Valence State Of Uranium Research Articles

Cs2K(UO)2Si4O12: A Mixed-Valence Uranium(IV,V) Silicate

The first mixed-valence uranium(IV,V) silicate is synthesized under high-temperature, high-pressure hydrothermal conditions. The structure contains chains of corner-sharing U(IV,V)O(6) octahedra which are interconnected by Si(4)O(12) four-membered rings to form a 3-D framework. XPS and XANES spectra were measured to identify the valence state of uranium.

XPS spectra of the [formula omitted] compounds [formula omitted], [formula omitted] and [formula omitted

Valence states of uranium in three ternary uranates KUO 3 , NaUO 3 and Ba 2 U 2 O 7 were studied using X-ray photoelectron spectroscopy (XPS). The analysis of the U4f photoelectron peak positions, peak widths at half maximum and satellite positions of U4f peaks, shows that the uranium in these compounds is present in a single valence state, namely U 5 + . Since the surface of these compounds is extremely susceptible to oxidation, the XPS spectra were measured after a carefully controlled etching of the surface using an Ar + ion beam so that the surface oxidation does not affect the XPS studies.

K3(U3O6)(Si2O7) and Rb3(U3O6)(Ge2O7): A Pentavalent-Uranium Silicate and Germanate

A new uranium(V) silicate, K3(U3O6)(Si2O7), and the germanate analogue, Rb3(U3O6)(Ge2O7), have been synthesized under high-temperature, high-pressure hydrothermal conditions and characterized by single-crystal X-ray diffraction. Their structures contain uranate columns formed of triple octahedral chains of the alpha-UF5 type linked by disilicate (or digermanate) units to form a 3-D framework structure. The valence state of uranium is confirmed by X-ray photoelectron spectroscopy, X-ray absorption spectroscopy, and magnetic susceptibility.

K(UO)Si2O6: A Pentavalent—Uranium Silicate.

AbstractFor Abstract see ChemInform Abstract in Full Text.

K(UO)Si2O6: A Pentavalent−Uranium Silicate

We report the first example of pentavalent-uranium silicate synthesized by a high-temperature, high-pressure hydrothermal method. The 3-D framework consists of four-membered single rings of corner-sharing SiO4 tetrahedra and 1-D UO4/1O2/2 chains. Magnetic susceptibility and XPS were measured to identify the valence state of uranium.

Determination of the uranium valence state in the brannerite structure using EELS, XPS, and EDX

In this study, the valence states of uranium in synthetic and natural brannerite samples were studied using a combination of transmission electron microscopy-electron energy loss spectroscopy, scanning electron microscopy-energy dispersive X-ray analysis (SEM-EDX), and X-ray photoelectron spectroscopy (XPS) techniques. We used a set of five (UO2, CaUO4, SrCa2UO6, UTi2O6, and Y0.5U0.5Ti2O6) U standard samples, including two synthetic brannerites, to calibrate the EELS branching ratio, M5/(M4 +M5), against the number of f electrons. The EELS data were collected at liquid nitrogen temperature in order to minimise the effects of electron beam reduction of U6+ and U5+. Test samples consisted of three additional synthetic brannerites (Th0.7U0.3Ti2O6, Ca0.2U0.8Ti2O6, and Th0.55U0.3Ca0.15Ti2O6) and three natural brannerites from different localities. The natural brannerite samples are all completely amorphous, due to cumulative alpha decay events over geological time periods (24–508 Ma). Our U valence calibration results are in reasonable agreement with previous work, suggesting possibly a non-linear relationship between the branching ratio and the number of f electrons (and hence the average valence state) of U in solids. We found excellent agreement between the nominal valence states of U and the average valence states determined directly by EELS and estimated by EDX analysis (with assumptions regarding stoichiometry) in two of the three synthetic brannerite test samples. The average U oxidation states of the five synthetic brannerite samples, as derived from XPS analyses, are also in good agreement with those determined by other techniques. The average valence states of U in three amorphous (metamict) natural brannerite samples with alpha decay doses ranging from 3.6×1016 to 6.9×1017 α/mg were found to be 4.4, 4.7, and 4.8, consistent with the presence of U5+ and/or U6+ as well as U4+ in these samples. These results are in general agreement with previous wet chemical analyses of natural brannerite. However, the average valence states inferred by SEM-EDX for two of the natural brannerite samples do not show satisfactory agreement with the EELS determined valence. This may be due to the occurrence of OH− groups, cation vacancies, anion vacancies, or excess oxygen in the radiation-damaged structure of natural brannerite.

