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Previous experiments on cubic UO2 have suggested that the temperature dependences of the nearest-neighbour U–O and U–U distances are different. We have acquired total-scattering neutron diffraction patterns out to Q = 23.5 Å−1 for 50 < T < 1023 K and produced via Fourier transform a pair-distribution function P D F ( r ) . The P D F ( r ) shows quite clearly that r ( U–O ) , defined by the maximum of the U–O peak in the P D F ( r ) , does in fact decrease with increasing temperature, whereas r ( U–U ) follows the lattice expansion as expected. We also observe that the r ( U–O ) contraction accelerates continuously above T ≈ 400 K, consistent with earlier experiments by others. Furthermore, by analysing the eigenvectors of the phonon modes, we show that the Δ 5 ( T O 1 ) phonon tends to separate the eight equivalent U–O distances into six shorter and two longer distances, where the longer pair contribute to a high-r tail observed in the U–O distance distribution becoming increasingly anisotropic at higher T. These results have significance for a wide range of materials in which heavy and light atoms are combined in a simple atomic structure.

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.

We report a simple technique for the synthesis of uniaxially textured, metastable hexagonal close-packed-like uranium thin films with thicknesses between 175-2800 \r{A}. The initial structure and texture of the layers have been studied via X-ray diffraction and reflectivity and the time-dependent transitions of the samples into various orientations of orthorhombic ${\alpha}$-U have been mapped by similar techniques. The final crystallographic orientations of each system and the timescales on which the transitions occur are found to depend on the lattice parameters of the original layer. The absence of the ${\alpha}$-U (001) orientation in the transition products suggests that the transitions in these layers are mediated by mechanisms other than the [110] transverse acoustic phonon mode previously suggested for the cubic ${\gamma}$-U(110) to hcp-U(00.1) to ${\alpha}$-U(001) displacive phase transition. Alternative transition pathways are discussed.

Previous experiments on cubic UO

To efficiently capture the energy of the nuclear bond, advanced nuclear reactor concepts seek solid fuels that must withstand unprecedented temperature and radiation extremes. In these advanced fuels, thermal energy transport under irradiation is directly related to reactor performance as well as reactor safety. The science of thermal transport in nuclear fuel is a grand challenge due to both computational and experimental complexities. Here, we provide a comprehensive review of thermal transport research on two actinide oxides: one currently in use in commercial nuclear reactors, uranium dioxide (UO2), and one advanced fuel candidate material, thorium dioxide (ThO2). In both materials, heat is carried by lattice waves or phonons. Crystalline defects caused by fission events effectively scatter phonons and lead to a degradation in fuel performance over time. Bolstered by new computational and experimental tools, researchers are now developing the foundational work necessary to accurately model and ultimately control thermal transport in advanced nuclear fuel. We begin by reviewing research aimed at understanding thermal transport in perfect single crystals. The absence of defects enables studies that focus on the fundamental aspects of phonon transport. Next, we review research that targets defect generation and evolution. Here, the focus is on ion irradiation studies used as surrogates for damage caused by fission products. We end this review with a discussion of modeling and experimental efforts directed at predicting and validating mesoscale thermal transport in the presence of irradiation defects. While efforts into these research areas have been robust, challenging work remains in developing holistic tools to capture and predict thermal energy transport across widely varying environmental conditions., Comment: Publication Date: December 17, 2021

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.

After an Introduction on the history of nuclear fission, we re-examine the scientific relationship between Otto Hahn and Lise Meitner in view of a number of newly published articles in the last two decades. We find the evidence overwhelming that they should have shared the Nobel Prize - rather than it being awarded to Hahn alone. In the post-war period Hahn's actions are often difficult to understand, but we find no compelling evidence that he acted dishonourably.

Isabelle suddenly passed away in her sleep, leaving a big hole in both science and the hearts of many friends, colleagues and collaborators. A passionate and motivated person who devoted her life t...

