Uranium Sites Research Articles

Influence of the Silicon Composition on the Crystallographic Properties of the New Ferromagnetic Ternary Silicides U 2Fe 17- ySi y (3.3 ≤ y ≤ 4.5) and U 2Co 15Si 2

After melting and quenching, the ternary silicides U 2Fe 17- y Si y exist far 3.3 ≤ y ≤ 4.5 but they partially decompose after annealing at 850-900°C. Their crystal structure, determined by X-ray diffractometry on both single crystals and powder, derives from the hexagonal Th 2Ni 17-type but depends strongly on the silicon content. For y = 3.7 some uranium sites are partially replaced by pairs of iron atoms and conversely. This structure shows some similarities to that observed for the binary compound Ho 2Fe 17. On the other hand, for y = 4.2, all the uranium atoms and pairs of iron atoms are statistically distributed. In contrast, the ternary silicide U 2Co 15Si 2, which is obtained as single phase after annealing at 850°C, adopts the Th 2Ni 17-type structure. In all compounds, iron or cobalt atoms of the pair are never substituted by silicon atoms.

Magnetization density in URhAl-evidence for hybridization effects

URhAl is one of a large class of uranium compounds with the hexagonal ZrNiAl structure. It becomes ferromagnetic at 27 K and the ordered moment is found to be 0.94 mu B per formula unit. The authors have performed experiments with polarized neutrons in order to measure the magnetization density in the unit cell. A crystallographic study gives atomic positions in agreement with previous work, but they find that the crystals, which were grown by a Czochralski method, are highly perfect and exhibit a large amount of extinction. Corrections for these effects are discussed. The magnetization density shows that the total moment as measured by magnetization is made up of four contributions. The moments (in mu B) on uranium, RhI, RhII, and from the conduction electrons are, respectively 0.94(3), 0.28(2), 0.03(2), and -0.11(3). The observation of a large moment at the RhI site, which lies in the (001) plane with the U atoms shows that strong anisotropic hybridization occurs between the U and RhI. This is the cause of the very large bulk anisotropy. At the uranium site they have analysed the contributions from the spin and orbital moments and find - mu L/ mu S=1.81(7). The free ion 5f3 value for this ratio if 2.6. The reduction gives one a quantitative measure of hybridization in the system. The results are compared with those calculated from density functional theory.

High-field magnetization processes in actinide intermetallics (invited)

In magnetically ordered intermetallics of uranium with d transition elements, the magnetic moment on the uranium site is often limited to values below 0.1μB, with, in some cases, extremely large magnetic anisotropies. Several approaches are followed for explaining these small uranium moments: opposite directions and almost compensation of the spin and orbital moments, reduction of the uranium 5f moment by strong hybridization effects between the 5f and conduction electrons, and very weak itinerant magnetism of the 5f electrons. In the Laves-phase compounds UFe2 and UNi2, the magnetic data have been explained in terms of opposite spin and orbital moments on the uranium sites. In the heavy-fermion compounds UPt3 and URu2Si2, on the contrary, a Kondo approach is followed, although coherence effects largely complicate a proper description. The experimental evidence for these different approaches will be reviewed.

Probing the f electrons in URu2Si2 with positrons: A theoretical investigation.

The two-photon momentum distribution for positron annihilation is computed for the heavy-fermion system ${\mathrm{URu}}_{2}$${\mathrm{Si}}_{2}$. A nonrelativistic approximation of the positron-electron overlap integrals is applied to wave functions obtained from a fully relativistic, self-consistent band-structure calculation. The crystal structure causes a preference of the positron for the uranium site, resulting in a considerable contribution to the two-photon momentum distribution of the f-electron states and marked structure related to the Fermi surface. Results are presented for integration of the two-photon momentum distribution along the [001] and [101] directions. Subsequent application of the Lock-Crisp-West theorem reveals Fermi-surface effects superimposed on long-wavelength structure due to the inhomogeneity of the positron wave function. It is concluded that positron annihilation can be used to study the nature of the f electrons in this heavy-fermion system.

