Recently, Sharma et al. [Adv. Sci. 9, 2203473 (2022)] claimed that thin films (around 20 nm) of UO2 deposited on perovskite substrates exhibit strongly enhanced paramagnetism (called induced ferromagnetism by the authors). Moments of up to 3 Bohr magneton/U atom were claimed in magnetic fields of 6 T. We have reproduced such films and, after characterisation, have examined them with X-ray circular magnetic dichroism (XMCD) at the uranium M edges, a technique that is element specific. We do not confirm the published results. We find a small increase, as compared to the bulk, in the magnetic susceptibility of UO2 in such films, but the magnetisation versus field curves, measured by XMCD, are linear with field and there is no indication of any ferromagnetism. The absence of any anomaly around 30 K (the antiferromagnetic ordering temperature of bulk UO2) in the XMCD signal suggests the films do not order magnetically.

We have conducted a series of scattering experiments at the uranium M4 absorption edge on low-symmetry uranium compounds (U2N3 and U3O8) produced as epitaxial films. At weak and forbidden reflections, we find a resonant signal, independent of temperature, with an energy dependence resembling the imaginary part of the scattering factor. Theory, using the FDMNES code, shows that these results can be reliably reproduced assuming that they originate from aspherical 5f electron charge distributions around the U nucleus. Such effects arise from the intrinsic anisotropy of the 5f shell and from the mixing of the 5f electrons of uranium with the outer 2p electrons of the anions. The good agreement between theory and experiment includes azimuthal scattering dependencies, as well as polarization states of the scattered photons. The methodology reported here opens the way for a deeper understanding of the role of the 5f electrons in the bonding in actinide compounds.

Thin layers of orthorhombic uranium ({\alpha}-U) have been grown onto buffered sapphire substrates by d.c. magnetron sputtering, resulting in the discovery of new epitaxial matches to Ti(00.1) and Zr(00.1) surfaces. These systems have been characterised by X-ray diffraction and reflectivity and the optimal deposition temperatures have been determined. More advanced structural characterisation of the known Nb(110) and W(110) buffered {\alpha}-U systems has also been carried out, showing that past reports of the domain structures of the U layers are incomplete. The ability of this low symmetry structure to form crystalline matches across a range of crystallographic templates highlights the complexity of U metal epitaxy and points naturally toward studies of the low temperature electronic properties of {\alpha}-U as a function of epitaxial strain.

Thin films based on silicon and transition-metal elements dominate the semiconducting industry and are ubiquitous in all modern devices. Films have also been produced in the rare-earth series of elements for both research and specialized applications. Thin films of uranium and uranium dioxide were fabricated in the 1960s and 1970s, but there was little sustained effort until the early 2000s. Significant programmes started at Oxford University (transferring to Bristol University in 2011), and Los Alamos National Laboratory (LANL) in New Mexico, USA. In this review we cover the work that has been published over the last ~20 years with these materials. Important breakthroughs occurred with the fabrication of epitaxial thin films of initially uranium metal and UO2, but more recently of many other uranium compounds and alloys. These have led to a number of different experiments that are reviewed, as well as some important trends. The interaction with the substrate leads to differing strain and hence changes in properties. An important advantage is that epitaxial films can often be made of materials that are impossible to produce as bulk single crystals. Examples are U3O8, U2N3 and alloys of U-Mo, which form in a modified bcc structure. Epitaxial films may also be used in applied research. They represent excellent surfaces, and it is at the surfaces that most of the important reactions occur in the nuclear fuel cycle. For example, the fuel-cladding interactions, and the dissolution of fuel by water in the long-term storage of spent fuel. To conclude, we discuss possible future prospects, examples include bilayers containing uranium for spintronics, and superlattices that could be used in heterostructures. Such applications will require a more detailed knowledge of the interface interactions in these systems, and this is an important direction for future research.

Understanding the role of disorder, and the correlations that exist within it, is one of the defining challenges in contemporary materials science. However, there are few material systems, devoid of other complex interactions, that can be used to systematically study the effects of crystallographic conflict on correlated disorder. Here, we report extensive diffuse x-ray scattering studies on the epitaxially stabilized alloy U1-xMox, showing that a new form of intrinsically tuneable correlated disorder arises from a mismatch between the preferred symmetry of a crystallographic basis and the lattice upon which it is arranged. Furthermore, combining grazing incidence inelastic x-ray scattering and state-of-the-art ab initio molecular dynamics simulations, we discover strong disorder-phonon coupling. This breaks global symmetry and dramatically suppresses phonon lifetimes compared to alloying alone, providing an additional design strategy for phonon engineering. These findings have implications wherever crystallographic conflict can be accommodated, and they may be exploited in the development of future functional materials.

