François Weill
Centre national de la recherche scientifique
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Journal of Materials Chemistry | 1999
François Cansell; B. Chevalier; A. Demourgues; J. Etourneau; Christophe Even; Vincent Pessey; S. Petit; A. Tressaud; François Weill
Supercritical fluids exhibit a range of unusual properties that can be exploited for new reactions which are qualitatively different from those involving classical solid state chemistry. After giving a brief introduction to these fluids we describe their use in inorganic chemistry and related fields. We then present two examples concerning different areas of solid state chemistry: (i) the formation of novel inorganic nanoparticles; (ii) the preparation of new open-structure oxy(hydroxy)fluorides, thus showing the advantages of this supercritical fluid processing that can be seen as an alternative method to regular solution chemistry or solid-gas reactions.
Journal of Applied Crystallography | 2000
Virginie Hornebecq; Catherine Elissalde; François Weill; Antoine Villesuzanne; M. Menetrier; J. Ravez
Electron diffraction analysis, nuclear magnetic resonance measurements, structure determinations and refinements were performed on two ferroelectric lead-containing ceramics (classical and relaxor), with tetragonal tungsten bronze structure, in order to investigate the origin of the relaxor behaviour in such compounds. In both compounds the lead atoms are found to be off-centred in the CN (coordination number) 15 (A2) site. The main difference between the two compositions is the proportion of lead in this site: in the relaxor composition, the A2 site is at least half occupied by lead atoms. Disorder in the repartition of lead and potassium atoms in the A1 (CN 12) and A2 sites was also studied via combinatorial calculations to correlate the evolution of the relaxor behaviour in the Pb5Ta10O30–Li10Ta10O30–K10Ta10O30 ternary system with composition. The combinatorial study demonstrated that the relaxor behaviour is enhanced by the disorder in cationic repartition.
Journal of Materials Chemistry | 1994
P. Gravereau; F. Mirambet; B. Chevalier; François Weill; L. Fournes; D. Laffargue; F. Bourée; J. Etourneau
The crystal structure of the new ternary stannide U2Pt2Sn has been investigated by both X-ray powder diffraction and electron diffraction. It crystallizes in a tetragonal unit cell with a= 768.1(1) pm and c= 739.1(1) pm. This crystal structure is a new superstructure of the tetragonal U3Si2-type which appears on account of the existence of short Pt–Pt distances. The crystal structure of U2Pt2Sn is described taking into consideration those of UPt and UPt2Sn.
Journal of Materials Chemistry | 1997
Peter D. Battle; Graeme R. Blake; Jacques Darriet; JonathanG. Gore; François Weill
The new phase Ba 6 CuIr 4 O 15 has been shown by powder X-ray diffraction to be a member of the perovskite-related structural family A 3n+3 A′ n B n+3 O 6n+9 withn=1; space group R32, a=10.1196(3), c=13.4097(4) A. The trigonal-prismatic A′ sites and the octahedral B sites are both occupied by a disordered distribution of Ir and Cu. A combination of electron microscopy and X-ray diffraction has shown that Ba 6 ZnIr 4 O 15 has a related incommensurate structure with a≈10.1, c≈4.4 A and a modulation vector q = a *+0.35 c *. Alternatively, Ba 6 ZnIr 4 O 15 can be considered as a composite of trigonal (cell 1) and rhombohedral (cell 2) substructures with a 1 =a 2 =10.1228(9) c 1 =4.4099(6), c 2 =2.6982(2) A. It is shown that the same structural formalism can be used to describe the incommensurate hexagonal phase of SrMn 1-x Co x O 3-y .
Journal of Materials Chemistry | 2003
Christophe Huber; Catherine Elissalde; François Weill; Mario Maglione
Nano-sized ferroelectric powders have been embedded for the first time in an amorphous silica shell. Massive composites made of these nano-objects have the same ferroelectric phase-transition temperature as the related pure ferroelectric phase, showing that chemical interdiffusion between the core and the shell is moderate. This is of key importance in building physical models for computing the electrical properties of the composites.
Inorganic Chemistry | 2008
B. Chevalier; Aleksandra A. Krolak; Jean-Louis Bobet; Etienne Gaudin; François Weill; Wilfried Hermes; Rainer Pöttgen
The intermediate valence compound Ce 2Ni 2Mg absorbs irreversibly hydrogen when exposed under 1 MPa of H 2 pressure at room temperature. The resulting hydride Ce 2Ni 2MgH 7.7 is stable in air and crystallizes as the deuteride La 2Ni 2MgD 8 in a monoclinic structure (space group P2 1 /c) with the unit cell parameters a = 11.7620(2), b = 7.7687(2), and c = 11.8969(2) A and beta = 92.75 degrees . The H-insertion in Ce 2Ni 2Mg induces a structural transition from a tetragonal to a monoclinic symmetry with an unit cell volume expansion Delta V m/ V m approximately 24.9%. The investigation of the hydride by magnetization, electrical resistivity, and specific heat measurements indicates a change from an intermediate valence behavior to a non-magnetic strongly correlated electron system. This transition results from a change of the coupling constant J cf between 4f(Ce) and conduction electrons induced by the hydrogenation.
