Uranium Transition Research Articles

Pressure modulated charge transfer and phonon interactions drive phase transitions in uranium–aluminum laves phases

Lanthanide AB2 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.

Towards an Intrinsic Doppler Correction for X-ray Spectroscopy of Stored Ions at CRYRING@ESR

We report on a new experimental approach for the Doppler correction of X-rays emitted by heavy ions, using novel metallic magnetic calorimeter detectors which uniquely combine a high spectral resolution with a broad bandwidth acceptance. The measurement was carried out at the electron cooler of CRYRING@ESR at GSI, Darmstadt, Germany. The X-ray emission associated with the radiative recombination of cooler electrons and stored hydrogen-like uranium ions was investigated using two novel microcalorimeter detectors positioned under 0∘ and 180∘ with respect to the ion beam axis. This new experimental setup allowed the investigation of the region of the N, M → L transitions in helium-like uranium with a spectral resolution unmatched by previous studies using conventional semiconductor X-ray detectors. When assuming that the rest-frame energy of at least a few of the recorded transitions is well-known from theory or experiments, a precise measurement of the Doppler shifted line positions in the laboratory system can be used to determine the ion beam velocity using only spectral information. The spectral resolution achievable with microcalorimeter detectors should, for the first time, allow intrinsic Doppler correction to be performed for the precision X-ray spectroscopy of stored heavy ions. A comparison with data from a previous experiment at the ESR electron cooler, as well as the conventional method of conducting Doppler correction using electron cooler parameters, will be discussed.

Selective hydroboration of terminal alkynes catalyzed by heterometallic clusters with uranium–metal triple bonds

Selective hydroboration of terminal alkynes catalyzed by heterometallic clusters with uranium–metal triple bonds

Time-resolved formation of uranium and silicon oxides subsequent to the laser ablation of U3Si2

The early-time kinetic behavior of species in a plume produced from laser ablation of U3Si2 in an environment containing 2% O2 is characterized using time-resolved absorption spectroscopy. The UO band around 593.55 nm and the SiO band around 230 nm is observed as well as various atomic and ionic uranium transitions. Temperatures and concentrations of these species are tracked and reported as well.

Hyperfine structures and isotopic shifts of uranium transitions using tunable laser spectroscopy of laser ablation plumes

We report isotopic shifts and hyperfine structures of selected U transitions employing tunable spectroscopy viz: laser-induced fluorescence and laser absorption spectroscopy of laser ablation plumes. The plasmas were produced during ns laser ablation on a natural U metal target which contains 0.73% 235U. Our results show that isotopic shifts between 238U and 235U are entangled with hyperfine structures of 235U. Measurements obtained using laser-induced fluorescence are affected by the high absorbance of 238U. Time-resolved laser absorption spectroscopy is carried out for evaluating the optical absorption and estimating the hyperfine constants.

Overview of the U3TGe5 family with T=Ti, V, Cr, Mn, Zr, Nb, Mo, Hf, Ta and W: Nine new members, phase formation, stability, structural and physical properties and electronic structures

Nine new ternary uranium transition metal germanides U3TGe5 with T = V, Cr, Mn, Zr, Nb, Mo, Hf, Ta and W have been discovered, in addition to the previously reported compound for T = Ti. These ten intermetallics adopt the anti-CuHf5Sn3-type of structure (hexagonal, P63/mcm, Z = 2). Scanning electron microscopy images, energy dispersive spectroscopy, differential thermal analyses and X-ray data refinements were employed for chemical, thermal and structural characterizations. For the purest samples, obtained for T = Ti, V and Cr, the investigation of the magnetic and transport properties were analyzed revealing the subsequent influence of the 3d metals on the ground-state properties. Ab initio calculations of the band structure of these three U3TGe5 compounds were performed, showing nested Fermi surface which may be associated to the presence of Dirac cones for the magnetically ordered compounds, i.e. for T = Ti and V.

Polymorph transitions in uranium at low temperatures: first-principles investigation

Polymorph transition from α to uranium was considered using the all-electron and pseudopotential approaches of density functional theory. The role of equilibrium lattice parameters of α uranium and the effect of a model for the atomic shift due to the considered transition were subjected to detailed investigation. Quasi-one-dimensional structures were observed by an increase in charge density between pairs of ions in the phase. A transition temperature of 39 K was predicted using the Peierls formula. A comparison of the structure between experimental and calculation data was discussed with regard to the accuracy of predicted interatomic forces.

