Uranate Ion Research Articles

Interaction of lanthanide and actinide with BaHfO3 perovskite: A case study with cerium and uranium

AbstractSpeciation of actinide (An) and lanthanide (Ln) in technologically important ceramics is very important from both fundamental as well as technological aspects. The intrinsic structural flexibility of perovskite containing AO6 and BO12 polyhedra makes them suitable and rich hosts for An and Ln. In this work, emphasis was given to deciphering information such as oxidation state, local dopant site, charge compensating defects, excited state kinetics, and so forth in BaHfO3 (BHO) related to dopant uranium (BHO‐U) and cerium (BHO‐Ce). Several spectroscopic techniques namely, time‐resolved photoluminescence (TRPL), positron annihilation lifetime spectra (PALS), and thermoluminescence (TL) spectroscopy coupled with density functional theory (DFT) were employed to probe the same. Ce and U though are distributed at both Ba and Hf sites, Ce prefers the former, while U prefers the latter site. Uranium on the other hand stabilizes as U6+ in the form of octahedral uranate ion giving green emission. PALS suggested the formation of defects in BHO‐Ce and BHO‐U with oxygen vacancies predominating in the former whereas BHO‐U perovskites are loaded with cation vacancies and vacancy clusters. These cation vacancies are responsible for lower TL output in BHO‐U. TL measurements also suggested cerium doping leads to a higher density of deeper traps in BHO‐Ce compared to uranium doping in BHO‐U which is in concurrence with DFT results and may have implications in designing afterglow phosphors based on perovskite. We believe this work would have a long‐term impact on exploring the potential of perovskite for nuclear waste host and afterglow phosphors applications.

Insight into the effect of A-site cations on structural and optical properties of RE2Hf2O7:U nanoparticles

Abstract A2B2O7 type pyrochlores have been recently proposed as a potential nuclear waste host due to their many interesting properties. To assess and understand the performance of these compounds as nuclear waste hosts, the speciation and structural investigations on actinide-doped RE2Hf2O7 are needed since both are imperative from their application perspective. In this work, we investigated the effect of uranium doping at different concentrations in the range of 0–10% on the structural and optical properties of RE2Hf2O7:U (RE = Y, Gd, Nd, and Lu) nanoparticles (NPs). The Y2Hf2O7 NPs exist in slightly disordered pyrochlore structure and the extent of disordering increases as a function of uranium doping while the structure reaches a cotunnite phase at 10.0% doping level. The Nd2Hf2O7 NPs also exist in a distorted pyrochlore structure and their distortion increases with increasing uranium doping inducing a phase transition into a disordered fluorite structure at 10.0% uranium doping. Both Gd2Hf2O7 and Lu2Hf2O7 NPs exist in a disordered fluorite structure and transforms into cotunnite structure at higher U concentrations (≥5.0%). Photoluminescence spectroscopy showed that uranium ions are stabilized in +6 oxidation state in all samples: in the form of uranate ion UO66− in the Y2Hf2O7, Nd2Hf2O7 and Lu2Hf2O7 NPs while in the form of uranyl ion UO22+ in the Gd2Hf2O7 NPs. Therefore, this work deepens the understanding of the behavior of uranium ions doped in different RE2Hf2O7 host matrices in the perspective of their application as nuclear waste hosts.

Fabrication and luminescence properties of U:YAG transparent ceramic

Uranium doped yttrium aluminum garnet (YAG) transparent ceramic with Ca2+ as charge compensator and sintering aid was firstly fabricated by vacuum sintering technique using the co-precipitation synthesis of raw powders. The structural, optical spectral and luminescence properties of the U:YAG ceramic have been investigated in detail. The experimental results show that uranium doped in YAG matrix has not changed the crystal structure. Vacuum sintering 15 h in 1800 °C, the sample has a pore-free structure with an average grain size of about 3.5 μm, and in-line transmittance reaches up to 78% in the visible wavelength region. The excitation spectrum showed a sharp band around 458 nm, and the emission spectrum revealed the intense green emission located at 520 nm. Further, with respect to the shape of emission band and luminescence color, uranium was inferred to exist as uranate ion (UO66−) in the YAG host where it replaces the ‘Y’ ions at dodecahedral site with surrounding defect centers created for charge compensation.

