Search

Resonant inelastic x-ray scattering (RIXS) using an incident energy tuned to the uranium N 4,5 absorption edges is reported from epitaxial films of α-U3O8 and UN. Theory shows that for U3O8 the multiplets associated with a 5f 1 configuration with a ground state of 2 F 5/2 and the excited state of 2 F 7/2 are observed. However, the strong transition predicted at a transfer energy of 1.67 eV is not observed. We assume this is a consequence of the intermediate state lifetime broadening due to interaction with continuum states when the transferred energy exceeds the onset of the continuum in the presence of the core hole. This hypothesis is supported by the results obtained for the 5f-itinerant system UN, where no sharp transitions have been observed, although the broad scattering response centered at ∼ 1 eV is considered a signature of a predominantly 5f 3 configuration in this band-like semi-metallic system. These experiments and theory add important information on these materials, both of which have been investigated since the 1960s, as well as whether RIXS at the uranium N edge can become a valuable tool for actinide research.

Synchrotron radiation based X-ray scattering techniques provide powerful bulk-sensitive probes for actinide materials. Here, selected experiments showing the potential of these techniques in advancing the knowledge of actinides and their compounds are reviewed., By reviewing a selection of X-ray diffraction (XRD), resonant X-ray scattering (RXS), X-ray magnetic circular dichroism (XMCD), resonant and non-resonant inelastic scattering (RIXS, NIXS), and dispersive inelastic scattering (IXS) experiments, the potential of synchrotron radiation techniques in studying lattice and electronic structure, hybridization effects, multipolar order and lattice dynamics in actinide materials is demonstrated.

Neptunium was the first actinide element to be artificially synthesized, yet, compared with its more famous neighbours uranium and plutonium, is less conspicuously studied. Most neptunium chemistry involves the neptunyl di(oxo)-motif, and transuranic compounds with one metal–ligand multiple bond are rare, being found only in extended-structure oxide, fluoride or oxyhalide materials. These combinations stabilize the required high oxidation states, which are otherwise challenging to realize for transuranic ions. Here we report the synthesis, isolation and characterization of a stable molecular neptunium(V)–mono(oxo) triamidoamine complex. We describe a strong Np≡O triple bond with dominant 5f-orbital contributions and σ u > π u energy ordering, akin to terminal uranium-nitrides and di(oxo)-actinyls, but not the uranium–mono(oxo) triple bonds or other actinide multiple bonds reported so far. This work demonstrates that molecular high-oxidation-state transuranic complexes with a single metal–ligand bond can be stabilized and studied in isolation. [Figure not available: see fulltext.]

Despite many years of research, the full complexity of the electron-lattice interactions in ${\mathrm{UO}}_{2}$ is not fully understood. We present x-ray inelastic scattering at low temperature showing that the interaction between electronic degrees of freedom and transverse acoustic phonons is strong only along the reciprocal space direction [100]. The anisotropy is reflected in the phonon-linewidth broadening, which persists also well above the N\'eel temperature. This intrinsic effect infers an anisotropy in the thermal conductivity, which has been observed, but which is formally forbidden in a cubic material. We have no model capable of connecting our experimental observations with the low thermal conductivity of ${\mathrm{UO}}_{2}$ below room temperature.

Neptunium was the first actinide element to be artificially synthesized, yet, compared with its more famous neighbours uranium and plutonium, is less conspicuously studied. Most neptunium chemistry involves the neptunyl di(oxo)-motif, and transuranic compounds with one metal-ligand multiple bond are rare, being found only in extended-structure oxide, fluoride or oxyhalide materials. These combinations stabilize the required high oxidation states, which are otherwise challenging to realize for transuranic ions. Here we report the synthesis, isolation and characterization of a stable molecular neptunium(V)-mono(oxo) triamidoamine complex. We describe a strong Np≡O triple bond with dominant 5f-orbital contributions and σ