High-temperature and high-pressure phase transitions in uranium

The phase diagram of uranium has been explored up to 100 GPa and 2000 K by means of ab initio molecular dynamics (AIMD) simulations. The lattice dynamics and energetics of the stable phases observed experimentally in this range of pressure and temperature are studied in this work. The phonon spectra of the $\ensuremath{\alpha}$ and $\ensuremath{\gamma}$ phases are shown to evolve strongly as a function of temperature, unveiling the huge anharmonic effects present in this material. If the elastics constants and the bulk and shear moduli of the $\ensuremath{\gamma}$ phase do not disclose any temperature effects, the shear modulus of the $\ensuremath{\alpha}$ phase decreases strongly as a function of temperature. Using the pressure- and temperature-dependent vibrational density of states and the Gibbs free energy of these two structures, we found a line transition between the $\ensuremath{\alpha}$ and $\ensuremath{\gamma}$ phases which slightly underestimates the experimental one. Coherently with experiments, the bct structure is never found stable between 0 and 100 GPa.

Features of structure and phase transitions in pure uranium and U–Mo alloys: atomistic simulation

We study structural properties of cubic and tetragonal phases of U-Mo alloys using atomistic simulations: molecular dynamics and density functional theory. For pure uranium and U-Mo alloys at low temperatures we observe body-centered tetragonal (bct) structure, which is similar to the metastable γ°-phase found in the experiments. At higher temperatures bct structure transforms to a quasi body-centered cubic (q-bcc) phase that exhibits cubic symmetry just on the scale of several interatomic spacings or when averaged over time. Instantaneous pair distribution function (PDF) differs from PDF for the time-averaged atomic coordinates corresponding to the bcc lattice. The local positions of uranium atoms in q-bcc lattice correspond to the bct structure, which is energetically favourable due to formation of short U-U bonds. Transition from bct to q-bcc could be considered as ferro-to paraelastic transition of order-disorder type. The temperature of transition depends on Mo concentration. For pure uranium it is equal to about 700 K, which is well below than the upper boundary of the stability region of the α-U phase. Due to this reason, bct phase is observed only in uranium alloys containing metals with low solubility in α-U.

Analysis of geological materials containing uranium using laser-induced breakdown spectroscopy

Laser induced breakdown spectroscopy (LIBS) is a rapid atomic emission spectroscopy technique that can be configured for a variety of applications including space, forensics, and industry. LIBS can also be configured for stand-off distances or in-situ, under vacuum, high pressure, atmospheric or different gas environments, and with different resolving-power spectrometers. The detection of uranium in a complex geological matrix under different measurement schemes is explored in this paper. Although many investigations have been completed in an attempt to detect and quantify uranium in different matrices at in-situ and standoff distances, this work detects and quantifies uranium in a complex matrix under Martian and ambient air conditions. Investigation of uranium detection using a low resolving-power LIBS system at stand-off distances (1.6m) is also reported. The results are compared to an in-situ LIBS system with medium resolving power and under ambient air conditions. Uranium has many thousands of emission lines in the 200–800nm spectral region. In the presence of other matrix elements and at lower concentrations, the limit of detection of uranium is significantly reduced. The two measurement methods (low and high resolving-power spectrometers) are compared for limit of detection (LOD). Of the twenty-one potential diagnostic uranium emission lines, seven (409, 424, 434, 435, 436, 591, and 682nm) have been used to determine the LOD for pitchblende in a dunite matrix using the ChemCam test bed LIBS system. The LOD values determined for uranium transitions in air are 409.013nm (24,700ppm), 424.167nm (23,780ppm), 434.169nm (24,390ppm), 435.574nm (35,880ppm), 436.205nm (19,340ppm), 591.539nm (47,310ppm), and 682.692nm (18,580ppm). The corresponding LOD values determined for uranium transitions in 7Torr CO2 are 424.167nm (25,760ppm), 434.169nm (40,800ppm), 436.205nm (32,050ppm), 591.539nm (15,340ppm), and 682.692nm (29,080ppm). The LOD values determine for uranium emission lines using the medium resolving power (10,000λ/Δλ) LIBS system for the dunite matrix in air are 409.013nm (6120ppm), 424.167nm (5356ppm), 434.169nm (5693ppm), 435.574nm (6329ppm), 436.205nm (2142ppm), and 682.692nm (10,741ppm). The corresponding LOD values determined for uranium transitions in a SiO2 matrix are 409.013nm (272ppm), 424.167nm (268ppm), 434.169nm (402ppm), 435.574nm (1067ppm), 436.205nm (482ppm), and 682.692nm (720ppm). The impact of spectral resolution, atmospheric conditions, matrix elements, and measurement distances on LOD is discussed. The measurements will assist one in selecting the proper system components based upon the application and the required analytical performance.