Neutron diffraction studies of the crystalline and magnetic properties of UFe2

Studies using both polarised and unpolarised neutrons have been performed on single crystals of the Laves phase intermetallic compound UFe2. The authors describe studies of the twinning characteristics (two crystals with a common (1, 1, 1) axis) of the sample, a feature that is probably common in these cubic compounds. They then describe studies with polarised neutrons to determine the magnitude of the spin and orbital moments on the uranium site. In agreement with a recent theoretical prediction these two moments are found to be almost equal, but oppositely directed in real space. The net moment at the U site is only 0.01 mu B. The polarised neutron experiments also allow them to put an upper limit on any possible anisotropy in the magnetisation. High-field magnetisation experiments up to (20 T) on the same crystal as used in the neutron experiments allow them to determine the conduction-electron polarisation.

Electronic structure of UPt 3 and UBe 13: An auger and electron energy loss study

The electronic structure of UBe 13 and UPt 3 heavy fermions were investigated with Auger and core level electron energy loss spectroscopy (CEELS). A comparison of the CCC, CCV and CVV Auger transitions of elemental thorium and uranium showed that the valence electrons ( V) involved in the uranium Auger peaks were predominately the 5f electrons. As a result the uranium O 5VV Auger line shape represents the self-fold of the U(5f) local density of states in uranium, UBe 13 and UPt 3. For UBe 13 the O 5VV line shape is nearly identical to that of uranium indicating a weak degree of hybridization. For UPt 3 the O 5VV line shape differed significantly from that of uranium or UBe 13 indicating the presence of an intense U(5f) tail state distribution suggestive of a strong degree of hybridization. Auger intensity ratios of CCV to CCC Auger transitions gave evidence of charge transfer of U(5f) electrons away from the uranium sites for both UBe 13 and UPt 3. The low loss energy component of the 5d CEELS spectra could be interpreted on the basis of 5d 95f n + 1 final state with n = 3 for UBe 13 and n = 2 for UPt 3. For both UPt 3 and UBe 13 this work suggests that the uranium ligand interactions are stronger than previously suspected.

Quantitative analysis of the electron paramagnetic resonance spectrum of a uranium(III) compound

The fundamental formula of the EPR lineshape of field-swept spectra has been modified for the case of a broad homogeneous line. As a consequence, a new formula has been obtained for the derivative of the absorption. For the case of an inhomogeneous line, the original formula of Pilbrow holds, these formulae lay the basis for the determination of the number of spins by integration of the experimental EPR spectrum. The Pilbrow lineshape theory is then combined with the standard results for a Kramers ion having an effective spin 1/2, an anisotropic g tensor and no hyperfine interaction. The assumption is made that the linewidth in frequency units σν is independent of the orientation. This assumption completes the model specifying the line position, intensity, shape and width for any given orientation. The model thus enables one to simulate a powder EPR spectrum and its derivative knowing the principal values of the g tensor and the linewidth σν. The simulation entails a double integration over the spherical coordinates of the magnetic field in the frame of the principal axis of the g tensor. The number of sampling points for numerical integration is determined as a function of gx, gy, gz and σν. Experimentally the EPR spectrum of a coordination compound of trivalent uranium in powder form, U3(BH4)9(dcc)2, in which there are two kinds of uranium sites, has been recorded at 10.5 K in X-band. The least-squares fit between the observed and simulated spectra has been achieved using the optimisation software BSOLVE, and excellent agreement was reached with a Laplace–Gauss lineshape. The adjusted parameters for the two uraniums were confirmed by an independent fit based on the least absolute values criterion. Based on the final parameters, the EPR spectrum predicted at higher frequencies is better resolved, whereas the actual resolution of the spectrum at 16 GHz is no better. Finally, the effective magnetic moment is predicted to be slightly above 2.

