[en] Two experimental approaches dealing with the determination of melting at high static pressures are described and analyzed. With the sample squeezed inside a diamond anvil cell, high temperatures up to the solid-liquid transition are obtained using Nd:YAG laser heating. Two methods have been investigated. In the first technique, the heating is accomplished with a pulsed laser and the brief radiation variations (t < 10 ms) emitted from the sample are recorded with two high-speed infrared detectors. The melting location is defined by a plateau or changes of slope of the signals, and the temperatures are calculated by assuming a constant value of emissivity factor at the end of the transition over the studied pressure range. The second system employs a continuous laser and a two-dimensional CCD detector to measure temperatures using multispectral pyrometry. Melting is detected from criteria related either to textural change in the sample involving interference contrast under a laser illumination or to the specific variations of temperatures and emissivity as a function of laser power. Thermal radiation is fitted to Planck's law with temperature and emissivity as the free parameters. Advantages and drawbacks are presented from results obtained on pure uranium

[en] Using a point charge model, the lattice contribution to the electric field gradient (EFG) was calculated for the isotypic hydrated neptunium fluorides MNpF₆ x 3H₂O (M = Mn, Fe, Co, Ni, Zn). The atomic coordinates in these compounds were computed from prescribed interatomic distances using a least squares procedure, the positional parameters in CoNpF₆ x 3H₂O and the ionic radii difference between the M²⁺ ions. With the authors structural determinations, calculations of the EFG and the experimental Moessbauer data, they can propose for the Sternheimer antishielding factor γ/sub infinity/ (Np³⁺) the value γ/sub infinity/ = -126(13). This result is in good agreement with the calculated value deduced from theoretical considerations

[en] The systematic study of Pu (or Am)-Mo (or W) -O systems by solid state reactions had led to the identification of seven new ternary oxides of transuranium elements. Conditions of synthesis and radiocrystallographic data for the compounds Tu₂MO₆ and Tu₂(MO₄)₃ are given; the latter exhibit structures closely related to that of tetragonal scheelite in accordance with the formula Tusub(0.66)MO₄. (orig.)

[en] To study actinide compounds , neptunium 237 has been studied by Moessbauer resonance. The different oxidation degrees of neptunium (7, 6, 5, 4 and 3) have a very important effect on isomeric displacements. In the study of chalcogenides, the isomeric displacement value of NpS₃ confirms the valency 4+ of neptunium in this compound. Results obtained with Np₃S₅ show two valency state +3 and +4 in this compound. There is a good agreement with the two crystalline sites determined by crystallography

[en] The authors report ²³⁷Np Moessbauer spectrometry measurements on some Np polychalcogenides: NpS₃, β-NpS₂, Np₃Se₅, NpTe₃, α-Np₂S₃ and γ-Np₂Se₃. The experiments have been performed at 4.2 K and 77 K. From the isomeric shift values, the oxidation state of the Np ion can easily be assigned in these kinds of compounds: Np(IV) in NpS₃, β-NpS₂ and Np(III) in NpTe₃, αNp₂S₃ and γNp₂Se₃. A mixture of Np(III) and Np(IV) ions has been observed in Np₃Se₅. Comparisons of the IS values have pointed out the influence of the electronegativity of the chalcogen atom and of the bond length Np-X (X = S, Se, Te). (Auth.)

[en] The influence of the neptunium ion environment on the ²³⁷Np Moessbauer isomer shifts has been studied in various metal coordination complexes: fluorides, oxides, oxide fluorides and polycarboxylates. A linear dependence between the isomer shift and the mean neptunium-ligand distance in a series of hexavalent Np compounds has been evidenced and the feasibility of overlapped isomer shift areas, namely Np(IV), Np(V) and Np(VI) has been established. (orig.)

No abstract available

[en] A new neptunium selenide Np₂Se₅ was obtained. It exhibits an orthorhombic pseudotetragonal structure of the Th₂S₅ type. The lattice parameters are a = 7.725(3) A, b = 7.725(3) A and c = 10.622(5) A. Magnetic susceptibility measurements performed on powder samples between 4.2 and 300 K show a ferromagnetic phase transition at 15 K. ²³⁷Np Moessbauer measurements at 4.2 K confirm the magnetic order. The Moessbauer spectrum at 77 K exhibits single quadrupole splitting with an isomer shift of 12.7(5) mm s^-1 relative to NpAl₂. (Auth.)

[en] The 59.5 keV Mossbauer resonance of ²³⁷Np has been measured in Np₃S₅ at 77 and 4.2 K. At 77 K, two different quadrupolar splittings are observed. The first one (delta₁ = - 5(1) mm sec^-1/NpAl₂) is assigned to the Np⁴⁺ ions and the second one (delta₂ = 28(1) mm sec^-1/NpAl₂) is assigned to the Np³⁺ ions. At 4.2 K the spectrum gives a ratio of areas S(Np³⁺)/S(Np⁴⁺) of 2.0. These results clearly confirm the crystallographic data of this mixed-valence material. (author)

[en] Neptunyl(VI) carbonate was precipitated at pH 5 from a neptunyl(VI) nitrate solution by the addition of solid lithium carbonate. The precipitate was characterized by thermogravimetric analysis, X-ray diffraction and ²³⁷Np Moessbauer measurements. NpO₂CO₃ decomposes at 350-400⁰C and was found to be isostructural with UO₂CO₃. The lattice parameters are a=4.82(1) A, b=9.17(5) A and c=4.24(2) A. The 4.2 K Moessbauer absorption spectrum can be described as a quadrupole interaction with the hyperfine parameters δ=-35.5(2) mm s^-1 (relative to NpAl₂), vertical strokee²qQvertical stroke=231(1) mm s^-1 and η=0.21(2). Both isomer shift and electric field gradient values are consistent with a neptunyl NpO₂²⁺ ion and confirm the crystal structure isotypic with UO₂CO₃. (orig.)