Solid state speciation of uranium and its local structure in Sr2CeO4 using photoluminescence spectroscopy

An effort was taken to carry our speciation study of uranium ion in technologically important cerate host Sr2CeO4 using time resolved photoluminescence spectroscopy. Such studies are not relevant only to nuclear industry but can give rich insight into fundamentals of 5f electron chemistry in solid state systems. In this work both undoped and varied amount of uranium doped Sr2CeO4 compound is synthesized using complex polymerization method and is characterized systematically using X-ray diffraction (XRD), Raman spectroscopy, photoluminescence spectroscopy and scanning electron microscopy (SEM). Both XRD and Raman spectroscopy confirmed the formation of pure Sr2CeO4 which has tendency to decompose peritectically to SrCeO3 and SrO at higher temperature. Uranium doping is confirmed by XRD. Uranium exhibits a rich chemistry owing to its variable oxidation state from +3 to +6. Each of them exhibits distinct luminescence properties either due to f-f transitions or ligand to metal charge transfer (LMCT). We have taken Sr2CeO4 as a model host lattice to understand the photophysical characteristics of uranium ion in it. Emission spectroscopy revealed the stabilization of uranium as U (VI) in the form of UO66− (octahedral uranate) in Sr2CeO4. Emission kinetics study reflects that uranate ions are not homogeneously distributed in Sr2CeO4 and it has two different environments due to its stabilization at both Sr2+ as well as Ce4+ site. The lifetime population analysis interestingly pinpointed that majority of uranate ion resided at Ce4+ site. The critical energy-transfer distance between the uranate ion was determined based on which the concentration quenching mechanism was attributed to electric multipolar interaction. These studies are very important in designing Sr2CeO4 based optoelectronic material as well exploring it for actinides studies.

Revealing the oxidation number and local coordination of uranium in Nd2Zr2O7 pyrochlore: A photoluminescence study

Uranium doped Nd2Zr2O7 is synthesized at 800°C (NUZO-800) using gel-combustion method and characterized using X-ray diffraction, scanning electron microscopy (SEM), X-ray photoelectron microscopy (XPS) and time-resolved photoluminescence (TRPL). As prepared sample was annealed further at 900, 1000, 1100 and 1200°C leading to increase in the size of the particle and the effect of same has been investigated on the photophysical properties of NUZO. Based on excitation and emission spectroscopy it was inferred that uranium stabilizes as +6 oxidation state in the form of UO66−. XPS measurement also establishes +6 oxidation state of uranium in Nd2Zr2O7. Both emission intensity and photoluminescence (PL) decay time increases with increase in temperature till 1000°C beyond which lifetime saturates whereas emission intensity decreases. The decay curve shows two different chemical environments for uranate ion in Nd2Zr2O7 pyrochlore matrix.

The electronic spectra of alkali metal uranates and band assignments: an analysis of their diffuse reflectance spectra

The diffuse reflectance spectra of alkali metal (Li, Na, K, Rb and Cs) mono- and diuranates, as well as several additional lithium polyuranates(VI), have been measured at 25 and − 196°C, and up to 500°C for the potassium uranates. The spectra of the mono- and diuranates of caesium and sodium were similar as were the spectra of the uranates of rubidium and potassium, because of identical site symmetries. Spectra recorded at − 196°C exhibited enough features to enable their resolution into individual overlapping bands. Existing theoretical models of the uranate spectra have been critically examined. The recorded spectra were analysed using the MO diagram for the octahedral UO66− moiety. The spectra were interpreted in terms of the various expected uranate ion site symmetries and the results obtained for all the alkali metal mono- and diuranates are reported here.

