Uranium hexafluoride (UF6) undergoes a rapid hydrolysis reaction when exposed to atmospheric water. In addition to producing hazardous HF gas, the hydrolysis reaction produces uranyl fluoride (UO2F2), a radioactive solid phase particulate material. Because of the technological utility of UF6 in the nuclear fuel cycle, understanding the transport properties of UO2F2 aerosol produced via UF6 hydrolysis is important for accident scenarios. Moreover, the fundamental chemical and physical properties of the UF6 hydrolysis reaction are not completely understood. Recently, several experiments on the aerosol phase properties of UO2F2 produced in this way have shown that under most relevant conditions, the particle size distribution (PSD) of UO2F2 can be extremely small, approximately 3 to 5 nm, which is well below the threshold that can be routinely observed via scanning electron microscopy (SEM). Although readily observable in the aerosol phase, observation of nanometer-sized particles in the condensed phase (i.e. deposited on surfaces) remains a challenge. Here, we have used atomic force microscopy (AFM) to study the PSD and morphological characteristics of UO2F2 deposited at low and high concentrations under different humidity conditions, a primary variable in the hydrolysis reaction. We find strong agreement between PSD measured in the aerosol phase via scanning mobility particle sizing and PSD measured via AFM, with particle sizes peaked below 4 nm for low-humidity conditions. At higher humidity, the distribution is centered around 5 to 10 nm but extends up to 20 nm. These results are in stark contrast to previous measurements using SEM that show PSD on the order of 300- to 1000-nm particle sizes; moreover, these are the first direct measurements of individual particles of UO2F2 having been produced via UF6 hydrolysis deposited on surfaces. These measurements, therefore, open a new avenue for collecting and detecting UO2F2 in the condensed phase and further refine the PSD, which is critical for environmental transport determinations.

Understanding the formation of uranium alloys with steel is important to advance nuclear technologies involving U metal fuels and machining U metal, and for nuclear forensics applications. No known phase diagram for the quaternary U-(M = Fe, Ni, Cr) system exists. We synthesize samples of U-304 L steel (nominal composition 70.1:18.3:10.4 at% Fe:Cr:Ni) across the U composition range 4.45-63.35 at%U by arc melting under inert conditions. Using the binary UFe phase diagram as a reference, we identify four U-steel alloy phases. We find the known U-Fe analogue phases UM2 and U6M, and two low-U composition phases with nominal compositions UM10 and U2M7. We apply a correlation length analysis to backscatter scanning electron microscopy images of sectioned and polished cross sections to quantify the domain formation length scale. We demonstrate that these depend heavily on the initial composition and range from 30 nm to 1.5 & mu;m. This result, in particular, could be applicable to theoretical predictions of transport properties. Furthering our understanding of U alloy phase formation with important structural elements such as steel primaries is foundational in developing future nuclear technology.1

Lanthanide AB(2) intermetallic compounds known as Laves phases have itinerant 3d and localized 4f electrons, which lead to interesting physical properties such as magnetic anisotropy and high Curie temperatures. Actinide Laves phases can display physical properties that are similarly intriguing. However, at reduced A-A spacing the C14 and C15 polytypes may exhibit larger wavefunction overlap for their 5f electron states and distinct characteristics for phases with more delocalized chemical bonding. The C36 polytype, on the other hand, is extraordinarily rare (<5% of known Laves phases). UAl2 is the only known actinide Laves phase to show a pressure-controllable C15 -> C36 transition. Here, we apply first principles calculations to determine the origin of the C15 -> C36 phase transition and reveal the differences between the corresponding properties of each phase. Pressure increases lead to bond compression-induced electron transfer from Al to U, which drives dynamic instability in the C15 phonon modes because of the uniform U-U bonding environment. The opposite phenomena is observed in C36: varied U-U bonding environments are vibronically more stable after charge transfer. We find that the interplay between charge transfer, chemical bonding, and phononic stability are central to predicting phase transitions and corresponding changes in physical properties for both C15 and C36 UAl2.

In a recent manuscript, Lawrence Brightet al(2023J. Phys.: Condens. Matter35175501) reported the resonant inelastic x-ray scattering spectra of U3O8, as well as UN. Their goal was to identify electronic multiplets associated with a 5f1configuration with ground state2F5/2. Complete active space self-consistent field with spin-orbit coupling (CASSCF-SOC) predicted that2F5/2transitions should be observable at 190 and 328 meV. However, these energies were not accessible in their experiment. They suggested that the recent inelastic neutron scattering results of Miskowiecet al(2021Phys. Rev.B103205101) could have been sensitive to these transitions. Here we show that transitions of this possible origin appear in that dataset near 198, 262, 362, and potentially 448 meV.