While x-ray absorption spectroscopy is mainly governed by electric-dipole transitions, the technique of nonresonant inelastic x-ray scattering (NIXS) offers the possibility to explore higher-order multipole transitions. These transitions obey different selection rules that can reach final states of higher angular momenta, opening complementary spectroscopic perspectives. Here, we investigate the suitability of NIXS to study transuranium compounds. We show that the $K$ edge of the Be encapsulation can be practically fully excluded by using the imaging capabilities of the technique arising from the position of the signals on the multidetector. Experimental results for the multipole transitions at the actinide ${O}_{4,5}$ edges (90--120 eV) in ${\mathrm{UO}}_{2}$, ${\mathrm{NpO}}_{2}$, ${\mathrm{PuO}}_{2}$, and ${\mathrm{Pu}}_{2}{\mathrm{O}}_{3}$ are compared with multielectronic calculations. The spectral features are shown to be very sensitive to the ratio of the triakontadipole and octupole transitions, which could potentially be used to assess the radial expansion of the $5f$ wave function, which is expected to occur in covalent mixing with the O $2p$ states.

A new neptunium intermetallic compound, NpPt 2 In 7 , has been synthesized in polycrystalline form and characterized by several macroscopic techniques. A Rietveld analysis of its powder X-ray diffraction pattern shows that NpPt 2 In 7 crystallizes in a tetragonal lattice with I 4/ mmm symmetry and lattice parameters a = 4.58471 ( 3 ) A, c = 21.5065 ( 3 ) A. Magnetic susceptibility, electrical resistivity, Hall effect, and heat capacity measurements indicate a metallic character and the occurrence of antiferromagnetic order below a Neel temperature T N = 23 K. The transition is exceptionally robust and T N decreases by 0.2 K under a magnetic field of 9 T. A modified Curie-Weiss fit of the high-temperature magnetic susceptibility curve χ ( T ) gives an effective magnetic moment μ e f f = 2.54 μ B close to the value expected for trivalent Np. Low temperature heat capacity measurements give a reduced Sommerfeld linear coefficient close to 25 mJ mol −1 K −2 and a Debye Temperature Θ D = 181 K. First principles, correlated-band electronic structure calculations suggest that the neptunium magnetic moment in NpPt 2 In 7 is localized and that a quasi-two-dimensional antiferromagnetic structure could result from the competition of very weak interlayer interactions leading to very anisotropic properties.

Na 3.16(2) U V,VI 0.84(2) O 4 is obtained from the reaction of sodium with uranium dioxide under oxygen potential conditions typical of a sodium-cooled fast nuclear reactor. In the event of a breach of the steel cladding, it would be the dominant reaction product forming at the rim of the mixed (U,Pu)O 2 fuel pellets. High-temperature X-ray diffraction measurements show that a distortion of the uranium environment in Na 3.16(2) U V,VI 0.84(2) O 4 results in a strongly anisotropic thermal expansion. A comparison with several related sodium metallates Na n -2 M n + O n -1 - including Na 3 SbO 4 and Na 3 TaO 4 , whose crystal structures are reported for the first time - has allowed us to assess the role played in the lattice expansion by the M n + cation radius and the Na /M ratio. On this basis, the thermomechanical behavior of the title compound is discussed, along with those of several related double oxides of sodium and actinide elements, surrogate elements, or fission products.

