1,721,005 research outputs found
Disorder in oxides
No full textIn this paper the intimate correlation among structure, defect thermodynamics and physical properties in oxides is briefly reviewed through few examples concerning high Tc superconductors and ionic conductors. In some cases the desired physical properties appear only when disorder is introduced in the structures, like in neodymium cuprate and doped ceria. Disorder can be even expressly introduced in order to tune the physical properties already present in the undoped compound, which is the case of Al-doped REBa2Cu3O6+d. It will be demonstrated that not only the point defects and the local distortions induced by them, but also the correlation lengths of disorder are to be taken into account in order to correctly understand the physical properties of interesting materials
LOCAL DISORDER IN DOPED CERIA: A CRYSTALLOGRAPHIC STUDY
Full text linkIn recent years, doped ceria compounds have attracted increasing attention as electrolytes for application in Solid Oxide Fuel Cell devices, thanks to their high performance at intermediate temperature. In cerium oxide, ionic conductivity is driven by the presence of oxygen vacancies, introduced for charge balance after doping with a lower valent cation. With increasing doping concentration the oxygen diffusion is impeded, probably because of the formation of aggregates that trap oxygen vacancies. Their nature is though still under debate.
Owing to the close interplay between transport properties and local structure, an accurate description of the defects on the atomic scale is of the utmost importance for understanding the mechanisms at work in doped ceria.
In this study we propose to unveil the complex disorder in doped ceria with a combined powder diffraction study. On the one hand, X-ray and Neutron powder diffraction are joint to couple their potentialities; on the other hand, two different crystallographic approaches are exploited.
The conventional crystallographic methods allow to study the long range structure modifications that occur with doping. The Pair Distribution Function is instead employed to study the local structure, in terms of deviations from the long range atomic ordering.
The goal of the thesis is to provide a crystallographic description of the atomic rearrangements induced by doping, looking for a relationship between the local structure and the transport properties
Local disorder in yttrium doped ceria oxides (Ce1-xYxO2-x/2) probed by joint X-ray and Neutrons Powder Diffraction
No full textIn recent years yttrium doped ceria oxides (Ce1-xYxO2-x/2) have attracted increasing attention as electrolyte for Solid Oxide Fuel Cells because their high ionic conductivity can considerably reduce the operating temperature of such devices therefore limiting the overall manufacturing costs.
At increasing doping concentration, the isothermal ionic conductivity reaches a maximum and then decreases. This behavior is attributed to the formation of defect clusters / domains, whose crystallographic structure is still under debate.
A precise knowledge of the local structure of these materials is then needed and can be obtained through the combined real space and reciprocal space analysis of diffraction patterns.
A complementary study can be performed due to the different X-rays and neutrons scattering lengths of the involved elements. In particular Neutrons allow for an accurate determination of oxygen related parameters, whose contribution to X-rays scattering is almost negligible compared to the cations.
To this purpose both X-Ray Powder Diffraction (ESRF, ID31 beamline) and Neutron Powder Diffraction (ILL, D20 and D4 instruments) studies have been performed.
The effect of doping on the average structure (i.e. on space group, mean atomic positions and atomic mean square displacements) was analyzed using Rietveld while Pair Distribution Function (PDF) Technique was also used to explore structural distortions as well as the spatial extent of disorder in the so called real space.
The reciprocal space analysis shows that the atomic mean square displacements increase as a function of the doping concentration. At the same time the local structure obtained by the PDF analysis is not consistent with the mean crystallographic model and it is better described by considering complex defect clusters
Local disorder in yttrium doped ceria (Ce1−xYxO2−x/2) probed by joint X-ray and Neutron Powder Diffraction
No full textYttrium doped ceria materials (Ce1-xYxO2-x/2) are widely studied for their application in Solid Oxide Fuel Cells devices. An anomalous decrease in the isothermal ionic conductivity at increasing Y3+ concentration above a critical value has been observed and attributed to the formation of defect clusters / domains at the nanometric scale, the crystallographic structure of
which is still under debate. In this context we present a combined Synchrotron Radiation and Neutron Powder Diffraction study. In particular, neutrons allow to determine accurately oxygen related parameters, the contribution of which in terms of X-ray scattering power is
almost negligible when compared to that of cations. The effect of doping on the average structure is investigated using conventional Rietveld analysis, while the Pair Distribution
Function (PDF) technique is used to explore structural distortions and the spatial extent of disorder as well. The local structure observed in the real space is not consistent with the mean
crystallographic one and is better modeled considering a biphasic model
Nanoscale inhomogeneities in Ce1-xGdxO2-x/2 probed by PDF analysis and anomalous scattering
No full textCe1-xMxO2-x/2 (M = Gd, Y, Sm) have been intensively studied in the last years as catalysts, structural and electronic promoters for heterogeneous catalytic reactions and oxide ion conducting electrolytes for electrochemical cells.
