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.     

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.     

Synthesis of New Crystalline Pu(V) Compounds from Solutions: II. Double Pu(V) Oxalates with Co(NH<Subscript>3</Subscript>)<Subscript>6</Subscript><Superscript>3+</Superscript> Ions in the Outer Sphere

Crystalline compounds of the general composition Co(NH³)⁶PuO₂(C₂O₄)²·nH₂O (n = 2, 3, 5) were isolated from freshly prepared neutral oxalate solutions of Pu(V) by addition of Co(NH₃)₆³⁺ ions. These compounds are fairly stable in storage in air and poorly soluble in water. Previously unknown double Np(V) oxalates Co(NH₃)₆NpO₂(C₂O₄)·nH₂O (n = 2, 5) were also synthesized and studied. All the compounds of Pu(V) and Np(V) of the same composition are mutually isostructural. The behavior of these compounds at heating was studied, and their IR spectra were measured. The optical spectra of new Np(V) compounds were measured.     

Oxidation of Np(V) with Xenon Trioxide in an HClO<Subscript>4</Subscript> Solution

Oxidation of Np(V) to Np(VI) with xenon trioxide in a 0.5–1.4 M HClO₄ solution was studied by spectrophotometry. The reaction rate is described by the equation–d[Np(V)]/dt = k[Np(V)][XeO₃], where k = 4.6 × 10^–3 L mol^–1 s^–1 in 1 M HClO₄ at 92°С. The activation energy is close to 92 kJ mol^–1. The activated complex is formed in contact of NpO ₂ ⁺ and ХеО₃ without participation of Н⁺ ions. The activated complex transforms into NpO ₂ ²⁺ and the products: ОН, Хе, and О₂. The ОН radical oxidizes Np(V). Admixtures of Со²⁺ and especially Fe³⁺ accelerate the Np(V) oxidation.     

Low temperature sintering and microwave dielectric properties of Zn<Subscript>3</Subscript>Mo<Subscript>2</Subscript>O<Subscript>9</Subscript> ceramic

Crystal structure and dielectric properties of Zn₃Mo₂O₉ ceramics prepared through a conventional solid-state reaction method were characterized. XRD and Raman analysis revealed that the Zn₃Mo₂O₉ crystallized in a monoclinic crystal structure and reminded stable up to1020 °C. Dense ceramics with high relative density (~ 92.3%) were obtained when sintered at 1000 °C and possessed good microwave dielectric properties with a relative permittivity (ε _r ) of 8.7, a quality factor (Q?×?f) of 23,400 GHz, and a negative temperature coefficient of resonance frequency (τ _f ) of around ??79 ppm/°C. With 5 wt% B₂O₃ addition, the sintering temperature of Zn₃Mo₂O₉ ceramic was successfully lowered to 900 °C and microwave dielectric properties with ε _r ?=?11.8, Q?×?f?=?20,000 GHz, and τ _f = ??79.5 ppm/°C were achieved.     

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.     

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.     

Electric-field effect on crystal growth in the Li<Subscript>3</Subscript>PO<Subscript>4</Subscript>-Li<Subscript>4</Subscript>GeO<Subscript>4</Subscript>-Li<Subscript>2</Subscript>MoO<Subscript>4</Subscript>-LiF system

We have studied the electric-field effect on crystallization processes in the Li₃PO₄-Li₄GeO₄-Li₂MoO₄-LiF system. In zero field, Li_3+x P_1?x Ge _x O₄ (x = 0.31) crystals were grown on the cathode under the conditions of this study. At low applied voltages (≤ 0.5 V), we obtained Li₂MoO₄, Li₂GeO₃, and Li_1.3Mo₃O₈. In the range V = 0.5–1 V, crystals of Li_3+x P_1?x Ge _x O₄ solid solutions with x = 0.17, 0.25, 0.28, 0.29, and 0.36 were obtained. An applied electric field was shown to reduce the melting temperature of the starting mixtures and the crystallization onset temperature.     

