J.P. Coutures

Two-dimensional magic-angle spinning isotropic reconstruction sequences for quadrupolar nuclei.

Two-dimensional magic-angle spinning (triple quantum, single quantum) correlation pulse sequences and phase cycles based on the technique of Frydman and Harwood for the reconstruction of the isotropic spectrum of half-integer spin quadrupolar nuclei broadened to second-order are described. These sequences provide pure absorption mode two-dimensional lineshapes and increased sensitivity. Experimental examples on spin I = 3/2 (87Rb in RbNO3) and I = 5/2 (27Al in NaSi3AlO8) are presented. The isotropic chemical shift and quadrupolar coupling parameters could be obtained from a simple analysis of the triple quantum filtered single quantum magic-angle spinning cross-sections.

Phase relations in the LaNiO system: Influence of temperature and stoichiometry on the structure of La2NiO4

Abstract This paper reports a study of the stability of the various phases in the La-Ni-O system and of the influence of temperature and stoichiometry on the crystal structure of La 2 NiO 4 . Several compositions have been synthesized, among which the second term of the homologous series La n +1 Ni n O 3 n +1 is stable in air up to 1200°C, this temperature probably being dependent on the oxygen partial pressure. Above 1200°C for La/Ni 2 NiO 4 and NiO, a result which is important for the preparation of compounds in this system. The observed crystal structure of La 2 NiO 4 is affected by the stoichiometry of the compound. Under reducing conditions the structure is orthorhombic rather than tetragonal with a = 5.4614 A, b = 5.5322 A, c = 12.527 A for the most reduced sample. The possibility is raised that this is the real structure of La 2 NiO 4 . The thermal expansion coefficient has been calculated from the evolution of the unit cell with temperature. Good agreement is obtained between microscopic X-ray data and macroscopic measurements giving a mean thermal expansion coefficient of approximately α¯= 15 × 10 −6 ° C −1 between 20 and 1100°C. The tetragonal c / a ratio has been used to analyze the deformation of the unit cell with temperature. The ratio attains a maximum for T ∼ 600K and this is interpreted as being associated with the transition from a semiconductor to a metallic state.

Characterization of mono- and diphasic mullite precursor powders prepared by aqueous routes. 27Al and 29Si MAS-NMR spectroscopy investigations

The structural evolution from amorphous to crystalline mullite, for different 3Al2O3 · 2SiO2 mono- and diphasic precursors, has been investigated by 29Si and 27Al magic angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy. The crystallization has also been studied by X-ray diffraction (XRD) and differential scanning calorimetry (DSC). The chemical composition in the aluminosilicate network of the diphasic precursors and in the crystallized phases has been determined from the 29Si NMR spectra. A close agreement is found with the composition deduced from the lattice parameters measured by XRD. For monophasic precursors the amount of hexa-coordinated aluminium atoms decreases when the temperature increases while Al(IV) and Al(V) increase. Al(VI) practically completely disappears just before the crystallization at 980 °C. An alumina-rich mullite 2Al2O3 · SiO2 (2∶1 mullite) is then formed through a strong exotherm. An enthalpy of 75 kJ per mol is determined for the crystallization of the 2∶1 mullite. At higher temperatures the segregated silica is progressively reincorporated into the mullite lattice. For diphasic precursors the 29Si NMR spectroscopy shows the segregation of silica. The aluminosilicate network is then richer in alumina and the amount of remaining AlO6 octahedra before the crystallization at 980 °C is higher. Spinel crystallizes and continues to become richer in alumina until it reacts with silica to form the stoichiometric 3∶2 mullite at 1260–1275 °C. The nature of the crystallization is related to the local composition of the amorphous alumino-silicate network and to the amount of AlO6 octahedra on approaching 980 °C.

New synthesis of mullite. Structural evolution study by 17O, 27Al and 29Si MAS NMR spectroscopy

Mullite has been prepared from a new combination of precursors. An aluminum alkoxide, aluminium isopropoxide, and silicon tetrachloride, are hydrolysed in tetrahydrofuran solution by 17O enriched water. The resulting powder is chemically homogeneous, crystallizing into mullite at 980°C. The structural evolution has been studied by DTA, TGA, XRD and 17O, 27Al and 29Si MAS NMR spectroscopy.