Journal of Materials Chemistry | 1999
Peter D. Battle; Jonathan C. Burley; Edmund J. Cussen; Jacques Darriet; François Weill
Polycrystalline samples of bulk composition Ba 5 Mn 3 PdO 12 , Ba 6 Mn 4 PdO 15 , and Ba 7 Mn 5 PdO 18 have been synthesised and characterised by X-ray diffraction, electron diffraction and magnetometry. These compounds adopt a 2H-related crystal structure consisting of [001] chains of MnO 6 and PdO 6 polyhedra, with Ba 2+ cations separating the chains. The three compounds, each of which is commensurate in the xy plane but incommensurate along the z axis, differ in the ratio of octahedra to trigonal prisms in the chains (3:1, 4:1 and 5:1 respectively). The lattice parameters of the four dimensional trigonal unit cell are typically a≈10.02 A, c 1 ≈4.3 A, c 2 ≈2.61 A. The homogeneity and crystallinity of the samples increases with the ratio of octahedra to trigonal prisms and is greater in the xy plane than parallel to z. The magnetic susceptibility of all three phases deviates from the Curie-Weiss law below 100 K, but no magnetic phase transition is apparent above 5 K.
Zeitschrift für Naturforschung B | 2006
Etienne Gaudin; François Weill; B. Chevalier
The title compound was discovered as an impurity phase in many GdCoSi samples. It crystallizes in the hexagonal space group P63/m with a = 11.7787(5) and c = 4.1640(2) Å . Using X-ray powder diffraction, an ordered distribution between Co and Si was found but one site is not fully occupied by Co for steric reasons. Magnetization measurements reveal that Gd6Co1.67Si3 exhibits a ferromagnetic transition at TC = 294(2) K, a Curie temperature similar to that reported for pure gadolinium. This magnetic ordering has been confirmed by electrical resistivity investigations.
Nuclear Instruments & Methods in Physics Research Section B-beam Interactions With Materials and Atoms | 1995
A. Demourgues; C. Dussarrat; R. Bontchev; B. Darriet; François Weill; Jacques Darriet
Abstract New bismuth cubic or pseudo-cubic perovskite type phases have been prepared by solid state reactions. Cubic symmetry ( F m 3 ¯ m ) was observed for Ba2(AxBi2−x)O6−x/2 (A = Sr2+, Ca2+) compounds, except for Ba2Sr1/3Bi5/3O5.83 compositions where the structure crystallizes with monoclinic symmetry (I2/m). Creation of oxygen vacancies contributing towards a gradual contraction of the unit cell, are competing with Ca2+ or Sr2+ substitutions regarded as the partial replacement of Bi3+ leading to an increase of the unit-cell. The XRD patterns of the Ba2(RuxBi2−x)O6 (x = 1/2, 2/3) pseudo-cubic solid solutions are indexed using a body-centered monoclinic cell (I2/m), the unit cell volume decreasing as the Ru content increases. X-ray absorption spectroscopy measurements of Bi LIII near edge structure and EXAFS oscillations have revealed evidence of mixed valencies in these compounds and have given complementary and new information about the environment of bismuth which can be five or six-fold coordinated to oxygen atoms. The Bi LIII edges exhibit a continuous evolution with increasing Ca or Sr doping level or with lowering Ru doping. The Bi LIII edges are dominated by two main features due to 2p3/2 ↦ 6d and 2p3/2 ↦ 6s transtions due to the unfilled 6s states of Bi5+ only. Such a feature which is indicative of the absence of the lone pair allows an unambiguous determination of the highest valence states of these elements and to discriminate between Bi3+ and Bi5+. Detailed analysis of XAFS reveals that with increasing Ca or Sr doping, or with lowering Ru doping, the average Bi−O distance decreases continuously, in good agreement with XANES and XRD analysis.
Ferroelectrics | 2003
Christophe Huber; Catherine Elissalde; François Weill; Mario Maglione
Non-linear dielectrics which display electric-field dependent dielectric properties can be used in microwave devices such as tunable resonators or phase shifters. The required performances are essentially moderate permitivity (100–1000), low dielectric losses (≤1%) in the GHz frequency range, good thermal stability and high tunability of the dielectric susceptibility (>10%). The mixed perovskite Ba0.6Sr0.4TiO3 [BST] is an attractive candidate as its ferroelectric transition occurs close to room temperature and it displays high tunability but also strong temperature sensitivity and high frequency dielectric losses. To overcome these two deficiencies, ceramic/ceramic composites based on BST and a non-ferroelectric oxide such as MgO, MgTiO3, SnO2 and SiO2 were synthesized and compared to BST. For a better control of the losses a new concept based on a core-shell principle is presented. Through soft chemistry routes, a core made of BST grains is coated by a simple binary oxide shell. Modulation of the dielectric properties is expected by changing the core-shell structure. High frequency experiments are underway in order to check the GHz losses of these composites.