Francis Abraham

The bimevox series: A new family of high performances oxide ion conductors

Abstract A new family of oxygen anionic conductors, exhibiting high performances at low temperature, has been prepared and characterized. They derive from Bi4V2O11 by partial substitution of vanadium by other metallic ions (BIMEVOX). Results dealing with the copper substituant (BICUVOX) are detailed.

Phase transitions and ionic conductivity in Bi4V2O11 an oxide with a layered structure

The structure of Bi 4 V 2 O 11 consists of Bi 2 O 2 layers interleaved with V 2 O 7 sheets. Differential thermal analysis showed three reversible phase transitions at 720, 840 and 1150 K. The cells of the low temperature forms derive from the high temperature tetragonal cell. Conductivity measurements confirm these transitions and reveal two further ones at 920 and 975 K. The conductivity is in the range 0.001–0.01 (Ω cm) −1 for the β phase and 0.1–1 (Ω cm) −1 for the γ phase. The mobile species are probably O 2− ions, at least above 720 K.

Structure and conductivity of Cu and Ni-substituted Bi4V2O11 compounds

The partially Cu- or Ni-substituted compounds (Bi4V2(1−x)M2xO11−3x;M=Cu, Ni) are highly oxygen-conducting. Three phases (α, β, γ) are observed in the unsubstituted compound; α is the low-conducting room temperature phase and γ the high-conducting phase at high temperature. Structure and conductivity are studied as a function of the substitution on the vanadium sites. Between 0 and 6% at room temperature, the Cu compound remains in the orthorhombic α phase and its ionic conductivity increases. A strong anisotropic conductivity is observed. For 0.07≤x≤0.12, the average structure is tetragonal (γ-type) at room temperature. The conductivity is very high and does not vary very much over this substitution range. Impedance spectroscopy measurements have also been carried out on the x=0.07 Ni-substituted compound. Commensurate or incommensurate superstructures are observed for all of these compounds.

La liaison metal-metal dans les clusters M12O36: I - Preparation et etude structurale des phases La4M6O19 (M = Ru, Os)

La/sub 4/Ru/sub 6/O/sub 19/ and La/sub 4/Os/sub 6/O/sub 19/ are prepared for the first time by the oxidation of stoichiometric mixtures of (La/sub 2/O/sub 3/ + RuO/sub 2/) and (La/sub 2/O/sub 3/ + Os) with excess KClO/sub 3/. The crystal structure of both compounds is solved by conventional Fourier and least-squares methods to R = 0.022 and R = 0.024, respectively, using automated four-circle diffractometer data. Both are isostructural with La/sub 4/Re/sub 6/O/sub 19/. M-M distances (2.488 A for M = Ru and 2.499 A for M = Os) show evidence of strong metal-metal interaction. Comparison of data for M = Ru, Os or Re reveals the evolution of some interatomic distances with Z.

An uranyl citrate coordination polymer with a 3D open-framework involving uranyl cation-cation interactions

A novel uranyl-organic compound incorporating citrate linker exhibits a 3D open-framework. It is built up from the connection of infinite chains of uranyl-centered polyhedra with citrate ligands, generating layers, which are further linked perpendicularly by isolated uranium tetranuclear motifs, involving uranyl cation-cation interactions.

X-Ray Diffraction and μ-Raman Investigation of the Monoclinic-Orthorhombic Phase Transition in Th1−xUx(C2O4)2·2H2O Solid Solutions

A complete Th(1-x)U(x)(C(2)O(4))(2).2H(2)O solid solution was prepared by mild hydrothermal synthesis from a mixture of hydrochloric solutions containing cations and oxalic acid. The crystal structure has been solved from twinned single crystals for x = 0, 0.5, and 1 with monoclinic symmetry, space group C2/c, leading to unit cell parameters of a approximately 10.5 A, b approximately 8.5 A, and c approximately 9.6 A. The crystal structure consists of a two-dimensional arrangement of actinide centers connected through bis-bidentate oxalate ions forming squares. The actinide metal is coordinated by eight oxygen atoms from four oxalate entities and two water oxygen atoms forming a bicapped square antiprism. The connection between the layers is assumed by hydrogen bonds between the water molecules and the oxygen of oxalate of an adjacent layer. Under these conditions, the unit cell contains two independent oxalate ions. From high-temperature mu-Raman and X-ray diffraction studies, the compounds were found to undergo a transition to an orthorhombic form (space group Ccca). The major differences in the structural arrangement concern the symmetry of uranium, which decreases from C2 to D2, leading to a unique oxalate group. Consequently, the nu(s)(C-O) double band observed in the Raman spectra recorded at room temperature turned into a singlet. This transformation was then used to make the phase transition temperature more precise as a function of the uranium content of the sample.

