F. Weill

Synthesis of nanostructured materials in supercritical ammonia: nitrides, metals and oxides

In this study, the synthesis of nanostructured particles of nitrides (Cr2N, Co2N, Fe4N, Cu3N, Ni3N), metal (Cu) and oxides (Al2O3, TiO2, Ga2O3) by using supercritical ammonia in the reaction medium is described. The elaboration process is based on the thermal decomposition of metal precursors in a supercritical ammonia–methanol mixture at a range of temperatures between 170 and 290 °C at about 16 MPa. Nitrides are obtained at relatively low temperatures in comparison with classical processes. It is shown that the chemical composition of the produced materials can be controlled by the adjustment of process operating conditions (pressure, temperature, metal precursor concentration and residence time in the elaboration reactor) and by the knowledge of the Gibbs free energy of oxide formation of the studied metal.

Electron microscopy study of electrochemically prepared La2NiO4+δ (0.17≤δ≤0.26)

Abstract A new material with composition of La2NiO4.26 exhibiting a large interstitial oxygen amount with regard to the basic K2NiF4- type structure has been prepared using an electrochemical process. This material has been examined by TEM and various super-structures either commensurate or incommensurate have been found. On the basis of the obtained ED patterns a structural model is proposed to account for the commensurate superstructures; they correspond to different oxygen stoichiometry, gd=0.17 and δ=0.25. In this structural model the interstitial oxygen atoms are located along the [111] directions of the K2NiF4-type unit cell. Two triclinic unit cells have been proposed for these two phases. Moreover an idealized structure involving an ordering of the nickel sites is depicted for La2NiO4.25. The ED patterns of the incommensurate structures have been described using a q ∗ modulation vector in addition to the three fundamental vectors q t ∗ , b t ∗ and c t ∗ of the I4/mmm structure. It was then pointed out that the modulation vector of the incommensurate structures lies between the two q 1 ∗ and q 2 ∗ modulation vectors that can be defined for the commensurate phases (δ=0.17 and δ=0.25 respectively) in the reciprocal plane corresponding to the [311] zone axis. We assume that a direct relation exists between q ∗ and δ.

Control of particle growth by chemical transformation in supercritical CO2/ethanol mixtures

The purpose of this work is to take advantage of specific properties of supercritical media to develop a new route for elaboration of fine metal powders. The process consists of the thermal decomposition of a copper precursor previously solubilized in a CO2/ethanol supercritical mixture. n It has been shown that low initial concentrations of precursor lead to spherical homogeneous nanostructurated particles with a mean size down to below 1 µm, well crystallized, and free from solvent contamination. A decomposition reaction has been performed in a polymer swollen by the supercritical fluid to quench aggregation. Particles sizes in the range 5–20 nm were obtained in polystyrene and in the range 150–250 nm in silicone. Short residence times in a tubular flow reactor (<30 s) allowed non-aggregated nanoparticles to be obtained (particles of size <50 nm were amorphous). n The mechanism of particle growth in supercritical nmedia has been examined assuming both total and partial coalescence (aggregation), since those mechanisms dominate the particle synthesis, according to experimental and simulation results.

Crystal chemistry of the G-phases in the systems Ti–{Fe, Co, Ni}–Al with a novel filled variant of the Th6Mn23-type

Abstract The crystal structures of the compounds Ti6M7Al17 with M=Fe,Co,Ni have been investigated by X-ray powder and single crystal, neutron powder and electron diffraction. These compounds crystallize with a new filled variant of the Mg6Cu16Si7-type (space group Fm 3 m ). A close structural relation is encountered for the series of crystal structures: Th6Mn16Mn7?, Mg6Cu16Si7?, Sc6Sc16Ir7Ir, Zr6Zn16Zn7Si, Ti6Al16M7Al, and Ti6Ni16Si7Si. Electrical resistivity of the Fe, Co-based compounds is typical metallic and temperature dependence follows the Bloch Gruneisen relation with a Debye temperature of about 300 K. No hydrogen was found to be absorbed in the Fe-, and Co-based compounds at room temperature under a H2 pressure of 5 MPa.

Hydrogen absorption properties of CeNiAl: influence on its crystal structure and magnetic behaviour

CeNiAl absorbs up to 1.93(5) hydrogen per formula unit at room temperature and at a pressure of 1 MPa. This hydride is stable in air. Hydrogenation induces both: (i) a structural transition hexagonal ZrNiAl-type→hexagonal AlB 2 -type; (ii) a valence transition for cerium. At low temperature for T<7 K. the hydride CeNiAlH 1.93 shows a spin fluctuation behaviour.

