Absence of Induced Ferromagnetism in Epitaxial Uranium Dioxide Thin Films
William Thomas, Fabrice Wilhelm, Sean Langridge, Lottie. M Harding, Christopher Bell, Ross Springell, Sven Friedemann, Roberto Caciuffo, Gerrard. H Lander
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 report an implementation of the constrained random phase approximation (cRPA) method within the projector augmented-wave framework. It allows for the calculation of the screened interaction in the same Wannier orbitals as our recent DFT+U and DFT+DMFT implementations. We present calculations of the dynamical Coulomb screened interaction in uranium dioxide and alpha and gamma cerium on Wannier functions. We show that a self-consistent calculation of the static screened interaction in DFT+U together with a consistent Wannier basis is mandatory for gamma cerium and uranium dioxide. We emphasiz…
A double-exchange mechanism for the emergence of ferromagnetism in cubic uranium compounds is proposed on the basis of a $j$-$j$ coupling scheme. The idea is {\it orbital-dependent duality} of $5f$ electrons concerning itinerant $Γ_8^-$ and localized $Γ_7^-$ states in the cubic structure. Since orbital degree of freedom is still active in the ferromagnetic phase, orbital-related quantum critical phenomenon is expected to appear. In fact, odd-parity p-wave pairing compatible with ferromagnetism is found in the vicinity of an orbital ordered phase. Furthermore, even-parity d-wave pairing with si…
In this work, the thermal transport properties of thorium dioxide (ThO$_2$, thoria) with low levels of substitutional uranium (U) doping are explored. We observe strong indications of resonant phonon scattering, an interaction between phonons and electronic degrees of freedom, induced by this doping in addition to common ``impurity'' scattering due to mass and interatomic force constant differences. Uranium doping levels of 6\%, 9\%, and 16\% were studied in a single hydrothermally synthesized U-doped thoria crystal with spatially-varying U doping levels. Within this crystal, isoconcentration …
We have investigated the grain boundary scattering effect on the thermal transport behavior of uranium dioxide (UO$_2$). The polycrystalline samples having different grain-sizes (0.125, 1.8, and 7.2 $μ$m) have been prepared by spark plasma sintering technique and characterized by x-ray powder diffraction (XRD), scanning electron microscope (SEM), and Raman spectroscopy. The thermal transport properties (the thermal conductivity and thermoelectric power) have been measured in the temperature range 2-300~K and the results were analyzed in terms of various physical parameters contributing to the …
A comprehensive investigation on point defects and their clustering behavior in nonstoichiometric uranium dioxide UO2+x is carried out using LSDA+U method based on density functional theory. Accurate energetic information and charge transfers available so far are obtained. With these energies that have improved more than 50% over that of pure GGA and LDA, we show the density functional theory predicts the predominance of oxygen defects over uranium ones at any compositions, which is possible only after treated the localized 5f electrons properly. Calculations also suggest an upper bound of x~0…
Formation Gibbs free energy of point defects and oxygen clusters in uranium dioxide at high-pressure high-temperature conditions are calculated from first principles, using the LSDA+U approach for the electronic structure and the Debye model for the lattice vibrations. The phonon contribution on Frenkel pairs is found to be notable, whereas it is negligible for the Schottky defect. Hydrostatic compression changes the formation energies drastically, making defect concentrations depend more sensitively on pressure. Calculations show that, if no oxygen clusters are considered, uranium vacancy bec…
Uranium dioxide (UO$_2$) remains a formidable challenge for first-principles approaches, due to the complex interplay among spin-orbit coupling, Mott physics, magnetic ordering, and crystal distortions. Here we use DFT+$U$ to explore UO$_2$ at zero temperature, incorporating all the aforementioned phenomena. The technical challenge is to navigate the many metastable electronic states produced by DFT+$U$, which is acomplished using $f$-orbital occupation matrix control to search for the ground state. We restrict our search to the high-symmetry ferromagnetic phase, including spin-orbit coupling,…
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…