Thermal behaviour of Bi4V2O11 : X-ray diffraction and impedance spectroscopy studies

Abstract Bi 4 V 2 O 11 powdered samples and single crystals were studied by high temperature X-ray diffraction and impedance spectroscopy to characterize the phase transitions. From high temperature X-ray diffraction on powders and single crystals, the α ⇆ β and β ⇆ γ reversible phase transitions were observed. The β ⇆ γ one is ferroelastic ⇆ paraelastic but surprisingly the α ⇆ β transition also exhibits a ferroelastic character, with a 90 ° switching of the a and b axis on cooling and/or, more scarcely, on heating. Impedance spectroscopy measurements were carried out using platelet shaped single crystals with well developed (001) faces. The corresponding σ ∥ (001 plane) and σ ⊥ ( c direction) bulk conductivities were obtained and compared with values from ceramic pellets, σ ∥ values are close to those characterizing the pellets, and the anisotropy of the conductivity is evidenced by σ ∥ values about 2 orders of magnitude larger than σ ⊥ ones. Slope changes observed in Arrhenius plots are in agreement with the phase transitions.

W-substituted Bi4V2O11

The introduction of tungsten in the [V] sites of Bi4V2O11 gives rise to a Bi2V1 − xWxO(11 + x)2 solid solution for x up to 0.25 on air quenched compounds and up to 0.125 on slow cooled ones. This produces the stabilization of either α, β or γ forms depending on the substitution ratio. These new phases have been characterized by X-ray diffraction, thermal analyses and conductivity measurements. Comparison between Mo- and W-substituted compounds is made.

Lanthanide-based 0D and 2D molecular assemblies with the pyridazine-3,6-dicarboxylate linker

Two distinct series of lanthanides-based coordination polymers involving the pyridazine-3,6-dicarboxylate (pzdc) ligand have been structurally characterized by means of single-crystal XRD analysis. The structure of the compounds Ln2(H2O)4(pzdc)3·(3/3.5)H2O with Ln = Pr (1), Nd (2), Sm (3), Eu (4) is built up from dinuclear bricks [Ln2O8N5(H2O)4] containing two μ3-oxo bridged nine-fold coordinated lanthanide cations, connected via the pzdc ligand, which acts as tetratendate, tridentate or monodentate linkers. It results in the generation of layers of mixed pzdc-Ln2O8N5(H2O)4 networks, intercalated by free water molecules. The second series with chemical formula Ln2(H2O)6(pzdc)3·3H2O (Ln = Gd (5), Tb (6), Dy (7), Er (8), Lu (9)) is a molecular assembly of discrete dinuclear bricks similar to those of the previous compounds. They only differ by the number of terminal water molecules within the dimer [Ln2O6N5(H2O)6]. Interactions viahydrogen bond occur between terminal water species attached the lanthanide and free intercalated water molecules. Europium- and terbium-based compounds 4 and 6 exhibit fluorescence emission in the visible region at room temperature.

Double substitutions in Bi4V2O11

Abstract Several double substitutions have been checked to compare the performances of these compounds with those of ICUVOX.10, which is used as a reference: (i) the introduction of two kinds of atoms in [V] sites (CuNi, CuZn, NiZn, CuMo); (ii) simultaneous substitution for [Bi] and [V] (BiPb and VCu, BiPb and VMo); (iii) substitution of Cu for [V] in the 2Bi 2 O 3 ·V 2 O 5 solid solution. In all cases the oxide anionic conductivity was not improved with respect to singly substituted V/Cu BICUVOX.10.