Uranium oxide hydrate frameworks with Er(III) or Y(III) ions: revealing structural insights leading to the low symmetry.

Two new mixed-valence uranium oxide hydrate frameworks (UOFs), incorporating either Er3+ or Y3+ ions, were successfully synthesised under hydrothermal conditions and characterised with single-crystal X-ray diffraction and a variety of other structural and spectroscopic techniques. Both frameworks are isostructural and crystallise in the triclinic P1̄ space group, consisting of ß-U3O8 type layers pillared by additional uranyl centres, with the Er3+/Y3+ ions lying in the channels of the framework. SEM-EDS analysis found that both materials existed in plate-like morphologies, with a U:Er/Y ratio of 5.5. Bond valence sum analysis revealed the possible existence of pentavalent uranium centres, which was confirmed with diffuse reflectance spectroscopy. Being the first reported UOFs in this space group, this work highlights the complex and flexible nature of these materials, and the broader uranium oxide hydrate systems which exist in the surrounds of spent nuclear fuel disposal in the underground repository.

Perspectives on Pyrochlores, Defect Fluorites, and Related Compounds: Building Blocks for Chemical Diversity and Functionality.

In this article we provide some perspectives on a range of pyrochlore and defect fluorite type compounds with nominal A2B2O7, A2BO5, ABC2O7, and other stoichiometries. Typically, the phase transformations and stability fields in these systems are mapped as a function of the ionic radii of the A and B-site cations, e.g., the A/B cation radius ratio (rA/rB). This provides a useful guide to compatible structures and compositions for the development of advanced materials. Pyrochlore commonly transforms to a defect fluorite structure at high temperature in many systems; however, it is not uncommon to observe defect fluorite as the initial metastable phase at low temperature. The patterns of order-disorder observed in these materials are primarily due to the energetics of layer stacking, the defect formation and migration energies of cations and anions, or modulations of the parent cubic structure in 3 + n dimensional space. The first lead to predominantly non-cubic derivatives of the parent defect fluorite structure (e.g., zirconolite polytypes), the second control the order-disorder processes, and the latter lead to a variety of subtle additional scattering features within the cubic parent structure. Although the energetics of cation disorder and anion-vacancy disorder have become more accessible via atomistic approaches (e.g., MD and DFT), we continue to find interesting physical-chemical problems in these materials. For example, although there are significant differences in composition (Tb/Zr ratio and O content) between Tb2Zr2O7 and Tb2ZrO5, both of which are defect fluorites, we note that the modulations found in these two compounds by electron scattering are virtually identical with regard to the direction and magnitude of displacement from the normal Bragg diffracted beams. This suggests that neither the A/B cation ratio nor the oxygen stoichiometry have a significant effect on the modulations. The general observations on the systems of compounds noted in this paper rest primarily in the context of industrial materials for nuclear waste disposal, potential applications in inert matrix fuel designs, and other important technological applications such as ionic conductivity, electrical conductivity, and magnetism. Scientific advances in these areas have been underpinned by recent advances in ion irradiation, synchrotron X-ray, neutron scattering, and modelling and simulation capabilities. Furthermore, there has been some renewed interest in natural samples, e.g., Th-U zirconolite and pyrochlore as analogues for potential host phases in nuclear waste forms. In particular, the natural pyrochlores have provided additional details with regard to radiation damage ingrowth, percolation transitions, and the relationships between accumulated dose and physical properties including hardness, elastic modulus. Specific details of the thermal annealing of these samples have also been elucidated in considerable detail.

Uranyl oxide hydrate frameworks with lanthanide ions.

Two uranyl oxide hydrate frameworks (UOFs) incorporating either Eu(iii) or Gd(iii) ions were synthesized hydrothermally and structurally studied. The uranyl oxide hydroxide layers similar to those in ß-U3O8 with both tetragonal and pentagonal bipyramidal uranium polyhedra are connected with pairs of pentagonal bipyramidal uranium polyhedra through uranyl cation-cation interactions to form three-dimensional frameworks with Eu(iii) or Gd(iii) ions inside the channels. Both SEM and TEM examinations revealed needle crystal morphologies and a U:Eu/Gd ratio of 5.5, with the TEM-SAED pattern indexed to the orthorhombic crystal structure C2221, as also determined using synchrotron single-crystal X-ray diffraction. Raman spectroscopy revealed the band splitting of uranyl symmetric stretching vibrations, reflecting the presence of a unique pentavalent uranium centre in octahedral coordination geometry. The presence of pentavalent uranium in both UOFs was confirmed with diffuse reflectance spectroscopy. Given that layer-structured uranyl oxide hydroxy hydrate phases are dominant for both light and heavy lanthanide ions under similar reaction conditions, the ionic radius plays an important role in controlling the structure types, with UOFs formed only for Eu(iii) and Gd(iii) ions in the lanthanide series. These new UOFs with lanthanide ions may have various implications especially in nuclear materials.