The local structure and chemical speciation of the mixed valence, fluorite-based oxides UO{sub 2+x} (0.00=

Analysis of X-ray absorption fine structure spectra of UO(2+x) for x = 0-0.20 (UO(2)--U(4)O(9)) reveals that the adventitious O atoms are incorporated as oxo groups with U--O distances of 1.74 A, most likely associated with U(VI), that occur in clusters so that the UO(2) fraction of the material largely remains intact. In addition to the formation of some additional longer U--O bonds, the U sublattice consists of an ordered portion that displays the original U--U distance and a spectroscopically silent, glassy part. This is very different from previous models derived from neutron diffraction that maintained long U--O distances and high U--O coordination numbers. UO(2+x) also differs from PuO(2+x) in its substantially shorter An-oxo distances and no sign of stable coordination with H(2)O and its hydrolysis products.

Pu L(3) X-ray near edge absorption spectra for Pu(0-VII) are reported for more than 60 chalcogenides, chlorides, hydrates, hydroxides, nitrates, carbonates, oxy-hydroxides, and other compounds both as solids and in solution, and substituted in zirconolite, perovskite, and borosilicate glass. This large database extends the known correlations between the energy and shape of these spectra from the usual association of the XANES with valence and site symmetry to higher order chemical effects. Because of the large number of compounds of these different types, a number of novel and unexpected behaviors are observed, such as effects resulting from the medium and disorder that can be as large as those from valence.

The discovery of superconductivity in single crystals of PuCoGa5 with transition temperature Tc = 18.5 K is discussed. The existing data lead to the speculation that the superconductivity in PuCoGa5 may be unconventional. In such a scenario the properties of PuCoGa5 would be intermediate between those of isostructural UCoGa5 and CeCoIn5, more heavily studied f-electron materials.

Determination of plutonium’s fluctuating electronic and magnetic ground state resolves long-standing “missing” magnetism puzzle., A central issue in material science is to obtain understanding of the electronic correlations that control complex materials. Such electronic correlations frequently arise because of the competition of localized and itinerant electronic degrees of freedom. Although the respective limits of well-localized or entirely itinerant ground states are well understood, the intermediate regime that controls the functional properties of complex materials continues to challenge theoretical understanding. We have used neutron spectroscopy to investigate plutonium, which is a prototypical material at the brink between bonding and nonbonding configurations. Our study reveals that the ground state of plutonium is governed by valence fluctuations, that is, a quantum mechanical superposition of localized and itinerant electronic configurations as recently predicted by dynamical mean field theory. Our results not only resolve the long-standing controversy between experiment and theory on plutonium’s magnetism but also suggest an improved understanding of the effects of such electronic dichotomy in complex materials.

X-ray diffraction has been used to probe the radiolytic corrosion of uranium dioxide. Single crystal thin films of UO2 were exposed to an intense X-ray beam at a synchrotron source in the presence of water, in order to simultaneously provide radiation fields required to split the water into highly oxidising radiolytic products, and to probe the crystal structure and composition of the UO2 layer, and the morphology of the UO2/water interface. By modeling the electron density, surface roughness and layer thickness, we have been able to reproduce the observed reflectivity and diffraction profiles and detect changes in oxide composition and rate of dissolution at the Ångström level, over a timescale of several minutes. A finite element calculation of the highly oxidising hydrogen peroxide product suggests that a more complex surface interaction than simple reaction with H2O2 is responsible for an enhancement in the corrosion rate directly at the interface of water and UO2, and this may impact on models of long-term storage of spent nuclear fuel.

Uranium and transuranium compounds are amongst the richest systems with regard to their magnetic properties. However, few experiments are conducted on transuranium materials because of the decreasing number of organizations capable to handle them. At ITU-Karlsruhe, we have developed an "Actinides user laboratory" concept in offering access to our infrastructure and equipment to external users. We illustrate the capabilities offered through SQUID magnetometry studies recently undertaken on a single crystal of the NpP monopnictide. This neptunium compound orders at TN ≈ 114 K into an incommensurate antiferromagnetic phase and undergoes a first order transition at Tic ≈ 73 K to a commensurate structure with an associated tetragonal distortion of the lattice. In this paper, we revisit and complement the intricate magnetic phase diagram. An additional metamagnetic phase is observed at low temperature and interpreted as the missing intermediate step between the low field (3+3-) magnetic structure and the high f...