Kinetic study of chemical transformation in supercritical media of bis(hexafluoroacetylacetonate)copper (II) hydrate

Abstract Thermal decomposition of bis(hexafluoroacetylacetonate)copper (II) hydrate in CO 2 /ethanol supercritical mixture is a new route to obtain homogeneous submicronic spherical copper particles without carbon contamination. A kinetic study of thermally activated decomposition reaction at 473 K of both hydrated and anhydrous complex by Visible and UV spectroscopies is reported in this paper. It was found that, at 473 K, the decomposition rate constant of the hydrated complex (1.670 min −1 ) is one order of magnitude higher than the dehydration rate constant (0.14 min −1 ).

SmCo5/Cu particles elaboration using a supercritical fluid process

Abstract Supercritical fluids exhibit a range of unusual properties that can be exploited for developing new processes. Thus, a new way for particles coating is presented. It consists of depositing on a core particle a thin layer of copper. The core particles are made of ground SmCo 5 . The evolution of the magnetic properties of SmCo 5 is studied as a function of the milling time. The copper source is the copper hexafluoroacetylacetonate (Cu(hfa) 2 .H 2 O), which is thermally decomposed in a supercritical medium. Then, the influence of the copper layer is evaluated. This process obtains new ‘core-shell structures’ as SmCo 5 /Cu. New interesting properties are expected for these structures more particularly in the magnetic recording field.

High pressure synthesis and structure of a novel clathrate-type compound: Te7+xSi20−x (x∼2.5)

Abstract A novel clathrate-type compound, Te 7+ x Si 20+ x ( x ∼2.5), has been synthesized from the elements under high pressure and high temperature conditions. Its structure has been solved by Rietveld analysis of the XRD powder patterns combined with the results of an electron diffraction study. This structure is closely related to that of a typexa0I clathrate, but with a different unit-cell parameter ( a ∼2× a 0 ) and space group ( Fd -3 c instead of Pm -3 n ). Its main feature is that it corresponds to a double clathration of a Te atom enclosed in a partially Te substituted (12.5%) Si 20 pentagonal dodecahedra, which is itself enclosed in a large Te 24 polyhedron, in form of a truncated octahedron. This outer Te network proved to be similar to the characteristic H 2 O host lattice of a type VII clathrate (HEF 6 ·(H 2 O) 5 ·HF (E=P, As, Te)), the distribution of the Si/Al atoms in minerals of the sodalite group ((Na,Ca) 8 (Al 6 Si 6 O 24 )(Cl,S,SO 4 ) 2 ) and to the arrangement of the Ba atoms in the structure of the superconducting compound, Ba 6 C 60 . This new clathrate structure of silicon is the first one which exhibits Si 20 pentagonal dodecahedra which are only linked by inter-cluster bonds, a result which opens new prospects in the domain of the chemistry of macro-ions such as Si 20 12+ or Si 28 4+ .

Incommensurate and commensurate magnetic structures of the ternary germanide CeNiGe3

The structural properties of CeNiGe3 have been investigated via electron diffraction and neutron powder diffraction (NPD). This ternary germanide crystallizes in the orthorhombic SmNiGe3-type structure (Cmmm space group). Electrical resistivity, ac- and dc-magnetization measurements show that CeNiGe3 orders antiferromagnetically below TN = 5.5(2) K and exclude the occurrence at low temperatures of a spin-glass state for CeNiGe3 as previously reported. Specific heat measurements and NPD both reveal two magnetic transitions, observed at TN1 = 5.9(2) K and TN2 = 5.0(2) K. Between TN1 and TN2, the Ce magnetic moments in CeNiGe3 are ordered in a collinear antiferromagnetic structure associated with the k1 = (100) wavevector and showing a relationship with the magnetic structure of the Ce3Ni2Ge7 ternary germanide. Below TN2, this k1 = (100) commensurate magnetic structure coexists with an incommensurate helicoidal magnetic structure associated with k2 = (00.409(1)1/2). This last magnetic structure is highly preponderant below TN2 (93(5)% in volume). At 1.5 K, the Ce atoms in CeNiGe3 carry a reduced ordered magnetic moment (0.8(2) μB). This value, smaller than that obtained in Ce3Ni2Ge7, results from an important hybridization of the 4f(Ce) orbitals with those of the Ni and Ge ligands.

Crystal structure of Bi2Sr2CuO6: A structure based on periodic crystallographic shear planes in the “2201” structure

Abstract The crystal structure of the insulating phase Bi 2 Sr 2 CuO 6 is reported. The refinements were performed on X-ray and neutron powder diffraction data. The R values were R i =10.56% (R p =6.67%, R wp =8.40%) for X-ray and R i =7.4%(R p =5.01%, R wp =6.45%) for neutron data. The structure is derived from that of the pseudo-tetragonal “2201” phase by a periodic crystallographic shear parallel to the c tetr. axis. Slabs of “2201” type with a thickness of eight octahedra are then formed which are parallel to (001). The β angle of the monoclinic cell results from the periodic crystallographic shear which is a simple translation along the c axis. This structural model has been confirmed by HRTEM where the slabs of “2201” have been observed.