We describe a method to produce U2O5 films in situ using the Labstation, a modular machine developed at JRC Karlsruhe. The Labstation, an essential part of the Properties of Actinides under Extreme Conditions laboratory (PAMEC), allows the preparation of films and studies of sample surfaces using surface analytical techniques such as X-ray and ultra-violet photoemission spectroscopy (XPS and UPS, respectively). All studies are made in situ, and the films, transferred under ultra-high vacuum from their preparation to an analyses chamber, are never in contact with the atmosphere. Initially, a film of UO2 is prepared by direct current (DC) sputter deposition on a gold (Au) foil then oxidized by atomic oxygen to produce a UO3 film. This latter is then reduced with atomic hydrogen to U2O5. Analyses are performed after each step involving oxidation and reduction, using high-resolution photoelectron spectroscopy to examine the oxidation state of uranium. Indeed, the oxidation and reduction times and corresponding temperature of the substrate during this process have severe effects on the resulting oxidation state of the uranium. Stopping the reduction of UO3 to U2O5 with single U(V) is quite challenging; first, uranium-oxygen systems exist in numerous intermediate phases. Second, differentiation of the oxidation states of uranium is mainly based on satellite peaks, whose intensity peaks are weak. Also, experimenters should be aware that X-ray spectroscopy (XPS) is a technique with an atomic sensitivity of 1% to 5%. Thus, it is important to obtain a complete picture of the uranium oxidation state with the entire spectra obtained on U4f, O1s, and the valence band (VB). Programs used in the Labstation include a linear transfer program developed by an outside company (see Table of Materials) as well as data acquisition and sputter source programs, both developed in-house.

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. Probing the chemistry of transuranic elements is notoriously challenging. Now, three neptunium(III) organometallic sandwich complexes have been prepared using a flexible macrocycle as ligand, and their molecular and electronic structures characterized, adding to our understanding of the behaviour of f-elements and suggesting that the lower oxidation state Np(II) may be chemically accessible.

The Raman features of neptunium dioxide (NpO2) and uranium–neptunium mixed oxides were investigated. Raman spectra of 16O and 17O-NpO2 enriched sample were measured with excitation energy range of Ei = 1.91–2.54 eV. Raman spectra of both NpO2 samples and the uranium–neptunium rich mixed oxides show an asymmetry at low energies of the Γ25′ (T2g) mode. The asymmetry was evidenced to originate from an electronic band at 431 cm–1 which originates from crystal field splitting. This assignment is in good agreement with theoretical models for NpO2 with the hypothesis that the electron–phonon bound state is decoupled at high temperatures. The asymmetry of the Γ25′ mode can be considered as a fingerprint of the NpO2 lattice.

Resonant inelastic x-ray spectroscopy at the uranium N4 absorption edge at 778 eV has been used to reveal the excitations in UO2 up to 1 eV. The earlier (1989) studies by neutron inelastic scattering of the crystal-field states within the 3H4 multiplet are confirmed. In addition, the first excited state of the 3F2 multiplet at ∼520 meV has been established, and there is a weak signal corresponding to the next excited state at ∼920 meV. This represents a successful application of soft x-ray spectroscopy to an actinide sample, and resolves an open question in UO2 that has been discussed for 50 years. The technique is described and important caveats are drawn about possible future applications.

Nonresonant inelastic x-ray scattering (NIXS) has been performed on single crystals of ${\mathrm{UO}}_{2}$ to study the directional dependence of higher-order-multipole scattering from the uranium ${O}_{4,5}$ edges (90--110 eV). By comparing the experimental results with theoretical calculations the symmetry of the ground state is confirmed directly as the crystal-field (CF) ${\mathrm{\ensuremath{\Gamma}}}_{5}$ triplet state within the $J=4$ manifold. The results demonstrate that the directional dichroism of the NIXS spectra is sensitive to the CF strength and establish NIXS as a tool for probing CF interactions quantitatively.

Thin films of the elusive intermediate uranium oxide U2O5 have been prepared by exposing UO3 precursor multilayers to atomic hydrogen. Electron photoemission spectra measured about the uranium 4f core-level doublet contain sharp satellites separated by 7.9(1) eV from the 4f main lines, whilst satellites characteristics of the U(IV) and U(VI) oxidation states, expected respectively at 6.9(1) and 9.7(1) eV from the main 4f lines, are absent. This shows that uranium ions in the films are in a pure pentavalent oxidation state, in contrast to previous investigations of binary oxides claiming that U(V) occurs only as a metastable intermediate state coexisting with U(IV) and U(VI) species. The ratio between the 5f valence band and 4f core-level uranium photoemission intensities decreases by about 50% from UO2 to U2O5, which is consistent with the 5f 2 (UO2) and 5f 1 (U2O5) electronic configurations of the initial state. Our studies conclusively establish the stability of uranium pentoxide.