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.

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,O-5 edges (90-120 eV) in UO2, NpO2, PuO2, and Pu2O3 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.

In 1966 Roger Cowley (together with Gerald Dolling) reported the first neutron inelastic scattering from the magnetic excitations from UO(2)below its antiferromagnetic ordering temperature of 30 K. They showed the strong magnon-phonon coupling in this material and that the excitations appeared to contain an additional mode that was not anticipated. Cowley never returned to UO2, but showed a keen interest in the developments. Forty years after this pioneering work, unambiguous evidence was found (using resonance x-ray techniques) for the ordering belowT(N)of the electric quadrupoles involving the anisotropy of the 5f chargedistribution around the uranium nuclei. A further 10 years later, now armed with a full theory for the excitation spectrum expected for phonons, magnons, and quadrupoles, we can identify the latter as the source of the 'extra' mode reported first in 1966. The story is a long winding one, with the expected serendipity and dead ends, but is now (almost) completed.

We examine the magnetic ordering of UN and of a closely related nitride, U2N3, by preparing thin epitaxial films and using synchrotron x-ray techniques. The magnetic configuration and subsequent coupling to the lattice are key features of the electronic structure. The well-known antiferromagnetic (AF) ordering of UN is confirmed, but the expected accompanying distortion at T-N is not observed. Instead, we propose that the strong magnetoelastic interaction below T-N causes substantial changes in the strain in the sample being measured. These strains vary as a function of the sample form. As a consequence, the accepted AF configuration of UN may be incorrect. In the case of cubic alpha-U2N3, no single crystals have been previously prepared, and we have determined the AF ordering wave vector. The AF T-N is close to that previously reported. In addition, resonant diffraction methods have identified an aspherical quadrupolar charge contribution in U2N3 involving the 5 f electrons.

We examine the magnetic ordering of UN and of a closely related nitride, U2N3, by preparing thin epitaxial films and using synchrotron x-ray techniques. The magnetic configuration and subsequent coupling to the lattice are key features of the electronic structure. The well-known antiferromagnetic (AF) ordering of UN is confirmed, but the expected accompanying distortion at Tn is not observed. Instead, we propose that the strong magneto-elastic interaction at low temperature involves changes in the strain of the material. These strains vary as a function of the sample form. As a consequence, the accepted AF configuration of UN may be incorrect. In the case of cubic a-U2N3, no single crystals have been previously prepared, and we have determined the AF ordering wave-vector. The AF Tn is close to that previously reported. In addition, resonant diffraction methods have identified an aspherical quadrupolar charge contribution in U2N3 involving the 5f electrons; the first time this has been observed in an actinide compound.

We report a series of measurements on a <1 mg sample of Cm-248 metal. Magnetization measurements are similar to those reported earlier. X-ray absorption and magnetic circular dichroism (XMCD) measurements at the ESRF synchrotron at the M-4,M-5 absorption edges show that the element has an orbital moment of similar to 6% of the spin moment, and the two are parallel. This confirms the predictions of intermediate coupling, which has been suggested for Cm by other spectroscopies. In an XMCD measurement 2.5 years later, a field of 17 T was found to induce a larger induced signal (by a factor of similar to 2) than that found earlier, indicating a change in the sample. We attribute this to the gradual loss of crystallinity with time since preparation, as a result of the damaged caused by fragments from the spontaneous fission within the sample itself. The final measurements aimed to determine the antiferromagnetic structure with resonant x rays were unable to observe any diffraction peaks from the sample. The ground-state antiferromagnetic structure remains unresolved.