Surface composition and morphology of a carbon matrix/Mo<Subscript>2</Subscript>C composite material

The surface composition and morphology of a carbon matrix/Mo₂C composite have been studied by scanning electron microscopy and X-ray photoelectron spectroscopy. The results demonstrate that the carbon matrix has the form of entangled filaments of carbon nanotubes. The surface layer of the composite contains 57 carbon atoms per molybdenum atom. Molybdenum is present here in the form of the Mo₂C carbide (39 at %) and the Mo₂O₅ (50 at %) and MoO₂ (11 at %) oxides, with E _b(Mo 3d _5/2) = 228.2, 229.6, and 231.9 eV, respectively. The presence of the Mo⁴⁺ and Mo⁵⁺ oxides in the surface layer is due to active reaction of the Mo₂C in the composite with atmospheric oxygen and moisture during the sample preparation process and can be accounted for by the small particle size of the material. Based on analysis of the structure of the C 2s and C 2p valence electron spectra, we assume that the carbon nanotubes of the composite are graphitelike carbon structures. The composite studied here does not become charged when exposed to an X-ray beam, which suggests that it is a weak dielectric.     

Stereochemistry of Neptunium in Oxygen-Containing Compounds

Coordination of Np(III–VII) atoms in the crystal structures of all the oxygen-containing compounds characterized with the R-factor lower than 0.1 was analyzed with the aid of Voronoi–Dirichlet polyhedra (VDPs). Nine types of NpO _n coordination polyhedra (6 ≤ n ≤ 12) are realized. The most characteristic of them are trigonal dodecahedra [Np(IV)], penta- and hexagonal bipyramids [Np(V) and Np(VI)], and octahedra [Np(VII)] based on square NpO₄^– cores. For Np atoms of a fixed oxidation state, the volume of their VDPs in the NpO _n complexes is virtually independent of the coordination number n. The VDP parameters can be used for determining the valence state of the Np atoms, finding compounds with the maximal nonlinearity of the NpO₂⁺ and NpO₂²⁺ dioxocations, and revealing errors in the crystal structure data. Anion–anion interactions involving NpO₄^– and OH^– ions are an important structure-forming factor in Np(VII) compounds. In sublattices consisting of Np atoms only (Np sublattices), the rule of 14 neighbors is fulfilled. Compounds in which binding Np···Np 5f interactions in crystal structures are possible were revealed by analysis of the VDPs of the atoms in the Np sublattices. In such compounds, the metal atoms form bent Np=O–Np bridging fragments and the Np^VO₇ bipyramids are combined in dimers sharing a common axial edge, with the Np atoms of the dimers being also bound via two carboxylate bridges.     

Electrical and thermodynamic properties of Cs<Subscript>0.97</Subscript> Rb<Subscript>0.03</Subscript>H<Subscript>2</Subscript>PO<Subscript>4</Subscript>

The transport properties of Cs_0.97Rb_0.03H₂PO₄ have been studied using polycrystalline samples and single crystals. The mixed salt is isostructural with cesium dihydrogen phosphate and has slightly smaller unitcell parameters. The cation substitution increases the low-temperature ionic conductivity of the material by about two orders of magnitude but has an insignificant effect on the conductivity of the high-temperature phase. The low-temperature conductivity of single-crystal samples exhibits significant anisotropy, with σ _a < σ _b±c . The conductivity of the polycrystalline material is close to σ _b±c . The substitution reduces the temperature of the superionic phase transition by 20°C and enhances the thermal stability of the high-temperature phase at low humidity (1 mol % H₂O).     

