Uranium Sites Research Articles

The cooperative Jahn-Teller effect in uranium (4+) hydroxy-sulphate

The magnetic susceptibility measurements of orthorhombic U(OH) 2SO 4 within the temperature range 4.2–300 K have revealed a magnetic anomaly at T D = 21 K associated with crystallographic transition induced by the cooperative Jahn-Teller effect. Above 21 K the magnetic susceptibility of the uranium (4+) ion corresponds to the electronic ground doublet ¡M J = ± 2〉 confirming thus the antiprismatic symmetry ( D 4d ) of the crystal field at the uranium site. Below T D the system of two singlets ( 1 √2 )|2〉 ± ( 1 √2 )| 2 〉 separated by δ( T) is the ground state of the uranium ion.

Electronic structure and properties of the actinides

We review the present state of understanding of the electronic structure and physical properties of actinide metals and intermetallic compounds as derived from relativistic APW energy band studies of some of the light (Th, U, Np, and Pu) and heavy (Am, Bk, and Cm) metals, the intermetallics URh 3 and UIr 3 and (NaCl structure compounds such as) UC. Emphasis is placed on the importance of Coulomb correlation and the role of actinide-actinide separation in determining the itinerancy (as in the light metals) or localization (as in the heavy metals) of the 5f electrons and in turn their resulting magnetic and other properties. The UX 3 systems considered are significant because the uranium sites are sufficiently separated to fall in the local moment f orbital range of the Hill superconductivity/magnetism plot but show no magnetic ordering. Comparisons to recent optical, de Haas-van Alphen and other data are given when available. The contrasting cases of the transition and rare-earth systems make it clear that the 5f electrons are a unique species which offer exciting challenges to both experimentalists and theorists.

Structure de PbU 2Se 5

PbU 2Se 5 crystallizes in a monoclinic unit cell a = 8,605 A ̊ ; b = 7,788 A ̊ ; c = 12,27 A ̊ ; β = 90° of space group P2 1 c , Z = 4. The crystal structure has been determined for 1251 independant reflections and refined to a final R value of 0,068. Two kinds of uranium sites are found; the average U-Se distances are 2,97 Å for eight-coordination and 2,88 Å in seven-coordination. The average Pb-Se distance for eight-coordination is 3,20 Å. This structure explains the solid solution between PbU 2Se 5 and U 3Se 5.

Optimization of solutions (decisions) in the development of uranium sites

Optimization of solutions (decisions) in the development of uranium sites

Studies of K-Ar dating and xenon from extinct radioactivities in breccia 14318; implications for early lunar history

The rare gases argon and xenon were studied intensively in lunar breccia 14318, one of a family of three Apollo 14 breccias exhibiting similarities, including substantial amounts of ‘parentless’ xenon from the spontaneous fission of extinct 244Pu. We made stepwise heatings on both unirradiated and pile-irradiated specimens. The isotopic composition of the xenon from fission was determined by a new method which invokes a minimum of assumptions; it is shown to be from 244Pu and almost certainly parentless. For example, the fission component, although not appreciably fractionated with respect to the trapped component during stepwise heating, has a low temperature character so that, relatively speaking, it appears to be more surficial than xenon emanating from uranium sites in the irradiated sample. We demonstrate that this effect is not an artifact of the neutron irradiation. The breccia contains abundant trapped argon with a high 40Ar 36Ar ratio for lunar material—~14. Otherwise the argon is radiogenic and gives a convincing K-Ar age of 3.69 ± 0.09 b.y. by the stepwise 40Ar- 39Ar method, nearly in agreement with ages for other Apollo 14 breccias obtained in our laboratory and elsewhere. Rock 14301, another of the family of breccias and one which has been studied in other laboratories, contains similar trapped argon and parentless xenon. Unlike 14318 it also contains a conspicuous excess of 129Xe from the radioactive decay of extinct 129I. Implications of the parentless xenon from extinct sources, as seen in these different rocks, depend upon the model adopted for its evolution and storage. We present four different models, all of which are unsatisfactory in some respects, so that we are presently unable to narrow the question. We must stress that other Apollo 14 breccias, such as 14321, contain fission xenon from 244Pu which was apparently produced in situ.

Spallation and fissiogenic xenon and krypton from stepwise heating of the Pasamonte achondrite; The case for extinct plutonium 244 in meteorites; Relative ages of chondrites and achondrites