Crystallization of Y3Al5O12 garnet from deep undercooled melt effect of the Al-Ga substitution

Abstract Melts of garnet Y 3 Al 5 O 12 and compositions lying on both sides of this compound were rapidly cooled in contactless conditions by using an aerodynamic levitation device coupled with a CO 2 laser. The crystallized phases were analyzed at room temperature by XRD and 27 Al MAS NMR. As the temperature reached by the supercooled melt decreases, the solidification was shown to lead successively to a mixture of YAlO 3 and Al 2 O 3 , a monophasic garnet type structure and a glass. The formation of Y 3 Al 5 O 12 starts at about 920 °C under the melting temperature and only 50 °C above the glass transition temperature. The solidification behaviour of liquid Y 3 Al 5− x Ga x O 12 ( x = 0.5, 1) was similar. Results clearly show the difficulties of garnet to nucleate. They are consistent with a rapid increase of the melt viscosity in the temperature range just above the formation of glass.

Implications of the Y2O3BaOCuO liquidus for processing pure YBa2Cu3O7−x material

Abstract Results relating to the liquids relationships in the CuO-rich corner of the 1 2 Y 2 O 3  BaO  CuO phase diagram were used to identify the partial meltings occurring in YBaCuO powders at temperatures below the peritectic decomposition of the YBa2Cu3O7−x compound. The production of this compound in three atmospheres with different oxygen potentials was followed by X-ray diffraction and differential scanning calorimetry. The origin of the partial meltings in these powders was found in the failure of the synthesis to be complete (i.e. to reach the equilibrium state) at temperatures below the solid-liquid phase transitions, implying the presence of the compounds YBa2Cu3O7−x, BaCuO2 and CuO. This difficulty is overcome if the powders are annealed in an O2 atmosphere which allows treatment temperatures as high as 940°C. Sintering of such powders up to 1000 °C is not promoted by a liquid phase. Differential scanning calorimetry proved to be an easy and valuable method for checking the purity of the YBa2Cu3O7−x compound.

Metastable solidification process of Ln3Al5O12 (Ln≡Dy, Y, Lu) garnets

Abstract The solidification process of liquid Ln 3 Al 5 O 12 (Ln ≡ Dy, Y, Lu) garnets was investigated by differential thermal analysis (DTA) and a critical temperature T cl was found. Cooling from temperatures below T cl leads to the crystallization of the single garnet phase. Cooling from temperatures above T cl leads to the solidification of a mixture of LnAlO 3 perovskite and α-Al 2 O 3 . For Ln ≡ Y, the results are in agreement with previously proposed stable and metastable phase diagrams. The variation of T cl with composition was investigated. Rapid cooling of liquid yttrium aluminium garnet in contactless conditions was performed, using an aerodynamic levitation device coupled with a CO 2 laser. The results are consistent with those obtained by DTA. In addition, cooling from temperatures above around 2400 °C leads to the crystallization of a metastable form of garnet.

YBa2Cu3O7−x single-crystal growth by a new “Y2O3 consumable crucible” technique

Abstract A new method of growing YBa 2 Ca 3 O 7−x single crystals is described which uses an yttria crucible containing an appropriate BaOCuO mixture. The yttria crucible supplies yttrium to the melted mixture and YBa 2 Cu 3 O 7−x crystallizes as the primary phase. Large single crystals are obtained which are superconducting at 40 K and the oxygen is probably homogeneously distributed. Almost complete reoxidation is achieved after 60 h of annealing at 450°C under oxygen, giving the superconducting phase at 88 K.

Synthesis and structural characterisation of Sr3Al10SiO20 by XRD and solid-state NMR

Abstract The strontium aluminosilicate Sr3Al10SiO20 has been synthesised by a spray-drying process and characterised by powder X-ray diffraction. It crystallises in the monoclinic I2/m space group with the cell parameters a=14.394(2) A ,b=11.189(2) A ,c=4.904(1) A ,β=90.793(1)°,Z=2 . Its structure is built of double AlO6 octahedra chains interconnected by AlO4 and SiO4 tetrahedra forming a three-dimensional channel-like network where the strontium cations are located. The distribution of aluminium and silicon cations in the different tetrahedral and octahedral sites of the structure was probed using 29 Si, 27 Al MAS and 3Q-MAS NMR.

Etude à haute temperature du diagramme d'equilibre du système La2O3Yb2O3

Abstract The high temperature phase diagram La 2 O 3 Yb 2 O 3 has been studied from 1400°C to the melting point, all together by direct thermal analysis and by X-ray diffraction. Particularly we observe, on the melting point, a cubic solution largely extending from La 2 O 3 near to Yb 2 O 3 . The LaYbO 3 perovskite-type compound which appears below 2040°C, is surrounded by two eutectoids.