The exchange bias effect was studied on antiferromagnetic-ferromagnetic UO 2 /permalloy (Ni 80 Fe 20 ) thin films. Films with a fixed thickness of UO 2 layer and variable thickness of the covering Ni 80 Fe 20 layer have been grown by reactive sputter deposition. The X-ray diffraction study showed epitaxial growth of the UO 2 layer on (100) CaF 2 substrates and a polycrystalline permalloy layer on top of it. The samples exhibited perpendicular exchange bias with the maximum magnitude of 22 mT found in UO 2 /Ni 80 Fe 20 with the thinnest permalloy of 177 A.

We report on the magnetic properties of the neptunium based ferromagnetic compound NpAl 2 . We used magnetization measurements and 27 Al NMR spectroscopy to access magnetic features related to the paramagnetic and ordered states ( T C =56 K). While very precise DC SQUID magnetization measurements confirm ferromagnetic ordering, they show a relatively small hysteresis loop at 5 K reduced with a coercive field H Co ~3000 Oe. The variable offset cumulative spectra (VOCS) acquired in the paramagnetic state show a high sensitivity of the 27 Al nuclei spectral parameters (Knight shifts and line broadening) to the ferromagnetic ordering, even at room temperature.

International audience; The hexavalent and heptavalent sodium neptunate compounds Na$_4$NpO$_5$ and Na$_5$NpO$_6$ have been investigated using X-ray powder diffraction, Mossbauer spectroscopy, magnetic susceptibility, and specific heat measurements. Na$_4$NpO$_5$ has tetragonal symmetry in the space group 14/m, while Na$_5$NpO$_6$ adopts a monoclinic unit cell in the space group C2/m. Both structures have been refined for the first time using the Rietveld method. The valence states of neptunium in these two compounds, i.e., Np(VI) and Np(VII), respectively, have been confirmed by the isomer shift values of their Mossbauer spectra. The local structural properties obtained from the X-ray refinements have also been related to the quadrupole coupling constants and asymmetry parameters determined from the Mossbauer studies. The absence of magnetic ordering has been confirmed for Na$_4$NpO$_5$. However, specific heat measurements at low temperatures have suggested the existence of a Schottky-type anomaly at around 7 K in this Np(VI) phase.

We measure x-ray magnetic circular dichroism (XMCD) spectra at the Pu M_{4,5} absorption edges from a newly prepared high-quality single crystal of the heavy-fermion superconductor ^{242}PuCoGa_{5}, exhibiting a critical temperature T_{c}=18.7 K. The experiment probes the vortex phase below T_{c} 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 superconducting quantum interference device. This observation is in agreement with theoretical models claiming that the Pu atoms in PuCoGa_{5} 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.

We present resonant x-ray emission spectroscopic (RXES) data from the uranium intermetallics UPd$_3$, USb, USn$_3$ and URu$_2$Si$_2$, at the U $M_{4,5}$ edges and compare the data to those from the well-localized $5f^2$ semiconductor UO$_2$. The technique is especially sensitive to any oxidation of the surface, and this was found on the USb sample, thus preventing a good comparison with a material known to be $5f^3$. We have found a small energy shift between UO$_2$ and UPd$_3$, both known to have localized $5f^2$ configurations, which we ascribe to the effect of conduction electrons in UPd$_3$. The spectra from UPd$_3$ and URu$_2$Si$_2$,are similar, strongly suggesting a predominant $5f^2$ configuration for URu$_2$Si$_2$. The valence-band resonant inelastic x-ray scattering (RIXS) provides information on the U $P_3$ transitions (at about $18$~eV) between the U $5f$ and U $6p$ states, as well as transitions of between $3$ and $7$~eV from the valence band into the unoccupied $5f$ states. These transitions are primarily involving mixed ligand states (O $2p$ or Pd, Ru $4d$) and U $5f$ states. Calculations are able to reproduce both these low-energy transitions reasonably well., 11 pages, 8 figures

