Wholepattern symmetry  Practical Electron Microscopy and Database   An Online Book  

Microanalysis  EM Book http://www.globalsino.com/EM/  


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Screwaxes and mirror planes can be deduced from CBED investigation by observing GjönnesMoodie (GM) lines (or called GM extinctions) and by determining wholepattern (WP) symmetries, respectively. Wholepattern symmetry [1] of electron diffraction or CBED patterns corresponds to the symmetry of the higherorder Laue zone (HOLZ) reflections and HOLZ Kikuchi lines shown in small camera length patterns and with a small convergence of beam, and presents mirror planes. Figure 1600a shows the wholepattern symmetry of the [0001] CBED pattern of T1 (Al_{2}CuLi) crystals. The pattern symmetry is 6mm.
Figure 1600b shows the CBED pattern of βpyrochlore oxide superconductor KOs_{2}O_{6} along the [001] zone axis. [3] The square array with small dark disks near the center is zeroorder Laue zone (ZOLZ) and the surrounding circle formed by the highly contrasted disks is firstorder Laue zone (FOLZ). The magnified image of the inset presents a fourfold rotational symmetry along the c* axis and two mirror symmetries m_{a} and m_{b}, indicating that the whole pattern (WP) has 4mm symmetry. Figure 1600b. CBED pattern taken from a KOs_{2}O_{6} crystal along [001] zone axis. [3] Note that the projectiondiffraction symmetry can never be lower than the wholepattern symmetry since the wholepattern symmetry reflects the symmetry of the full threedimensional (3D) crystal. Table 1600. Examples of wholepattern applications.
[1] B. E Buxton, J.A. Eades, J.W. Steeds, and G.M. Rackham: Phil. Trans. Roy. Soc., vol. 281, A1301, pp. 17194.


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