Plutonium aging leads to the creation of decay products, such as americium, uranium, helium, but also to self-irradiation defects such as vacancies and interstitials. These self-irradiation defects can affect the properties of plutonium; as an example, swelling of PuGa or PuAl alloys has been observed at room temperature. This work is devoted to the study of the effect of aging on the magnetic susceptibility of pure and alloyed plutonium. An anomaly has been revealed at low temperatures (T < 75K) for magnetic fields lower than 4.5 T. This anomaly decreases after heat treatment. A tentative explanation, based on results from molecular dynamics calculation in δ-plutonium, is given to explain the origin of the anomaly.

Synthesis and resistivity measurements on URu 2 Si 2 -NpRu 2 Si 2 solid solutions are reported. U 1 − x Np x Ru 2 Si 2 systems, with x = 0.01, 0.1, 0.3, 0.5, 0.7 and 0.9, crystallize in the tetragonal ThCr 2 Si 2 -type structure. Resistivity measurements indicate a marked variation of the transport properties with regard to the Np substitution. For low Np dilution (≤ 0.1) considerable changes are observed from the x = 0 Cr-like anomaly and a Kondo-like minimum appears below 10 K. In the systems for 0.3 ≤ x ≤ 0.7, an increasing T N is clearly seen and the Cr-like anomaly destroyed in favor of a strong increase of the resistivity suggesting a gap opening. At higher Np-concentration the resistivity is very similar to the pure Np compounds but a large increase in the pure Np-compound 6 K anomaly is observed.

X-ray pair distribution function (pdf) and U L${}_{3}$ extended x-ray absorption fine structure (EXAFS) and neutron pdf measurements that give identical results for UO${}_{2}$ show U(VI)-oxo moieties with x rays for mixed valence U${}_{4}$O${}_{9}$ and U${}_{3}$O${}_{7}$, in contrast to the neutron data that indicate only U(V) sites with no short U-O bonds as well as other differences. In addition, although the EXAFS spectra of UO${}_{2}$ are essentially identical at 30, 100, and 200 K, those of the UO${}_{2+x}$ compounds exhibit different nearest-neighbor U-O distributions at each temperature. Further tunneling polaron-type behavior is found in the broadening of the features of the O K-edge x-ray absorption spectra (XAS) of the UO${}_{2+x}$ compounds. Raman spectra of powders also show a large increase in scattering cross section with increasing O content that would originate in a change in the electronic structure that increases the overall polarizability. The XAS and Raman also show that U${}_{4}$O${}_{9}$ does not behave as a linear combination of the UO${}_{2}$ and U${}_{3}$O${}_{7}$ fluorite endpoints. The properties induced by mobile rather than static charged quasiparticles were explored by optical pumping of the metal-to-metal charge-transfer transition. The temperature dependence of 4.71 eV pump--1.57 eV probe reflectivity on UO${}_{2}$ that initially populates the U 6$d$-dominated portion of the upper Hubbard band (UHB) shows a sharp 28-\ensuremath{\mu}sec lifetime peak at 25 K that may be associated with the fluctuations of its antiferromagnetic transition. Pumping at 3.14 eV into the 5$f$-dominated portion of the UHB shows an analogous 2.8-\ensuremath{\mu}sec peak but also a plateau bracketing this peak that ends in a cusp at 50--60 K and an abrupt change in the hardening rate of a novel 12--15 GHz phonon that is the signature for the quasiparticle quantum phase. The different results for the different excitation channels indicate a highly specific nonthermal relaxation mechanism. These results constitute the first observation of a distinct phase of photoinduced quasiparticles that is sufficiently coupled to the lattice to undergo a gap-opening transition. When the intragap state is probed with a terahertz time domain spectroscopy (TTDS) measurement 33 psec after a 3.14 excitation pulse, it shows increased absorption in the 0.5--1.1 THz range with a decrease in temperature from \ensuremath{\sim}30 to 10 K instead of the expected decrease, a result consistent with the presence of a condensate. These results are too extreme to originate in the dynamical, nonadiabatic, coupled charge-transfer--phonon/tunneling polaron scenario previously used for doped Mott-Hubbard insulators with intermediate electron-phonon coupling and therefore indicate novel physics. One possibility that could cause all of these behaviors is that a collective, dynamical, charge transfer-coupled Peierls distortion involving the 2 U(V) \ensuremath{\leftrightarrow} U(IV)+U(VI)-oxo excitation occurs coherently over an entire domain to cause the atoms in this domain to condense into a system with Bose-Einstein or Bose-Einstein-Hubbard properties.