The reduction of UVI uranyl halides or amides with simple LnII or UIII salts forms highly symmetric, linear, oxo-bridged trinuclear UV /LnIII /UV , LnIII /UIV /LnIII , and UIV /UIV /UIV complexes or linear LnIII /UV polymers depending on the stoichiometry and solvent. The reactions can be tuned to give the products of one- or two-electron uranyl reduction. The reactivity and magnetism of these compounds are discussed in the context of using a series of strongly oxo-coupled homo- and heterometallic poly(f-block) chains to better understand fundamental electronic structure in the f-block.

We have performed high-resolution powder x-ray diffraction measurements on a sample of $^{242}$PuCoGa$_{5}$, the heavy-fermion superconductor with the highest critical temperature $T_{c}$ = 18.7 K. The results show that the tetragonal symmetry of its crystallographic lattice is preserved down to 2 K. Marginal evidence is obtained for an anomalous behaviour below $T_{c}$ of the $a$ and $c$ lattice parameters. The observed thermal expansion is isotropic down to 150 K, and becomes anisotropic for lower temperatures. This gives a $c/a$ ratio that decreases with increasing temperature to become almost constant above $\sim$150 K. The volume thermal expansion coefficient $\alpha_{V}$ has a jump at $T_{c}$, a factor $\sim$20 larger than the change predicted by the Ehrenfest relation for a second order phase transition. The volume expansion deviates from the curve expected for the conventional anharmonic behaviour described by a simple Gr\"{u}neisen-Einstein model. The observed differences are about ten times larger than the statistical error bars but are too small to be taken as an indication for the proximity of the system to a valence instability that is avoided by the superconducting state., Comment: 6 pages, 4 figures

The crystal size dependence of the bulk modulus of nano-crystalline uranium dioxide is investigated by means of in situ high pressure X-ray diffraction. Samples with average particle sizes of 4, 6 and 34 nm as measured from high resolution TEM images were studied up to 27 GPa revealing that all three nano-crystalline UO2 samples show a drastic decrease of bulk modulus with decreasing particle size. The influence of choosing the right EOS to fit the compression data is also discussed.

The thermal recovery of self-irradiated and damaged U 0.80 Am 0.20 O 2− x solid solution has been studied in situ using high-temperature x-ray diffraction between 25 °C and 1300 °C. The cumulative α decay dose of the damaged material was equal to 2.2 displacements per atom, leading to a lattice expansion of 0.31%. This value is close to the saturated lattice expansion of self-irradiated polycrystalline UO 2 and AmO 2 . The thermal recovery process is characterized by three stages, as revealed by the temperature derivative of the lattice parameter showing two maxima at 600 °C and 1000 °C. The thermal recovery is completed at about 1200 °C. These data are compared with existing studies on other oxide fuels. In addition, the lattice expansion of the damaged U 0.80 Am 0.20 O 2− x sample is also discussed and compared to that of defect free UO 2 , NpO 2 and AmO 2 materials. The thermal energy necessary to initiate the annealing of the defects is reached at about 400 °C, whilst at 1100 °C, a defect free U 0.80 Am 0.20 O 2− x sample is obtained.

