Behavior of Np(VI) and Np(V) Ions in NaHCO<Subscript>3</Subscript> Solutions Containing H<Subscript>2</Subscript>O<Subscript>2</Subscript>

The behavior of Np(VI) and Np(V) in NaHCO₃ and NaHCO₃ + Na₂CO₃ solutions containing H₂O₂ was studied spectrophotometrically. In 0.75–1.0 M NaHCO₃, hydrogen peroxide oxidizes Np(V) to Np(VI). The kinetics curves of Np(V) oxidation into Np(VI) have a complex shape and are characterized either by an induction period of up to tens of minutes or by a period of steady-state Np(VI) concentration, followed by an increase in the Np(VI) concentration. When Np(VI) initially exists in the solution, the induction period is lacking. The process character changes when the bicarbonate concentration decreases, or when Na₂CO₃ is added. In 1.0 M Na₂CO₃, 0.5 M NaHCO₃ + 0.5 M Na₂CO₃, or 0.01–0.5 M NaHCO₃, hydrogen peroxide completely reduces Np(VI) into Np(V). The probable mechanisms of this process were discussed. Accumulation of Np(VI) in NaHCO₃ solutions can be accounted for by assuming that Np(VI) itself participates in the transformations. Initially, the reaction of Np(VI) with H₂O₂ yields the excited *Np(V) ion. Then it reacts with another H₂O₂ molecule and forms a carbonate-peroxide complex. In the collision of the latter with unexcited Np(V), two electrons from two Np(V) ions are transferred onto the O ₂ ^2? ligand with formation of two Np(VI) ions.     

Reduction of Np(V) with Fe(II) in weakly acidic solutions containing H<Subscript>2</Subscript>C<Subscript>2</Subscript>O<Subscript>4</Subscript>

The kinetics of the reaction of Np(V) with Fe(II) in dilute perchloric and nitric acid solutions containing H₂C₂O₄ was studied by spectrophotometry. In the range pH 1–2, the reaction rate is described by the equation d[Np(V)]/dt = k[Np(V)][Fe(II)][H₂C₂O₄]²[H⁺]^−1.6, k = 182 mol^−1.4 l^1.4 s⁻¹. The activation energy in the range 25–45°C is 26 kJ mol⁻¹. The reaction mechanism involves formation of Fe(II) and Np(V) oxalate complexes, followed by their reaction with the participation of the H⁺ ion.     

Oxidation of Np(IV) with hydrogen peroxide in carbonate solutions

Oxidation of Np(IV) with hydrogen peroxide in NaHCO₃-Na₂CO₃ solutions was studied by spectrophotometry. In NaHCO₃ solution, Np(IV) is oxidized to Np(V) and partially to Np(VI). It follows from the electronic absorption spectra that Np(IV) in 1 M Na₂CO₃ forms with H₂O₂ a mixed peroxide-carbonate complex. Its stability constant β is estimated at 25–30. The Np(IV) bound in the mixed complex disappears in a first-order reaction with respect to [Np(IV)]. The first-order rate constant k’ is proportional to [H₂O₂] in the H₂O₂ concentration range 2.5–11 mM, but further increase in [H₂O₂] leads to a decrease in k′. The bimolecular rate constant k = k′/[H₂O₂] in solutions containing up to 11 mM H₂O₂ increases in going from 1 M NaHCO₃ to 1 M Na₂CO₃ and significantly decreases with a further increase in the carbonate content. The activated complex is formed from Np(IV) peroxide-carbonate and carbonate complexes. Synchronous or successive electron transfer leads to the oxidation of Np(IV) to Np(V). Large excess of H₂O₂ oxidizes Np(V) to Np(VI), which is then slowly reduced. As a result, Np(V) is formed in carbonate solutions at any Np(IV) and H₂O₂ concentrations.     

Reaction of Np(VI) with cyclohexanediaminetetraacetic acid in HClO<Subscript>4</Subscript> solutions

The stoichiometry of the reaction of Np(VI) with cis-cyclohexanediaminetetraacetic acid (CHDTA, H4chdta) in 0.05 M HClO₄ solution was studied by spectrophotometry. With Np(VI) in excess, 1 mol of the complexone converts 4 mol of Np(VI) into Np(V). In 0.115–0.98 M HClO₄ solutions (the ionic strength of 1.0 was supported with LiClO₄) containing 3–29 mM CHDTA at 20–45°С, Np(VI) at a concentration of 0.2–3.3 mM is consumed in accordance with the first-order rate law until less than 40% of Np(VI) remains. After that, the reaction decelerates. The reaction rate has first order with respect to [CHDTA] and the order of–1.2 with respect to [H⁺]. The activated complex is formed with the loss of one and two Н⁺ ions. The activation energy is 82.3 ± 3.8 kJ mol^–1.     

