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January 24, 2013
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January 24, 2013
Abstract
Electron microscopy techniques yield information for crystal structure analysis that is remarkably complementary to that obtained from X-ray powder diffraction data. Structures of polycrystalline materials that resist solution by either method alone can sometimes be solved by combining the two. For example, the intensities extracted from an X-ray powder diffraction pattern are kinematical and can be interpreted easily, while those obtained from a typical selected area electron diffraction (SAED) or precession electron diffraction (PED) pattern are at least partially dynamical and therefore more difficult to use directly. On the other hand, many reflections in a powder diffraction pattern overlap and only the sum of their intensities can be measured, while those in an electron diffraction pattern are from a single crystal and therefore well separated in space. Although the intensities obtained from either SAED or PED data are less reliable than those obtained with X-rays, they can be used to advantage to improve the initial partitioning of the intensities of overlapping reflections. However, it is the partial crystallographic phase information that can be extracted either from high-resolution transmission electron microscopy (HRTEM) images or from PED data that has proven to be particularly useful in combination with high-resolution X-ray powder diffraction data. The dual-space (reciprocal and real space) structure determination programs Focus and Superflip have been shown to be especially useful for combining the two different types of data.
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Open Access
October 22, 2012
Abstract
Intergrowth and stacking disorders are often found in minerals and synthetic zeolites and inorganic open-frameworks. Structure elucidation of stacking disorders in these materials have been difficult and structure solution of stacking disorders in unknown zeolites and open-frameworks has been challenging. There exist no standard methods for structure analysis of such disordered materials. In this review we present various stacking disorders and intergrowth in a number of representative zeolite families containing stacking disorders. These include zeolite beta, SSZ-26/SSZ-33, ITQ-39, ABC-6, ZSM-48, SSZ-31, UTD-1, faujasite FAU/EMT, pentasil ZSM-5/ZSM-11, ITQ-13/ITQ-34, ITQ-22/ITQ-38 etc. Stacking disorders in open-frameworks containing mixed coordinations, including titanosilicates ETS-10 and ETS-4, and the silicogermanate SU-JU-14 are also described. Various crystallographic methods used for solving disordered structures are summarized. The methods include powder X-ray diffraction (PXRD), electron diffraction, high resolution transmission electron microscopy (HRTEM), and single-crystal X-ray diffraction. Examples of model building combined with simulations of PXRD and single crystal X-ray diffraction to verify the structure models are given.
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January 24, 2013
Abstract
The crystal structure of SrCo 0.7 Fe 0.2 Nb 0.1 O 2.72 was determined using a combination of precession electron diffraction (PED), high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) and spatially resolved electron energy loss spectroscopy (STEM-EELS). The structure has a tetragonal P 4/ mmm symmetry with cell parameters a = b = a p , c = 2 a p ( a p being the cell parameter of the perovskite parent structure). Octahedral BO 2 layers alternate with the anion-deficient BO 1.4 layers, the different B cations are randomly distributed over both layers. The specific feature of the SrCo 0.7 Fe 0.2 Nb 0.1 O 2.72 microstructure is a presence of extensive nanoscale twinning resulting in domains with alignment of the tetragonal c -axis along all three cubic direction of the perovskite subcell.
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January 11, 2013
Abstract
We present several examples of crystal structures solved by precession electron diffraction which cover a great domain of different structures from very small to very large unit cells, having very high or low symmetry, containing light or heavy elements or both. A common point of these samples is that they were synthesized as powders. X-ray powder diffraction encountered difficulties in solving these structures because the powders were a mixture of phases or because the peak overlap was too severe due to large unit cell parameters. The influence of various experimental parameters and data exploitation procedures on the successful structure solution are investigated. It is shown in particular that the amount of data available is the most influential parameter, whereas the precision of the intensity measurement or of the determination of the structure factor amplitudes from the intensities is of less importance provided that the experimentally strong reflections correspond to the high structure factor amplitudes. This is precisely the achievement of the precession electron diffraction method, which reduces multiple scattering, while in standard selected area electron diffraction multiple scattering leads to intensities that do not show a clear correlation between intensities and structure factor amplitudes.
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December 10, 2012
Abstract
We report initial findings on improvements to precession electron diffraction (PED) achieved through aberration correction of the probe-forming lens and by energy-filtering. Using current-generation aberration correctors, we show that PED patterns can, in principle, be acquired with sub-nm spatial resolution. We present initial experimental results that illustrate aberration-corrected PED of nanostructured alloys. We show also that zero loss energy filtering minimizes the inelastic background in a PED pattern, important for weak reflections, and leads to an improvement in the refinement of a crystal structure using elastic-only intensities.
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January 24, 2013
Abstract
Structure solutions of CaFe 2 O 4 from energy filtered and unfiltered precession electron diffraction tomography and rotation electron diffraction tomography data, collected on two different microscopes, are reported. The collected data are analysed with three available software packages (ADT3D, PETS and EDT-PROCESS) and the obtained results are compared. In all cases the structure solution is successfully achieved. Energy filtered precession electron diffraction tomography, performed here for the first time, gives sharper diffraction peaks and less background compared to the unfiltered data and the final recovered model is closer to the X-ray refinement. Simultaneously the first crystal structure solution obtained from the rotation electron diffraction tomography data is reported.
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January 11, 2013
Abstract
Three ternary intermetallic phases were revealed in Mg-20 at%Al—20 at%Ag alloy: Mg 59 Al 34 Ag 7 (body-centered cubic, a = 10.2 Å, probably isotypical to Mg 17 Al 12 phase); Mg 62 Al 11 Ag 27 (body centered cubic, a = 14.5 Å) and new Mg 19 Al 13 Ag 6 compound. The geometry and the symmetry of the unit cell of these phases were investigated by electron diffraction methods. It was found that the structure of the new Mg 19 Al 13 Ag 6 phase belongs to the orthorhombic crystal system with lattice parameters of a = 8.87, b = 16.48 and c = 19.48 Å. The space group was evaluated by Precession Electron Diffraction technique in “descan-off” mode as Pbcm (No. 57).