Raman and infrared spectra of rubidium and caesium uranates(VI) and some problems assigning diuranate site symmetries

The vibrational spectra of rubidium and caesium mono- and diuranates, M 2UO 4 and M 2U 2O 7 (M=Rb or Cs), have been measured in the infrared range, 4000–200 cm −1, and the Raman shift range of 1100–50 cm −1. The Raman spectra of these uranates are reported for the first time. The uranate ion site symmetries were assigned, based on the analysis of the spectra, as D 4h for Rb 2UO 4, D 2h for Cs 2UO 4. Assigning definite site symmetries for the diuranates presented some difficulties because it was not possible to do so unambiguously from spectral data but the likeliest for Rb 2U 2O 7 is C 2h and either C 2 or C s for Cs 2U 2O 7. The electronic spectra of molecules with a centre of symmetry are predicted to exhibit a greater temperature effect than acentric species. This is here shown experimentally, for the first time from diffuse reflectance spectra, by comparing the spectra of caesium mono- and diuranates at liquid nitrogen and room temperature; the much greater temperature effect of centrosymmetric over acentric species is clearly revealed by this technique. Crystallographic cell parameters of the uranates are reported, from X-ray powder diffraction studies.

Vibrational spectra of alkali metal (Li, Na and K) uranates and consequent assignment of uranate ion site symmetry

Vibrational (Raman and infrared) spectra of nine alkali metal (lithium, sodium and potassium) uranates have been measured in the infrared range of 4000–250 cm −1 and the Raman shift range of 1100–50 cm −1. The Raman spectra of sodium and potassium uranates, and lithium polyuranates are reported for the first time. From these spectra the site symmetries of intrinsic uranate groups are deduced by comparing the number of observed and resolved bands with the number predicted by the various site symmetries subgroups possible for each uranate crystal symmetry. The following could then be assigned: C 2 h for Li 2UO 4; D 2 h for α-Na 2UO 4; D 4 h for K 2UO 4; D 2 for Na 2U 2O 7 and C 2 h for K 2U 2O 7. For the lithium polyuranates, Li 2O·1.6UO 3, Li 2O·1.75UO 3, Li 2U 2O 7 and Li 2U 3O 10, as their crystal symmetries are not fully known, no definite conclusions concerning uranate site symmetry could be drawn, except that primitive monoclinic symmetry point groups must be excluded.

Thermally stimulated luminescence and electron paramagnetic resonance studies on uranium doped K2Ca2(SO4)3

Thermally stimulated luminescence (TSL) studies of gamma-irradiated uraniumdoped K2Ca2(SO4)3 revealed two glow peaks around 400 K and 435 K. Electron paramagnetic resonance (EPR) studies carried out on these samples have shown the formation of the radical ions SO 4 − , SO 3 − , SO 2 − and O 3 − . From the study of the thermal stabilities of these radical ions, it was found that the thermal destruction of SO 2 − and SO 4 − radical ions are associated with the glow peaks observed around 400 K and 435 K respectively. Uranate ion was identified as the luminescent centre for the observed TSL glow. The trap depth values for the glow peaks have been determined from TSL data.

Role of radical ions in the thermally stimulated luminescence of uranium doped BaCO3 system

Electron paramagnetic resonance (EPR) studies of γ-irradiated uranium doped BaCO3 have shown the formation of CO3−, CO2−, O3− and O2− ions. Thermally stimulated luminescence (TSL) glow curves of the γ-irradiated samples in the 300–600 K range have exhibited an intense peak around 360 K and a weak one around 440 K. The trap parameters for these peaks have been determined from TSL data. Spectral studies of the glow have revealed emission around 566, 583 and 590 nm characteristic of the uranate ion. From studies on the thermal stabilities of the radical ions, it has been inferred that the glow peak around 360 K is associated with the thermal destruction of O2− ion and the peak around 440 K is associated with the thermal destruction of CO3− ion.

Photophysical studies of uranyl complexes 5. Luminescence spectrum of K 4UO 2(CO 3) 3

The photoluminescence of K 4UO 2(CO 3) 3 has been studied under conditions of high resolution at cryogenic temperatures. The origin corresponding to the pure electronic transition was located at 4774 Å (20945 cm -1), and it was found that the totally symmetric uranyl stretching mode was coupled to this transition. A progression of four band systems thus resulted, and from an examination of the energies of corresponding peaks in each system, a value of 813 cm -1 for the U-O stretching mode was determined. Two lattice modes (34 and 80 cm -1) and two molecular vibrational modes (205 and 276 cm -1) were also found to couple with the pure electronic transition, thus yielding approximately 15 major peaks in each band system. The 205 cm -1 vibration corresponded to a CO 2- 3 vibration, while the 276 cm -1 vibration was a UO 2+ 2 deformation. The low values obtained for the force constant and totally symmetric stretching frequency of the U-O bond suggested that in UO 2(CO) 4- 3, the uranium atom is bound in a complex species that may be considered as an intermediate between that of a uranyl (UO 2+ 2) and a uranate (UO 10- 8) ion.