Kinetics of Np(V) transformation in concentrated HNO<Subscript>3</Subscript> solutions containing potassium phosphotungstate K<Subscript>10</Subscript>P<Subscript>2</Subscript>W<Subscript>17</Subscript>O<Subscript>61</Subscript>

The behavior of Np(V) in concentrated HNO₃ solutions containing potassium phosphotungstate K₁₀P₂W₁₇O₆₁ (KPW) at various concentrations of HNO₃ (1.0–3.0 M) and KPW [(1–5) × 10^?3 M] was studied. Under the examined experimental conditions, the final products of Np(V) transformation are Np(IV) and Np(VI). The reaction follows a first-order rate equation with respect to the Np(V) concentration.     

Synthesis,crystal structure,and absorption spectra of a complex neptunium(v) chromate,Na<Subscript>3</Subscript>[NpO<Subscript>2</Subscript>(CrO<Subscript>4</Subscript>)<Subscript>2</Subscript>](H<Subscript>2</Subscript>O)<Subscript>5</Subscript>

A new Np(V) chromate complex with outer-sphere sodium cations, Na₃[NpO₂(CrO₄)₂](H₂O)₅ (I), was synthesized from aqueous solution. Its composition and structure were determined by single crystal X-ray diffraction. The structure of I is based on anionic chains of the composition [NpO₂(CrO4)2] _n ³ⁿ⁻, running along [010] and forming layers parallel to the (101) plane. The Na⁺ ions and water molecules of crystallization are arranged between the layers. The coordination polyhedra of the Np atoms (pentagonal bipyramids) are combined pairwise by sharing common equatorial edges formed by two bridging oxygen atoms of bidentate chelate-bridging CrO₄ groups. The absorption spectra of I in the IR and visible ranges are presented.     

Formation mechanism of NaNbO<Subscript>3</Subscript> powders during hydrothermal synthesis

Hydrothermal synthesis of NaNbO₃ fine powders was investigated, and the formation mechanism was revealed. The lowest temperature to form NaNbO₃ powders was about 140 °C. An intermediate hexaniobate, Na₈Nb₆O₁₉·13H₂O, was formed first before the precursors were eventually converted to the perovskite phase. The step of dissolving Nb₂O₅ powders in OH⁻ solution and forming Nb₆O₁₉ ⁸⁻ ion was very important to the synthesis of NaNbO₃. For [OH⁻] = 3.0 M, Nb₂O₅ had the highest yield. There was another dissolvable sodium niobate in the hydrothermal system, which was stable when [OH⁻] = 1.2 M. The reaction mechanism was in situ transformation. The reaction speed first increased then decreased with [OH⁻]. High [OH⁻] is not always favorable in preparing perovskite NaNbO₃, and there is an optimum [OH⁻].     

Formal oxidation potentials of actinide couples in K<Subscript>10</Subscript>P<Subscript>2</Subscript>W<Subscript>17</Subscript>O<Subscript>61</Subscript> solutions

The formal oxidation potentials of the M(VI)/M(V), M(V)/M(IV), and M(IV)/M(III) couples for actinides from U to No and of the M(IV)/M(III) couples for some actinides in 1 M H⁺ or 1 M Na⁺ (pH ～5–5.5) solutions containing K₁₀P₂W₁₇O₆₁ were calculated from the data on stability of complexes of f element ions with the unsaturated heteropolytungstate anion P₂W₁₇O ₆₁ ^10? . In some cases, the previously accepted values were subjected to major revision, especially the potentials of the An(V)/An(IV) couples. Problems arising in measuring the potentials of the couples involving Np(III) and Pu(III) which react with the heteropolyanion to form a heteropoly blue are discussed. The potentials of some M(III)/M(II) couples are estimated.     