Plutonium is notorious for its radioactive properties, but in his Perspective, [Lander][1] draws attention to another property of this element: its unusual electronic properties that have long defied explanation. A recent theoretical study has overcome the problems associated with simulating the electron correlations in solid plutonium. The simulation shed light on the movement of atoms in the solid. Now, [ Wong et al .][2] report experimental results that agree extremely well with the simulations. Together, the two studies provide important insights into the elastic and thermodynamic properties of plutonium. [1]: http://www.sciencemag.org/cgi/content/full/301/5636/1057 [2]: http://www.sciencemag.org/cgi/content/short/301/5636/1078

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, plutonium's 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 plutonium's 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-T(c) copper oxides. We suggest that the mechanism of superconductivity is unconventional; seen in that context, the fact that the transition temperature, T(c) approximately 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.

Synthesis and resistivity measurements on URu 2 Si 2 -NpRu 2 Si 2 solid solutions are reported. U 1 − x Np x Ru 2 Si 2 systems, with x = 0.01, 0.1, 0.3, 0.5, 0.7 and 0.9, crystallize in the tetragonal ThCr 2 Si 2 -type structure. Resistivity measurements indicate a marked variation of the transport properties with regard to the Np substitution. For low Np dilution (≤ 0.1) considerable changes are observed from the x = 0 Cr-like anomaly and a Kondo-like minimum appears below 10 K. In the systems for 0.3 ≤ x ≤ 0.7, an increasing T N is clearly seen and the Cr-like anomaly destroyed in favor of a strong increase of the resistivity suggesting a gap opening. At higher Np-concentration the resistivity is very similar to the pure Np compounds but a large increase in the pure Np-compound 6 K anomaly is observed.

The high-temperature behavior of an intrinsically localized mode (ILM) in $\ensuremath{\alpha}\text{\ensuremath{-}}\mathrm{U}$ was measured using inelastic neutron scattering. The mode, which forms above $450\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ on the [010] boundary, becomes undetectable at $675\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. Thermodynamic and transport anomalies that develop with the ILM persist to temperatures above $675\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, but mechanical and electronic anomalies show changes at both 450 and $675\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. Anisotropic thermal expansion shows that ILMs drive the structure toward hexagonal symmetry. On the $[\frac{1}{2}\phantom{\rule{0.3em}{0ex}}\frac{1}{2}\phantom{\rule{0.3em}{0ex}}0]$ zone boundary, which becomes equivalent to [010] under a hexagonal distortion, a normal mode shows a softening coincident with the disappearance of the ILMs. We argue that the symmetry local to the ILMs becomes hexagonal above $600\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, causing ILMs to hop between equivalent orientations and putting the structure on a path toward the high-temperature $\ensuremath{\gamma}$ phase (bcc).

We report pressure studies on the magnetic and superconducting properties of PuTGa5 (T=Co, Rh) and Americium metal down to 0.4K. Recent discovery of superconductivity in the Plutonium based compounds PuTGa5 (T=Co, Rh) has shed a new light on the study of Actinides compounds. The fact that these systems present lots of similarities with the CeTIn5 (T=Co, Rh, Ir) must not hide some fascinating features of these materials: they display superconducting characteristics one order of magnitude higher than their Cerium counterparts or all 5f Superconducting Heavy. Moreover they present at ambient pressure a Non Fermi Liquid described by a T4/3 law which seems to maintain up to the highest pressure achieved (∼20 GPa) for the 2 Plutonium superconductors. Finally we report superconducting pressure diagram of americium metal under pressure up to 27 GPa and down to 0.4K. The complex superconducting diagram Tc(p) related to the several phases transition led us to examine the localization delocalization aspect of the 5f electrons in this metal through the Mott transition.