Syntheses, Crystal Structures, and Spectroscopic Studies of Uranyl Oxide Hydrate Phases with La(III)/Nd(III) Ions.

We have synthesized two uranyl oxide hydrate (UOH) phases incorporating La(III) or Nd(III) ions under hydrothermal conditions. Investigations with scanning electron microscopy and transmission electron microscopy (TEM) revealed thin-plate morphologies with a U-to-Ln atomic ratio of 2:1 (Ln = La or Nd), while single-crystal X-ray diffraction and TEM electron diffraction studies confirmed that both UOH phases crystallized in the trigonal P31m space group with uranyl oxide layered structures incorporating La(III)/Nd(III) ions as interlayer species. Vibrational spectroscopic studies revealed typical vibrational modes for U ions, with the derived U═O bond lengths being comparable to the values reported on other UOH phases. Bond-valence-sum calculations suggest hexavalent uranium in the uranyl form, which was confirmed by the results of diffuse-reflectance and X-ray absorption near-edge structure spectroscopies. This work reports the first single-crystal structural investigation of UOH phases with Ln ions, which has significant implications in the weathering products of uraninite mineral in nature as well as the alteration products of spent nuclear fuels during interim storage and safe disposal over geological timespans.

Effect of Transition Metal Cations on Stability Enhancement for Molybdate-Based Hybrid Supercapacitor.

The race for better electrochemical energy storage systems has prompted examination of the stability in the molybdate framework (MMoO4; M = Mn, Co, or Ni) based on a range of transition metal cations from both computational and experimental approaches. Molybdate materials synthesized with controlled nanoscale morphologies (such as nanorods, agglomerated nanostructures, and nanoneedles for Mn, Co, and Ni elements, respectively) have been used as a cathode in hybrid energy storage systems. The computational and experimental data confirms that the MnMoO4 crystallized in ß-form with α-MnMoO4 type whereas Co and Ni cations crystallized in α-form with α-CoMoO4 type structure. Among the various transition metal cations studied, hybrid device comprising NiMoO4 vs activated carbon exhibited excellent electrochemical performance having the specific capacitance 82 F g-1 at a current density of 0.1 A g-1 but the cycling stability needed to be significantly improved. The specific capacitance of the NiMoO4 electrode material is shown to be directly related to the surface area of the electrode/electrolyte interface, but the CoMoO4 and MnMoO4 favored a bulk formation that could be suitable for structural stability. The useful insights from the electronic structure analysis and effective mass have been provided to demonstrate the role of cations in the molybdate structure and its influence in electrochemical energy storage. With improved cycling stability, NiMoO4 can be suitable for renewable energy storage. Overall, this study will enable the development of next generation molybdate materials with multiple cation substitution resulting in better cycling stability and higher specific capacitance.

Uranium sorption on various forms of titanium dioxide--influence of surface area, surface charge, and impurities.

Titanium dioxide (TiO(2)) has often served as a model substrate for experimental sorption studies of environmental contaminants. However, various forms of Ti-oxide have been used, and the different sorption properties of these materials have not been thoroughly studied. We investigated uranium sorption on some thoroughly characterized TiO(2) surfaces with particular attention to the influence of surface area, surface charge, and impurities. The sorption of U(VI) differed significantly between samples. Aggressive pretreatment of one material to remove impurities significantly altered the isoelectric point, determined by an electroacoustic method, but did not significantly impact U sorption. Differences in sorption properties between the various TiO(2) materials were related to the crystallographic form, morphology, surface area, and grain size, rather than to surface impurities or surface charge. In-situ attenuated total reflection Fourier-transform infrared (ATR FT-IR) spectroscopic studies showed that the spectra of the surface species of the TiO(2) samples are not significantly different, suggesting the formation of similar surface complexes. The data provide insights into the effect of different source materials and surface properties on radionuclide sorption.