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

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A crystal of a specific Laue symmetry or point group may give rise to different diffraction groups at different zone-axis orientations, and thus there may be several possible space groups. The choice may be reduced through use of "systematic extinctions" in diffractions indicating glide planes or screw axes.

Even though some techniques can be used to determine the space groups and point group of crystals, the determination procedure is usally not simple. The diffuculties can be significantly reduced if we know some relavent characteristics of the materials such as listed in Table 1625. However, if one wants to be an expert on  electron crystallography, he should get familiar with all the methods for point group determination.

Table 1625. Materials’ characteristics related to their space groups and point groups.

Substance

Impossible Possible
L-amino acid Centrosymmetric space groups Non-centrosymmetric space groups
Centrosymmetric molecule Centrosymmetric space groups Non-centrosymmetric space groups
Racemic mixture Centrosymmetric space groups Non-centrosymmetric space groups
Piezoelectric crystals Centrosymmetric space groups, 432 point groups Non-centrosymmetric space groups
Pyroelectric crystals Centrosymmetric space groups Non-centrosymmetric space groups, only the presence of a polar axi
Ferroelectric crystals Centrosymmetric space groups Non-centrosymmetric space groups
Crystals with second-harmonic generation in laser irradiation Centrosymmetric space groups, 422 point group, 622 point group Non-centrosymmetric space groups

 

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