Compounds WAl<Subscript>4</Subscript>, WAl<Subscript>3</Subscript>, W<Subscript>3</Subscript>Al<Subscript>7</Subscript>, and WAl<Subscript>2</Subscript> and Al-W-N combustion products

X-ray diffraction data are presented for combustion products in the Al-W-N system. New, nonequilibrium intermetallic compounds have been identified, their diffraction patterns have been indexed, and their unit-cell parameters have been determined. The phases α-and β-WAl₄ are shown to exist in three isomorphous forms, differing in unit-cell centering. The phases α′-, α″-, and α?-WAl₄ are monoclinic, with a ₀ = 5.272 Å, b ₀ = 17.770 Å, c ₀ = 5.218 Å, β = 100.10°; point groups C12/c1, A12/n1, I12/a1, respectively. The phases β′-, β″-, and β?-WAl₄ are monoclinic, with a ₀ = 5.465 Å, b ₀ = 12.814 Å, c ₀ = 5.428 Å, β = 105.92°; point groups A112/m, B112/m, I112/m, respectively. The compounds WAl₂ and W₃Al₇, identified each in two isomorphous forms, differ in cell metrics (doubling) but possess the same point group: P222. WAl ₂ ^′ : orthorhombic, a ₀ = 5.793 Å, b ₀ = 3.740 Å, c ₀ = 6.852 Å. WAl ₂ ^″ : orthorhombic, a ₀ = 11.586 Å, b ₀ = 3.740 Å, c ₀ = 6.852 Å. W₃Al ₇ ^′ : orthorhombic, Pmm2, a ₀ = 6.225 Å, b ₀ = 4.806 Å, c ₀ = 4.437 Å. W₃Al ₇ ^″ : orthorhombic, Pmm2, a ₀ = 12.500 Å, b ₀ = 4.806 Å, c ₀ = 8.874 Å. The new phase WAl₃: triclinic, P1, a ₀ = 8.642 Å, b ₀ = 10.872 Å, c ₀ = 5.478 Å, α = 104.02°, β = 64.90°, γ = 107.15°.     

Crystallization from high-temperature solutions in the K<Subscript>2</Subscript>O-P<Subscript>2</Subscript>O<Subscript>5</Subscript>-V<Subscript>2</Subscript>O<Subscript>5</Subscript>-Bi<Subscript>2</Subscript>O<Subscript>3</Subscript> system

We have studied general trends of crystallization from high-temperature solutions in the K₂O-P₂O₅-V₂O₅-Bi₂O₃ system at P/V = 0.5?2.0, K/(P + V) = 0.7?1.4, and Bi₂O₃ contents from 25 to 50 wt % and identified the stability regions of BiPO₄, K₃Bi₅(PO₄)₆, K₂Bi₃O(PO₄)₃, and K₃Bi₂(PO₄)_{3 ? x} (VO₄) _x (x = 0?3) solid solutions. The synthesized compounds have been characterized by X-ray powder diffraction and IR spectroscopy, and the structure of two solid solutions has been determined by single-crystal X-ray diffraction (sp. gr. C ₂/c): K₃Bi₂(PO₄)₂(VO₄), a = 13.8857(8), b = 13.5432(5), c = 6.8679(4) Å, β = 114.031(7)°; K₃Bi₂(PO₄)_1.25(VO₄)_1.75, a = 13.907(4), b = 13.615(2), c = 6.956(2) Å, β = 113.52(4)°.     

Electric field effect on the short-range order in amorphous Bi<Subscript>2</Subscript>S<Subscript>3</Subscript> films

The short-range order in thin amorphous Bi₂S₃ films grown under ordinary conditions and in an applied electric field of 3000 V/cm has been studied. The interatomic distances obtained are r ₁ = 0.23 nm, r ₂ = 0.345 nm, and r ₃ = 0.45 nm, and the corresponding coordination numbers are n ₁ = 4, n ₂ = 6, and n ₃ = 5.88. The coordination numbers and the ratios r ₃/r ₁ = 1.96 and r ₂/r ₁ = 1.5 indicate that the atoms in amorphous Bi₂S₃ are in tetrahedral and octahedral coordination. In the films grown in an electric field, the near neighbor distances are slightly shorter.     