As solid electrolytes, they display an ion conductivity higher than conventional Yttria-Stabilized-Zirconia-based ones and would operate at lower temperatures.
In functional materials competing internal forces produce complex structures. If two or more forces are competing, then a small external force can sometimes destroy the balance causing the system to respond with a very large effect.
The electronic properties of these materials are sensitive to the nanoscale rather than to the average structure
Probing complex disorder in Ce1-xGdxO2-x/2 using the pair distribution function analysis
No full textIn this work the first Pair Distribution Function (PDF) study on Ce1-xGdxO2-x/2 (CGO) electrolytes for solid oxide fuel cells is presented, aiming to unveil the complex positional disorder induced by gadolinium doping and oxygen vacancies formation in these materials. The whole range of Gd concentration x(Gd) (0 similar to 10 angstrom. The same models applied at lower r values fails to reproduce the experimental curves. A clear improvement of the fit quality in the 1.5 < r < similar to 6 angstrom range was obtained for all the CGO samples applying a biphasic model encompassing both a fluorite CeO2-like and a C-type Gd2O3-like phases. This provides evidence that extended defects at local scale exist in the CGO system. Gd-rich and Ce-rich droplets coexist in the subnanometric range
Structural characterization of Tb-and Pr-doped ceria
No full textThe crystallographic structure of Tb- and Pr-doped ceria is investigated through X-ray and neutron powder diffraction, combining pair distribution function and UV-vis spectroscopy. Compared to trivalent dopants, Tb and Pr show peculiar optical and crystallographic properties: whilst Tb have a mixed +3/+4 valence state, Pr is stable mostly in the +4 valence state up to similar to 50% dopant concentration. For larger Pr amounts, doping promotes a fluorite to monoclinic phase transformation. A straightforward method for extracting the dopant oxidation state by the cell parameter dependence on the ionic radius is also proposed
Defect clustering in Ce1-xGdxO2-x/2 using the PDF Analysis
Full text linkAim of this work is to unveil the complex positional disorder induced by gadolinium doping and oxygen vacancies formation in Ce1-xGdxO2-x/2 (CGO) electrolytes for solid oxide fuel cells by means of the Pair Distribution Function (PDF) analysis.
The whole range of Gd concentration xGd (0 ~10 Å. The same models applied at lower r values fails to reproduce the experimental curves. A clear improvement of the fit quality in the 1.5 < r < ~6 Å range was obtained for all the CGO samples applying a biphasic model encompassing both a fluorite CeO2-like and a C-type Gd2O3-like phases. This provides evidence that extended defects at local scale exist in the CGO. Gd-rich and Ce-rich droplets coexist in the sub-nanometric range
Comparison of the photocatalytic activityof TiO2-WO3 materials in oxidation and reduction reactions
No full textIn order to minimize the rate of the fast and undesired recombination of photoproduced e-/h+ pairs on the TiO2 surface, the effect induced by coupling TiO2 with different amounts of WO3, according to an alkaline-catalyzed sol-gel method followed by an incipient wetting procedure, has been investigated in both an oxidation and a reduction process, i.e. in formic acid (FA) mineralization and in Cr(VI) reduction in aqueous suspensions at pH 3.7 under UV-vis irradiation. A different photoactivity trend of TiO2-WO3 materials in the two investigated test reactions was clearly observed. While the progressive increase of W/Ti content limited the surface properties of titania in FA oxidation, the coupling of TiO2 with relatively small amounts of WO3 (0.2-1.0 mol.%) was beneficial in increasing the rate of Cr(VI) photoreduction, pointing to an essential role of WO3 domains in photopromoted electron transfer efficiency
Morphological characterization of shape-controlled TiO2 anatase through XRPD analysis
No full textPreferential growth of anatase crystallites along different directions is known to deeply affect their photocatalytic properties, especially with respect to the exposure of the reactive {001} facets. Its extent can be easily quantified through simple geometric calculations, on the basis of crystal sizes extracted for specific directions by means of X-Ray Diffraction data analysis. Nevertheless, the actual results depend on the method employed for such a quantification. Here we report on a comparative morphological investigation, performed by employing the Scherrer equation and the line profile from Rietveld refinements, on shape-controlled anatase photocatalysts produced by employing HF as capping agent. Compared to the Rietveld-based method, the use of the Scherrer equation produces a systematic underestimation of crystallite dimensions, especially concerning the [100] direction, which in turn causes the percentage of exposed {001} crystal facets to be underestimated. Neglecting instrumental-related effects may further reduce the estimate
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