Sylvie Malo

Hollow core@mesoporous shell boron nitride nanopolyhedron-confined ammonia borane: a pure B–N–H composite for chemical hydrogen storage

Ammonia borane-derived boron nitride nanopolyhedra with a hollow core@mesoporous shell structure displaying a specific surface area of 200.5 m2 g−1, a total pore volume of 0.287 cm3 g−1 and a bimodal pore size distribution were successfully prepared and used as nanoscaffolds to improve the dehydrogenation properties of ammonia borane by an exclusive nanoconfinement effect.

The dehydration of SrTeO3(H2O) - a topotactic reaction for preparation of the new metastable strontium oxotellurate(IV) phase ε-SrTeO3

Microcrystalline single-phase strontium oxotellurate(IV) monohydrate, SrTeO(3)(H(2)O), was obtained by microwave-assisted hydrothermal synthesis under alkaline conditions at 180 °C for 30 min. A temperature of 220 °C and longer reaction times led to single crystal growth of this material. The crystal structure of SrTeO(3)(H(2)O) was determined from single crystal X-ray diffraction data: P2(1)/c, Z = 4, a = 7.7669(5), b = 7.1739(4), c = 8.3311(5) Å, β = 107.210(1)°, V = 443.42(5) Å(3), 1403 structure factors, 63 parameters, R[F(2)>2σ(F(2))] = 0.0208, wR(F(2) all) = 0.0516, S = 1.031. SrTeO(3)(H(2)O) is isotypic with the homologous BaTeO(3)(H(2)O) and is characterised by a layered assembly parallel to (100) of edge-sharing [SrO(6)(H(2)O)] polyhedra capped on each side of the layer by trigonal-prismatic [TeO(3)] units. The cohesion of the structure is accomplished by moderate O-H···O hydrogen bonding interactions between donor water molecules and acceptor O atoms of adjacent layers. In a topochemical reaction, SrTeO(3)(H(2)O) condensates above 150 °C to the metastable phase ε-SrTeO(3) and transforms upon further heating to δ-SrTeO(3). The crystal structure of ε-SrTeO(3), the fifth known polymorph of this composition, was determined from combined electron microscopy and laboratory X-ray powder diffraction studies: P2(1)/c, Z = 4, a = 6.7759(1), b = 7.2188(1), c = 8.6773(2) Å, β = 126.4980(7)°, V = 341.20(18) Å(3), R(Fobs) = 0.0166, R(Bobs) = 0.0318, Rwp = 0.0733, Goof = 1.38. The structure of ε-SrTeO(3) shows the same basic set-up as SrTeO(3)(H(2)O), but the layered arrangement of the hydrous phase transforms into a framework structure after elimination of water. The structural studies of SrTeO(3)(H(2)O) and ε-SrTeO(3) are complemented by thermal analysis and vibrational spectroscopic measurements.

Cationic substitution in mercury-based cuprates: generation of five superconductors

Five mercury-based superconductors, Hg1–xMxBa2CuO4+δ with MMn, Nb, Hg1–xMxBa2CaCu2O6+δ with M = Mn, Ge and Hg1–xNbxBa2Ca2Cu3O8+δ, have been synthesized. A significant decrease of the c parameter of the tetragonal cells of the Ge and Mn phases with respect to the unsubstituted mercury cuprates is observed for the Ge and Mn phases, whereas the Nb phases exhibit a ‘c’ parameter close to that of the unsubstituted cuprates, in agreement with the relative sizes of MnIV, GeIV and NbV. EDS analyses confirm that Mn, Ge and Nb have entered into the matrix of the cuprate in a homogeneous way. Magnetic studies of the as-synthesized phases and of the samples after argon and oxygen annealing show sharp transitions except for the 1201-Nb phase. The critical temperatures, Tc(midpoint), of these phases after optimisation are close to those of the unsubstituted mercury cuprates: 91 K for the 1201-Mn phase, 123–124 K for the 1212 Mn and Ge phases and 128 K for the 1223 Nb phase. The investigation of the AI-substituted mercury cuprates is also discussed.

Nanocomposites through the Chemistry of Single-Source Precursors: Understanding the Role of Chemistry behind the Design of Monolith-Type Nanostructured Titanium Nitride/Silicon Nitride

Monolith-type titanium nitride/silicon nitride nanocomposites, denoted as TiN/Si3 N4 , have been prepared by a reaction of polysilazanes with a titanium amide precursor, warm pressing of the resultant polytitanosilazanes, and subsequent pyrolysis of the green bodies at 1400 °C. Initially, a series of polytitanosilazanes was synthesized and the role of the chemistry behind their synthesis was studied in detail by using solid-state NMR spectroscopy, elemental analysis, and molecular-weight measurements. The intimate relationship between the chemistry and the processability of these precursors is discussed. Polytitanosilazanes display the appropriate requirements for facile processing in solution and as a melt, but they must be treated with liquid ammonia to be adapted for solid-state processing, that is, warm-pressing, to design dense and mechanically stable structures after pyrolysis. We provide a comprehensive mechanistic study of the nanocomposite conversion based on solid-state NMR spectroscopy coupled with thermogravimetric experiments. HRTEM images coupled with XRD and Raman spectroscopy confirmed the unique nanostructural features of the nanocomposites, which appear to be a result of the molecular origin of the materials. The as-obtained samples are composed of an amorphous Si3 N4 matrix, in which TiN nanocrystals are homogeneously formed in situ in the matrix during the process. The hardness and Young moduli were measured and are discussed.

