International
Tables for
Crystallography
Volume B
Reciprocal space
Edited by U. Shmueli

International Tables for Crystallography (2006). Vol. B. ch. 2.5, pp. 291-292   | 1 | 2 |

Section 2.5.3.5.1. Stages of procedure

P. Goodmanb

2.5.3.5.1. Stages of procedure

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  • (i) Zone-axis patterns . The first need is to record a principal zone-axis pattern. From this, the rotational order X of the vertical axis and associated mirror (including glide-line) components are readily observed (see all examples).

    This pattern may include part of the higher-order Laue zone; in particular the closest or first-order Laue zone (FOLZ) should be included in order to establish the presence or absence of horizontal glide planes, as illustrated in Fig. 2.5.3.3[link]. The projection approximation frequently applies to the zone-axis pattern, particularly when this is obtained from thin crystals (although this cannot apply by definition to the FOLZ). This is indicated by the absence of fine-line detail in the central beam particularly; identification of the projected symmetry is then straightforward.

  • (ii) Laue circle patterns . Next, it is usual to seek patterns in which discs around the Laue circle include the line [m_{R}] (Fig. 2.5.3.2[link]). The internal disc symmetries observed together with those from the zone-axis pattern will determine a diffraction group, classifying the zero-layer symmetry. [Fig. 6(c) of Goodman & Whitfield (1980)[link] gives an example of Laue-circle symmetries.]

  • (iii) Alternative zone axes or higher-order Laue zones . Finally, alternative zone or higher-order Laue-zone patterns may be sought for additional three-dimensional data: (a) to determine the three-dimensional extinction rules, (b) to test for centrosymmetry, or (c) to test for the existence of mirror planes perpendicular to the principal rotation axis. These procedures are illustrated in the following examples.

References

First citation Goodman, P. & Whitfield, H. J. (1980). The space group determination of GaS and Cu3As2S3I by convergent beam electron diffraction. Acta Cryst. A36, 219–228.Google Scholar








































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