162,085 research outputs found
The valence of Bi and Pb in BaPb1-xBixO3 superconducting oxide: a x-ray absorption study
No full textX-ray diffraction, extended x-ray absorption fine structure and Raman spectroscopy studies of WO3 powders and, (1-x)WO3-y center dot xReO(2) mixtures
No full textPure ground tungsten trioxide WO3 and (12x)WO32y xReO2 mixtures were studied by x-ray absorption spectroscopy, x-ray powder diffraction and Raman spectroscopy in comparison with
hydrogen bronzes HxWO3 and hydrogenated calcium tungstate CaWO4:H. It was found that a grinding of pure WO3 leads to a decrease of the crystallites size and a development of the bluish
coloration. The color change was found to be reversible under moderate heat treatment or after storage in oxidizing atmosphere and is attributed to the reduced W51 ions, located at the surface of freshly ground powder. The (12x)WO32y xReO2 mixtures were found to be composed of monoclinic/orthorhombic WO3 and orthorhombic ReO2 phases with a grain boundary containing reduced W5.71 ions which are mainly responsible for the compound color at low rhenium ion concentrations. In both cases, the W(62z)1 (0,z<1) color centers are responsible for strong optical absorption resulting in the dramatic decrease of the total Raman intensity. The structural models of free surface in pure ground WO3 and bulk WO3 /ReO2 intragrain boundary in
(12x)WO32y xReO2 mixtures are proposed and discussed
X-ray-absorption spectroscopy of Nd3+-exchanged beta-alumina crystal
No full textThe results of x-ray-absorption spectroscopy investigations of Nd31 ions in a nearly fully exchanged ~93%! sodium b-alumina crystal are reported. The Nd31 local environment is reconstructed using a multishell best fit procedure and is compared with existing structural models. The experimental extended x-ray-absorption finestructure ~EXAFS! spectra are also compared with model ab initio FEFF6 calculations based on the x-raydiffraction data for the Nd31 ions located at the BR~2d) and interstitial A(6h) sites of b-alumina. Neodymium ions are mainly found near the BR~2d) sites of the conduction plane, where they are strongly bonded to three O~5! oxygens located in the same plane and shifted by ;0.6 Å from the crystallographic positions occupied in sodium b-alumina. The atoms located at the spinel blocks below and above the conduction plane produce negligible contribution to the total Nd L3 edge EXAFS signal due to the polarization effect and, mainly, to thermal and/or static disorder. The similarity of the local structure around the Nd31 ions in the b- and b9-alumina crystals is also discussed
X-RAY-ABSORPTION SPECTROSCOPY OF A ND3+-EXCHANGED BETA''-ALUMINA CRYSTAL
No full textWe present an x-ray-absorption spectroscopy study of the electronic and crystallographic structure around the Nd3+ ions in 60% Nd-exchanged sodium beta''-alumina crystal. Using a multishell best-fit analysis procedure, we reconstructed the Nd3+ local environment and compared it with existing structural models. We found that neodymium ions are mainly located near the mO(9d) sites of the conduction region; they are strongly bonded to two O(5) oxygens which are located in the same plane and shifted from their ideal crystallographic positions in sodium beta''-alumina crystal. In particular, with respect to crystallographic parameters of sodium beta''-alumina, the Nd-O(5) distance is shortened by 0.36 angstrom, while the Nd-Al(6c) is slightly increased by 0.13 angstrom. Comparative analysis of Debye-Waller factors obtained for different coordination shells shows evidence of correlation in the vibrational motion of neodymium with the O(5) oxygens and with the Al(6c) aluminum
[Report to Chief J. E. Curry, by an unknown author #1]
No full textReport to Chief J. E. Curry, by an unknown author. The report contains a list of officers who gave depositions to the United States Attorney
[Report to Chief J. E. Curry, by an unknown author #2]
No full textReport to Chief J. E. Curry, by an unknown author. The report contains a list of officers who gave depositions to the United States Attorney
EXAFS STUDY OF ND3+-EXCHANGED BETA''-ALUMINA CRYSTAL
No full textThe local structure around the Nd3+ ions in 60%-exchanged sodium beta''-alumina was studied by X-ray absorption spectroscopy. The analysis of the Nd L3-edge experimental data shows that neodymium ions are mainly located near mid-oxygen (9d) sites. The local environment of Nd is slightly changed compared to crystallographic positions: the Nd-O(5) distance is shortened by 0.36 angstrom, while the Nd-Al (6c) is increased by 0.13 angstrom. The values of Debye-Waller factors of different coordination shells are discussed in terms of vibrational correlation
Low-temperature polymorphism in tungsten trioxide powders and its dependence on mechanical treatments
No full textThe polymorphism of WO3 powder samples, resulting from
mild mechanical treatments and from temperature changes between 30 K and room temperature, has been investigated by
using Raman spectroscopy and X-ray di¤raction. A transition
from the monoclinic (I) c-phase to the triclinic d-phase after
moderate mechanical treatments has been observed for untreated
powder, just what happens when the same samples are rapidly
cooled to low-temperature. Evidences of the low temperature
monoclinic (II) polar e-phase have been found at room temperature in samples after a stronger milling treatment. The sequence of the low-temperature phase transitions appears to be strongly dependent on the mechanical history of the powders. A new lowtemperature N-phase has been observed below about 200 K in di¤erent samples: it is the main phase in commercial untreated powders, having the monoclinic (I) c-phase at room temperature, but constitutes only a small fraction in moderately treated powders, having the triclinic d-phase at room temperature
Color Centres and Polymorphism in Pure WO(3) and Mixed (1-x)WO(3-y)center dot xReO(2) Powders
No full textThe relationship between structural transformations and colour centres creation is discussed for deeply coloured hydrogen tungsten bronzes and for pure WO) powders, acquiring less intense colour after mechanical treatments of variable duration. A comparative study on coloration is made also for mixed compounds (l-x)WO].y·xRe01, where anevidence of a resonance effect for a particular Raman band at 970 ern", attributed to the color centres, is observed. Besides, it is found that even moderate milling treatments result in a quite different structural evolution of tungsten trioxide uponcooJing
- …