Measurement of photoexcitation cross-sections of uranium by saturation method

We report the measurement of photoexcitation cross-sections of three first-step uranium transitions (0 → 16900.38 cm −1, 0 → 17361.89 cm −1 and 620 → 17361.89 cm −1) using saturation method. These measurements were performed on a resonance ionization mass spectrometry (RIMS) set-up consisting of Nd:YAG-pumped dye lasers, a reflectron time-of-flight mass spectrometer and high-temperature atomic vapour source. The uranium vapours were excited and photoionized by two-colour, three-photon photoionization scheme using Nd:YAG-pumped dye laser system. The resultant photoion signal was monitored as a function of dye laser fluence used for first step excitation to measure the excitation cross-section values. A new approach was adopted to overcome the large uncertainties associated with such measurements. With this approach the cross-section of transitions whose value is already reported in the literature was measured as a bench mark. By normalizing the measured value to the reported value, a scaling factor was derived. This scaling factor was used to scale up the cross-section values of other transitions measured by this method.

Doppler-tuned Bragg spectroscopy of excited levels in He-like uranium: A discussion of the uncertainty contributions

We present the uncertainty discussion of a recent experiment performed at the GSI storage ring ESR for the accurate energy measurement of the He-like uranium 1s2p3P2→ 1s2s3S1 intra-shell transition. For this propose we used a Johann-type Bragg spectrometer that enables to obtain a relative energy measurement between the He-like uranium transition, about 4.51 keV, and a calibration x-ray source. As reference, we used the Kα fluorescence lines of zinc and the Li-like uranium 1s22p2P3/2 → 1 s22s 2S1/2 intra-shell transition from fast ions stored in the ESR. A comparison of the two different references, i.e., stationary and moving x-ray source, and a discussion of the experimental uncertainties is presented.

Synthesis and structure of [{N(CH2CH2NSiMe3)3}URe(η5-C5H5)2]: a heterobimetallic complex with an unsupported uranium–rhenium bond

The first structurally authenticated molecular uranium-transition metal bond is reported; DFT studies show sigma- and pi-components in the U-Re bond and this is the first time that the latter component has been reported in an unsupported f-element-transition metal bond.

Electronic band-structure and X-ray photoemission spectra of ternaries APtGe (A=Th, U)

UPtGe as a representative of uranium transition metal germanides, UTGe, reveals a unique character of crystal, electronic and magnetic properties. We attempt to elucidate them starting from band-structure calculations of this compound and its non-5f-electron counterpart ThPtGe in the paramagnetic state. The obtained theoretical electron densities of states (DOS) are compared with measured X-ray photoemission spectra (XPS). For both compounds, we achieve good agreement between the theoretical and experimental XPS, particularly concerning positions of peaks in the range of valence band spectra. Thus, in the case of UPtGe some important delocalisation of 5f electrons is well predicted by the LDA theory.

Study of intra-L shell transitions in Be-like uranium

The x-ray spectra produced in collisions of 98 MeV/u Li-like uranium ions with N2 molecules have been studied. By measuring the decay photons associated with the capture process, experimental information for the formation of the excited [1s22s2p3/2]1P1 and [1s22s2p3/2]3P2 states in Be-like uranium has been obtained. It was found that relative probability for production of the P-states is close to statistical population law 2J + 1.

