u/Ok_Course1403

I am performing Rietveld refinement on a material in which a site is shared between Iron and Nickel. Fullprof is not capable of refining the occupation of those atoms since their atomic number is too similar, however would it be possible to refine the occupation of the site, while keeping the Fe/Ni ratio constant? I tried fidgeting with the constraints, but I'm not very good with those.