Optical absorption properties and valence states of uranium in CaF2 crystals grown by TGT

Calcium fluoride single crystals doped with uranyl nitrate were grown by an improved temperature gradient technique under different conditions. Absorption spectra, energy levels and unit cell parameters were studied to analyze the possible color centers and valence states of uranium ions in as-grown U:CaF2 crystals. Uranium in U:CaF2 crystals grown in the presence of PbF2 as an oxygen scavenger is trivalent. F-centers and other defects related to oxygen, with respective absorption lines at 604 and 526nm, and impure valence states of uranium ions exist in U3+:CaF2 when the molar ratio of PbF2 to U is less than 25. In the absence of PbF2, U:CaF2 crystals are multicolor, consisting of red, cerise, yellow and green volumes from inside to outside where the red part in the core is still U3+:CaF2. Mixed valence states of uranium ions exist in the crystal. The valences of uranium ions are inferred to gradually increase from +3 to +6 according to the graded changes of the absorption spectra and unit cell parameters.

Examination of U valence states in the brannerite structure by near-infrared diffuse reflectance and X-ray photoelectron spectroscopies

The valence state of uranium doped into a f 0 thorium analog of brannerite (i.e., thorutite) has been examined using near-infrared (NIR) diffuse reflectance (DRS) and X-ray photoelectron (XPS) spectroscopies. NIR transitions of U 4+, which are not observed in spectra of brannerite, have been detected in the samples of U x Th 1− x Ti 2O 6, and we propose that strong specular reflectance is responsible for the lack of U 4+ features in UTi 2O 6. Characteristic U 5+ bands have been identified in samples in which sufficient Ca 2+ has been added to nominally effect complete oxidation to U 5+. XPS results support the assignments of U 4+ and U 5+ by DRS. The presence of residual U 4+ bands in the spectra of the Ca-doped samples is consistent with segregation of Ca 2+ to the grain boundaries during high temperature sintering.

Effects of pH and Uranium Valence State on the Aqueous Dissolution of Brannerite

ABSTRACTThe dissolution of the thorium analogue of brannerite (ThTi2O6-I) and U(IV)/U(V) doped Th-brannerite (Th0.97U0.03Ti2O6-II and Th0.955U0.03Ca0.015Ti2O6-III) in aqueous media under atmospheric conditions has been studied to elucidate the effects of pH and uranium valence state on the dissolution rate.The dissolution of I is nearly stoichiometric but slightly preferential release of U occurs for II and preferential release of Ca and U occur for III. The V-shape pH dependence previously observed for U-brannerite only occurs for U (not other matrix elements) for II, indicating that the pH dependence is related to the U oxidation state upon dissolution. The normalised U dissolution rates of III are nearly an order of magnitude higher than those of II for pH values over 3, suggesting brannerite is less durable with U(V) doping. TEM examination of specimens after leaching revealed few surface alteration products, which is consistent with the nearly stoichiometric dissolution of thorium brannerite.

The use of electromigration as a qualitative technique to study the migration behaviour and speciation of uranium in the Boom Clay

Summary Under the geochemical conditions prevailing in situ in the Boom Clay Formation (pH, Eh, …), calculations predict that U(OH)4 is the dominant uranium species present in the interstitial water and the concentration is solubility limited. However the boundary of the domain where the non solubility limited UO2(CO3)3 4− species dominates is very close. It is therefore of prime interest to know the correct speciation of uranium during the migration process. Electromigration was used as technique with the advantage that it can provide information on the speciation because the movement of the species towards the electrodes depends on its charge and speciation. Electromigration experiments have been performed with preconditioned 233UO2(CO3)3 4− sources, starting from the hypothesis that this species should migrate without retardation towards the anode. Despite relatively long electromigration times, sufficient to displace strong retarded tracers, no displacement of the migration profile towards the anode was observed. All 233U remained near the source position, but the electropherograms clearly showed the presence of species moving towards the cathode. This indicates the presence of neutral or positively charged uranium species. These electropherograms are interpreted as a change in uranium valence state: reduction of UO2(CO3)3 4− and precipitation of U(IV) oxy-hydroxides near the source position. The solubility limited species, U(OH)4(aq), are carried with the pore water towards the cathode. The electromigration experiments indicate, in support of the speciation calculations, that the dominant migrating U-species is probably the solubility limited U(OH)4.