X-Ray Spectral Methods in Investigation of the UO<Subscript>2</Subscript> Electronic Structure

Fine structure of the X-ray photoelectron spectrum of UO₂ at electron binding energies from 0 to ～40 eV is primarily due to electrons of outer (0–15 eV) and inner (15–40 eV) valence molecular orbitals formed from unoccupied U5f, 6d, 7s and O2p and occupied low-energy U6p and O2s shells of the neighboring uranium and oxygen atoms, respectively. This is consistent with the results of the relativistic calculation of the electronic structure of the UO ₈ ^12? cluster with O _h symmetry, simulating the nearest surrounding of uranium in UO₂, and is confirmed by the data of X-ray spectroscopy (conversion electron, nonresonance and resonance X-ray O_4,5(U) emission, O_4,5(U) XANES, photoelectron resonance, and Auger spectroscopy of oxygen). The fine structure of the X-ray photoelectron spectra, associated with electrons from outer valence and inner valence molecular orbitals, allows estimation of the degree of participation of U6p, 5f electrons in chemical bonding, as well as the structure of the nearest surrounding of the uranium atom and the bond length in its oxides. The total contribution from electrons of inner valence molecular orbitals to the absolute value of the chemical bonding energy can be compared with the corresponding contribution of the electrons from outer valence molecular orbitals to bonding of the atoms. Inner valence molecular orbitals can be formed in compounds of any elements, and this is an important new fact in chemistry and physics of condensed state.     

Phase relations in the Ag<Subscript>8</Subscript>SnS<Subscript>6</Subscript>-Ag<Subscript>2</Subscript>SnS<Subscript>3</Subscript>-AgBr system and crystal structure of Ag<Subscript>6</Subscript>SnS<Subscript>4</Subscript>Br<Subscript>2</Subscript>

The T-x phase diagram of the Ag-Sn-S-Br system has been studied in the composition region Ag₈SnS₆-Ag₂SnS₃-AgBr, and a compound of composition Ag₆SnS₄Br₂ has been identified. Ag₆SnS₄Br₂ has a new structure, closely related to that of Ag₆GeS₄Br₂: sp. gr. Pnma, a = 6.67050(10), b = 7.82095(9), c = 23.1404(3) Å, Z = 4, R _B = 0.0519, R _wp = 0.0782, χ² = 1.36.     

Interaction of neptunium and technetium with UO<Subscript>2+<Emphasis Type="Italic">x</Emphasis></Subscript>

Interaction of uranium dioxide with highly mobile radionuclides ²³⁷Np and ⁹⁹Tc was studied under oxidative conditions. Sorption of these radionuclides at different pH was measured, and the mechanism of redox reaction occurring in the course of their sorption were determined. In alkaline solution, Np(V) is reduced on the UO_2+x surface and is sorbed in the form of tetravalent species. In neutral solutions, Np is sorbed in the form of Np(V). This is due to the fact that the stoichiometry of the UO_2+x surface corresponds to U₄O₉. In acid solution, U(VI) is leached to form surface UO₂. Although the free surface area of a UO_2+x sample is low, the Np distribution coefficients K _d at pH > 6 are relatively high: log K _d > 2. Unlike Np, Tc(VII) is not reduced on the UO_2+x surface. However, the sorption capacity of uranium dioxide for Tc(IV) is high.     

Solid-state reactions in the system Li<Subscript>2</Subscript>CO<Subscript>3</Subscript>-Na<Subscript>2</Subscript>CO<Subscript>3</Subscript>-Nb<Subscript>2</Subscript>O<Subscript>5</Subscript>-Ta<Subscript>2</Subscript>O<Subscript>5</Subscript>

The formation mechanisms of Li _x Na_{1 ?x} Ta _y Nb_{1 ? y} O₃ perovskite solid solutions in the Li₂CO₃-Na₂CO₃-Nb₂O₅-Ta₂O₅ system have been studied by x-ray diffraction, differential thermal analysis, thermogravimetry, IR spectroscopy, and mass spectrometry at temperatures from 300 to 1100°C. The results indicate that the synthesis of Li _x Na_{1 ? x} Ta _y Nb_{1 ? y} O₃ solid solutions involves a complex sequence of consecutive and parallel solid-state reactions. An optimized synthesis procedure for Li _x Na_{1 ? x} Ta _y Nb_{1 ? y} O₃ solid solutions is proposed.     

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.