A dramatic difference in the ability of the reducing An(III) center in AnCp3 (An=U, Np, Pu; Cp=C5 H5 ) to oxo-bind and reduce the uranyl(VI) dication in the complex [(UO2 )(THF)(H2 L)] (L="Pacman" Schiff-base polypyrrolic macrocycle), is found and explained. These are the first selective functionalizations of the uranyl oxo by another actinide cation. At-first contradictory electronic structural data are explained by combining theory and experiment. Complete one-electron transfer from Cp3 U forms the U(IV) -uranyl(V) compound that behaves as a U(V) -localized single molecule magnet below 4 K. The extent of reduction by the Cp3 Np group upon oxo-coordination is much less, with a Np(III) -uranyl(VI) dative bond assigned. Solution NMR and NIR spectroscopy suggest Np(IV) U(V) but single-crystal X-ray diffraction and SQUID magnetometry suggest a Np(III) -U(VI) assignment. DFT-calculated Hirshfeld charge and spin density analyses suggest half an electron has transferred, and these explain the strongly shifted NMR spectra by spin density contributions at the hydrogen nuclei. The Pu(III) -U(VI) interaction is too weak to be observed in THF solvent, in agreement with calculated predictions.

We report an experimental and theoretical investigation of the lattice dynamics and thermal properties of the actinide dioxide NpO$_2$. The energy-wavevector dispersion relation for normal modes of vibration propagating along the $[001]$, $[110]$, and $[111]$ high-symmetry lines in NpO$_2$ at room temperature has been determined by measuring the coherent one-phonon scattering of X-rays from a $\sim$1.2 mg single-crystal specimen, the largest available single crystal for this compound. The results are compared against ab initio phonon dispersions computed within the first-principles density functional theory in the generalized gradient approximation plus Hubbard $U$ correlation (GGA+$U$) approach, taking into account third-order anharmonicity effects in the quasiharmonic approximation. Good agreement with the experiment is obtained for calculations with an on-site Coulomb parameter $U = 4$ eV and Hund's exchange $J= 0.6$ eV in line with previous electronic structure calculations. We further compute the thermal expansion, heat capacity, thermal conductivity, phonon linewidth, and thermal phonon softening, and compare with available experiments. The theoretical and measured heat capacities are in close agreement with another. About 27% of the calculated thermal conductivity is due to phonons with energy higher than 25 meV ($\sim$ 6 THz ), suggesting an important role of high-energy optical phonons in the heat transport. The simulated thermal expansion reproduces well the experimental data up to about 1000 K, indicating a failure of the quasiharmonic approximation above this limit., 12 pages, 10 figures

Rare-earth-based compounds R2T17 (R=Rare earth; T=Transition metal) have been extensively studied and developed for applications as permanent magnets. The actinide-based analogues, however, are much less documented and we report here about the magnetic properties of Np2Co17 and Np2Ni17, as inferred from 237Np Mossbauer spectroscopy, the best resonance in actinides, and specific heat.

The physical and chemical properties at low temperatures of hexavalent disodium neptunate α-Na2NpO4 are investigated for the first time in this work using Mössbauer spectroscopy, magnetization, magnetic susceptibility, and heat capacity measurements. The Np(VI) valence state is confirmed by the isomer shift value of the Mössbauer spectra, and the local structural environment around the neptunium cation is related to the fitted quadrupole coupling constant and asymmetry parameters. Moreover, magnetic hyperfine splitting is reported below 12.5 K, which could indicate magnetic ordering at this temperature. This interpretation is further substantiated by the existence of a λ-peak at 12.5 K in the heat capacity curve, which is shifted to lower temperatures with the application of a magnetic field, suggesting antiferromagnetic ordering. However, the absence of any anomaly in the magnetization and magnetic susceptibility data shows that the observed transition is more intricate. In addition, the heat capacity measurements suggest the existence of a Schottky-type anomaly above 15 K associated with a low-lying electronic doublet found about 60 cm(-1) above the ground state doublet. The possibility of a quadrupolar transition associated with a ground state pseudoquartet is thereafter discussed. The present results finally bring new insights into the complex magnetic and electronic peculiarities of α-Na2NpO4.