Following the discovery by Rowe and Kuroda that there are striking excesses of the xenon ratios 134/132 and 136/132 in the Pasamonte achondrite, we performed stepwise heating experiments with this meteorite. Xenon results from three experimental systems were concordant. The ratio 124/130 is linearly correlated with the ratio 126/130, showing a (cosmogenic) spallation component. Ratios reported by other laboratories for total xenon from various achondrites and a mesosiderite fall on this correlation line. The ratio 134/132 is linearly correlated with the ratio 136/132, showing a component presumed to be from fission. The line so defined is a gross but accurate extrapolation of the ‘Pepin line,’ which is now seen to be applicable to both chondrites and achondrites. Our Pasamonte data, taken alone, permit calculation of the isotopic composition of the fission and spallation components which are: fission, 131/132/134/136 = 0.285/1.00/1.06/1.13; spallation, 124/126/130/131/132 (assumed zero) = 0.285/0.995/1.00/4.48/0. Our Pasamonte data combined with Berne data for the highly cosmogenic Stannern achondrite provide an alternative but similar set of values: fission, 131/132/134/136 = 0.31/1.00/0.98/1.02; spallation, 124/126/128/130/131/132 = 0.58/1.00/1.34/0.73/2.76/0.08. Krypton data from the stepwise heating experiment provide ratios 80/84, 82/84, and 83/84 that are linearly correlated, showing a spallation component. Results from other laboratories for total krypton from achondrites and a mesosiderite fall on the correlation line. A preferred composition of the spallation krypton results from Berne measurements on Stannern and our measurements on Pasamonte: 80/82/83/84 = 0.52/0.75/1.00/0.42. The ratio 86/84 does not correlate with the others, suggesting detectable fission krypton in Pasamonte, but background interference makes this a weak conclusion at present. Results for the amounts and compositions of the spallation components agree generally well with calculations based on a formula due to Rudstam and a cosmic-ray spectrum given by Arnold and co-workers. The case for 82-m.y. Pu244 as an extinct radioactivity in meteorites rests on the following evidence: (1) the fission component in xenon differs isotopically from any possible fission component from uranium; (2) the concentration of fission xenon exceeds that attributable to uranium in Pasamonte by at least a factor of 15; (3) arguments are given that rule out other sources of the fission component such as primitive irradiations by slow neutrons, fast neutrons, or γ rays. Previous experiments on ordinary and carbonaceous chondrites can be reexamined in light of the Pu244 hypothesis. At face value, the fission components for chondrites are very large, suggesting that ordinary chondrites cooled at least 168 m.y. earlier than achondrites, and carbonaceous chondrites cooled at least 416 m.y. earlier than achondrites. An initial Pu/U ratio in carbonaceous chondrites would have to exceed 0.26, a result that would require that there was extensive local nucleosynthesis of the transbismuth elements shortly before the origin of the solar system, conflicting with present theories about the evolution of uranium in the solar system. We propose an experiment to test possible correlation of the fission component at retentive sites in the meteorite with fission xenon produced at uranium sites by pile neutrons.

Nonstoichiometry in Uranium Dioxide

An extension of Anderson's formulation of the defect theory of nonstoichiometry is used with an oxygen—interstitial oxygen-vacancy model to calculate the composition of the uranium dioxide phase at the lower phase boundary and the relative partial molar thermodynamic functions for oxygen at compositions ranging from the lower phase boundary to UO2.08. The model is used as a basis on which to argue that differing experimental results near UO2.00 are more likely due to differing, nonequilibrium, concentrations of uranium vacancies than to failure to reach oxygen equilibrium. The present form of the model, with an interstitial site density of one per uranium site fails sharply at about UO2.08 rather than gradually as the oxygen content is increased from UO2.00. Discussion of the effects of alternate site densities is used to explain the observed values of the partial molar entropy of the oxygen in the range from UO2.02 to UO2.24.

Temperature Dependence of Vibronic Transitions in Crystals and Spectroscopic Observation of a Phase Transition

The temperature dependence of vibronic absorption bands, corresponding to either one phonon absorbed or emitted simultaneously with an electronic transition in a crystal having an octahedral UCl62− complex was measured and found to agree with the theoretically expected results. A phase transition was observed by the weak appearance of the pure electronic transition above a transition temperature. The latter exhibits hysteresis favoring a first-order transition. Selection rules and general symmetry arguments show that the distortion must be one of D2, D3, S4, D2d (special) about the uranium site. The particular arrangement of atoms enables a further reduction in the possible distortions to D2d or D3. From the relative intensity of pure electronic to vibronic transitions one can estimate the size of the distortion to be of the order of 2×10−3 times the lattice constant.

Electronic Specific Heat Associated with Localized States in Actinide Metals

Motivated by the observations that the substitution of U for Th in the intermetallic compound ThAl2 results in the formation of localized magnetic moments at the uranium sites, we have made measurements of the low-temperature specific heat of a range of compounds, Th1−xUxAl2, in the temperature range 1.7° to 20°K. We have observed that the value of γ, the electronic contribution, increases with the addition of U; which increases we associate with the formation of localized 5f-like states on the U atoms and close to the Fermi energy. We calculate from the observed values of γ that the change in density of states per U atom ranges from 140 states eV−1 per U atom on the addition of 0.5 at.% U to 80 states eV−1 per U atom on the addition of 18 at.% U. That these states contribute to the specific heat indicates that they form close to the Fermi energy, a condition, within the framework of current theories, favorable to their being magnetized.

Structures of UO2, UO2+x andU4O9 by neutron diffraction

Single crystals have been examined by neutron diffraction in the range of temperatures 20 °C to 1 100 °C and of compositions UO2 to U4O9. The investigations divide into three groups, relating to UO2, UO2+x (where 0 (for O') and along (for O) from the large interstices in the fluorite structure. The interstices themselves are not occupied. At the composition UO 2.25 the oxygen atoms are ordered to form the new phase U4O9. The main neutron reflexions of U4O9, corresponding to the fluorite-type peaks of UO2, are much stronger than the superlattice reflexions, which arise from the ordering of the interstitial oxygen atoms within the fluorite framework. The analysis of the intensities of the main reflexions shows that there are two kinds of interstitial sites, as in UO2+x : the 0' atoms are located 0.85 A along the directions from the centres of the large interstices, and the atoms 1.05 A along . Vacancies occur in the fluorite-type oxygen sites (0) but not in the uranium sites, and the chemical formula UO2.25 is expressed more fully as U1.00 O1.77 O'0.29 O0.19. It is concluded that oxygen atoms cannot enter the fluorite structure individually but are incorporated in small groups containing O' and atoms and O vacancies. A complete structure determination of U4O9 must await the analysis of the superlattice reflexions.