This paper reports experiments investigating the reaction of H-2 with uranium metal-oxide bilayers. The bilayers consist of <= 100 nm of epitaxial alpha-U (grown on a Nb buffer deposited on sapphire) with a UO2 overlayer of thicknesses of between 20 and 80 nm. The oxides were made either by depositing via reactive magnetron sputtering, or allowing the uranium metal to oxidise in air at room temperature. The bilayers were exposed to hydrogen, with sample temperatures between 80 and 200 C, and monitored via in-situ x-ray diffraction and complimentary experiments conducted using Scanning Transmission Electron Microscopy - Electron Energy Loss Spectroscopy (STEM-EELS). Small partial pressures of H-2 caused rapid consumption of the U metal and lead to changes in the intensity and position of the diffraction peaks from both the UO2 overlayers and the U metal. There is an orientational dependence in the rate of U consumption. From changes in the lattice parameter we deduce that hydrogen enters both the oxide and metal layers, contracting the oxide and expanding the metal. The air-grown oxide overlayers appear to hinder the H-2-reaction up to a threshold dose, but then on heating from 80 to 140 C the consumption is more rapid than for the as-deposited overlayers. STEM-EELS establishes that the U-hydride layer lies at the oxide-metal interface, and that the initial formation is at defects or grain boundaries, and involves the formation of amorphous and/or nanocrystalline UH3. This explains why no diffraction peaks from UH3 are observed. Crown Copyright (C) 2018 Published by Elsevier B.V. All rights reserved.

Nonresonant inelastic x-ray scattering (NIXS) has been performed on single crystals of UO2 to study the directional dependence of higher-order-multipole scattering from the uranium O-4,O-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) 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.

We report experiments to determine the effect of radiation damage on the phonon spectra of the most common nuclear fuel, UO2. We irradiated thin (similar to 300 nm) epitaxial films of UO2 with 2.1 MeV He2+ ions to 0.15 displacements per atom and a lattice swelling of Delta a/a similar to 0.6% and then used grazing-incidence inelastic x-ray scattering to measure the phonon spectrum. We succeeded in observing the acoustic modes, both transverse and longitudinal, across the Brillouin zone. The phonon energies, in both the pristine and irradiated samples, are unchanged from those observed in bulk material. On the other hand, the phonon linewidths (inversely proportional to the phonon lifetimes) show a significant broadening when comparing the pristine and irradiated samples. This effect is shown to increase with phonon energy across the Brillouin zone. The decreases in the phonon lifetimes of the acoustic modes are roughly consistent with a 50% reduction in the thermal conductivity.

Epitaxial thin films have been utilised to investigate the radiolytic dissolution of uranium dioxide interfaces. Thin films of UO$_2$ deposited on single crystal yttria stabilised zirconia substrates have been exposed to water in the presence of a high flux, monochromatic, synchrotron x-ray source. In particular, this technique was applied to induce dissolution of three UO$_2$ thin films, grown along the principle UO$_2$ crystallographic orientations: (001), (110) and (111). Dissolution of each film was induced for 9 accumulative corrosion periods, totalling 270s, after which XRR spectra were recorded to observe the change in morphology of the films as a function of exposure time. While the (001) and (110) oriented films were found to corrode almost linearly and at comparable rates, the (111) film was found to be significantly more corrosion resistant, with no loss of UO$_2$ material being observed after the initial 90s corrosion period. These results distinctly show the effect of crystallographic orientation on the rate of x-ray induced UO$_2$ dissolution. This result may have important consequences for theoretical dissolution models, as it is evident that orientation dependence must be taken into consideration to obtain accurate predictions of the dissolution behaviour of UO$_2$.

We have performed high-resolution powder x-ray-diffraction measurements on a sample of (PuCoGa5)-Pu-242, the heavy-fermion superconductor with the highest critical temperature of T-c = 18.7K. The results show that the tetragonal symmetry of its crystallographic lattice is preserved down to 2 K. Marginal evidence is obtained for an anomalous behavior 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 similar to 150 K. The volume thermal expansion coefficient alpha(V) has a jump at T-c, a factor similar to 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 behavior described by a simple Gruneisen-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.

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.

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.

In their recent paper [Söderlind, P. et al., Sci. Rep. 5, 15958 (2015)], Söderlind et al. discuss two subjects from a theoretical point of view: the phonon spectra and the possible magnetic structure of δ-plutonium (Pu). Here, we comment on the second subject. Söderlind et al. compare the Pu magnetic form factor F(Q) calculated via density functional theory (DFT) with measurements of F(Q) by neutron spectroscopy [Janoschek, M. et al., Sci. Adv. 1, e1500188 (2015)]. In particular, this comparison does not consider a number of experimental facts established in the neutron spectroscopy study.