P. Berthet

Synthesis of various crystalline gold nanostructures in water: The polyoxometalate β-[H4PMo12O40]3− as the reducing and stabilizing agent

This paper reports a facile, one-pot, room-temperature synthesis, in water, of various Au nanostructures using, as reducing agent, the mixed-valence polyoxometalate β-H3[H4P(MoV)4(MoVI)8O40]3−. By modifying the initial concentrations of the polyoxometalate and chloroauric acid, the morphology of the synthesized Au nanostructure can be tuned. The whole process is a “green-chemistry-type” synthesis of Au0 nanostructures.

Phonon and crystal field excitations in geometrically frustrated rare earth titanates

The phonon and crystal field excitations in several rare earth titanate pyrochlores are investigated. Magnetic measurements on single crystals of Gd(2)Ti(2)O(7), Tb(2)Ti(2)O(7), Dy(2)Ti(2)O(7), and Ho(2)Ti(2)O(7) are used for characterization, while Raman spectroscopy and terahertz time domain spectroscopy are employed to probe the excitations in the materials. The lattice excitations are found to be analogous across the compounds over the whole temperature range investigated (295-4 K). The resulting full phononic characterization of the R(2)Ti(2)O(7) pyrochlore structure is then used to identify crystal field excitations observed in the materials. Several crystal field excitations have been observed in Tb(2)Ti(2)O(7) in Raman spectroscopy, among which all of the previously reported excitations. The presence of additional crystal field excitations, however, suggests the presence of two inequivalent Tb(3+) sites in the low-temperature structure. Furthermore, the crystal field level at approximately 13 cm(-1) is found to be both Raman and dipole active, indicating broken inversion symmetry in the system and thus undermining its current symmetry interpretation. In addition, evidence is found for a significant crystal field-phonon coupling in Tb(2)Ti(2)O(7). The additional crystal field information on Tb(2)Ti(2)O(7) adds to the recent discussion on the low temperature symmetry of this system and may serve to improve its theoretical understanding.

A new family of sodium ion conductors: the diphosphates and diarsenates Na7M3 (X2O7)4; (M=Al, Ga, Cr, Fe; X=P, As)

Abstract Seven monoclinic isotypic compounds with the general formula Na7M3 (X2O7)4 have been synthesized by crystallization in a flux of sodium phosphates or arsenates. They exhibit high ionic conductivity (σ≈10−3-10−2ω−1cm−1 at 300°C) and good cation exchange properties particularly with Ag+ ions. At temperatures varying from −20°C to 240°C, depending on the particular compound, they undergo a reversible phase transition, α⇌β, which is associated with a loss of long range ordering of the sodium ions and results in an increase of the conductivity. The crystal structures of β-Na7Fe3 (As2O7)4 and α-Na7Fe3 (P2O7)4 have recently been determined and a model of sodium diffusion paths has been proposed. This model indicates a two-dimensional conduction within planes parallel to (001). Conductivity measurements performed on single crystals confirm this two-dimensional character with a σ|/σ⊥ ratio of about 2×103.

Manganous heteropolytungstates. Synthesis and heteroatom effects in Wells–Dawson-derived sandwich complexes

A tetranuclear manganous Wells–Dawson sandwich-type polyoxometalate has been synthesized by the reaction of α-Na12(As2W15O56) with an aqueous solution of MnCl2·4H2O. The structure of this complex, αββα-Na16(MnIIOH2)2MnII2(As2W15O56)2·55H2O (Na1), was determined by single-crystal X-ray crystallography (a = 14.5230(12) A, b = 14.7104(13) A, c = 19.8927(17) A, α = 84.326(2)°, β = 81.709(2)°, γ = 65.584(2)°, triclinic, R1 = 6.2%, based on 26721 independent reflections) and is similar to the phosphorus analogue, αββα-Na16(MnIIOH2)2MnII2(P2W15O56)2 (Na2). Magnetization studies confirm that both Na1 and Na2 show antiferromagnetic coupling of the four Mn(II) centers. Despite the structural similarities, electrochemical studies reveal that the presence of arsenic shifts the Mn waves to more positive potentials. Catalytic studies confirm that 1 is a significantly better catalyst than 2 for the H2O2-based epoxidation of cis-cyclooctene, cyclohexene, and 1-hexene.

