Convergent beam

Figure Bl.8.7. A convergent beam diffraction pattern of the fivefold axis of a quasicrystal, as in figure Bl.8.6. The diffraction rings show that the synnnetry is fivefold, not tenfold. (Courtesy of L Bendersky.)...

Tanaka M and Terauchi M 1985 Convergent-Beam Eleotron Diffraotion JEOL, Tokyo... 

Lattice parameters to four significant figures using convergent beam diffraction... 

J. Mansfield. Convergent Beam Electron Dijfraction of Alloy Phases, (by the Bristol Group, under the direction of J. Steeds, and compiled by J. Mansfield) Hilger, Bristol, 1984. This book is an atlas of CBED patterns that may be used to identify phases by comparing published patterns with experimental patterns. 

Transmission Electron Microscopy Transmission Electron Microscope Conventional Transmission Electron Microscopy Scannir Transmission Electron Microscopy High Resolution Transmission Electron Microscopy Selected Area Diffraction Analytical Elearon Microscopy Convergent Beam Elearon DifFraaion Lorentz Transmission Electron Microscopy... 

Mile. Cauchois20 used in transmission with a convergent beam a thin crystal bent cylindrically with the Bragg planes about as shown in Figure 4-11. Mica, gypsum, and quartz proved suitable materials. The action of the crystal is shown graphically in Figure 4-11, and it can be demonstrated analytically as well.22... 

The diffraction pattern obtained in the detector plane when the beam scan in a STEM instrument is stopped at a chosen point of the image comes from the illuminated area of the specimen which may be as small as 3X in diameter. In order to form a probe of this diameter it is necessary to illuminate the specimen with a convergent beam. The pattern obtained is then a convergent beam electron diffraction (CBED) pattern in which the central spot and all diffraction spots from a thin crystal are large discs rather than sharp maxima. Such patterns can normally be interpreted only by comparison with patterns calculated for particular postulated distributions of atoms. This has been attempted, as yet, for only a few cases such as on the diffraction study of the planar, nitrogen-rich defects in diamonds (21). 

Gamma knife A device that uses multiple converging beams of gamma radiation from cobalt-60 to highly focus radiation on small tumors within the brain. 

Zuo, J.M. and Spence, J.C.H. (1991) Automated structure factor refinement from convergent-beam patterns, Ultramicroscopy, 35, 185-196. 

Vincent, R., Bird, D.M. and Steeds, J.W. (1984) Structure of augeas determined by convergent-beam electron diffraction - II. Refinement of structural parameters, Phil. Mag. A, 50,765-786. 

Bird, D.M. and Saunders, M. (1992) Inversion of convergent-beam electron diffraction patterns, Acta Cryst. A, 48, 555-562. 

In a TEM with STEM attachment it is possible to ob-Q tain diffraction patterns from areas from 50 200 A. That allows in most cases to obtain patterns from individual particles. In order to study the crystal structure of the particle is more convinient to use a non-convergent beam 10 3 rad). This produces sharp spots and avoids interference effects such as the ones described by Roy et al. (9) that makes the interpretation of the data more complicated. Again in this case the operation conditions must be as clean as possible. 

For single crystal substrates which are not in the form of thin films, the techniques of transmission microscopy and nanodiffraction can not be used. For such cases, the techniques of reflection electron microscopy (REM) or its scanning variant (SREM) and reflection high energy electron diffraction (RHEED), in the selected area or convergent beam modes, may be applied (18). 

The use of a STEM instrument allows the controlled movement of a very fine electron beam in relation to the specimen and the efficient detection of the scattering and energy losses of the beam. We have outlined here a few of the possible applications arising from this capability. Other applications, of increasing sophistication and power will undoubtedly follow in time. In particular a range of phenomena, resulting from coherent interference effects in diffraction patterns produced by coherent convergent beams, have been observed (26) but not yet exploited. 

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. 

Several other techniques, such as electron holography (Lichte, 1986) and convergent beam electron diffraction (CBED) have also been developed for structure analysis. CBED can provide information not only on the lattice parameters and the S5mimetry of crystals, but also accurate structure-factor amplitudes and phases (Hoier et al, 1993). Accurate structure factor determination by CBED can provide information on the location of valence electrons. However, it is more favourable for thick crystals (> 500 A) with small unit cells (< 10 A). Structure analysis by CBED has been summarized in two review articles (Spence, 1993 Tanaka, 1994). 

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. 

For a long time, Selected-Area Electron Diffraction (SAED) performed with a parallel incident beam and a selected-area aperture was the only experimental method available. During the three last decades, new diffraction techniques based on a convergent electron incident beam (CBED Convergent-Beam Electron Diffraction, LACBED Large-Angle... 

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. 

Dijfraction with a large-angle convergent beam... 

Figure 5. Large Angle Convergent Beam Eleetron Diffraetion (LACBED).

Abstract Symmetry determinations performed from Microdiffraction, Convergent-Beam... 

Electron Diffraction (CBED) and Large-Angle Convergent-Beam Electron Diffraction (LACBED) allow the identification of the crystal system, the Bravais lattice and the point and space groups. These crystallographic features are obtained at microscopic and nanoscopic scales from the observation of symmetry elements present on electron diffraction patterns. 

Convergent Beam Electron Diffraction, M. Tanaka and M. Terauchi, Vol. I, JEOL, Tokyo, 1985... 

Abstract This chapter introduces quantitative convergent-beam electron diffraction... 

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