An X-ray Spectral Study of the Electronic Structure of Uranyl Fluoride UO<Subscript>2</Subscript>F<Subscript>2</Subscript>

The fine structure of the X-ray photoelectron spectrum of low-energy electrons and O _4,5(U) X-ray emission spectrum of uranyl fluoride UO₂F₂ (at binding energies from 0 to 40 eV) was identified on the basis of the electronic structure calculations for the [(UO₂)F₆]⁴⁻ cluster with D _6h point symmetry, simulating the nearest surrounding of uranium in UO₂F₂, by the relativistic X _α discrete variation method. It was shown that the U5f electrons can directly participate in chemical bonding, and U6p electrons, in formation of not only inner valence but also outer valence molecular orbitals. The sequence of the inner valence molecular orbitals in the binding energy region from 12 to 40 eV was established, which is essential for development of a procedure for determining the uranium-ligand bond length in the axial direction and in the equatorial plane of the uranyl compounds, based on the characteristics of their X-ray photoelectron spectra.__________Translated from Radiokhimiya, Vol. 47, No. 4, 2005, pp. 305-314.Original Russian Text Copyright © 2005 by Utkin, Yu. Teterin, Terekhov, Ryzhkov, A. Teterin, Vukvcevic.     

Dissolution of UO<Subscript>2</Subscript> in the N<Subscript>2</Subscript>O<Subscript>4</Subscript>-H<Subscript>2</Subscript>O system

The kinetics of the UO₂ dissolution in the N₂O₄-H₂O system was studied. At 25°C, the process is kinetically controlled, whereas at 55°C the process occurs initially under kinetic control (3 min) and then under diffusion-kinetic control. At 80°C, the process occurs exclusively under diffusion-kinetic control. The apparent activation energy was estimated at ∼39 kJ mol⁻¹.     

High-temperature crystal chemistry of Na<Subscript>6</Subscript>(UO<Subscript>2</Subscript>)<Subscript>2</Subscript>O(MoO<Subscript>4</Subscript>)<Subscript>4</Subscript>

Thermal deformations of Na₆(UO₂)₂O(MoO₄)₄ were studied by high-temperature powder X-ray diffraction. The compound crystallizes in the triclinic system, space group Р\(\bar 1\), a = 7.636(7), b = 8.163(6), c = 8.746(4) Å, α = 72.32(9)°, β = 79.36(4)°, γ = 65.79(5)°, V = 472.74(4) ų. It is stable in the temperature interval 20–700°С. The thermal expansion coefficients (TECs) are α₁₁ = 25.5 × 10^–6, α₂₂ = 7.8 × 10^–6, and α₃₃ = 1.1 × 10^–6 (°C)^–1. The orientation of the TEC pattern relative to the crystallographic axes is a₃₃^Z = 45°, a₃₃^X = 122°, a₂₂^Z = 59°, and a₂₂^X = 66°. The anisotropy of the thermal expansion is due to specific features of the crystal structure of the compound.     

Nature of the Chemical Bond in Uranium Dioxide UO<Subscript>2</Subscript>

Fine structure of the X-ray photoelectron and conversion spectra of low-energy (0–40 eV) electrons of uranium dioxide UO₂ was analyzed based on the electronic structure calculations for the UO ₈ ¹²⁻ cluster with O _h symmetry, simulating the nearest surrounding of uranium in UO₂, by the relativistic X _α discrete variation method. It was predicted theoretically and validated experimentally that, in UO₂, the U5f electrons (∼1 U5f electron) can directly participate in chemical bonding: ∼2 U5f electrons weakly contributing to chemical bonding are localized at −1.9 eV; ∼1 U5f electron participating in chemical bonding is delocalized in the range of outer valence molecular orbital energies from −4 to −9 eV; and unfilled U5f states are localized mostly at low (from 0 to 5 eV above zero) energies. It was shown experimentally that the U6p electrons actively participate in formation of not only inner valence but also outer valence (0.6 U6p electron) molecular orbitals. The density of the U6p states in UO₂ was estimated experimentally. The composition and sequence of the inner valence molecular orbitals at energies within 13–40 eV were also elucidated.__________Translated from Radiokhimiya, Vol. 47, No. 3, 2005, pp. 193–202.Original Russian Text Copyright © 2005 by Yu. Teterin, Maslakov, Ryzhkov, Traparic, Vukcevic, A. Teterin, Panov.     