Preparation of textured alumina films by the sol-gel route

Alumina films were fabricated by the sol-gel route under various preparation conditions. The peptization rate and the structure of colloidal particles depend on these conditions. In particular, the peptization of a dried boehmite precipitate prepared from aluminum alkoxide was achieved within a few seconds without heating. The resulting sol is made of highly dispersed crystallites; this allows the formation of well-textured xerogel films as evidenced by the Weissenberg x-ray method.

One-step synthesis and stabilization of gold nanoparticles in water with the simple oxothiometalate Na2[Mo3(μ3-S)(μ-S)3(Hnta)3]

Na2[Mo3(μ3-S)(μ-S)3(Hnta)3] serves both as a reducing and a capping agent in the synthesis of Au nanoparticles in water at room temperature in a “fully green chemistry-type process”, thus avoiding the usual two-step preparation of thiolate-monolayer-coated gold nanoparticles. The nanoparticles were characterized by TEM, DLS, UV-vis spectroscopy, XPS and XRD analyses and cyclic voltammetry.

Structure of metastable strontium ferrite

The metastable compound SrFe2O4 was prepared by crystallization at 630°C of a glass prepared by melt-spinning. It decomposes above 900°C. It has an orthorhombic unit cell at room temperature with a = 8.031(10) A, b = 18.208(18) A, c = 5.454(6) A, Z = 8, space group Pbc21.

Substrate influence on the growth of Co-doped La0.5Sr0.5TiO3−δ epitaxial thin films

We report on the growth of thin films of the diluted magnetic oxide Co-doped La0.5Sr0.5TiO3−δ on SrTiO3 and LaAlO3 substrates. The films are ferromagnetic at room temperature, with no indications of parasite phases from high-resolution x-ray diffraction experiments. The structural mismatch with the substrate is shown to have a strong impact on the growth mechanisms and, consequently, on the surface morphology. In this sense, to grow the films on SrTiO3, for which the mismatch is minimized, appears to be more appropriate for the future integration of Co-doped La0.5Sr0.5TiO3−δ layers into heterostructures for perpendicular transport.

Direct and improved synthesis of the tri-nickel sandwich-type polyoxoanion [Ni3Na(H2O)2(PW9O34)2]11−

Abstract The direct use of preformed B,α-Na 9 PW 9 O 34 was successful in improving the synthesis of Na 11 [Ni 3 Na(H 2 O) 2 (PW 9 O 34 ) 2 ]·14H 2 O. The complex was characterized by elemental analysis, IR spectroscopy, magnetic properties and cyclic voltammetry. The yield was increased to 30%, which is four times more than that obtained in the initial synthesis. The delay before the appearance of the first crystals is drastically shortened, from months to hours, and these crystals are predominantly those of the yellow compound of interest. Keys to these improvements are the use of freshly prepared B,α-Na 9 PW 9 O 34 through prolonged heating of A,β-Na 9 PW 9 O 34 and the increase of the concentrations of the reactants in a close-to-neutral pH medium. Starting the synthesis with several concentrations of preformed A,β-Na 9 PW 9 O 34 revealed the importance of the concentrations of reactants in speeding up the formation of crystals and suggested strongly that the [A-PW 9 O 34 ] 9− →[B-PW 9 O 34 ] 9− isomerization might be the slow step in the whole process. Cyclic voltammograms as well as X-ray diffraction structure determination confirm that the synthesized compound is indeed Na 11 [Ni 3 Na(H 2 O) 2 (PW 9 O 34 ) 2 ]·14H 2 O. A study of the magnetic properties of Na 11 [Ni 3 Na(H 2 O) 2 (PW 9 O 34 ) 2 ]·14H 2 O indicates that the three nickel centres exhibit ferromagnetic exchange interactions.

Structure and ion transport properties of Na2OGa2O3P2O5 glasses

Abstract The structure of glasses prepared in the Na 2 OGa 2 O 3 P 2 O 5 system was investigated by vibrational (IR, Raman) and X-ray absorption spectroscopies. For the orthophosphate compositions, gallium is found in four-fold coordination, whereas it is found in six-fold coordination in a metaphosphate glass. For pyro-, tri and tetraphosphate glasses, gallium atoms are distributed on tetrahedral and octahedral sites. Gallium atoms found on octahedral sites are likely chelated by pyrophosphate groups. The ionic conductivity of the glasses depends mainly on their sodium content.