The structural behavior of UO2 under high pressure up to 300GPa has been studied by first-principles calculations with LSDA+U approximation. The results show that a pressure-induced structural transition to the cotunnite-type (orthorhombic Pnma) phase occurs at 38GPa. It agrees well with the experimentally observed ~42 GPa. An isostructural transition following that is also predicted to take place from 80 to 130GPa, which has not yet been observed in experiments. Further high compression beyond 226GPa will result in a metallic and paramagnetic transition. It corresponds to a volume of 90A^3 pe…
Uranium dioxide (UO$_2$), one of the most important nuclear fuels, can accumulate excess oxygen atoms as interstitial defects, which significantly impacts thermal properties. In this study, thermal conductivities and inelastic neutron scattering measurements on UO$_2$ and UO$_{2+x}$ (x=0.3, 0.4, 0.8, 0.11) were performed at low temperatures (2-300 K). The thermal conductivity of UO$_{2+x}$ is significantly suppressed compared to UO$_2$ except near the Néel temperature TN= 30.8 K, where it is independent of x. Phonon measurements demonstrate that the heat capacities and phonon group velocities …
D. J. Antonio et al. report an x-ray diffraction experiment on uranium dioxide at high-magnetic field and low temperature. The authors have apparently not realized that the diffraction data actually shows unambiguously the presence of a rhombohedral distortion induced by the field at low temperature and the presence of more than one domain. Our note correctly analyses their data. The crystallographic analysis we perform is based on simple arguments with respect to plane spacings in slightly distorted cubic materials. Similar studies have been reported in many materials over the last 50 years. …
We provide a methodology for generating interatomic potentials for use in classical molecular dynamics simulations of atomistic phenomena occurring at energy scales ranging from lattice vibrations to crystal defects to high energy collisions. A rigorous method to objectively determine the shape of an interatomic potential over all length scales is introduced by building upon a charged-ion generalization of the well-known Ziegler-Biersack-Littmark universal potential that provides the short- and long-range limiting behavior of the potential. At intermediate ranges the potential is smoothly adju…
We present a study of the structural phase transition, mechanical and thermodynamic properties of UO$_{2}$ by means of the local density approximation (LDA)$+U$ approach. A phase transition pressure of 40 GPa, which agrees well with the experimental value of 42 GPa, is obtained from theory at 0 K. Pressure-induced enhancements of elastic constants, elastic moduli, elastic wave velosities, and Debye temperature of the fluorite phase are observed. Phonon spectrums of both the ground state fluorite structure and high pressure cotunnite structure calculated by the supercell approach show that the …
Self-defect clusters in bulk matrix might affect the thermodynamic behavior of fission gases in nuclear fuel such as uranium dioxide. With first-principles LSDA+U calculations and taking xenon as a prototype, we find that the influence of oxygen defect clusters on the thermodynamics of gas atoms is prominent, which increases the solution energy of xenon by a magnitude of 0.5 eV, about 43% of the energy difference between the two lowest lying states at 700 K. Calculation also reveals a thermodynamic competition between the uranium vacancy and tri-vacancy sites to incorporate xenon in hyper-stoi…
Our series of articles is devoted to high-precision molecular dynamics simulation of mixed actinide-oxide (MOX) fuel in the rigid ions approximation using high-performance graphics processors (GPU). In this article we assess the 10 most relevant interatomic sets of pair potential (SPP) by reproduction of the Bredig superionic phase transition (anion sublattice premelting) in uranium dioxide. The measurements carried out in a wide temperature range from 300K up to melting point with 1K accuracy allowed reliable detection of this phase transition with each SPP. The λ-peaks obtained are smoother …
A uniform distribution of point defects in an otherwise perfect crystallographic structure usually describes a unique pseudo phase of that state of a non-stoichiometric material. With off-stoichiometric uranium dioxide as a prototype, we show that analogous to a conventional phase transition, these pseudo phases also will transform from one state into another via changing the predominant defect species when external conditions of pressure, temperature, or chemical composition are varied. This exotic transition is numerically observed along shock Hugoniots and isothermal compression curves in U…