Synthesis and crystal structure of [UO<Subscript>2</Subscript>(OH)(CO(NH<Subscript>2</Subscript>)<Subscript>2</Subscript>)<Subscript>3</Subscript>]<Subscript>2</Subscript>(ClO<Subscript>4</Subscript>)<Subscript>2</Subscript>

The complex [UO₂(OH)(CO(NH₂)₂)₃]₂(ClO₄)₂ (I) was synthesized. A single crystal X-ray diffraction study showed that compound I crystallizes in the triclinic system with the unit cell parameters a = 7.1410(2), b = 10.1097(2), c = 11.0240(4) Å, α = 104.648(1)°, β = 103.088(1)°, γ = 108.549(1)°, space group \(P\bar 1\), Z = 1, R = 0.0193. The uranium-containing structural units of the crystals are binuclear groups [UO₂(OH)· (CO(NH₂)₂)₃] ₂ ²⁺ belonging to crystal-chemical group AM²M ₃ ¹ [A = UO ₂ ²⁺ , M² = OH^?, M¹ = CO(NH₂)₂] of uranyl complexes. The crystal-chemical analysis of nonvalent interactions using the method of molecular Voronoi-Dirichlet polyhedra was performed, and the IR spectra of crystals of I were analyzed.     

Phase transitions and high-temperature crystal chemistry of polymorphous modifications of Cs<Subscript>2</Subscript>(UO<Subscript>2</Subscript>)<Subscript>2</Subscript>(MoO<Subscript>4</Subscript>)

Phase transitions and thermal deformations of - and -Cs₂(UO₂)₂(MoO₄)₃ were studied by high-temperature X-ray diffraction analysis. In heating of -Cs₂(UO₂)₂(MoO₄)₃ to 625 ± 25°C, the reconstructive phase transition proceeds. -Cs₂(UO₂)₂(MoO₄)₃ is stable up to 700 ±25°C. The thermal expansion of both phases is sharply anisotropic: ₁₁ = 10 × 10^–6, ₂₂ = 33 × 10^–6, ₃₃ = 10 × 10^–6, _V = 53 × 10^–6 deg^–1 for -Cs(UO₂)₂(MoO₄)₃ and ₁₁ = 13 × 10^–6, ₃₃ = 3 × 10^–6, _V = 31 × 10^–6 deg^–1 for -Cs₂ (UO₂)₂ (MoO₄)₃. The anisotropy of thermal expansion is explained by features of the crystal structure of the compounds.Translated from Radiokhimiya, Vol. 46, No. 5, 2004, pp. 405–407.Original Russian Text Copyright © 2004 by Nazarchuk, Krivovichev, Filatov.     

Crystal structure of [CH<Subscript>3</Subscript>NH<Subscript>3</Subscript>][(UO<Subscript>2</Subscript>)(H<Subscript>2</Subscript>AsO<Subscript>4</Subscript>)<Subscript>3</Subscript>]

The crystal structure of a previously unknown compound [CH₃NH₃][(UO₂)(H₂AsO₄)₃] was solved by direct methods and refined to R ₁ = 0.038 for 3041 reflections with |F _hkl | >-4σ |F _hkl |. The compound crystallizes in the monoclinic system, space group P2₁/c, a = 8.980(1), b = 21.767(2), c = 7.867(1) Å, β = 115.919(5)°, V = 1383.1(3) ų, Z = 4. In the structure of the compound, pentagonal bipyramids of uranyl ions, sharing bridging atoms with tetrahedral [H₂AsO₄]^? anions, form strongly corrugated layered complexes [(UO₂)(H₂AsO₄)₃]^? arranged parallel to the (100) plane. The protonated methylamine molecules [CH₃NH₃]⁺ form unidimensional tapelike packings parallel to the c axis and linked by hydrophilic-hydro-phobic interactions. The topology of the layered uranyl arsenate complex [(UO₂)(H₂AsO₄)₃]^? is unusual for uranyl compounds and was not observed previously. A specific feature of this topology is the presence of monodentate arsenate “branches” arranged within the layer.     