Chemical bonding and magnetism in 3d-5f intermetallics

The authors report self-consistent energy band calculations for UFe2 and UCo2 in four approximations: both semi-relativistic and fully relativistic for the paramagnetic ground states; both spin-polarised and spin-polarised with spin-orbit coupling included self consistently for the ferromagnetic ground state of UFe2. They compare ab initio calculated and measured lattice constants, bulk moduli, linear specific heat coefficients, pressure dependence of the magnetic moment and magnetic form factor. UCo2 is found to be an enhanced paramagnet and UFe2 an itinerant ferromagnet with a magnetic moment due mainly to the iron moments. However, the magnetism on the uranium sites is unusual in that they calculate both spin and orbital contributions to the moment that are large, antiparallel and almost equal.

Electrical resistivity of the heavy-fermion compounds (U, Th)Pt3

Substitution effects of the uranium site by thorium in heavy-fermion compound UPt3 have been investigated. We observed that anomalous behaviors reflecting antiferromagnetic order at 4.56 at%Th, a coherence effect as in concentrated Kondo system above 17 at%Th and anomalously large increase of the residual resistivity with increasing Th contents below 10.5 at%Th.

Effect of Substitutions on Heavy-Fermion UPt3

Substitution effects of either uranium or platinum site by several foreign elements (Y, Ce, Th, Rh, Pd, Os, Ir and B) on the electrical resistivity of UP3 have been investigated in order to study systematic trends in the heavy-fermion superconductor UPt3. Resistivities at high temperatures show a clear saturation behavior, while they behave rather differently at low temperatures. An anomalously large increase of the residual resistivity was found especially in Th and Pd (about 20 µΩcm/at.%) and Y (about 10 µΩcm/at.%) substitutions. A rapid increase of the coefficient of the quadratic term in temperature of the resistivity is noteworthy especially in the case of boron addition.

Spin-Polarized Energy Bands of UFe2

The spin-polarized energy bands, density of states and calculated equation of state of UFe2 are analysed. The calculated lattice parameter is in good agreement with experiment and both the iron 3d and uranium 5f electrons are found to be strongly bonding. Hence we conclude that the uranium 5f electrons are hybridizing with both iron 3d and other conduction electrons. In the spin-polarized calculations a self-consistent moment of 0.7 μB on an iron site was obtained, in reasonably good agreement with the experimental value of 0.6 μB. However, on the uranium site a moment of -0.6 μB was calculated in apparent conflict with the experimental value of 0.06 μB. The anomaly is attributed to the effect of spin-orbit coupling.

Delocalized versus localized unoccupied 5f states and the uranium site structure in uranium oxides and glasses probed by x-ray-absorption near-edge structure.

X-ray-absorption near-edge structure (XANES) spectroscopy at the ${M}_{3}$,4,5 and ${L}_{3}$ edges of uranium and thorium using synchrotron radiation has been used to probe the unoccupied 5f electronic states and local structure of uranium sites in oxides and glasses. The uranium sites in hyperstoichiometric ${\mathrm{UO}}_{2+\mathrm{x}}$ (x\ensuremath{\sim}0.25, x\ensuremath{\sim}0.66) oxides have been studied. The multiple-scattering resonance in the direction of the linear uranyl group ${\mathrm{UO}}_{2}$${\mathrm{}}^{2+}$ is identified in the ${L}_{3}$ XANES spectra of uranyl nitrate hexahydrate. The localization of unoccupied 5f states in uranium and thorium oxides and glasses has been probed by ${M}_{4}$- and ${M}_{5}$-edge spectra. The presence of narrow 5f localized unoccupied states in Th- and U-containing glasses is indicated by the symmetric sharp white line due to 3d5${f}^{n+1}$ final states. The variable width of the white line indicates that the bandwidth of unoccupied 5f states increases by 4 eV going from glasses to oxides. The presence of 5f components in the U 6d conduction band of oxides, due to hybridization between the U 6d,7s and U5f states is shown by a long asymmetric tail of the white line at its high-energy side extending up to 12 eV. No evidence of many-body effects in XANES spectra, due to the configuration interaction between localized 5f configurations which was observed in mixed-valence rare-earth-metal compounds, has been found in Th and U oxides.

