h
k

|F| phi

|F| phi

In this section we will look at the effect of symmetry centering and pseudo-centering on the diffraction pattern.

The map and structure factors now show a 3-atom structure with no symmetry.

Select each reflection for which h is an odd number in turn, and use the 'Clear SF' button to set the structure factor to zero. What happens to the electron density?

Could we describe the new structure with a smaller unit cell? Taking into account the reciprocal relationship between distances in real and reciprocal space, how would this affect the reciprocal lattice?

Reset the map. What do you expect will happen if you zero all the reflections with k odd? Try it and check.

Reset the map. Now try zeroing all the reflections for which the sum of h and k is odd (i.e. in a checker-board pattern). What happens to the map? Can you think of a crystallographic spacegroup analagous to this case?

What would happen if alternate rows in the diffraction pattern were weak rather than absent? What would this tell you about the structure?