Partially ordered organic-inorganic nanocomposites in the system UO<Subscript>2</Subscript>SeO<Subscript>4</Subscript>-H<Subscript>2</Subscript>O-NH<Subscript>3</Subscript>(CH<Subscript>2</Subscript>)<Subscript>9</Subscript>NH<Subscript>3</Subscript>

The compound [NH₃(CH₂)₉NH₃]₂[(UO₂)₃(SeO₄)₅(H₂O)₂](H₂O)_x (1) was prepared by isothermal evaporation from aqueous uranyl selenate solutions containing 1,9-diaminononane. A structural study showed that the compound is a partially ordered organic-inorganic nanocomposite. The structural model of the inorganic complex was determined by single crystal X-ray diffraction a = 19.5572(5), c = 47.878(2) Å, V= 15859.1(9) ų, Z= 12; R₁ = 0.1318, wR₂ = 0.3186 for 2808 reflections with |F_o| ≥ 4σ_F). The structure consists of double hydrogen-bonded [(UO₂)₃(SeO₄)₅(H₂O)₂]^2- layers parallel to the (001) plane. The disordered protonated 1,9-diaminononane molecules and water molecules are arranged between the layers. The inorganic layered complex [(UO₂)₃(SeO₄)₅(H₂O)₂]^2- belongs to a new type that was not observed previously in the structures of inorganic and organometallic compounds.     

Thermodynamic simulation of phase equilibria in the UO<Subscript>2</Subscript>-Gd<Subscript>2</Subscript>O<Subscript>3</Subscript> system at high temperatures

Thermodynamic models of a UO₂-based melt and cubic solid solution were developed on the basis of the most accurate measurements of the UO₂-Gd₂O₃ liquidus in a range of 0–30 mol. % of Gd₂O₃. These models enabled computing the phase diagram of this system in a temperature range of 1900–3200 K for all composition varieties.     

Crystal structure of KNa<Subscript>3</Subscript>[(UO<Subscript>2</Subscript>)<Subscript>5</Subscript>O<Subscript>6</Subscript>(SO<Subscript>4</Subscript>)]

The crystal structure of a previously unknown compound KNa₃[(UO₂)₅O₆(SO₄)] [space group Pbca, a = 13.2855(15), b = 13.7258(18), c = 19.712(2) Å, V = 3594.6(7) ų] was solved by direct methods and refined to R ₁ = 0.055 for 3022 reflections with |F _hkl | ≥ 4σ |F _hkl |. In the structure there are five sym-metrically nonequivalent uranyl cations. They are linked by cationcation (CC) interactions to form a pentamer whose central cation is U(2)O ₂ ²⁺ forming two three-centered CC bonds. All the uranyl ions are coordinated in the equatorial plane by five O atoms, which leads to the formation of pentagonal bipyramids sharing common edges to form layers parallel to the (100) plane. The sulfate tetrahedron links the uranyl layers into a 3D framework. The K⁺ and Na⁺ cations are arranged in framework voids. A brief review of CC interactions in U(VI) compounds is presented.     

Crystal and molecular structure of uranyl acetylacetonate dimer, [UO<Subscript>2</Subscript>(C<Subscript>5</Subscript>H<Subscript>7</Subscript>O<Subscript>2</Subscript>)<Subscript>2</Subscript>]<Subscript>2</Subscript>

The crystal and molecular structure of uranyl acetylacetonate dimer was determined by single crystal X-ray diffraction. The compound crystallizes in the tetragonal system, a = 7.9420(2), c = 40.1240(13) Å (at 100 K), Z = 4, space group P4₁2₁2. Dimeric uranyl acetylacetonate molecules in the crystal are formed by bridging bonding of one of O atoms of the acetylacetonate ligands with U atoms, so that the coordination polyhedra of U atoms (distorted pentagonal bipyramids) share a common equatorial edge. The dimer has a nonplanar structure, being significantly bent along the conventional line connecting the bridging O atoms.     