Spin-density study of UCl 4 — discovery of covalency effects

Neutron evperiments have been performed to measure the induced spindensity distribution in single crystals of uranium tetrachloride at low temperature. A structural refinement has also established the atomic positions and thermal parameters at low temperature and these are in agreement with earlier studies. The spin-density distribution, determined with polarived neutrons, shows small but significant deviations from that evpected from the single-ion crystal-field model. Fourier difference maps show a small depletion of the 5f density near the uranium site and a small positive increase of about 1 Å from the uranium site and principally along the shorter U-Cl bonds. We ascribe this to population of a 6 d ( v 2 − y 2) orbital. No significant spin density is associated with the chlorine site. Model calculations are presented which vualitatively account for the observed effect and involve electron transfer from the 5f to the 6d antibonding orbital formed as a result of a covalent interaction between the U 6d and Cl p states. This is the first direct observation of covalency in systems involving 5f 6d electrons.

Phase relationships in the uranium-palladium-sulfur system. II. Characterisation and crystal structure of U xPd 3S 4 (0.9 < x < 1)

The new ternary uranium-palladium-sulfide U x Pd 3S 4 (0.9<x<1) crystallizes in the cubic system a =6.639(2) Å, space group Pm 3 n, with a platinum bronze type structure. Refinement of the structure of a single crystal grown by the chemical vapour transport method revealed an incomplete uranium site filling in the crystal lattice, corresponding to the formula: U o.92Pd 3S 4. The crystal radius of the uranium is found to be intermediate between U 3+ and U 4+. so that the uranium is expected to be in an intermediate valence state.

Applications of UThPb isotope systematics to the problems of radioactive waste disposal

Concentrations of U, Th and Pb, and the isotopic composition of Pb for whole-rock samples of granitoids show: (1) that open-system behavior is nearly universal in the surface and near-surface environment; and (2) that elemental mobility is possible to depths of several hundred meters. Several identified or at least postulated factors that control U and/or Pb mobility include: (1) the mineralogical sites for U and its daughter products; (2) access of groundwater to these sites; (3) the volume of circulating water; and (4) the chemistry of the groundwater. Studies of granitic samples from peralkaline complexes in the Arabian Shield have shown that most samples lost less than 20% of their U during recent exposure to the near-surface environment. Most of the U in these samples appears to be firmly bound in zircons. In contrast, most surface and shallow drill-core samples of the granite of Lankin Dome (Granite Mountains, Wyoming) have lost ∼ 70% of their U. Most of the U in these samples is weakly bound in biotite and epidote-family minerals. The granite recovered during the Illinois Deep Drill Hole Project (Stephenson County, Illinois) is mineralogically similar to the granite of Lankin Dome, but this granite lost radiogenic Pb rather than U, probably as a result of exposure to groundwater that had a markedly different chemistry from that in the Granite Mountains. Studies of the Sherman Granite (Wyoming) and the Götemar Granite (southeastern Sweden) have shown that U and/or Pb mobility is greatest in and near fractured rock. The greater mobility is interpreted to be the result of both a larger water/rock ratio in the fractured rock and exposure to water over an increased surface area (and consequently a greater number of uranium sites). Several types of geochemical and mineralogic data can be used to identify rock—water interaction in granites; however, if rock samples have favorable radiogenic to common Pb ratios, both the amount and approximate timing of U or Pb mobility can be obtained through the use of isotopic studies. Such information can be extremely important in the search for favorable hosts for containment of radioactive waste. Rocks such as the Götemar Granite have undergone considerable rock—water interaction, most of which occurred ∼ 400 Myr. ago and little in recent times. Thus a search for zones that have experienced only a little interaction with water may provide a misleading prediction as to the ability of such zones to shield radioactive wastes from the modern biosphere. From an isotopic point of view, an ideal candidate for evaluation as a host rock for radioactive wastes would have the following characteristics: (1) a high ratio (> 2) of radiogenic to common Pb in order to optimize precision of the results; (2) a simple two-stage geologic history so that results could be interpreted without multiple working hypotheses; and (3) an originally high percentage (> 50%) of labile U so that the results would be highly sensitive to even small amount of rock—water interaction. These characteristics should produce rocks with marked radioactive disequilibrium in surface samples. The disequilibrium should grade to radioactive equilibrium with increasing depth until zones in which water has not circulated are found. Extensive regions of such zones must exist because UThPb systematics of most analyzed granitoids demonstrate closed-system behavior for almost all of their history except for their recent history in the near-surface environment.

