Twofold Axis (2)

2

The symbol above shows a twofold axis perpendicular to the plane of the screen, i.e parallel to Z. A twofold axis is equivalent to a rotation of 180° about a line. A twofold rotation about the c-axis, i.e. about the line 0,0,z, will have the corresponding symmetry operator -x,-y,z. This axis has the written symbol "2".
2
The symbol shown above corresponds to a twofold axis, but now parallel to the Cartesian X direction. A twofold rotation about the x-axis will have the corresponding symmetry operator x,-y,-z. A fractional value next to the symbol indicates the height of the axis of rotation above the XY plane.
2
In space group diagrams for cubic symmetry, twofold axes may appear inclined at an angle of 45° to the XY plane. The symbol for these is shown above, the dot indicating the exact position of intersection with the plane.


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© Copyright 1995-2006.  Birkbeck College, University of London. Author(s): Jeremy Karl Cockcroft