[en] The complexes of Np(IV) and U(IV) with unsaturated heteropolyanion SiW₁₁O₃₉^8- have been prepared in single crystal form, their structures have been determined by X-ray structural analysis. Actinide atoms in [An(SiW₁₁O₃₉)₂]^12- complex anions are eight-coordinated. Their coordination polyhedra can be described as distorted square anti-prisms. The main distortion of the coordination polyhedron in both complexes is twisting of one base of the square anti-prism from its ideal position by about 9 angle around the fourfold inversion axis. (authors)

[en] (NpO₂)₂SiO₄.2H₂O (I) and (PuO₂)₂SiO₄.2H₂O (II) have been prepared by hydrothermal synthesis technique. Chemical composition of (I) was attributed to the compound from its isostructurality with (UO₂)₂SiO₄.2H₂O; crystal unit cell parameters of (NpO₂)₂SiO₄.2H₂O have been refined as orthorhombic, a = 8.365(5) A, b = 11.089(5) A, c = 18.640(9) A with the space group Fddd. (PuO₂)₂SiO₄.2H₂O has not been found isostructural with uranyl(VI) and neptunyl(VI) analogues. Chemical composition of (II) was determined by chemical analysis and proved by the formula volume calculated from powder X-ray data. Indexing of X-ray powder pattern of the prepared compound gave the tetragonal unit cell with a = 6.912(3), c = 18.563(3) A, possible space group I4₁/amd. Visible-near IR and IR absorption spectra of the prepared compounds have been measured in NaCl and KBr. f-f absorption band of PuO₂²⁺ in the visible spectrum of crystal (PuO₂)₂SiO₄.2H₂O is not splitted but its maximum is essentially shifted (up to 858 nm) to the long wave region in comparison with hydrated PuO₂²⁺ ions. Visible-near IR absorption spectrum of crystal (NpO₂)₂SiO₄.2H₂O is close to that of aqueous neptunyl(VI) solution. Thermal behavior of synthesized compounds has been studied up to 800 C in air. (orig.)

No abstract available

[en] The data published on values of redox potentials E of heptavalent Np, Pu, Am, Ru and Mn and octavalent Ru and Pu were analyzed. The half-life (τ_1/2) for reduction reactions of heptavalent metal by water in aqueous alkaline solutions was also analyzed. The decrease in the half life stability of actinide in water with the redox potential for 3- charged anions of Np(VII), Pu(VII), and Am(VII) was found. At the same potential, a charge decrease from 3- to 1-reduces the stability approximately two fold. For the redox couple Pu(VIII)/Pu(VII), the estimated value is E > 1.15 V/NHE. If Pu(VIII) existed as PuO₄, it would be very unstable (τ_1/2 ∼ 10^-2 s) in 1 M NaOH. For the anionic forms of Pu(VIII), PuO₄(OH)₂^2- and PuO₆^4-, the half life was estimated to be very low (τ_1/2 < 1 s). (orig.)

No abstract available

[en] A formation of Am(VI) carbonate complexes has been observed under ozonation of Am(III) in guanidinium carbonate solutions. New [C(NH₂)₃]₄[AmO₂(CO₃)₃].2H₂O tricarbonate complex has been isolated and studied by the X-ray analysis. The Am atom has oxygen environment as distorted hexagonal bipyramid with three CO₃^2- ions in equatorial plane and oxygen atoms of the AmO₂ group in the apical positions. Averaged Am-O_CO3 bond length is 2.431(13) A, the length of Am=O bond is 1.750(12) A. In crystal hydrogen bonds between [C(NH₂)₃]⁺ cations, water molecules and [AmO₂(CO₃)₃]^4- complex anions link the fragments of structure into the three-dimensional network. To compare the peculiarities of the structure of tricarbonate complexes it was prepared [C(NH₂)₃]₄[AnO₂(CO₃)₃], where An = U(VI), Np(VI) and Pu(VI). In structure of [C(NH₂)₃]₄[AnO₂(CO₃)₃] an actinide contraction appears as a slight shortening of the bond lengths in the actinyl groups AnO₂: 1.80 A for U, 1.78 A for Np 1.76 A Pu. The An-O_CO3 bond length in the equatorial plane of hexagonal bipyramids remains almost unchanged in the series from U to Am. An analysis of the structures showed that hydrogen bonding played an important role in crystal formation. (orig.)

No abstract available

[en] The hydrolysis and interaction of Np(IV) and Pu(IV) with orthosilicic acid, Si(OH)₄, were studied in 0.1-1.0 M ionic strength aqueous solutions. Spectrophotometry was used to study these reactions at about 10^-4 M Np(IV) and Pu(IV) concentrations. The first hydrolysis constants, K_hydr, agree with the majority of earlier spectrophotometric and potentiometric data. The absorption spectra of NpOH³⁺ and PuOH³⁺ were obtained by spectral deconvolution. Reasons to explain the overestimation of K_hydr obtained by other methods [by extraction of trace amounts of Np(IV) and Pu(IV) and by solubility] are discussed. Formation of the complexes NpOSi(OH)₃³⁺ and PuOSi(OH)₃³⁺ is demonstrated in the presence of 0.005-0.016 M Si(OH)₄ in the p[H⁺] range 1.0-2.2 and 0.3-1.4, respectively. Measured values of equilibrium constants of the reaction M⁴⁺ + Si(OH)₄ <=> MOSi(OH)₃³⁺ + H⁺ at ionic strength I = 1.0 are lg K₁ = 0.41 ± 0.02 and 1.04 ± 0.04, respectively, for Np(IV) and Pu(IV). The stability constants of the NpOSi(OH)₃³⁺ and PuOSi(OH)₃³⁺ complexes, recalculated to zero ionic strength, are lg β₁⁰ = 11.2 and 11.8, respectively. The correlation between K_hydr and K₁, as observed for all earlier studied metal ions, also occurs for both Np(IV) and Pu(IV). (orig.)

[en] Nitrogen-bearing ligands have received a lot of attention focused primarily on liquid-liquid extraction and separation of lanthanides and actinides. To modelise the complexation mechanism and to understand more deeply the electronic and thermodynamic properties of complexes formed in solution, compound structural data are needed. The present work deals with the synthesis and the crystal structure of three Np(V) compounds: [(NpO₂)(NO₃)Tpy(H₂O)], [(NpO₂)(4-(OOC)NC₅H₄)Tpy(H₂O)]2H₂O formed with terpyridine (Tpy) and [NpO₂(2-(OOC)NC₅H₄)(H₂O)₂], a new Np(V) complex formed with picolinate-ion. Far-IR spectroscopic properties are investigated and discussed in terms of deformation vibrations in the neptunyl groupings. (author)

[en] Optical and far-IR spectroscopy techniques have shown that the spectroscopic properties of Np(VI) and Pu(VI) compounds formed with different tetrahedral anions XO₄^2- (X = S, Se, Cr) are depending on the nature of the central atom X in the oxoanions. Changes in electronic transition energies within the ground state configuration are mainly due to crystal field perturbations induced by oxygen charge distribution variations in the actinide ion surrounding depending upon the nature of X. Changes may be related to covalence variation in An-O bonding or distortions in the ligand arrangement. (orig.)