Centre of Inversion on Twofold Screw Axis (21 / m)
Return link to the symbol table
The symbol above shows a two-one screw axis
perpendicular to the plane of the screen, i.e parallel to Z, with a centre
of inversion on the axis.
This combination results when there is a mirror plane
perpendicular to the two-one screw axis,
hence the written symbol "21 / m".
A fractional value next to the symbol indicates the height of the inversion
centre above the XY plane.
The mirror plane is separated from the centre of inversion by 1/4 of the repeat
distance in the lattice along the axis direction.
© Copyright 1997-1999.
Birkbeck College, University of London.