213 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
Study of N2O formation over Rh- and Pt-based LNT catalysts
In this paper, mechanistic aspects involved in the formation of N2O over Pt-BaO/Al2O3 and Rh-BaO/Al2O3 model NOx Storage-Reduction (NSR) catalysts are discussed. The reactivity of both gas-phase NO and stored nitrates was investigated by using H2 and NH3 as reductants. It was found that N2O formation involves the presence of gas-phase NO, since no N2O is observed upon the reduction of nitrates stored over both Pt- and Rh-BaO/Al2O3 catalyst samples. In particular, N2O formation involves the coupling of undissociated NO molecules with N-adspecies formed upon NO dissociation onto reduced Platinum-Group-Metal (PGM) sites. Accordingly, N2O formation is observed at low temperatures, when PGM sites start to be reduced, and disappears at high temperatures where PGM sites are fully reduced and complete NO dissociation takes place. Besides, N2O formation is observed at lower temperatures with H2 than with NH3 in view of the higher reactivity of hydrogen in the reduction of the PGM sites and onto Pt-containing catalyst due to the higher reducibility of Pt vs. Rh
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
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
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