Behavior of UO<Subscript>2</Subscript> in a room-temperature ionic liquid in the presence of AlCl<Subscript>3</Subscript>

A study was made of interaction between AlCl₃ and room-temperature ionic liquid (RTIL) [C₈H₁₅N₂][N(SO₂CF₃)₂], or BuEtIm-Tf₂N, and of anodic dissolution in RTIL of UO₂ and of a simulated oxide fuel at 297–302 K, depending on the AlCl₃ concentration. It was shown that anodic dissolution of UO₂ pellets and a UO₂-Al mixture in RTIL yields soluble uranium species. Potentiostatic electrolysis of the resulting solutions can yield uranium compounds at the cathode, though with low current efficiencies. The role of AlCl₃ in these processes was suggested. A heterophase reaction between UO₂ and AlCl₃ was studied, depending on the content of AlCl₃ in solution. It was found that the exchange reaction products, soluble uranium species, are accumulated in solution only at the molar ratio AlCl₃/Tf₂N > 1.Translated from Radiokhimiya, Vol. 46, No. 6, 2004, pp. 536–539.Original Russian Text Copyright © 2004 by Smolenskii, Bove, Borodina, Bychkov, Osipenko.     

Synthesis and crystal structure of Cu<Subscript>2</Subscript>{(UO<Subscript>2</Subscript>)<Subscript>3</Subscript>[(S,Cr)O<Subscript>4</Subscript>]<Subscript>5</Subscript>}(H<Subscript>2</Subscript>O)<Subscript>17</Subscript>

Cu₂{(UO₂)₃[(S,Cr)O₄]₅}(H₂O)₁₇ crystals were prepared by evaporation of aqueous solutions. The crystal structure was solved by the direct method and refined to R ₁ = 0.064 (wR ₂ = 0.177) for 8120 reflections with ¦F _hkl¦ 4 ¦F _hkl¦. Rhombic system, space group Pbca, a = 18.0586(8), b = 19.9898(9), c = 20.5553(8) Å, V = 7420.2(6) ų. The structure is based on {(UO₂)₃[(S,Cr)O₄]₅}^4– anionic layers, formed by combination of UO₇ pentagonal bipyramids and TO₄ tetrahedra through common vertices. The { (UO₂)₃ [(S,Cr)O₄]₅}^4– layers are parallel to the (010) plane. The Cu²⁺ (H₂O)₆ octahedra and additional water molecules are located in the interplanar space and provide binding of the layers in the structure by hydrogen bonds. Based on the occupancy of tetrahedral positions, more accurate chemical formula of the compound should be written as Cu₂{(UO₂)₃[(S_0.804 Cr_0.196)O₄]₅} (H₂O)₁₇.Translated from Radiokhimiya, Vol. 46, No. 5, 2004, pp. 408–411.Original Russian Text Copyright © 2004 by Krivovichev, Burns.     

X-Ray luminescence of polycrystalline TbO<Subscript>2</Subscript>-doped Li<Subscript>2</Subscript>B<Subscript>4</Subscript>O<Subscript>7</Subscript>

This paper examines the effect of doping level on the X-ray luminescence of TbO₂-doped polycrystalline lithium tetraborate. It is shown that, when interpreting such spectra, it is convenient to proceed from the terms of free activator and constituent ions. We demonstrate that the emission lines of Tb³⁺ in doped polycrystalline lithium tetraborate are effectively excited in the band between 350 and 650 nm, which is predominantly due to electron transitions from the ⁵ D ₃ and ⁵ D ₄ excited states to spin-orbital levels of the ⁷ F _J ground multiplet. The emission lines of lithium and boron in single-crystal and polycrystalline undoped lithium tetraborate are effectively excited in the band between 274 and 550 nm.     

Synthesis and structure of (NH<Subscript>4</Subscript>)<Subscript>3</Subscript>[UO<Subscript>2</Subscript>(CH<Subscript>3</Subscript>COO)<Subscript>3</Subscript>]<Subscript>2</Subscript>(NCS)

The compound (NH₄)₃[UO₂(CH₃COO)₃]₂(NCS) (I) was synthesized and examined by single crystal X-ray diffraction analysis. The compound crystallizes in the rhombic system with the unit cell parameters a = 11.5546(4), b = 18.5548(7), c = 6.7222(3) Å, V = 1441.19(10) ų, space group P2₁2₁2, Z = 2, R = 0.0345. The uranium-containing structural units of crystals of I are isolated mononuclear groups [UO₂(CH₃COO)₃]^? belonging to crystal-chemical group AB ₃ ⁰¹ (A = UO ₂ ²⁺ , B⁰¹ = CH₃COO^?) of uranyl complexes. The specific features of packing of the uranium-containing complexes in the crystal structure are considered.     