Carbon fiber composites have gained attention as a structural material because of their high strength-to-weight ratio, and understanding the effect of defects on reactivity and mechanical properties is important for the longevity and safety of the composite. Although it is known that strain causes the underlying graphitic vibrational modes to redshift, it is not clear how strain may alter reactivity and defect-induced vibrational changes. To investigate the strain-induced phonon changes of defective carbon fiber composites, density functional theory calculations of graphite are used, including intercalated hydrogen and fluorine defects. By comparing changes in the bond lengths, formation energies, and phonon density of states for uniaxially and biaxially strained graphite, strain was found to generally make defect formation more favorable and the specific behavior changes are dependent on the strain direction and defect identity. Specifically, intercalated fluorine phonons are more sensitive to strain than hydrogen intercalation phonons, and strain applied along the zigzag direction alters the calculated properties more than strain along the armchair direction. These results highlight the importance of understanding the microstructural effect of deviations from the ideal material because small changes in strain or defect type can significantly alter the behavior of the carbon fiber composite core.

Atomic-level defects dictate the mechanical properties of carbon fibers and strong correlations have been established between the crystallite sizes and mechanical properties. We recently demonstrated similar correlations with hydrogen content, but reliably quantifying the hydrogen content is not possible using only inelastic neutron scattering experiments. Here, we present prompt-gamma activation analysis (PGAA) experiments collected on 20 commercially available carbon fibers to quantify the hydrogen content of carbon fibers and find correlations between fiber modulus and hydrogen content. We then evaluate the role of hydrogen defect type and connect the PGAA results to both newly acquired and recently reported inelastic neutron scattering experiments. We find that intercalated hydrogen defects are preferentially removed at carbonization temperatures required for high-modulus fibers, potentially giving rise to voids within the carbon fibers that undermine their tensile strength.

Carbon fibers and carbon fiber composites are applied in high-performance applications, but a key consideration for application is their relative sensitivity to oxidative environments. To enable in-situ characterization of carbon fibers exposed to oxidative conditions, the Raman spectral response of T700 carbon fibers that have been exposed to a variety of dwell temperatures is reported herein with dwell times reaching up to 1 month. We evaluate the spectra holistically by using integrated absolute difference analysis. By combining this analysis with straightforward kinetic models, we connect the total Raman spectral response to the temperature-time curve that could yield such a shift in spectral parameters. Our work connects the Raman spectral response of carbon fibers to their thermal history and can easily be extended to other graphitic materials, such as nuclear graphite.

Optical vibrational spectroscopy has shown promise as a noninvasive means of monitoring the mechanical properties of carbon fiber (CF), which is increasingly used for industrial and consumer purposes. However, interpretation of optical vibrational spectra for solid materials is inferential, particularly when defects are present. Because inelastic neutron scattering (INS) spectroscopy is not subject to selection rules, the full vibrational spectra can be measured. And, identifying correlations between INS features and tensile properties can assist in the interpretation of spectra from more commonly used optical vibrational spectroscopic techniques, such as Raman and infrared (IR) spectroscopy. Recent INS experiments on high-performance commercial carbon fibers showed features near 900 and 1100 cm-1 in addition to a broad feature near 3000 cm-1 that increased in in-tensity with decreasing tensile strength. These features were assigned to hydrogen defects. In the present work, we use density functional theory to simulate the INS spectra of several hydrogen defect geometries in graphite as a model for carbon fiber structure units, confirming the experimental assignment of these peaks to hydrogen modes and providing insights into the structure and lattice dynamics of the defects.