Kinetics of Pu(V) disproportionation in CH<Subscript>3</Subscript>COOH-CH<Subscript>3</Subscript>COOLi aqueous solutions

The behavior of Pu(IV–VI) in CH₃COOH-CH₃COOLi solutions was studied by spectrophotometry. The Pu(VI) absorption spectrum changes essentially with an increase in the CH₃COOLi concentration. Owing to formation of Pu(VI) acetate complexes, the maximum of the main absorption band is shifted from 830.6 (in HClO₄ solution) to 845 nm, with the band intensity decreasing by a factor of approximately 8. The Pu(V) and Pu(IV) absorption spectra at low concentrations of acetate ions vary insignificantly relative to the spectra in noncomplexing media. With an increase in the acetate concentration in the system to 1–3 mM, the effect of Pu(V) complexation on its absorption spectrum becomes noticeable (the absorption intensity considerably decreases), whereas the Pu(IV) absorption spectra remain essentially unchanged. Solutions containing 1–2 mM Pu(V) and 0.2–0.5 M CH₃COOLi remain unchanged at 18–25°C for 2 days. In solutions with [CH₃COOLi] = 1–3 M, Pu(V) disproportionates with the formation of soluble Pu(VI) complexes and a suspension of Pu(IV) hydroxide. Introduction of CH₃COOH to a concentration of 0.1–1.0 M prevents the formation of a suspension of Pu(IV) hydroxide, but only up to a temperature of 45°C. The Pu(V) loss follows a second-order rate law, with the reaction products, Pu(IV) and Pu(VI), accelerating the Pu(V) consumption. The reaction rate at a constant concentration of acetate ions is proportional to [H⁺]. The reaction order with respect to Ac⁻ ions is close to 1.6. The activation energy of the Pu(V) disproportionation in the range 19–45°C is estimated at 74.5 kJ mol⁻¹. It is assumed that the disproportionation mechanism involves the formation of dimers from Pu(V) acetate complexes and aqua ions, their protonation, and decomposition with the transformation into Pu(IV) and Pu(VI).     

Reaction of Np(VI) with <Emphasis Type="Italic">N</Emphasis>-(2-Hydroxyethyl)ethylenediaminetriacetic acid in perchloric acid solutions

The stoichiometry of the reaction Np(VI) + H₃hedta [hedta is N-(2-hydroxyethyl)ethylenediaminetriacetate, HEDTA, anion] in a 0.05 M HClO₄ solution was studied by spectrophotometry. With Np(VI) in excess, 1 mol of HEDTA reduces 4 mol of Np(VI) to Np(V). In 0.125–1.0 M HClO₄ solutions (the ionic strength of 1.0 was maintained constant by adding LiClO₄), containing 3–29.2 mM HEDTA, at 20–45°С Np(VI) at a concentration of 0.4–3.5 mM is consumed in accordance with a first-order rate law until approximately 40% of Np(VI) remains. Then, the reaction decelerates. The reaction rate has first order with respect to [HEDTA] and the order of–1.6 with respect to [Н⁺]. The activated complex arises with the loss of one and two Н⁺ ions. The activation energy is 88.4 ± 5.3 kJ mol^–1.     

Effect of secondary heat treatment on the spectroscopic properties of Na<Subscript>2</Subscript>O-Nb<Subscript>2</Subscript>O<Subscript>5</Subscript>-P<Subscript>2</Subscript>O<Subscript>5</Subscript> glasses

We have studied the optical absorption and luminescence spectra of 45Na₂O · xNb₂O₅ · (55 − x)P₂O₅ glasses containing 5, 10, 20, 25, 30, and 35 mol % Nb₂O₅. The results indicate that the absorption band around 26000 cm⁻¹, responsible for the yellow color of the glasses, is due to the [Nb^(5+)--O⁻] center and disappears upon secondary heat treatment. Heat treatment of europium-doped glasses increases the concentration of Eu³⁺ centers in an asymmetric environment, which is accompanied by an increase in luminescence efficiency. The reason for this is that the Eu³⁺ ions are located outside the niobate subsystem of the glass matrix. The europium in the glasses studied acts as a protector ion.     

Reaction of Am(VI) with Na<Subscript>4</Subscript>XeO<Subscript>6</Subscript> in Alkaline Solutions in the Presence of Ozone

The reaction of Am(VI) with perxenate ions XeO ₆ ^4? in 1 M NaOH solutions was studied. A solid compound [Am(VI) : Na₄XeO₆ = 1 : 1] is formed in reaction of 1 mM Am(VI) with solid Na₄XeO₆; its ozonation in a thin layer yields an Am(VII) compound.     

