Precession electron diffraction

Stmcture determination of unknown crystals by electron diffraction was performed by several research groups, on Al-Fe alloys by Gjonnes et al. (1998), on metal-cluster compounds by Weirich et al. (2000) and on zeolites by Wagner et al. (1999). Selected area electron diffraction or electron diffraction collected by a precession technique were used and the structure factor phases were deduced by direct methods, Patterson method or from convergent beam electron diffraction. 

Key words electron diffraction, precession method, electron diffractometry, structure determination... 

M.Geinmi Precession electron diffraction in EUROSUMMER SCHOOL Electron Crystalloggraphy and cryoelectron microscopy on inorganic materials and organic and biological molecules , Barcelona (Spain), July 23-27,2001... 

Another goal in this area is the recent ab initio determination of the framework stmcture of the heavy-metal oxide CsxNb2.54W2 460i4 from 100 kV precession electron diffraction data using the same approach [27] (for the... 

Most of the applications of electron diffraction intensities for structure analysis rely on a kinematical approximation and thus do not account for the effects of dynamical multiple diffraction. The use of intensities which may be strongly perturbed by multiple scattering results in many cases in poor or misleading structure indications in the direct methods results. One approach which can be shown to reduce dynamical effects somewhat is to use precession electron diffraction (RED) [67] which involves conical rotation of the incident beam about a zone axis direction and thus avoids the strongly dynamical direct zone axis orientation. Although the intensities collected with this technique are still significantly perturbed by dynamical effects [68, 69] results obtained by this approach for zeoHtes are encouraging [70-72]. 

Precession electron diffraction 1 multislice simulation. Acta Crystallogr. 

Application of a 2-beam model for improving the structure factors from precession electron diffraction intensities. Ultramicroscopy, 107, 543-550. 

Cone-angle dependence of Ab-initio structure solutions using precession electron diffraction. AIP Conf Proa, 999, 53-65. 

The number of structures solved using 3-D electron diffraction data sets is relatively small because of the experimental difficulties associated with the collection of 3-D diffraction data and dynamic scattering effects on diffraction intensities. A major development toward solving these difficulties is the precession electron diffraction technique, which allows a large number of reflections to be collected in a single orientation and averages over different beam directions to minimize dynamic effects. Progress is under way for more information see Refs. 40 and 41. 

Electron Diffraction (SAED), Microdiffraction, Convergent-Beam Eleetron Diffraetion (CBED), Large-Angle Convergent-Beam Eleetron Diffraetion (LACBED) and electron precession. They produee spot, ring, disk or line patterns at microseopie or nanoseopie seales in eorrelation with the image of the diffracted area. An overview of the main applieations is given. 

Convergent-Beam eleetron Diffraction and Microdiffraction) become available on analytical transmission electron microscopes. Most of the electron diffraction techniques use a stationary incident beam, but some specific methods like the precession method take advantage of a moving incident beam. 

Electron diffraction can also be performed with a moving incident beam. This is the case with the rocking beam method [2] and the precession method [3] which are schematically described on figure 7. Both methods use an incident beam which is angularly scanned within a cone or at constant angle on a specimen. The precession method is particularly useful for ab-initio structure determination since it allows to measure integrated intensities. 

Figure 3. (a)Beam movement in a TEM after applying conical dark field/precession mode (b)electron diffraction (ED) pattern of a LixMn204 nanocrystal in (111) orientation, with no application and after application of precession mode as it can be observed dynamical contribution to all reflections is greatly reduced. 

Figure 4. (left )Nd A13(B03)4 electron diffraction pattern near zone axis orientation, without precession (right) same orientation with precession... 

Electron diffractometry system with the combination of the precession technique can be very perspective experimental instrumentation for precise structural investigations. The technique can now be adapted in a commercial TEM (previously applied uniquely to electron diffraction cameras) taking advantage of the small beam size and can measure reflections in the ED pattern with same required precision for structure analysis. 

Gjonnes, J., Hansen, V., Berg, B. S., Runde, P., Cheng, Y. F., Gjonnes, K., Dorset D. L., Gilmore, C. J. (1998) Structure Model for the Phase Al Ee Derived from Three-Dimensional Electron Diffraction Intensity Data Collected by a Precession Technique. Comparison with Convergent-Beam Diffraction.", Hcta Cryst. A54, 306-319. 

Gemmi, M., Zou, X. D., Hovmdller, S., Migliori, A., Vennstrom, M., Andersson, Y. (2003) Structure of Ti2P solved hy three-dimensional electron diffraction data collected with the precession technique and high-resolution eleetron mieroseopy", Acta Cryst. A59, 117-126. 

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