Investigation of the Thermodynamic and Electronic Properties of Double Perovskite Ca<Subscript>2</Subscript>CoNbO<Subscript>6</Subscript>

In the present work, a self-consistent ab initio calculation using the full-potential linearized augmented plane wave (FP-LAPW) method within the framework of the spin-polarized density functional theory (DFT) was used to study the structural, electronic, and thermodynamic properties of Ca₂CoNbO₆ double perovskite compound. The generalized gradient approximation (GGA) described by Perdew–Burke–Ernzerhof (PBE) and GGA + U were used. The results obtained for the electronic properties show a ferrimagnetic and half-metallic behavior of the compound. The novelty of our work is the study of the thermodynamic properties of Ca₂CoNbO₆ double perovskite such as heat capacity and Debye temperature which showed an important effect of pressure compared to the temperature.     

Electronic Structure of Superoxygenated La<Subscript>2</Subscript>NiO<Subscript>4</Subscript> Domains with Ordered Oxygen Interstitials

The electronic structures of La₂NiO_4+δ , where additional interstitial oxygens are forming stripes along (1,1,0), are presented. Spin-polarized calculations show that ferromagnetism on Ni sites is reduced near the stripes and enhanced far from the stripes. Totally, the magnetic moment becomes reduced because of interstitial oxygens. It is suggested that the oxygen interstitial concentration in oxygen-rich domains in nickelates suppress magnetism and give multiband metallic domains.     

Structure of multilayer ZrO<Subscript>2</Subscript>/SrTiO<Subscript>3</Subscript>

Multilayered oxide heteroepitaxial systems, including that of a 1-nm-thick Y₂O₃-stabilised ZrO₂ (YSZ) sandwiched between layers of SrTiO₃ (STO) [1], have been a subject of much interest lately due to their significantly enhanced ionic conductivities as compared to the bulk materials. We aim to provide the foundation for understanding this increase in conductivity by considering the atomic configurations at the interfaces of such systems, specifically a ZrO₂/STO multilayer system. Possible stable lattice structures of pure ZrO₂ in the system are explored using a genetic algorithm in which the interatomic interactions are modelled by simple pair potentials. The energies of several of the more stable of these structures are then evaluated more accurately within density functional theory (DFT). We find that the fluorite ZrO₂ phase is unstable as a coherently strained epitaxial layer in the multilayer system. Instead, anatase-, columbite-, rutile-, and pyrite-like ZrO₂ epitaxies are found to be more stable, with the anatase-like epitaxy being the most stable structure over a wide range of chemical potential of the components. We also find a high energy metastable structure resembling the tetragonal fluorite structure which is predicted by DFT to be stabilised by SrO-terminated STO but not by TiO₂-terminated STO.     

Synthesis and study of uranyl phosphates (UO<Subscript>2</Subscript>)<Subscript>3</Subscript>(PO<Subscript>4</Subscript>)<Subscript>2</Subscript>·<Emphasis Type="Italic">n</Emphasis>H<Subscript>2</Subscript>O

Uranyl phosphate (UO₂)₃(PO₄)₂·8H₂O was synthezied. Its dehydration was studied by X-ray diffraction, IR spectroscopy, and thermal and chemical analysis. The dehydration products were isolated and characterized by X-ray diffraction and IR spectroscopy. Their structural features were determined.     

Structure of the double formate KNpO<Subscript>2</Subscript>(OOCH)<Subscript>2</Subscript>

KNpO₂(OOCH)₂ was isolated from neutral Np(V) solutions with a high concentration of potassium formate. The crystal structure of this compound was determined. The structure consists of infinite anionic chains [NpO₂(OOCH)₂] _n ^n? . Potassium cations are located between these chains. The Np coordination polyhedron is a hexagonal bipyramid whose equatorial plane is formed by the oxygen atoms of four HCOO^? ions. The Np bipyramids in the chains are bound via common equatorial edges. The anionic chains in the structures of KNpO₂(OOCH)₂ and NH₄NpO₂(OOCH)₂ studied previously have similar composition but different structure.