There are two seperate issues here - coding the restraint and then having enough information to theoretically solve what is a effectively a simultaneous equation problem with 3 or more variables. The CCP14 website tells you how to code it (the same approach works just as well in programs other than GSAS, e.g. Topas). However, expecting to get freely refine occupancies from 3 atoms on a site from only a single lab XRD dataset is unreasonable as there isn't enough information to do it - honest. As Alan says, XRD gives you an average site scattering power - you have to work out the rest somehow. If people read the whole CCP14 posting from Marc Joubert, he says (not just me!) that you need two or more datasets (have to have different contrast) to have a sound theoretical footing for a 3 atom refinement. Additional diffraction information can come from neutron diffraction or resonant scattering, but simple elemental analysis is also useful, as it gives you a constraint to stabilize things - the overall composition. In some instances some other tidbits of information can come from other sources. Pam
________________________________ From: Alan Hewat [mailto:[EMAIL PROTECTED] Sent: Tue 30/01/2007 3:03 AM To: rietveld_l@ill.fr Subject: Re: occupations and GSAS > I have another situation. i.e. three atoms at two equivalent sites > in La( Mg0.5Sn0.25Ti0.25)O3. This is really not complicated. All you can expect is to be able to refine the average scattering power of each site. To do that you could fix an arbitrary scattering power and refine the site occupation number; their product is then the site scattering power. Then write down any relations you have between the average scattering powers of the sites, the numbers of different atoms (stoichiometry) and their individual scattering powers, and you may or may not have enough information to obtain the occupancy of each site by each kind of atom. Don't just rely on refining parameters without understanding what they mean. _____________________________________________________________ Dr Alan Hewat, ILL Grenoble, FRANCE <[EMAIL PROTECTED]>fax+33.476.20.76.48 +33.476.20.72.13 (.26 Mme Guillermet) http://www.ill.fr/dif/people/hewat/ _____________________________________________________________