Angular correlations in the two-photon decay of aligned hydrogenlike ions

The two-photon decay of highly charged hydrogenlike ions is studied within the framework of second-order perturbation theory, based on Dirac's equation. Attention has been paid to the angle-angle correlations between the two emitted photons. In particular, the density matrix theory is applied in order to explore how these angle-angle correlations are affected by the alignment of the excited states of the ions. Detailed computations have been carried out for the 3d{sub 5/2}{yields}1s{sub 1/2} transition in hydrogenlike uranium U{sup 91+} ions for different (degrees of) initial alignment. It is shown that the angular correlation function is very sensitive to the alignment of the 3d{sub 5/2} state, an effect which has to be accounted for in the analysis of all (future) two-photon decay experiments, for which the excited state of the ions is produced by relativistic ion-atom collisions.

QED corrections to the4p−4dtransition energies of copperlike heavy ions

Quantum electrodynamic (QED) corrections to 4p-4d transition energies of several copper-like ions with Z = 70-92 are calculated non-perturbatively in strong external fields to all orders in binding corrections. Dirac-Kohn-Sham potentials are used to account for screening and core-relaxation effects. For the 4p{sub 1/2}-4d{sub 3/2} transition in copperlike bismuth, thorium and uranium, results are in good agreement with empirical QED corrections deduced from differences between transition energies obtained from recent high-precision electron-beam ion-trap (EBIT) measurements and those calculated with the relativistic many-body perturbation theory (RMBPT). These comparisons provide sensitive tests of QED corrections for high angular momentum states in many-electron heavy ions and illustrate the importance of core-relaxation corrections. Comparisons are also made with other theories and with experiment on the 4s-4p transition energies of high-Z Cu-like ions as accuracy checks of the present RMBPT and QED calculations.

Structure and Dynamics of High-Z Ions Studied at the ESR Storage Ring

The ESR storage ring offers unique conditions for precision investigations in the realm of atomic structure and collisions research at highest nuclear charges. For atomic structure studies, a very recent experiment will be discussed aiming on a precise determination of the electron–electron QED contribution to the groundstate ionization potential in He-like uranium. For the regime of collision dynamics, the close interplay between dynamics and atomic structure is outlined. Here, for the case of the 2p3/2→1s1/2 transition in H-like uranium, a strong interference between the E1 and M2 transition amplitudes has been identified which gives rise to a remarkable modification of the angular distribution of emitted photons.

Non-selective photoionization of uranium atoms by the thirdharmonic of a Nd-YAG laser: experiment and simulation

Time of flight mass spectrometry in combination with laserablation (LA) can be successfully applied to low-isotope-ratiomeasurements of metallic samples. In this connection, thenon-selective quasi-resonant photoionization by Nd-YAG pulselaser third-harmonic radiation of uranium atoms created byeither LA on a metallic target or by radiative heatingin an oven was studied in a wide range of laser fluence bothexperimentally and with computer simulation. In the latter caseboth the level structure of the uranium atom and the experimentallyobtained profiles of spectral and spatial distributions of thethird-harmonic radiation of the Nd-YAG laser in the photoionization zonewere taken into account. Comparison of experimental andsimulated results was made. Experimental and simulated resultsfor the LA case were found to be in agreement only with theassumption that the ionization yield from the metastable level ofthe uranium atom is strongly suppressed. This could be explained bylow population of the metastable level in the photoionization volumedue to the expansion and subsequent population cooling ofuranium atom vapour created by LA. The unknownparameters of uranium transitions (lifetimes and ionizationcross sections) involved in the quasi-resonant photoionizationprocess under study are estimated.

Polygonized structure formation in polymorphic transitions in uranium

Polymorphic transformations in uranium are considered in relation to the formation of polygonized structures in the α and β phases. Dislocational polygonized structures arise in γ→β and β→α transitions in a fashion that on the whole is subject to the laws found for other polymorphic metals, while the differences are associated in the main with the uniquely complicated crystal structures of the uranium β and α phases. Metallography has been applied to determine the conditions of continuous cooling that produce a polygonized structure in the β grains in the γ→β transition (network substructure), and also the conditions for subgrains to arise in the α grains. The continuous cooling conditions govern whether the β→α transition occurs with normal kinetics or martensite kinetics. A bainite type of transition is characteristic of the γ→β transition. The defectiveness in the polygonized structure in α-uranium is such that the gridded substructure of the β grains is retained after cooling from the γ region (grain boundaries and subboundaries containing segregations of impurity particles), and this favors hardening of the α phase, but it reduces the plasticity.