The physical and chemical properties at low temperatures of hexavalent disodium neptunate α-Na2NpO4 are investigated for the first time in this work using Mössbauer spectroscopy, magnetization, magnetic susceptibility, and heat capacity measurements. The Np(VI) valence state is confirmed by the isomer shift value of the Mössbauer spectra, and the local structural environment around the neptunium cation is related to the fitted quadrupole coupling constant and asymmetry parameters. Moreover, magnetic hyperfine splitting is reported below 12.5 K, which could indicate magnetic ordering at this temperature. This interpretation is further substantiated by the existence of a λ-peak at 12.5 K in the heat capacity curve, which is shifted to lower temperatures with the application of a magnetic field, suggesting antiferromagnetic ordering. However, the absence of any anomaly in the magnetization and magnetic susceptibility data shows that the observed transition is more intricate. In addition, the heat capacity measurements suggest the existence of a Schottky-type anomaly above 15 K associated with a low-lying electronic doublet found about 60 cm-1 above the ground state doublet. The possibility of a quadrupolar transition associated with a ground state pseudoquartet is thereafter discussed. The present results finally bring new insights into the complex magnetic and electronic peculiarities of α-Na2NpO4.

The crystal structure of the cheralite—CaTh(PO4)2—has been revisited by neutron diffraction and its behaviour under high pressure investigated by X-ray diffraction up to 36 GPa. The neutron diffraction data at ambient pressure gave a more accurate determination of the Ca/Th cation position than previous XRD data, taking advantage that the neutron scattering lengths of calcium and thorium are of same order of magnitude. The nuclear density distribution was also determined using the maximum entropy method (MEM) confirming that the two cations are not located at the same position in the unit cell but are slightly displaced from one another along a specific direction in order to minimize the electrostatic repulsion with the surrounding phosphorus atoms. At high pressure, the compound did not show any phase transition or amorphization. From the evolution of the unit-cell volume as a function of the pressure, the zero-pressure bulk modulus B0 and its pressure derivative B0 ′ have been determined by fitting the experimental compressibility curve to the Birch–Murnaghan equation of state. The results are B0 = 140(2) GPa and B 0 ′ = 4.4(4) GPa.

Actinide materials play a special role in condensed matter physics, spanning behaviours of itinerant d-electron and localized 4f-electron materials. This duality of the 5f electrons confer to actinide-based intermetallic compounds a broad variety of physical properties such as magnetic or multipolar ordering, heavy fermion behaviour, quantum criticality, unconventional superconductivity... 237Np Mossbauer spectroscopy is a unique microscopic tool for gaining information on the electronic and magnetic properties of Np systems.

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.

The structural behavior under pressure of three lanthanide pyrochlore zirconates Ln2Zr2O7 (Ln3+ = Ce, Nd and Gd) has been investigated by X-ray diffraction up to 50 GPa. For all three compounds, a symmetry reduction from cubic to monoclinic is observed under increasing pressure dependant on a pressure value that increases with the ionic radius of the lanthanide ions, r Ln. The cubic and monoclinic phases coexist over a wide pressure range which increases with r Ln. The zero-pressure bulk modulus of the cubic phase, B 0, and its pressure derivative, B 0′, have been determined by fitting the experimental compressibility curves to the Birch–Murnaghan equation of state.

Inelastic neutron scattering, probing the temporal spin–spin correlation at the microscopic scale, is a powerful technique to study the magnetic behaviour of molecular nanomagnets. Experiments at different energy scales and different energy-transfer resolution allow precise determinations of the parameters defining the effective Hamiltonians used to model the diverse physical properties exhibited by this class of materials. The intrinsic disadvantage of the technique (low flux, requiring a sample mass in the gram scale) is over-compensated by the large amount of information that can be straightforwardly extracted. Zero-field splittings and exchange interactions can be determined with a large degree of confidence, shedding light on important issues such as magnetization tunnelling in giant-spin clusters, or the occurrence of quantum coherence phenomena and their consequences on macroscopic observables.

