Even with neutron diffraction :-) you can only refine the scattering power
(length) of each site. When you refine the occupancy of two atoms on a
single site you are adding additional "information" by assuming that the
site is fully occupied by one or the other. That may be true, or not. If
the scattering power changes significantly with wavelength (or angle with
x-rays) you may obtain the necessary additional information.
But can you do a chemical or other quantitative analysis to determine the
relative proportions of eg cations ? Neutron activation analysis may also
be possible. Don't believe that everything can be determined by powder
diffraction :-)
Alan
______________________________________________________
Dr Alan Hewat, NeutronOptics, Grenoble, FRANCE
from my telephone
<alan.he...@neutronoptics.com> +33.476.98.41.68
http://www.NeutronOptics.com/hewat
______________________________________________________
On Sat, 31 Aug 2019, 19:59 Larry Finger, <larry.fin...@lwfinger.net> wrote:
> 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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