Last Update 27/ 03/ 2001
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The interactive figure shows the improper symmetry element named three fold screw axis, with symbol 31. Each blue sphere represents an equivalent position related with the mentioned axis.
The rotation angle of the three fold screw axis like the three fold axis is 120°. It is possible to interact with this applet by a push and drag mouse action on the green square in the bottom of the figure, to change the visual angle of the set. The three line segments connecting the blue spheres to the long line representing the screw axis are 120° from each other and displaced 1/3 of the repetition parameter of the crystal unity to which the symmetry element belongs. The long line supporting the three line segments described above represents the31 axis. It causes systematic diffracted rays extinctions, for example the diffracted rays with Miller indices (0 0 1), (0 0 2), (0 0 4), (0 0 5), (0 0 7), (0 0 8), etc. can not be observed due to the extinction or destructive interference when the 31 axis is parallel to direction z, measured with parameter c of the unit cell.
Bibliography
1. Stout, G.H. and Jensen, L.H., X-ray Structure Determination, Macmillan, London, 1972.
2. Buerguer, M.J., X-ray Crystallography, John Wiley & Sons, New York, 1966.
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