Our electron photoemission experiments demonstrate that the magnetization of the ferromagnetic state of UTe is proportional to the binding energy of the hybridized band centered around 50 meV below EF. This proportionality is direct evidence that the ferromagnetism of UTe is itinerant; i.e., the 5f electrons are not fully localized close to the atomic core. This mechanism of itinerant ferromagnetism differs from the traditional picture for 5f-electron magnetism in an essential and a novel way. We propose a simple model for the observed proportionality between the temperature dependence of the magnetization and the binding energy of the hybridized band near EF. This model allows us to estimate the effective magnetic interaction and to identify signatures of itinerant ferromagnetism in other materials. DOI: 10.1103/PhysRevLett.93.267205

Single crystals of $\mathrm{USb}$, $\mathrm{NpSb}$, $\mathrm{PuSb}$, $\mathrm{UTe}$, $\mathrm{NpTe}$ and $\mathrm{PuTe}$ were investigated by photoelectron spectroscopy, this included angular-resolved studies on the U compounds. The spectral features show little dependence on incident photon energy, suggesting they all contain comparable amounts of $5f$ and conduction character. In the case of $\mathrm{USb}$ interesting (and unexpected) momentum dependent effects are observed in the angular-resolved studies. In $\mathrm{PuTe}$, we confirm the presence of a strong three-peak structure near ${E}_{F}$, which has been interpreted as the signature of an intermediate valence state in similar materials. Hybridization of the $5f$ electrons with the conduction band is found within the series and the level of localization is shown to increase from $\mathrm{Te}$ to $\mathrm{Sb}$. A surprising correlation between the binding energy of the peak bearing most of the $5f$ weight in the photoemission spectrum and the magnetic moment is discovered within the series, for which some explanations are suggested.

Single crystals of the heavy-fermion superconductor UBe 1 3 doped with 3.5% Th have been studied by neutron scattering techniques. This material undergoes two superconducting transitions at T c 1 ≅0.55 and T c 2 ≅0.40 K.Using elastic neutron scattering we found no evidence for either short- or long-range magnetic order down to temperatures of 0.15 K. Similarly, exhaustive scans at reasonably high values of the momentum transfer found no evidence of a recently predicted internal lattice rearrangement of the Be nuclei at low temperatures. However, using neutron inelastic scattering we did find weak signals with a propagation direction q= , indicating antiferromagnetic fluctuations that are short range in real space, and are also short lived. These correlations disappear only above about 30 K, similar to effects found in pure UBe 1 3 . They also show no variation on passing through T c .

The other articles in this issue are concerned with the variety of science that goes on at reactor‐based neutron sources. Such sources, based on nuclear fission, have been available since 1942. It has been known since the 1930s that neutrons could be produced through spallation by sending a charged beam of accelerated protons or electrons into a target. However, it has only been in the last few years that the intensity of these spallation sources has been sufficient for the range of sophisticated experiments required to study the properties of condensed matter. Today there are a number of operating spallation neutron sources, and more with higher intensity are planned (see the table). In this article we want to explain the difference between doing experiments at steady‐state and at pulsed sources, illustrate what has been done with the modest sources now available, and speculate on some future experimental efforts. We do not have space to describe how a spallation source works: this information is present...

A single crystal neutron diffraction experiment has been performed on NpO2 to search for anomalies in the Debye-Waller factors associated with the as yet unexplained 25 K phase transition. Data were collected out to a maximum momentum transfer of 16 A−1. A detailed analysis shows no significant changes of the thermal parameters when cooling below 25 K. This result effectively eliminates the most recent proposal that a dynamical distortion of the oxygen sublattice occurs.