On 8/31/19 9:44 AM, Ahmed Subrati wrote:
Dear all,
I wanted to ask how three atoms, sharing the same xyz position, could be refined
in terms of their occupancies /via /FullProf. We know that for the case of two
atoms, the first atom is set as 11.0 and the second one as -11.0 so that their
sum is unity, but how would be the case for three atoms.
It does not matter what program you use. If you have X-ray data, and were to
normalize the scattering curve for each type of atom by dividing by the atomic
number, the curves would very nearly overlap. The minute differences would be
the only information allowing you to determine those occupancies uniquely. If
you merely constrain the sum of the occupancies, and the composition of the
sample, the results will converge to nonsense results because the errors in the
data would overwhelm those small differences.
If you had neutron scattering data, then you might be able to do it, depending
on how different the scattering for the 3 types of nuclei.
Your only possibility is to add additional constraints. Perhaps average bond
lengths will provide such information, or there may be other information
available for your material. In any case, the diffraction data will not be
sufficient for Reitveld, or single-crystal data.
Larry
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