Thermal recovery of irradiated uranium dioxide lattice have been investigated by experimental groups and defects related to each recovery stages are estimated based on the indirect observations like activation energies. In this study, thermal recovery mechanisms of both neutron and alpha-particle irradiated uranium dioxide lattice is investigated by atomistic simulation methods. Isochronal and isothermal annealing have been applied to the irradiated samples. Following the same procedures of the experimental groups, activation energies are calculated with molecular dynamics simulations and comp…
Pressure Dependence of Ferromagnetism in Uranium Hydride UH$_3$ up to 20 GPa Revealed by NV Centers Magnetometry
CEA DAM, DIF；Université Paris-Saclay（巴黎萨克雷大学）；CEA（法国原子能和替代能源委员会）；Laboratoire Matière en Conditions Extrêmes（极端条件物质实验室）；CNRS（法国国家科学研究中心）；ENS Paris-Saclay（巴黎萨克雷高等师范学院）；Centralesupelec（中央理工-电院）
Swelling of uranium dioxide with ingrowth of defects by irradiation is not fully understood. Experimental and theoretical groups have attempted to explain this phenomenon with various complex theories. In this study, experimental lattice expansion and super saturation of the lattice were well reproduced by molecular dynamics simulation method. From the resemblance with the experimental data, here it is manifested that only oxygen frenkel pairs were created in the fission induced lattice while alpha particle radiation causes both oxygen and considerable amount of uranium defects. Moreover, in t…
α-particle irradiated single-crystal isostructural oxide lattices (CmO2, AmO2, CeO2, UO2, NpO2, PuO2, ThO2) were modeled by molecular dynamics simulation method. Lattice parameter changes with IFP cation defects displayed exponential increases. These data were compatible with the available lattice parameter changes with α-particle dose experiments. Pre- and post- peaks emerged around principal peaks of irradiated oxide cation-cation radial distribution functions, which indicate obstruction and distortion type defects, respectively. Dependence of lattice dilatations on the number of obstruction…
Magnetic response of uranium dioxide (UO2) has been investigated through temperature and magnetic field dependent dc magnetization measurements. UO2 is a paramagnet at room temperature. The magnetic susceptibility, however, deviates from Curie-Weiss (CW) like paramagnetic behavior below T = 280 K. Further down the temperature UO2 undergoes phase transition to an antiferromagnetic state below TN = 30.6 K. The zero field cooled (ZFC) and field cooled (FC) magnetizations exhibit some distinct thermomagnetic irreversibility below TN. The temperature dependence of the FC magnetization is more like …
Interatomic potentials of uranium dioxide are investigated on their applicability to model structural stabilities beyond fluorite phase by comparing with ab-initio results. A high pressure cotunnite phase and loosely stacking virtual crystal are involved in order to get a primary confidence for large-scale deformation modelings of UO2 under irradiation damages. The behavior of Xe atoms in UO2 fuel is studied with molecular dynamics simulations. Besides the ground state bubble geometry, we find that a planar distribution is also (meta-) stable for xenon under thermodynamic perturbations. The Xe…
The FAST Framework: Developing a Data-Efficient Machine Learning Potential to Decode Superionic Transition-Induced Thermophysical and Kinetic Anomalies in UO2 under Extreme Conditions
State Key Laboratory for Strength and Vibration of Mechanical Structures, Shaanxi Engineering Laboratory for Vibration Control of Aerospace Structures, School of Aerospace, Xi'an Jiaotong University（西安交通大学航天学院）；Aircraft Strength Research Institute of China（中国飞机强度研究所）
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 …
Commercial nuclear power plants extensively rely on fission energy from uranium dioxide (UO2) fuel pellets that provide thermal energy; consequently, generating carbon-free power in current generation reactors. UO2 fuel incurs damage and fractures during operation due to large thermal gradients that develop across the fuel pellet during normal operation. The underlying mechanisms by which these processes take place are still poorly understood. This work is a part of our combined experimental and computational effort for quantifying the UO2 fuel fracture behavior induced by thermal shock. In th…
