The following example creates a DELF (difference) map to try and
find a missing Oxygen atom in cubic Y2O3. Though maps such as FOBS and FCALC are
done just as easily.
This example uses Cubic Y2O3 giving the two main Y atoms with the Oxygen atom missing and to be found using the Fourier Difference Map. I A 3 a=b=c=10.6032 Angstrom site Y x 0.25 y 0.25 z 0.25 occ Y+3 num_posns 8 site Y x 0.4673 y 0 z 0.25 occ Y+3 num_posns 24 The trick here with GSAS, Fourier Maps and Powder Diffraction data is not only to set up the Fourier Map options but also go into Least Squares Controls and use E (Change powder data Fobs extraction flag to ON) before doing the cycles of least squares. NOTE: If wanting to do a difference map focussing on existing atoms. Rather than delete the atoms; another way to do this would be to set the FRAC (occupancy) of the atom(s) of interest to zero; fixing atomic refinement parameters, then doing zero cycles of refinement. The advantage of this is that the atoms of interest will still be present (can be labelled) in the VRML/Forplot plot files.




Peak search of the Fourier Map