Electrochemical synthesis of a lithium-rich rock-salt-type oxide Li5W2O7 with reversible deintercalation properties.

Starting from the ribbon structure Li2W2O7, the lithium-rich phase Li5W2O7 with an ordered rock-salt-type structure has been synthesized, through a topotactic irreversible reaction, using both electrochemistry and soft chemistry. In contrast to Li2W2O7, the lithium-rich oxide Li5W2O7 shows reversible deintercalation properties of two lithium molecules per formula unit: a stable reversible capacity of 110 mAh/g at 1.70 V is maintained after 10 cycles. The exploration of the lithium mobility in this system shows that Li2W2O7 is a cationic conductor with σ = 4.10(-4) S/cm at 400 °C and Ea = 0.5 eV.

The modulated structure of the calcium aluminate Ca6(AlO2)12·Bi2O3

Bismuth calcium aluminate, Bi(2)Ca(6)Al(12)O(27), has been prepared as a ceramic and a single crystal. Analysis of reciprocal space using both electron and X-ray diffraction show an R-centred hexagonal unit cell: a = b = 17.3892 (4), c = 6.986 (1) Å. Additional weak reflections are observed; they require the introduction of a modulation wavevector q = 0.0453 (2)c* for indexing. The modulated structure has been solved using the superspace formalism [superspace group X3(00γ)0]. A framework of corner-sharing AlO(4) tetrahedra forms corrugated sixfold rings and uncommon triple rings. The Ca(2+) cations exhibit an eightfold coordination sphere; edge-sharing CaO(8) polyhedra form intertwinned zigzagging rows along c creating a three-dimensional net. Bi atoms are located in large hexagonal tunnels parallel to c and form Bi(2)O(3) pairs, which adopt a trigonal bipyramidal configuration. The 6s(2) lone-electron pairs (Lp) point along c, in the opposite direction to the three Bi-O strong bonds to form two BiO(3)Lp tetrahedra with a common base. Different orientations of the Bi(2)O(3)Lp(2) pairs, rotated by 60° around c, are observed. Their stacking modes in each of the hexagonal tunnels are described. The sequence of the stacking varies along c in each of the tunnels.

Changes of phase composition of Zr1-x(Ce/Y)xO2 complex oxides induced by thermal treatment

The medical prosthesis components made from (ZrO2)-based ceramics present a good biocompatibility as well as especially mechanical properties. Much more, the problems of nondestructive evaluation for these elements, which assure both comfort and maximum safety to the patient, are imperative for these medical implants. In this study, we investigate the structure and the mechanical properties of Zr1-x(Ce/Y)xO2 , (x=0.0;0.09;0.13;0.17) materials as well as the modification of their crystallographic structure due to various thermal treatments and variation of Ce/Y concentrations in the samples. The substitution of Zr with Ce and the thermal treatment at 1000°C produced important transformation in the phase composition and the microstructure of the sample. A large decrease of the microstrains was observed at the treated samples. Combining characterization techniques based on XRD and ND with non-destructive evaluation methods based on Resonant Ultrasound Spectroscopy (RUS) and Acoustic Emission (AE), we emphasize a unique approach on evaluating the physical properties of these ceramics. Performed evaluation tests of Zr1-x(Ce/Y)xO2 ceramics have shown big influence of composition on their fracture behavior and resulting strength due to different damage mechanisms.

Sr21Bi8Cu2(CO3)2O41, a Bi5+ Oxycarbonate with an Original 10L Structure

The layered structure of Sr21Bi8Cu2(CO3)2O41 (Z = 2) was determined by transmission electron microscopy, infrared spectroscopy, and powder X-ray diffraction refinement in space group P6₃/mcm (No. 194), with a = 10.0966(3)Å and c = 26.3762(5)Å. This original 10L-type structure is built from two structural blocks, namely, [Sr15Bi6Cu2(CO3)O29] and [Sr6Bi2(CO3)O12]. The Bi(5+) cations form [Bi2O10] dimers, whereas the Cu(2+) and C atoms occupy infinite tunnels running along c⃗. The nature of the different blocks and layers is discussed with regard to the existing hexagonal layered compounds. Sr21Bi8Cu2(CO3)2O41 is insulating and paramagnetic.