We present a perturbative model for crystal-field calculations in f-electron compounds, which keeps into account the mixing between states labelled by different total angular momentum J. We use this model to analyze the results of inelastic neutron scattering experiments on actinide dioxides accumulated over a number of years. By comparing our calculations with the published results of other experiments, we point out that a unified crystal-field picture for this series of compounds clearly emerges.

The anisotropy of magnetic fluctuations propagating along the high-symmetry directions in cubic systems with 3- k magnetic order is analyzed within the random-phase approximation assuming anisotropic exchange interactions. Both transverse and longitudinal structures are considered, with reference to the UO 2 and USb compounds, respectively. In the case of UO 2 , the spin-waves polarizations calculated for acoustic and optical branches are favorably compared with three-dimensional polarization analysis experiments carried out on a triple axis spectrometer. The overall spin-waves polarization behavior emerges as a consequence of the 3- k nature of the magnetic order, whatever the strength of the exchange coupling assumed.

We present the results obtained by superconducting quantum interference device (SQUID) magnetometry, specific heat, and M\"ossbauer spectroscopy measurements carried out on ${\mathrm{Np}}_{2}{\mathrm{Ni}}_{17}$ polycrystalline samples. We show that long-range magnetic order, with a moment ${\ensuremath{\mu}}_{(2b)}\ensuremath{\sim}2.25$ ${\ensuremath{\mu}}_{B}$, occurs below ${\mathrm{T}}_{N}=17.5$ K on the Np $(2b)$ sites. A nontrivial situation is observed in that the other Np sites $(2d)$ do not take part to the order transition and carry only an induced moment of about 0.2 ${\ensuremath{\mu}}_{B}$ below ${\mathrm{T}}_{N}$. A combined analysis of the different experimental data sets allowed us to determine key parameters associated with the electronic structure of the system. The experimental results are discussed against first-principles electronic structure calculations based on the spin-polarized local spin density approximation plus Hubbard interaction.

The structures of the title compounds are determined by Rietveld refinement of powder XRD data.

Exchange interaction through interface between a ferromagnet and an antiferromagnet may result in the magnetic exchange bias (EB) effect [1]. The EB manifests itself as a shift of a magnetic hysteresis loop along the field direction. This property has become of a great technological value for applications in magnetic sensors based on spin-valves or tunnel junctions [2]. In this work, we studied crystal structure and magnetic EB effect in bilayers of antiferromagnetic UO 2 (bulk Neel temperature 30.8 K) with ferrimagnetic Fe 3 O 4 . A reference UO 2 /Fe sample has been also prepared and studied. We used reactive sputter deposition from metallic U and Fe targets to prepare a set of samples with different Fe 3 O 4 thicknesses while the thickness of the UO 2 layer was kept constant. The samples were grown on commercially available CaF 2 (100) and (111) substrates (CaF 2 has an identical crystal structure and a lattice parameter within 0.5% of UO 2 ). The layer of UO 2 was deposited onto the substrate at elevated temperature (850 K), using a partial oxygen pressure of 1.2×10−6 mbar (Ar pressure of 6×10−3 mbar). Fe 3 O 4 was deposited on the top of UO 2 at the same partial oxygen pressure but at room temperature, in order to avoid interdiffusion. The stoichiometry of each deposited layer was controlled by X-ray Photoelectron Spectroscopy (XPS). A magnesium cap was deposited for protection on top of each sample. The samples have been characterized using X-ray diffraction.

Trivalent americium has a nonmagnetic (J=0) ground state arising from the cancellation of the orbital and spin moments. However, magnetism can be induced by a large molecular field if Am^{3+} is embedded in a ferromagnetic matrix. Using the technique of x-ray magnetic circular dichroism, we show that this is the case in AmFe_{2}. Since ⟨J_{z}⟩=0, the spin component is exactly twice as large as the orbital one, the total Am moment is opposite to that of Fe, and the magnetic dipole operator ⟨T_{z}⟩ can be determined directly; we discuss the progression of the latter across the actinide series.