Laves phase alloys possess unique thermal and electrical conduction properties, yet the factors governing phase stability in these systems remain an open question. The influence of phonons in particular has been broadly overlooked. Here, we investigate the UCo2x Ni2(1-x) chemical space using density functional theory, which offers a unique opportunity to explore the factors influencing Laves phase stability as all three primary Laves phases (C14, C15, C36) can be stabilized by changing the ratio of Co to Ni. Calculations of the thermodynamic and dynamical stability of pure UCo2 and UNi2 in each of three primary Laves phases confirm the stability of experimentally known Laves phases for UNi2 and UCo2. A decrease in bonding strength is identified in UNi2 compared to UCo2, aligned with redshifts observed in the UNi2 phonon density of states and a decoupling of the U and Ni vibrational modes. Phonon calculations of C14 UCo2 reveal dynamical instabilities. Efforts to remove the unstable mode at the Γ point in UCo2 via atomic displacements break the symmetry of the C14 phase, revealing a lower energy P2/c structure. Vibrational contributions to the free energy were calculated and did not change the thermodynamically stable Laves phase below 1000 K. The temperature-dependent free energies of single phase UCo2 and UNi2 were used to interpolate the relative stability of ternary UCo2x Ni2(1-x) in each of the three Laves phases at varying temperatures and stoichiometries. The ternary C36 phase is only predicted to be thermodynamically stable over a narrow stoichiometric range below 600 K.

The polymer network structure of epoxy thermosets playsa significantrole in its final material properties. However, the effects of mildthermal exposure on these network structures are poorly studied. Inthis work, wide-angle X-ray scattering was used to investigate thepolymer network structure of two epoxy thermosets: homopolymerizedbisphenol A (BPA) epoxy resin and BPA epoxy resin cured with a polyetheramine hardener (BPA/T-403). Using density functional theory and wide-angleX-ray scattering, insights into the polymer network structure wereobtained. Diffraction features were determined to originate from hardener-to-hardenermolecular distance, perpendicular & pi;-& pi; stackingof aromatic p-phenylene rings, and the average carbon-carbondistance in the polymer. Thermal exposure was found to permanentlyalter these structural features for both thermosets, with an increasein the & pi;-& pi; stacking distance. Homopolymerized BPAhad an additional decrease in the hardener-to-hardener distance. Thesestructural alterations were found to be detectable using Fourier transforminfrared spectroscopy and Raman spectroscopy, with changes in thehardener-to-hardener distance having the largest variations in theresulting spectra specifically at the aromatic and ether frequencies.

Uranium trioxide (UO3) is a stable chemical form of uranium oxide with multiple polymorphs found throughout the nuclear fuel cycle. The pressure-induced changes in the structure and lattice dynamics of four of these polymorphs are simulated with density functional perturbation theory and analyzed. Two phases, alpha- and delta-UO3 are found to exhibit an isotropic response to pressure and do not undergo any changes in coordination geometry up to similar to 40 GPa. In contrast, the other two phases investigated, beta- and gamma-UO3, exhibit an anisotropic response to pressure. Decomposition of the phonon eigenvectors allows us to assign specific pressure-induced structural changes to individual phonon modes. This analysis has been performed on a per atom basis for the relatively simple alpha- and delta-UO3 structures, which have one symmetrically unique uranium site, and on a per coordination environment basis for beta- and gamma-UO3, which have multiple U sites.

The mechanical strength properties of carbon fibers are generally thought to be correlated with the presence of underlying atomic-level defects. These defects serve to break the underlying translational symmetry of the graphitic or graphene subunits of the fiber construction, resulting in the emergence of new Raman-active spectral features. However, historical attempts to classify the precise origin of defect contributions to the Raman spectra have been challenging because of indistinct and overlapping features in the carbon fiber Raman spectra. Further, while substantial research exists on high-temperature exposure in inert atmospheres for carbon fiber composites, comparatively less addresses microscale behaviors and Raman spectral alterations to monofilament carbon fibers exposed to high-temperature atmospheric environments. Here, we report Raman spectral responses of nine commercially available high-performance, polyacrylonitrile-based carbon fibers exposed to various atmospheric heat treatments. We introduce a model-independent characterization method, the integrated absolute difference, to quantify the spectral responses to heat treatment of different fiber modulus classes. We combine this new method with a newly reported strategy to standardize spectral fitting for carbon fibers. With this approach, we show that atomic-scale defects in the underlying fiber microstructure manifest in measurably distinct manners and have distinct responses to thermal perturbation. These combined approaches may lay the foundation for disentangling contributions from specific defects in the Raman spectra of carbon fibers.