Oxygen adsorption on UNi 2 studied by HREELS, AES and XPS

HREELS, AES and XPS have been jointly used to study the adsorption of O 2 on the UNi 2 surface. It shows that oxygen interacts mainly with the uranium sites, leading to Ni precipitation through the surface underlayers. The HREELS results suggest the presence of two oxygen-uranium sites having different adsorption kinetics.

An Investigation of Uranium L-Edges of Metamict and Annealed Betafite

AbstractThe uranium site in naturally occurring metamict minerals of the pyrochlore group (A1–2B2O6Y0–1) has been investigated using x-ray absorption spectroscopy (XAS). Pyrochlore structures are common phases in proposed polycrystalline waste forms. Betafite, a member of the pyrochlore group (B = 2Ti ≥ Nb+Ta), exhibits U-O bond lengths of 1.94 and 2.37 A for the metamict state, and 2.03 and 2.51 A for the crystalline (annealed) state. The U-O bond lengths decrease (∼0.1 A) and there is a disruption in the second nearest neighbor periodicidy as material is converted from the crystalline to the metamict state.

Determination of concentrations of selected radionuclides in surface soil in the U.S.

Background radionuclide concentrations in surface soil across the U.S. have been measured by the Remedial Action Survey and Certification Activities Group of the Health and Safety Research Division at Oak Ridge National Laboratory (ORNL). These measurements have been made as part of the ORNL program of radiological surveillance at inactive uranium mills and sites formerly utilized during Manhattan Engineer District and early Atomic Energy Commission projects. The background soil sampling program involved determination of 226Ra, 232Th and 238U concentrations in surface soil samples for comparative purposes to determine the extent of contamination present at the survey sites and surrounding off-site areas. The sampling program to date has provided background information at 356 locations in 33 states. The nationwide average concentrations of 226Ra, 232Th and 238U in surface soil were determined to be 1.1, 0.98 and 1.0 pCi/g, respectively. This paper summarizes the results of these background measurements and provides a brief analysis of regional differences and similarities in data values.

Petrographic relations and uranium distribution in the Skaloti granitoid complex, Paranestion District, Greek Macedonia

