Considering, that I have a set of atoms:
Fe1, Ti2, Fe3, Ti4
I want to set following occupancy constraints in Rietveld refinement in GSAS/EXPGUI.
First two are related to chemical composition:
Fe1+Fe3=1
Ti2+Ti4=1
Other two are related to the same position in the structure:
Fe1+Ti2=1
Fe3+Ti4=1
Unfortunately, when I try to input this constrains, I receive an error which says that atomic parameter can appear in only one constraint.
Do you know any solution or trick on how to solve this?
Constraints in GSAS do not fix the sum of the occupancies. Instead, the relative speed of change of the different values is fixed. Using this, you can create a complex constraint including all 4 atoms. If the multiplicities of the positions are the same, create 4 columns for each atom and set multipliers equal to 1 for Fe1 and Ti4 and equal to -1 for Fe3 and Ti2. If the multiplicities are different, you will have to make a corresponding correction to the values keeping the signs.
Thank you a lot Alexander. It works!
Also, I made a mistake in my question so finally I set
multipliers of Fe1 and Ti4 to 1 and Fe2 and Ti3 to -1.
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