Bulk ac conductivity studies of lithium substituted layered sodium trititanates (Na<Subscript>2</Subscript>Ti<Subscript>3</Subscript>O<Subscript>7</Subscript>)

Lithium mixed sodium trititanates with 0.3, 0.5 and 1.0 M percentage of Li₂CO₃ (general formula Na_2−X Li _X Ti₃O₇) have prepared by a high temperature solid-state reaction route. EPR analysis, high temperature range (473–773 K) and variable frequency range (100 Hz–1 MHz) ac conductivity measurements were carried out on prepared sample. The lithium ions are accommodated with the sodium ions in the interlayer space. The EPR specta of lithium mixed sodium Trititanates confirm the partial reduction of Ti⁴⁺ ions to Ti³⁺. Four distinct regions have identified in the LnσT versus 1,000/T plots. Various conduction mechanisms which dependence on concentration, frequency and temperature are reported in this paper for lithium mixed layered sodium Trititanates. The dilation of interlayer space has further been proposed to occur due to inclusion of lithium ions in the interlayer space. The conductivity increases as the concentration of lithium increases. The increase of ionic conductivity in these compounds is due to accommodation of lithium ions with sodium ions in interlayer space.     

An alternative approach of solid-state reaction to prepare nanocrystalline KNbO<Subscript>3</Subscript>

A simple preparation of KNbO₃ powders was proposed by an alternative approach of solid-state reaction. Stoichiometric niobium oxalate and potassium acetate were mixed in water and then dried. It was demonstrated that an ion-exchange reaction occurred with the formation of K[NbO(C₂O₄)₂]·nH₂O intermediate. The single-phase KNbO₃ powders were synthesized when K[NbO(C₂O₄)₂]·nH₂O intermediate was calcined between 500 and 800 °C for 3 h. KNbO₃ powders obtained at 500 °C are determined as orthorhombic structure with an average particle size of 20–50 nm by X-ray diffraction, scanning electron microscope (SEM), and transmission electron microscopy (TEM) analysis. The morphologies of KNbO₃ obtained at different temperatures were observed by SEM and TEM analysis. The average band gap energy is estimated to be 3.16 eV by UV–vis diffuse reflectance spectra.     

Giant dielectric and electrical properties of sodium yttrium copper titanate: Na<Subscript>1/2</Subscript>Y<Subscript>1/2</Subscript>Cu<Subscript>3</Subscript>Ti<Subscript>4</Subscript>O<Subscript>12</Subscript>

The electrical properties and dielectric response in Na_1/2Y_1/2Cu₃Ti₄O₁₂ ceramic prepared by conventional solid-state reaction method and sintered at 1,090 °C for 5 h were investigated as functions of frequency and temperature. Main phase of Na_1/2Y_1/2Cu₃Ti₄O₁₂ with CaCu₃Ti₄O₁₂-like crystallographic structure and CuO secondary phase were observed in the X-ray diffraction pattern. Abnormal grain growth was observed just as observed in CaCu₃Ti₄O₁₂ ceramics. The Na_1/2Y_1/2Cu₃Ti₄O₁₂ ceramic exhibits a high ε′ of ~2.04 × 10⁴ at 20 °C and 1 kHz and low tan δ (with the minimum 0.080 at 5 kHz). Impedance spectroscopy analysis reveals that Na_1/2Y_1/2Cu₃Ti₄O₁₂ ceramic is electrically heterogeneous, consisting of semiconducting grains and insulating grain boundaries. Giant ε′ response in Na_1/2Y_1/2Cu₃Ti₄O₁₂ ceramic is therefore attributed to an internal barrier layer capacitor effect.     