The Local Uranium Environment in Cesium Uranates: A Combined XPS, XAS, XRD, and Neutron Diffraction Analysis

The cesium uranates Cs2UO4, Cs2U2O7, Cs4U5O17 and Cs2U4O12 were studied using X-ray Diffraction (XRD), neutron diffraction, X-ray Photoelectron Spectroscopy (XPS) and X-ray Absorption Spectroscopy (XAS) in an attempt to couple the crystallographic structure to the uranium valence state using the local uranium environment. The diffraction spectra were used for Rietveld refinement to determine the atomic positions and interatomic distances. These distances were subsequently used in Bond Valence Sum (BVS) calculations to determine the uranium valences. The XAS spectra give direct information on the local uranium environment regarding the U–O distances and the arrangement of the oxygen atoms around the central uranium. The difference between the monovalent uranates and the multivalent Cs2U4O12 is clearly established in all spectra, as well as in the crystal structures. The different valences present can be assigned to individual uranium lattice sites, but some amount of disorder is required to balance the charges.

The Effects of Uranium on the Structure of Iron Phosphate Glasses

AbstractBecause of their high chemical durability and waste loading capacity, iron phosphate glasses are a natural candidate for a nuclear waste disposal medium. We have studied the effects of uranium on the structure of iron phosphate glasses with both neutron and high-energy x-raydiffraction. The results of neutron scattering, which is mostly sensitive to pair correlations involving light atoms i.e. O-O, Fe-O and P-O, indicate the main structural features of the base glass are largely unaffected by the addition of UO2. The nearest-neighborbour P-O, Fe-O and O- O peaks remain at the same position in real space and their intensities scale approximately with concentration. These findings are consistent with earlier results using Raman scattering and EXAFS on the Fe-K edge, where in both cases the spectra remain similar to the base glass. The results of high-energy x-ray scattering, which is sensitive to correlations involving the heavier atoms and thus complements the neutron measurements, are also consistent with the overall picture of uranium occupying interstitial sites in the relatively undisturbed base glass structure. Combining the neutron and x-ray data for a 10 mol% UO2 glass suggests the intriguing possibility of a U6+ uranyl ion configuration although further work is needed to establish the precise local structure and valence state of uranium in these glasses.

Oxidation state of uranium: an XPS study of alkali and alkaline earth uranates

X-ray photoelectron spectroscopy (XPS) studies are performed on some mixed oxides of uranium with alkali and alkaline earth metals (Na, Li, Rb, Tl, Sr, Ba) to investigate the presence of multiple valence states of uranium in these compounds. Photoelectron peak positions, peak widths and satellite positions were measured to identify the chemical states. The analysis of the shake up satellites and the deconvolution of the principal peaks are used to quantify the chemical states. The domination of U(V) chemical state is inferred in Na 2U 3O 9 and Sr 2U 3O 10.

Valence states of uranium and gamma irradiation defects in sodaphosphate glasses

The investigated samples belong to the xUO 3 .(1-x)[2P 2 O 5 .Na 2 O] glass system with 0<x≤0.2

Determination of oxidation states of uranium and plutonium in cooling solution of spent fuel element assemblies from NPP A-1

The knowledge of oxidation states of uranium and plutonium is necessary from the point of view of the proposed vitrification technology for the final liquidation of the cooling solution as a high radioactive waste containing alpha activity. The valence states of uranium and plutonium have a major influence on the subsequent leaching rate of these elements from vitrification matrices. Using the various chemical behaviours of these elements, in accordance with their valence states, we made an attempt to establish their oxidation state in the original solution.

Neutron-Diffraction Study of Antiferromagnetism in UO2

Antiferromagnetism in U${\mathrm{O}}_{2}$ has been investigated in a detailed single-crystal neutron-diffraction study. Work on this material was prompted by a general interest in unpaired $5f$ and $6d$ electron distributions for different valence states of uranium in simple compounds. The form factor determined in the present case indicates a $5{f}^{2}$ electronic configuration with an effective moment slightly less than $1.8{\ensuremath{\mu}}_{B}$ for the ${\mathrm{U}}^{4+}$ ion. The magnetic ordering of the first kind, reported earlier by Henshaw and Brockhouse, is confirmed, but their proposed [111] orientation for the magnetization axis is not. The axis is found to be within the alternating ferromagnetic sheets. In the course of the diffraction study it was found that the paramagnetic-to-antiferromagnetic transition is an unusually sharp one, especially for such a simple magnetic system. Accordingly, the temperature dependence of magnetic neutron intensities was investigated through the N\'eel point. From the sharpness of the transition (50% of the magnetization is established within 0.03\ifmmode^\circ\else\textdegree\fi{} below the N\'eel temperature), and the absence of a critical scattering peak, it is concluded that the transition is of the first order.