If you have any decent Rietveld software this should be straightforward
enough. Give the atomic positions for the two ions the same codewords so
that when these are refined they will stay the same. Refine the ionic
occupancies and temperature factors for the two ions with separate codewords.
You should then be able to find out the quantities of your two ions.
-- tony bell A.M.T.Bell@dl.ac.ukcrystallographer and marathon runner (but not necessarily in that order :-))engineering and physical science research councildaresbury laboratory, daresbury, warrington, cheshire, UK. WA4 4AD.