Adding a little bit to the previous answers,aside from using neutron diffraction, on can also use anomalous diffraction close to the absorption edge of the respective chemical elements. This requires a synchrotron source in most cases (unless one happens to have just the right exotic X-ray tube). Anomalous scattering changes the scattering power of the elements, too, adding extra information quite similar to the neutron case. In principle this can be done for each chemical species involved in the site disorder, thus allowing to refine simultaneously all occupation factors independently.
In praxis such a refinement will be ridiculously unstable in most cases.Fullprof in particular allows, in addition to the hard parameter coupling you mentioned in the initial mail below, soft linear restraints. For this you put NLI = 1 (or higher, for multiple restraits), and add a CHEMCOMP line at the very end of the .pcr-file. This looks like this:
! Set of 1 linear restraints: ! Identifier, number of coeff., value, sigma / List of coeff & ParametersChemcomp 3 28.000000 0.000010 -> Current calculated value: 27.999981
1.0000 8 2.0000 9 1.0000 10This example would be for three parameters P8, P9, P10 to be coupled in a way that their values are P8 + 2.0*P9 + P10 = 28.0. Negative multiplier are also possible. This chemcomp approach is quite flexible and allows to restrict the refinement to total composition or charge neutrality. If you use the same parameter number multiple times in your refinement, make sure which value is selected.
Good Luck, Daniel Többens
On 31 Aug 2019, at 16:44, Ahmed Subrati <[email protected] <mailto:[email protected]>> 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.Thank you very much. King regards, Ahmed Subrati PhD student NanoBioMedical Centre | Adam Mickiewicz University in Poznan, Poland ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++Please do NOT attach files to the whole list <[email protected] <mailto:[email protected]>> Send commands to <[email protected] <mailto:[email protected]>> eg: HELP as the subject with no body text The Rietveld_L list archive is onhttp://www.mail-archive.com/[email protected]/++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
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