The Skaloti granitoid complex consists, for the most part, of biotite granites. The granites are composed of quartz, K-feldspar (mainly microcline), plagioclase, and biotite, with less common muscovite and rare hornblende. Common accessories are allanite, apatite, sphene, epidote, monazite, and garnet. The complex is divided on the basis of texture into two groups--granular and foliated--which have distinct accessory mineralogy. Epidote and sphene coexisting with biotite are relatively abundant in the foliated types; garnet is found only in the granular types. The granular granitoid group includes a leucocratic facies that occurs along the central part of the granitoid terrain. The leucocratic rocks show alteration and mechanical deformation and consist of quartz, feldspar, minor amounts of mica, garnet, and disseminated sulfides (pyrite, chalcopyrite). Sulfide mineralization and hematitization are associated with the microfractures.The Skaloti whole-rock chemical compositions indicate a metaluminous to peraluminous character, a potassium affinity, moderate to high thorium contents, apparently high Th/U ratios, and a tendency toward alkali enrichment. A shallow to moderate depth of emplacement and fractional crystallization paths highly sensitive to variations in total pressure, fluid pressure, and temperature are inferred from a consideration of the chemical data and appropriate phase relations. On the basis of present evidence, the Skaloti granitoid can be tentatively regarded as an S-type granite.The alkali enrichment, potassium affinity, moderate to high thorium contents, and presence of uranium-bearing accessory minerals render the complex a possible uranium source rock. The two groups of granitoids (granular, foliated) identified in Skaloti differ in the crystal chemical character of the uranium that they contain. In the foliated granites, the accessory minerals contain most of the uranium and represent refractory uranium sites. Granular granites, on the other hand, contain relatively high soluble uranium; these uranium sites in which uranium is not firmly chemically bound, are related to fragmentation and hydrothermal alteration undergone by the leucocratic granites, and to deuteric and low-temperature alteration which, overall, has affected the rocks of this group.The total uranium contents (U T ) of the analyzed surface samples range between 1 and 39 ppm. High total uranium values related to refractory uranium sites are attained by rocks of the foliated group and also by some aplitic-pegmatitic rocks; the extractable to total uranium ratios (U Ex /U T ) of these rocks fall below 0.39. Anomalously high total uranium values not attributable to refractory uranium sites and accompanied by anomalously high (U Ex /U T ) ratios (>0.50) are found in rocks of the leucocratic facies and also in samples from areas where dikes invade country rocks, and define localized uranium anomalies.The field petrographic and chemical relations of the Skaloti rocks suggest that fractional crystallization of the silicate melt, undersaturated in its aqueous phase, proceeded in the isobaric quinary granitic system (albite-anorthite-orthoclase-quartz-H 2 O) producing sodic plagioclase followed by a single alkali feldspar. This process of crystallization under hypersolvus conditions was terminated by an event of vapor saturation, related to the formation of the aplitic-pegmatitic rocks in the area. Conceivably, myrmekite and associated deuteric alteration and resorption are the textural evidence of boiling. The expelled fluid phase caused deuteric alteration and created areas rich in soluble uranium. These areas correspond to those radio-active-geochemical anomalies defined by exploration in Skaloti, which are located within or peripheral to the granitoid terrain and are associated with aplitic-pegmatitic rocks. These uranium anomalies, which show no structural control but can be identified as alteration areas, may represent the surface expression of uranium-rich, late-stage magmatic alteration channels at depth.Within the Skaloti granitoid terrain, significant concentration of uranium may be anticipated to occur in two types of setting: in uranium anomalies that may signify the presence of late-stage magmatic alteration channels at depth and in anomalies affected by way of secondary remobilization and deposition processes that are controlled by Tertiary faulting and fracturing.

Uranium mobility during interaction of rhyolitic obsidian, perlite and felsite with alkaline carbonate solution: T = 120° C, P = 210 kg/cm 2

Well-characterized samples of rhyolitic obsidian, perlite and felsite from a single lava flow are leached of U by alkaline oxidizing solutions under open-system conditions. Pressure, temperature, flow rate and solution composition are held constant in order to evaluate the relative importance of differences in surface area and crystallinity. Under the experimental conditions U removal from crushed glassy samples proceeds by a mechanism of glass dissolution in which U and silica are dissolved in approximately equal weight fractions. The rate of U removal from crushed glassy samples increases with decreasing average grain size (surface area). Initial rapid loss of a small component (≈ 2.5%) of the total U from crushed felsite. followed by much slower U loss, reflects variable rates of attack of numerous uranium sites. The fractions of U removed during the experiment ranged from 3.2% (felsite) to 27% (perlite). An empirical method for evaluating the relative rate of U loss from contemporaneous volcanic rocks is presented which incorporates leaching results and rock permeability data.