Physicochemical simulation of fusion processes of basalt and diabase with Na<Subscript>2</Subscript>CO<Subscript>3</Subscript> and Na<Subscript>2</Subscript>CO<Subscript>3</Subscript> + CaO

Fusion processes of basalt and diabase with sodium carbonate and its mixture with calcium oxide are investigated by methods of physicochemical simulation. Equilibrium compositions of the Si-Al-Fe-Ca-Mg-Na system are calculated at various ratios Na₂CO₃: basalt (diabase) and (Na₂CO₃ + CaO): basalt (diabase) in the temperature range of 1270–1470 K. It is shown that, on fusion with sodium carbonate, depending on conditions, the main components of fusion products are sodium metasilicate (Na₂SiO₃) and sodium metaaluminate (NaAlO₂), magnesium orthosilicate (CaMgSiO₄), sodium ferrite(III) (NaFeO₂), iron(III) oxide (Fe₂O₃), and sodium-aluminum orthosilicate (Na₂AlSiO₄). On fusion with the mixture of sodium carbonate and calcium oxide, respectively, the products are sodium metaaluminate, calcium pyrosilicate (Ca₃Si₂O₇), calcium-magnesium orthosilicate, and calcium ferrite (CaFe₂O₄).     

Synthesis and crystal structure of new Np(VI) and Pu(VI) phthalates, Na4{AnO2[(OOC)2C6H4]3} · n H2O

Double phthalates of Np(VI) and Pu(VI) of the composition Na₄[AnO₂L₃] · nH₂O [L = (OOC)₂C₆H₄] were synthesized in the crystalline form and studied by X-ray diffraction. The compounds are isostructural, their crystals consist of complex anions [AnO₂L₃]⁴⁻ and Na⁺ cations, forming neutral layers; water molecules are located between the layers. The coordination polyhedra of An(VI) are hexagonal bipyramids, whose average planes are formed by oxygen atoms of three phthalate anions. In passing from Np(VI) to Pu(VI), the actinide contraction is reflected to a greater extent in the variation of the bond lengths with apical oxygen atoms of the bipyramids, whereas the An-O bond lengths in the equatorial plane of the bipyramids vary insignificantly. Original Russian Text ? I.A. Charushnikova, N.N. Krot, I.N. Polyakova, Z.A. Starikova, 2007, published in Radiokhimiya, 2007, Vol. 49, No. 2, pp. 106–110.     

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.     

Microstructure and H<Subscript>2</Subscript>S gas sensing properties of nanocrystalline perovskite-type La<Subscript>0.6</Subscript>Co<Subscript>0.4</Subscript>Mn<Subscript>0.5</Subscript>Ni<Subscript>0.5</Subscript>O<Subscript>3</Subscript>

Nanocrystalline La_1−x Co _x Mn_1−y Ni _y O₃ (x = 0.2 and 0.4; y = 0.1, 0.3, and 0.5) thick films sensors prepared by sol–gel method were studied for their H₂S gas sensitivity. The structural and morphological properties have been carried out by X-ray diffraction (XRD) and transmission electron microscopy (TEM). Average particle size estimated from XRD and TEM analyses was observed to be 30–35 nm. The gas response characteristics were found to depend on the dopants concentration and operating temperature. The maximum H₂S gas response of pure LaMnO₃ was found to be at 300 °C. In order to improve the gas response, material doped with transition metals Co and Ni on A- and B-site, respectively. The La_0.6Co_0.4Mn_0.5Ni_0.5O₃ shows high response towards H₂S gas at an operating temperature 250 °C. The Pd-doped La_0.6Co_0.4Mn_0.5Ni_0.5O₃ sensor was found to be highly sensitive to H₂S at an operating temperature 200 °C. The gas response, selectivity, response time and recovery time were studied and discussed.     

A facile synthesis of MInSe<Subscript>2</Subscript> (M = Cu,Ag) via low temperature pyrolysis of single source molecular precursors, [(R<Subscript>3</Subscript>P)<Subscript>2</Subscript>MIn(SeCOAr)<Subscript>4</Subscript>]

The reaction of KSeCOAr with InCl₃ and [MCl(PR₃)₂] in benzene afforded bimetallic complexes, [(R₃P)₂MIn(SeCOAr)₄] (PR₃ = PEt₃ or PPh₃; M = Cu or Ag; Ar = −C₆H₅ (phenyl) or 4-MeC₆H₄ (tolyl)). The triethylphosphine complexes decomposed rapidly when M = Ag while slowly when M = Cu. All these complexes were characterized by elemental analysis, IR, UV-VIS, NMR (¹H, ³¹P) spectral data. Pyrolysis in a furnace at 300°C gave tetragonal MInSe₂ (M = Cu, Ag) structure. Solvothermal decomposition of [(PPh₃)₂CuIn(SeCOAr)₄] in boiling ethylene glycol gave nanorods of CuInSe₂ which were characterized by XRD, EDAX, SEM and TEM.