The symbol above shows a fourfold axis perpendicular to the plane of the screen,
i.e parallel to Z.
A fourfold axis is equivalent to an anticlockwise rotation of 90°
about a line.
A fourfold rotation about the c-axis, i.e. about
the line 0,0,z, will have
the corresponding symmetry operator -y,x,z.
This axis has the written symbol "4".
In space group diagrams for cubic symmetry, fourfold axes
are shown along both X and Y directions as well as along the Z direction
when present.
The symbol for these is shown above.
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