Electron microscopy
 
Determination of Crystal Symmetry
- Practical Electron Microscopy and Database -
- An Online Book -
Microanalysis | EM Book                                                                                   http://www.globalsino.com/EM/        

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In practice, it is often impossible to determine the full symmetry of a crystal with a single symmetry element. In this case, the simplist way for such determination is that we record CBED patterns along a proper zone axis and then analyze the resulting symmetries of the whole-pattern, projection-diffraction pattern, and bright-field (BF) (000) disk.

In order to determine symmetry elements in crystals, some common rules in CBED can be used:
         i) Rotational axes can be observed directly in CBED patterns when the electron beam is aligned with the ratational axis.
         ii) Mirror planes can be observed directly as mirror lines in the CBED pattern if the electron beam lies in the mirror plane of the symmetry.
         iii) A vertical glide plane also results in a mirror line in the CBED pattern. At Bragg condition, a horizontal twofold axis or twofold screw axis in the ZOLZ along g presents a mirror line of symmetry onto disk g, and this line runs normal to g.
         iv) At Bragg condition, a horizontal mirror plane or glide plane leads to a centric distribution of intensity in every CBED disk.
         v) The odd order reflections in the direction of the axis will be forbidden if the glide plane is parallel to the electron beam.
         vi) For a screw axis or glide plane, if the projection of the unit cell in the beam direction has a symmetry, then the forbidden reflections would not be fully forbidden but would obviously be very weak.
         vi) Horizontal three-, four-, and sixfold axes do not induce useful CBED symmetries.

 

 

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