Photophysical studies of uranyl complexes 5. Luminescence spectrum of K 4UO 2(CO 3) 3

The photoluminescence of K4UO2(CO3)3 has been studied under conditions of high resolution at cryogenic temperatures. The origin corresponding to the pure electronic transition was located at 4774 A (20945 cm−1), and it was found that the totally symmetric uranyl stretching mode was coupled to this transition. A progression of four band systems thus resulted, and from an examination of the energies of corresponding peaks in each system, a value of 813 cm−1 for the U-O stretching mode was determined. Two lattice modes (34 and 80 cm−1) and two molecular vibrational modes (205 and 276 cm−1) were also found to couple with the pure electronic transition, thus yielding approximately 15 major peaks in each band system. The 205 cm−1 vibration corresponded to a CO32− vibration, while the 276 cm−1 vibration was a UO22+ deformation. The low values obtained for the force constant and totally symmetric stretching frequency of the U-O bond suggested that in UO2(CO)34−, the uranium atom is bound in a complex species that may be considered as an intermediate between that of a uranyl (UO22+) and a uranate (UO810−) ion.

Concentration dependencies in the luminescence of Li 4WO 5: U

The uranate ion luminescence in a sequence of (Li 1− x Na x ) 4W 1− y O 5:U y phosphors was examined at cryogenic temperatures under conditions of high resolution. It was found that as the ratio of U/W or Na/Li was increased, certain features in the sharp-line emission spectra decreased in intensity and other features increased at the same time. Careful examination of several photophysical parameters led to the conclusion that two different uranate sites yielded the observed luminescence features. One of the U(VI) ions occupies a site characterized by the presence of an inversion center, and the other ion occupies a site lacking such symmetry. The lower symmetry site may be considered a defect site, and was found to be an efficient quencher of the higher symmetry site.

Concentration dependencies in the luminescence of Li 4WO 5:U

The uranate ion luminescence in a sequence of (Li1 − xNax)4W1 − yO5:Uy phosphors was examined at cryogenic temperatures under conditions of high resolution. It was found that as the ratio of U/W or Na/Li was increased, certain features in the sharp-line emission spectra decreased in intensity and other features increased at the same time. Careful examination of several photophysical parameters led to the conclusion that two different uranate sites yielded the observed luminescence features. One of the U(VI) ions occupies a site characterized by the presence of an inversion center, and the other ion occupies a site lacking such symmetry. The lower symmetry site may be considered a defect site, and was found to be an efficient quencher of the higher symmetry site.

Observation of Ionic Sodium Uranate Precursors in a Laser Simulation of the Conditions inside the Bubble Formed in an HCDA of an LMFBR

The conditions inside the bubble formed in a hypothetical core disruptive accident (HCDA) of a liquid-metal fast breeder reactor have been simulated with a LAMMA 500 laser microprobe mass analyzer. Results for Na/sub 2/U/sub 2/O/sub 7/ show that negative diuranate and positive sodium uranate ions are produced. Higher laser powers favor greater fragmentation to U/sup +/, (UO)/sup +/, and (UO/sub 2/)/sup +/. The Na/sub 2/O/UO/sub 2/ results indicate vapor phase reactions result in the formation of positive and negative sodium uranate ion intermediates. Positive hydrogen ions are observed in some spectra. Higher laser energies (higher HCDA temperatures) favor sodium uranate ion formation. These data support the view that sodium uranate ionic precursors are formed in the vapor phase, bubble, of a simulated HCDA reaction. A prior argon-ionexcited secondary ion mass spectroscopy investigation of Na/sub 2/O/UO/sub 2/ and Na/sub 2/U/sub 2/O/sub 7/ showed no sodium uranate species, only the formation of U/sup +/, (UO)/sup +/, and (UO/sub 2/)/sup +/.