130 research outputs found
Observation of the cluster spin-glass phase in La_{2-x}Sr_{x}CuO_{4} by anelastic spectroscopy
An increase of the acoustic absorption is found in La_{2-x}Sr_{x}CuO_{4} (x =
0.019, 0.03 and 0.06) close to the temperatures at which freezing of the spin
fluctuations in antiferromagnetic-correlated clusters is expected to occur. The
acoustic absorption is attributed to changes of the sizes of the quasi-frozen
clusters induced by the vibration stress through magnetoelastic coupling.Comment: LaTeX, 2 PostScript figures, submitted to Phys. Rev.
Determination of the high-pressure crystal structure of BaWO4 and PbWO4
We report the results of both angle-dispersive x-ray diffraction and x-ray
absorption near-edge structure studies in BaWO4 and PbWO4 at pressures of up to
56 GPa and 24 GPa, respectively. BaWO4 is found to undergo a pressure-driven
phase transition at 7.1 GPa from the tetragonal scheelite structure (which is
stable under normal conditions) to the monoclinic fergusonite structure whereas
the same transition takes place in PbWO4 at 9 GPa. We observe a second
transition to another monoclinic structure which we identify as that of the
isostructural phases BaWO4-II and PbWO4-III (space group P21/n). We have also
performed ab initio total energy calculations which support the stability of
this structure at high pressures in both compounds. The theoretical
calculations further find that upon increase of pressure the scheelite phases
become locally unstable and transform displacively into the fergusonite
structure. The fergusonite structure is however metastable and can only occur
if the transition to the P21/n phases were kinetically inhibited. Our
experiments in BaWO4 indicate that it becomes amorphous beyond 47 GPa.Comment: 46 pages, 11 figures, 3 table
High-pressure structural study of the scheelite tungstates CaWO4 and SrWO4
Angle-dispersive x-ray diffraction (ADXRD) and x-ray absorption near edge
structure (XANES) measurements have been performed in the AWO4 tungstates CaWO4
and SrWO4 under high pressure up to approximately 20 GPa. Similar phase
transitions and phase transition pressures have been observed for both
tungstates using the two techniques in the studied pressure range. Both
materials are found to undergo a pressure-induced scheelite-to-fergusonite
phase transition under sufficiently hydrostatic conditions. Our results are
compared to those found previously in the literature and supported by ab initio
total energy calculations. From the total energy calculations we have also
predicted a second phase transition from the fergusonite structure to a new
structure identified as Cmca. Finally, a linear relationship between the charge
density in the AO8 polyhedra of ABO4 scheelite-related structures and the bulk
modulus is discussed and used to predict the bulk modulus of other materials,
like zircon.Comment: 52 pages, 9 figure, 4 table
Direct dehydration of 1,3-butanediol into butadiene over aluminosilicate catalysts
The catalytic dehydration of 1,3-butanediol into butadiene was investigated over various aluminosilicates with different SiO2/Al2O3 ratios and pore architectures. A correlation between the catalytic performance and the total number of acid sites and acid strength was established, with a better performance for lower acid site densities as inferred from combined NH3-TPD, pyridine adsorption and 27Al-NMR MAS spectroscopy. The presence of native Brønsted acid sites of medium strength was correlated to the formation of butadiene. A maximum butadiene yield of 60% was achieved at 300 °C over H-ZSM-5 with a SiO2/Al2O3 ratio of 260 with the simultaneous formation of propylene at a BD/propylene selectivity ratio of 2.5. This catalyst further exhibited a slight deactivation during a 102 h run with a decrease in the conversion from 100% to 80% due to coke deposition as evidenced by XPS and TGA-MS, resulting in a 36% loss of the specific surface area
Long-term follow-up of children with Helicobacter pylori-associated nonulcer dyspepsia after eradication of the infection.
Surface and Structure Characterization of Some Perovskite-type Powders To Be Used As Combustion Catalysts
Some perovskite-type transition-metal mixed oxide powders active in the combustion catalysis, with formulas LaMO(3) (M = Fe, Cr, Co, and Mn), have been prepared by different methods. Similarly, potential supports for combustion catalysts nearly isostructural with the above ones have been prepared, with compositions SrZrO3 and LaAlO3. Mixed phases with formulas Sr1-xLaxZr1-xMnxO3 have also been investigated. The bulk properties have been studied by XRD, far-IR spectroscopy, and DTA-TG analyses. Morphological properties have been determined by XRD, SEM, and surface area measurements. These phases crystallize after heating at 970 K (except LaFeO3, which is already crystalline after calcination at 770 K) with surface area ranging 10-20 m(2)/g. The solubility of LaMnO3 into SrZrO3 is limited to near 10%. Their surface properties have been investigated by IR spectroscopy that showed a predominantly basic surface character of all these solids
Attività in vitro di nuovi fluorochinoloni ed altri farmaci antibatterici nei confronti di Helicobacter pylori di recente isolamento
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