Convergent beam electron diffraction CBED

The volume of specimen contributing to a CBED pattern is much smaller than that contributing to an SAD pattern. Thus, there is less likelihood that the effects of strain, specimen bending, or crystal defects will influence the nature of a CBED pattern. Consequently, Kikuchi lines are usually more often observed and are usually clearer in CBED patterns than in SAD patterns. 

The nature and origin of Kikuchi lines that arise from planes of the ZOLZ were discussed in Section 3.9. Kikuchi lines can also arise from HOLZ planes and are observed outside the diffraction disks of a CBED pattern. However, within the disks there are the so-called HOLZ lines, which are continuous with the HOLZ Kikuchi lines. 

K-M and Kossel patterns are extremely sensitive to crystal orientation, and are therefore particularly useful for tilting a crystal into a precise orientation. However, their most important use is in obtaining three-dimensional crystal symmetry information, including the complete determination of the point group and space group of a crystal. Extensive reviews of 

CBED has not been used to any appreciable extent in minerals research. However, because of its many advantages over the SAD mode and the ease with which CBED patterns can be formed in modern microscopes, it is likely to be applied to many mineralogical problems in the near future. 

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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). 

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). 

Various electron diffraction techniques are available on modem transmission electron microscopes. Selected-Area Electron Diffraction (SAED) and Microdiffraction are performed with a parallel or nearly parallel incident beam and give spot patterns. Convergent-Beam Electron Diffraction (CBED) and Large-Angle Convergent-Beam Electron Diffraction (LACBED) are performed with a focused and defocused convergent beam... 

Figure 6. A 3D rendering that reveals the details of chemical bonding and dz2 orbital-like holes in Cu20. The amount of charge redistribution is very small and its detection requires a high degree of experimental accuracy. In this picture, the small charge differences between the measured crystal charge density derived from convergent-beam electron diffraction (CBED) and that derived from superimposed spherical 02- and Cu+ ions are shown. The red and blue colors represent excess electrons and holes, respectively.

There is another diffraction mode, called convergent beam electron diffraction (CBED), in which the incident electron beam is focused to a fine spot on the specimen. If the convergence angle is appropriately chosen, the diffraction pattern consists of an array of nonoverlapping disks. For thin specimens ( SO nm) the CBED disks are featureless, but... 

Figure 10. (a) TEM bright-field image, (b) selected area diffraction (SAD) pattern, and (c) convergent beam electron diffraction (CBED) pattern of the optimally reannealed (r, 83 K) Tl-2201 thin film. 

Basis and Applications of Convergent Beam Electron Diffraction (CBED) for the Investigation of Phases in Ceramics and Glass-Ceramics... 

Convergent beam electron diffraction (CBED) strictly refers to diffraction using an electron probe in a TEM with very large convergence angle. When this convergence angle becomes small the conditions are similar to the spot pattern obtained with parallel illumination in the conventional Selected Area Diffraction Patterns (SADP) as is shown in Fig. 1. 

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