There are several things to comment on here.
One is the presence of centers of symmetry at the corners of the cell and all the half-way points in all directions. Here's a schematic showing how two objects related by a center at the origin of the cell are also related by all the other centers. (The outer box is the unit cell. The lines at 1/2 in x and y are there simply as guides.) Objects at A and B are related by the first center. Copies of B related to the first by unit translations are related to A by centers of symmetry half way along the cell edges of in the middle of the cell. This principle also holds in the third dimension. And the idea also holds for other symmetry operations as we'll see later.
Another set of symmetry elements in the cell are the two fold screw axes parallel to the b axis. They aren't located at a z coordinate of zero though. They're up at 1/4 and 3/4. They also show up at 1/2 in x.
There's also a c glide in the cell with the glide planes located at 1/4 and 3/4 in y with the glide direction coming out of the board.back to index next topic previous topic
copyright © Ron Stenkamp firstname.lastname@example.org Most recent update 1/17/97