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.

Epitaxial thin films of ThO2 and (UxTh1−x)O2 mixed oxides (MOX) have been synthesised by DC magnetron sputtering. The samples were characterised using x-ray diffraction and spectroscopic ellipsometry. Three epitaxial ThO2 samples of different crystallographic orientations have been synthesised and confirmed by x-ray diffraction analysis. The samples are [1 1 1], [1 1 0] and [0 0 1] oriented, with lattice parameters determined to be 5.714 ± 0.001 Å, 5.707 ± 0.001 Å and 5.624 ± 0.001 Å respectively. Four [0 0 1] oriented epitaxial (UxTh1−x)O2 samples have been fabricated, across 0≤x≤1, with all samples found to have a unique specular direction. The mixed oxides were found to obey Vegard’s law, with lattice parameter varying linearly with ThO2 content. Values for the optical band gap and optical constants of the epitaxial ThO2 and MOX samples have also been determined by spectroscopic ellipsometry. The band gap determined for the three ThO2 samples were in the 3.9 - 4.6 eV range, which is a slight underestimate compared to other experimental values. The optical band gap was found to vary approximately linearly with ThO2 content, across the sample series. These films can be implemented in the research of thorium-based nuclear fuels, mixed actinide oxide nuclear fuel and the long term storage of nuclear waste forms.

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.

Epitaxial single crystal thin films of U3Si, U3Si5, α−USi2, and USi3, alongside poly-crystalline U3Si2 have all been synthesised using DC magnetron sputtering. These idealised samples provide the bases on which fundamental studies can be conducted for the understanding of advanced technology fuel (ATF) candidates: U3Si, U3Si2, and U3Si5. The silicon-rich phases, USi2 and USi3 are of interest as intermediate oxidation products, forming as a result of the surface oxidation of the fuel candidates. Films were characterised using x-ray diffraction (XRD), and x-ray photoelectron spectroscopy (XPS), with XRD results indicating the stabilisation of [001]-oriented surfaces for all epitaxial phases with the exception of hexagonal U3Si5, which was found to be [100]-oriented. The XPS area analysis results from the U-4f and Si-2s core levels indicate that all phases are stoichiometric within error.

The oxidation of uranium mononitride is a potential issue when considering this material as a nuclear fuel. This work investigates the rate and mechanism of this reaction at room temperature, giving insight into fuel surface oxidation during manufacture and storage. Chemical and structural investigation of the ideal surface of an epitaxial (001) UN thin film, shows that a UO2+xNy layer forms on the surface, highlighting the need for better understanding of the U-N-O system. Below this, a U2N3 interlayer is detected. The topotactic formation of these layers is predicted to play a critical role in the passivation measured during long-duration structural investigation.

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.

Magnetometry measurements of Fe/U and Ni/U bilayer systems reveal a non-monotonic dependence of the magnetic anisotropy for U thicknesses in the range 0 nm - 8 nm, with the Fe/U bilayers showing a more prominent effect as compared to Ni/U. The stronger response for Fe/U is ascribed to the stronger 3d-5f hybridization of Fe and U. This non-monotonic behaviour is thought to arise from quantum well states in the uranium overlayers. Estimating an oscillation period from the non-monotonic data, and comparing it to Density Functional Theory calculations, we find that wavevector matches to the experimental data can be made to regions of high spectral density in (010) and (100) cuts of the electronic structure of{\alpha}-U, consistent with the measured texture in the films. Unexpectedly, there are also indications of perpendicular magnetic anisotropy in a subset of Fe/U samples at relatively large U thickness.

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.

This work describes a methodology for producing high quality metallic surfaces from uranium primarily for characterisation and investigations involving electron backscatter diffraction. Electrochemical measurements have been conducted to inform ideal polishing conditions to produce surfaces free from strain, induced by mechanical polishing. A commonly used solution for the electropolishing of uranium, consisting in part of phosphoric acid, was used to conduct the electrochemical experiments and polishing. X-ray diffraction techniques focusing on the surface show low stresses and strains are exhibited within the material. This is mirrored in good quality electron backscatter diffraction.

X-ray magnetic critical scattering measurements and specific heat measurements were performed on the perovskite iridate [Formula: see text]. We find that the magnetic interactions close to the Néel temperature [Formula: see text] are three-dimensional. This contrasts with previous studies which suggest two-dimensional behaviour like Sr2IrO4. Violation of the Harris criterion ([Formula: see text]) means that weak disorder becomes relevant. This leads a rounding of the antiferromagnetic phase transition at [Formula: see text], and modifies the critical exponents relative to the clean system. Specifically, we determine that the critical behaviour of [Formula: see text] is representative of the diluted 3D Ising universality class.

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.

Uranium mononitride, UN, is considered a potential accident tolerant fuel due to its high uranium density, high thermal conductivity, and high melting point. Compared with the relatively inert UO2, UN has a high reactivity in water, however, studies have not considered the significant effect of radiation, which is known to cause corrosion of UO2. This study uses 0.1 M H2O2 to simulate the effects of water radiolysis in order to compare the radiolytic corrosion rates of UO2, UN, and U2N3 thin films at room temperature. X-ray reflectivity was used to investigate the changes in film morphology as a function of H2O2 exposure time, allowing changes in film thickness and roughness to be observed on the Angstrom length-scale. Results showed significant differences between UO2, UN, and U2N3, with corrosion rates of 0.083(3), 0.020(4), and 0.47(8) A/s, respectively, showing that UN corrodes more slowly than UO2 in 0.1 M H2O2.

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.

Very little is known about the electronic properties of the heavy actinide as thin films mixed into non-magnetic and magnetic materials. The availability of thin film growth systems with these materials is very limited: here we use an actinide-dedicated DC magnetron sputtering system at the University of Bristol to investigate their possible application in spintronic applications. 1. Research Objectives Current spintronics research is focussed on heavy metals as a source of highly spin polarized currents. They can give rise to large spin Hall effects which can be used for magnetization switching. Initial investigations of the transport properties of dilute-U/Cu, were begun to compare and contrast with previous studies of the spin-orbit coupling of Bi and Ir atoms in Cu, as shown in Fig. 1 below [1]. A key question in the community is what materials are most suitable to provide the largest signals. Actinide materials are of interest in the question, since in the most naive picture the spin-orbit coupling strength scales as a function of the atomic number to the fourth power. The objectives of this project are to continue this work and include Th as an additional type of impurity dopant as well as U. We will synthesize thin films of copper with dilute uranium or thorium content, investigating several different Th concentrations (typically in the range of a few %). Fig. 1: Spin Hall resistivity in Cu as a function of induced resistivity due to Ir impurities. Taken from Ref. [1]. The behavior of actinide materials in proximity to ferromagnetic materials should also be understood, both to allow integration in more complex spintronic devices, and also for fundamental studies. A key point is that previous x-ray magnetic circular dichroism studies of uranium/ferromagnet thin film superlattices have indicated an induced moment in the U, which is especially big when the ferromagnet is iron, approaching 0.1 Bohr magnetons per atom just at the interface [2, 3], also see Fig. 2. 2. Research