We present the vibrational spectra of polyacrylonitrile-based carbon fibers collected using inelastic neutron scattering. We ascertain the behavior of a broad range of vibrational spectra that are optically silent, and demonstrate a direct connection between these modes and thermomechanical properties of the fibers. We show directionally dependent coupling of hydrogen in the carbon fiber matrix, which is directly linked to mechanical properties. Further, we show hydrogen preferentially couples to the midband of the vibrational spectrum, and that there are higher overall mode populations in the traditional Raman D--G intervalley region, suggesting involvement of these modes in tensile strength reduction and transport properties.

The CemMnIn3m+2n (m = 1, 2; n = 0, 1) family has been one of the most studied families of heavy fermion compounds. This family has revealed many interesting low-temperature physics phenomena, like quantum critical points, heavy fermion superconductivity, and non-Fermi liquid behavior, when these materials are exposed to pressure, magnetic fields, and/or chemical substitution. Here we provide a thorough investigation of the Ce1-xNdxIn3 phase diagram through single crystal synthesis, x-ray diffraction, energy-dispersive spectroscopy, magnetic susceptibility, and electrical resistivity measurements. Previous electrical resistivity measurements on CeIn3 reveal a broad maximum, T-max similar to 50 K, which has been associated with the Kondo lattice coherence crossover and/or the crystal electric field depopulation effect as the 4f electrons condense from the high-energy quartet down to the ground state doublet. Our findings show that in the most disordered substitution region, x = 0.4-0.5, these features disjoin to reveal two distinct broad humps in electrical resistivity measurements. Magnetic susceptibility and electrical resistivity data on Ce1-xNdxIn3 also reveal the antiferromagnetic ordering competition between CeIn3 and NdIn3, where the T-N of CeIn3 is linearly suppressed to a critical concentration of x(Nd) similar to 0.6. This concentration is slightly lower than what was previously reported in nonmagnetically substituted Ce1-xLaxIn3. Our magnetic susceptibility measurements and subsequent simulations show that in the CeIn3 antiferromagnetic regime, x(Nd) <= 0.4, the Nd ions act as free paramagnets. The large magnitude of the associated paramagnetic response then masks the overlapping antiferromagnetic ordering signature of the Ce ions. Overall our study further sheds light on the underlying crystal electric field and Kondo lattice coherence interactions within the CemMnIn3m+2n family and could stimulate further studies of these systems via neutron diffraction or under applied pressure.

Determining the correct electronic structure of U3O8 remains a formidable experimental and theoretical challenge. In the low-temperature phase, two crystallographic U sites are separated into a distinct 2U(V)+1U(VI) oxidation configuration. At low temperatures, the U(V) sites form a distorted honeycomb lattice, but the U(VI) sit on a triangular sublattice, suggesting potential for magnetic frustration effects. The spin configuration of the unpaired f electrons on the U(V) sites is likely antiferromagnetic (AFM) from susceptibility measurements, but this has not been confirmed. Here, we present a neutron scattering investigation of the structure and dynamics of U3O8 from 1.7 to 600 K. We confirm static AFM ordering onset at between 22 and 25 K, which is present down to at least 1.7 K with AFM peaks corresponding to [0.5 1 1] and [0.5 2 2] in the orthorhombic phase. These measurements rule out static AFM order along the a axis of the Amm2 phase, a configuration previously suggested by theory. Above 100 K a quasielastic scattering channel opens that we speculate arises from a lattice relaxation response to thermally activated electron hopping. This term does not conform to a magnetic form factor, so it is not related to spin relaxations. If correct, this mechanism stabilizes a continuous valence transition from 2U(V)+1U(VI) in the low-temperature (T < 600 K) orthorhombic phase to the hexagonal phase that contains only one degenerate U site, wherein the U valence can be dynamically stabilized between U(V) <-> U(VI) by phonon-assisted electron hopping.

Although the literature on the Raman spectra of carbon fibers is vast, no consistent, robust predictive relationship between mechanical properties of carbon fibers and spectral parameters exists. This shortcoming is due to the use of numerous fitting functions to evaluate Raman spectra of carbon fibers and the inconsistencies in establishing the best fitting models in a statistically robust fashion. To address this gap, we present a comprehensive work on the Raman spectra of carbon fibers that combines a vast library of experimental data with a robust numerical analysis and a statistical evaluation of a wide range of suggested fitting models. This manuscript begins with a brief review of the commonly applied fitting models. Then, the Raman spectra of 32 commercially available polyacrylonitrile-based carbon fibers collected at excitation wavelengths 532, 633, and 785 nm are presented and the best fit for all fibers is evaluated based on several statistical criteria in conjunction with numerical calculations and physical arguments. The results suggest that high-performance fibers must be fit with at least five peaks, whereas high-tensile modulus fibers are best fit with at least six distinct peaks. Finally, we employ simultaneous fitting of the Raman spectra of specific fibers and wavelengths and demonstrate that strong correlations exist between mechanical properties and the D1 peak position and shape across the range of evaluated mechanical properties. We suggest straightforward improvements in fitting analysis procedures that can be implemented to increase coherency in the understanding of the underlying carbon fiber microstructure intuited from Raman spectroscopy.

