommodated in the UO2 fluorite structure by charge compensation with U(V) only in limited amounts. Still, the feasibility of the method has been demonstrated for an Am-content relevant to transmutation fuels. Thus, U1-xAmxO2 (x = 0.1 and 0.2) and U0.75Pu0.20Am0.05 nanosized powders were produced by hydrothermal decomposition of the corresponding mixed oxalate. Typical oxidation states detected for the cations were U(IV)/U(V) for nanometric and sintered materials, respectively, where both exhibit mixed and Am(III)/Am(IV) oxidation state. Their sintering ability was tested and the volumetric expansion was measured by powder XRD as a function of storage time. A low swelling with no indications of amorphization has been derived over a period of four years

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

Variations of Electronic Properties in High-Density Pu Systems

Electronic properties of Pu intermetallics were traditionally understood as dominated by the Pu-Pu spacing. While very low spacing should lead to formation of a 5f band, its increase should drive the system to strong e-e correlation regime and eventually to a magnetic order. The large difference between the Sommerfeld coefficient of electronic specific heat , being 17 mJ/mol K2 for -Pu and 64 mJ/mol K2 for -Pu 1 (40 mJ/mol K2 given later) was understood in this framework. However, theoretical works over last years indicated that relatively small variations of 5f occupancy can have equally (or perhaps more) important effect on magnetic and other properties. A possibility to test whether the Pu-Pu spacing plays only a secondary role is to check properties of Pu systems with low Pu-Pu spacing for occurrence of species with high value of and magnetic susceptibility. We investigated several U-Pu alloys, forming a high-density -phase, and showed that -values reach the range of -Pu. Magnetic susceptibility (T) even higher. Most recently we turned the attention to -Pu, which may actually be the most strongly correlated Pu phase. The monoclinic structure of -Pu cannot be stabilized to low T by doping, but we noticed that the crystal structure of Pu19Os (especially its high-T variety -Pu19Os) is almost identical as to volume and coordination. The reported room-temperature density 18.02 g/cm3 is only slightly lower than that of -Pu, if the 2 latter is corrected for the thermal expansion (18.20 g/cm3), and is very different from those of -Pu (19.82 g/cm3) or -Pu (15.92 g/cm3). This phase was found thermodynamically stable between 468 and 707 K. Below that range, the structure transforms into the other variety, -Pu19Os, with structure of slightly higher density (18.12 g/cm3). We succeeded to synthesize both phases. (T) weakly increases with decreasing T for both phases, from 8*10-9 m3/mol Pu at 300 K to 9*10-9 m3/mol Pu in the low-T limit (higher than for - and -Pu, both of 6-7*10-9 m3/mol). T- dependence of specific heat exhibits quite a non-Debye type of behaviour, similar to -Pu alloys.1,2 The lowest T achieved (<4 K) was nevertheless sufficient to extrapolate reliably to T 0. The obtained 's, (552) mJ/mol Pu K2 for -Pu19Os and (742) mJ/mol Pu K2 for -Pu19Os, are considerably higher than for -Pu. The slopes of the Cp/T vs. T2 plot, which determine the Debye temperatures, are even higher than for -Pu. It reveals D even lower than for -Pu, but the values D = 96 K for -Pu19Os and 101 K for Pu19Os in fact fall within the spread of -Pu stabilized by 6.1 % Ce (103 K) and by 8% Am (95 K). The results show that the Pu properties are not primarily controlled by the Pu-Pu spacing and the description cannot be based on the 5f-band approach only. ACKNOWLEDGMENT This work was supported by the Grant agency of the Academy of Sciences of the Czech Republic (IAA100100912), and the Grant Agency of the Czech Republic (P204/10/0330). Participation in the EC JRC-ITU Actinide User Laboratory program through the support of the EC-Transnational Access to Research Infrastructures Action of the “Structuring the European Research Area” specific program, contract RITA-CT-2006-026176, is acknowledged.JRC.E.6-Actinide researc