M. Pagès

Crystal structure of a neptunyl cation-cation complex (NpO2+) with mellitic acid: Na4(NpO2)2Cl12O12·8H2O

The crystal structure of the compound formed by the neptunyl(V) ion (237NpO2+) and mellitic acid was determined. The stoichiometric formula of the compound was Na4(NpO2)2C12O12·8H2O with the following crystal data: Z = 4; Vc = 2486.7 A3;Dcalc = 2.96 g cm−3; a = 12.53(1) A; b = 11.58(2) A; c = 17.81(2) A; β = 105.79°; space group, C2/c. The structure was obtained using classical methods with R = 0.044 and Rw = 0.056 for 2372 independent reflections. The most interesting result was the evidence for neptunyl dimers in the lattice, possibly as a result of cation-cation interactions. The two neptunium atoms in such dimers are 3.4824(8) A apart. The neptunyl dinier is associated with four mellitate ligands. The sodium ions and the water molecules serve as bridging species to increase the crystalline cohesion.

Phase transformation of AnX compounds under high pressure (An ≡ Np, Pu; X ≡ Sb, Te)

Abstract High pressure X-ray diffraction studies were performed on AnX compounds (An ≡ Np, Pu; X ≡ Sb, Te) in the pressure range up to about 57 GPa, at room temperature, using a diamond anvil cell in an energy-dispersive X-ray diffraction facility. These AnX compounds have an f.c.c., NaCl-type structure at ambient pressure and undergo first-order phase transitions under pressure. Whereas NpTe and PuTe transform to a primitive cubic cell of the CsCl type at 13 GPa and 15 GPa respectively, a tetragonal structure, space group P 4/ mmm , is observed for NpSb above 10 GPa. This tetragonal structure can be regarded as a “distorted” CsCl-type structure, the atomic positions being the same as in the cubic CsCl cell. In PuSb two transitions are detected: the CsCl-type structure is observed above 17 GPa; then PuSb transforms to the tetragonal P 4/ mmm structure at about 42 GPa. In addition, the compressibility of the low pressure phase was determined from the V ( p ) data for each compound.

Crystal structure and Mössbauer studies of neptunium(V) complexes with polycarboxylic acids

Abstract Complexes of the neptunyl ion ( 237 NpO 2 + ) with pyromellitic acid and mellitic acid were prepared with the formulae (Na 3 NpO 2 (C 10 O 8 H 2 )) 2 ·11H 2 O and Na 4 (NpO 2 ) 2 (C 12 O 12 )·8H 2 O respectively. These solids were studied using X-ray diffraction, Mossbauer spectroscopy and magnetic susceptibility measurements. The crystal structure of (Na 3 NpO 2 (C 10 O 8 H 2 )) 2 ·11H 2 O was refined to R = 0.027 for 6793 independent reflections (structural data: a = 14.46(1) A ; b = 9.404(8) A ; c = 6.768(4) A ; α = 95.08°; β = 103.23°; γ = 87.47°; space group, P1; Z = 1). The NpO 2 + ion is coordinated to four pyromellitate anions. The Mossbauer spectra exhibited magnetic hyperfine splitting which was attributed to paramagnetic relaxation phenomena.

High-pressure X-ray diffraction on neptunium compounds: Recent results for NpAs

Abstract High-pressure X-ray diffraction studies have been performed at room temperature on neptunium monoarsenide up to 53 GPa. This compound undergoes a phase transformation which starts around 25 GPa and is completed at 40 GPa. The high-pressure phase was indexed in the CsCl structure type. On pressure decrease, hysteresis to transform is observed down to 16 GPa. The bulk modulus B0 = 70 GPa and its pressure derivative B′0 = 6.2 have been calculated for the low-pressure phase by fitting the V(P) data to the Birch and Murnaghan equations.

Complexation of Neptunium(V) by Halo- and Hydroxycarboxylate Ligands

The complexation of N p O j ion by haloacetates and hydroxy acids has been studied in 2.0 M (NaClOi) by spectrophotometry. The enhancement in complexation by a-hydroxyacids was attributed to chelate formation involving the a-hydroxy group. Other orthocarboxyphenolates acted as monodentate ligands. Data analysis of complexation by haloacetates suggest a dominant inner-sphere nature of bonding.

Crystal structure of uranyl benzene 1,2,4,5-tetracarboxylate dihydrate: UO2C10O8H4•2H2O

The crystal structure of the complex in the title was determined by X-ray crystallography from single-crystal diffractometer data. Crystal data: UO2C10O8H4 · 2H2O, triniclic, P1, a = 6.355(6) A, b = 11.531(9) A, c = 5.548(5) A, α = 104.07 °, β = 109.06 °, γ = 81.85 °, M = 558.192, Z = 1, V =371.875 A3, dcalc = 2.49 g cm−3, R (F) = 0.106, Rw = 0.134 for 3413 independent reflections. The hexagonal dipyramid around the uranium atom contains two carboxyl ligands and two water molecules in the equatorial plane of the uranyl ion. The U—O (uranyl bond) is 1.70(2) A, 〈U—O〉 in the equatorial plane is 2.48 A, and other distances and angles are similar to those in the literature.

BULK MODULUS AND P– V RELATIONSHIP UP TO 52 GPa OF NEPTUNIUM METAL AT ROOM TEMPERATURE

Abstract High purity neptunium metal was studied under high pressure using a diamond anvil cell in an energy dispersive X-ray facility. No phase transition was observed up to 52 GPa. A bulk modulus B 0 = 118 GPa and its pressure derivative B ′ 0 = 6.6. were calculated by fitting the V ( P ) data to the Birch and Murnaghan equations. The results are discussed in the context of recent compressibility measurements of other actinides.

Cation – Cation Complexes of NpO2+

The stability constants of two cation —cation complexes, N p 0 2 + U 0 | + and N p O j N p O j , were measured by spectrophotometry and the enthalpies of formation obtained by the temperature coefficient method. The thermodynamic parameters for the N p O j U O l + complex are 0 = 2.25 + 0.03, AH= 1 2 . 0 + 1.7 kJ/mol and AS= 3 4 ± 6 J/mol-K. For the N p O j N p O Í complex, values of/5 = 1.41 ±0.14, Λ / / = 0 + k J/ mol, and AS = 3±2 J/mol-K were found. The small or negative values of the enthalpy and entropy changes would seem to indicate the formation of outer-sphere complexes. If the complexes are inner-sphere, the thermodynamic parameters may mean that the complexes are more hydrated than the individual reactants.

Systematic trends in the237neptunium Mössbauer isomer shift: Overlap of IV, V and VI neptunium oxidation states and correlation between isomer shift and crystal structure

The influence of the neptunium ion environment on the237Np Mössbauer 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.

237 Np Mössbauer isomer shifts in neptunium chalcogenides

We report237Np Mössbauer spectrometry measurements on some Np polychalcogenides: NpS3,β-NpS2, Np3Se5, NpTe3,α-Np2S3 andγ-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,α-Np2S3 andγ-Np2 Se3. A mixture of Np(III) and Np(IV) ions has been observed in Np3Se5. Comparisons of theIS values have pointed out the influence of the electronegativity of the chalcogen atom and of the bond length Np-X(X≡ S, Se, Te).