Two-dimensional van der Waals magnets with multiple functionalities are becoming increasingly important for emerging technologies in spintronics and valleytronics. Application of external pressure is one method to cleanly explore the underlying physical mechanisms of the intrinsic magnetism. In this paper, the magnetic, electronic, and structural properties of van der Waals-layered, Fe-deficient ${\mathrm{Fe}}_{3\ensuremath{-}x}\mathrm{Ge}{\mathrm{Te}}_{2}$ are investigated. Magnetotransport measurements show a monotonic decrease in the Curie temperature $({T}_{C})$ and the magnetic moment with increasing pressure up to 13.9 GPa. The electrical resistance of ${\mathrm{Fe}}_{3\ensuremath{-}x}\mathrm{Ge}{\mathrm{Te}}_{2}$ shows a change from metallic to a seemingly nonmetallic behavior with increasing pressure. High-pressure angle dispersive powder x-ray diffraction shows a monotonic compression of the unit cell and a reduction of the volume by $\ensuremath{\sim}25%$ with no evidence of structural phase changes up to 29.4(4) GPa. We suggest that the decrease in the ${T}_{C}$ due to pressure results from increased intralayer coupling and delocalization that leads to a change in the exchange interaction.

We have performed pressure-dependent x-ray diffraction and resonant x-ray emission spectroscopy experiments on USb2 to further characterize the antiferromagnetic-ferromagnetic transition occurring near 8 GPa. We have found the magnetic transition coincides with a tetragonal to orthorhombic transition resulting in a 17% volume collapse as well as a transient f-occupation enhancement. Compared to UAs2 and UAsS, USb2 shows a reduced bulk modulus and transition pressure and an increased volume collapse at the structural transition. Except for an enhancement across the transition region, the f occupancy decreases steadily from 1.96 to 1.75.

Uranyl peroxide minerals are important materials in the nuclear fuel cycle. Despite nearly 150 years of research, conflicting literature reports about the thermal stability of uranyl peroxides coupled with poorly understood reaction kinetics have led to pressurization and explosion of uranium storage containers. To understand the influence of applied heating rate on phase transition temperatures, synthesized uranyl peroxide minerals studtite (UO2O2·4H2O) and metastudtite (UO2O2·2H2O) are systematically monitored via powder X-ray diffraction (PXRD), Raman spectroscopy, and non-isothermal thermogravimetric analysis across a set of heating rates (1, 2, 5, and 10 °C/min). A strong linear correlation between sample heating rate and phase transition temperature is observed from PXRD, Raman spectroscopy, and thermogravimetric analysis. Distinct kinetic mechanisms are shown to govern the phase transition as a function of heating rate for studtite, whereas a consistent kinetic model describes the transition from metastudtite to UOx regardless of heating rate. Insight into mechanisms of phase transformation is provided, and significant water retention during dehydration from studtite to metastudtite is indicated by PXRD, Raman spectroscopy, and thermogravimetric analysis. Mixed phase uranium oxide products are produced by calcination of metastudtite with a likely formation mechanism involving conversion of some uranyl centers from hexagonal to pentagonal bipyramidal coordination units via structural destabilization induced by peroxide liberation.

We have performed an extensive pressure-dependent structural, spectroscopic, and electrical transport study of LaCrSb_3. The ferromagnetic phase (T_C = 120 K at p = 0 GPa) is fully suppressed by p = 26.5 GPa and the Cr-moment decreases steadily with increasing pressure. The unit cell volume decreases smoothly up to p = 55 GPa. We find that the bulk modulus and suppression of the magnetism are in good agreement with theoretical predictions, but the Cr-moment decreases smoothly with pressure, in contrast to steplike drops predicted by theory. The ferromagnetic ordering temperature appears to be driven by the Cr-moment.