Dear Othman, I agree with previous comments and warnings from colleagues.
But regarding your specific question, you need to check the manuals for GSAS II or FULLPROF on how to setup the constraints on the site occupancies of all the atoms and sites involved. It is not sufficient to constrain the occupancies of each site add up to 1. You also need to cross-constrain occupancies in such a way that the sum of occupancy*multiplicity for each element equals your known chemical composition. For example, if you have 2 sites a and b for B/C, the two sites have multiplicities MULTa and MULTb (as defined in the International Tables Vol. A), your composition is B0.5C2.5 and the number of formula units A2B/C3O12 is Z, your constrains should look something like this: (I will call SOF-Ba and SOF-Bb the fractional occupancies of B in a and b sites respectively and SOF-Ca and SOF-Cb the fractional occupancies of C in a and b sites respectively): SOF-Ba+SOF-Ca=1 SOF-Bb+SOF-Cb=1 (you said you already set up these two and those make sure you don´t have more or less than 3*Z atoms in the sites) SOF-Ba*MULTa+SOF-Bb*MULTb=0.5*Z SOF-Ca*MULTa+SOF-Cb*MULTb=2.5*Z these constraints keep your B and C composition in the expected values. Be careful with the number of formula units per unit cell or your constrain will be wrong. If you have more than two sites you need to proceed accordingly adding a third constraint for the total occupancy of the third site and adding the corresponding term in the two sum constraints. Depending on the specific software this will be written differently but the idea should be to keep these relations. Best luck with your refinement, Leo 2016-12-19 5:54 GMT-03:00 Othman Al Bahri <[email protected]>: > Dear all, > > > I've made a series of solid solution powders using a solid state reaction > in the form A2B3-xCxO12 at x= 0.5 steps. A2B3O12 is orthorhombic while A2C > 3O12 is monoclinic. I'm refining the XRD data to find the atomic > distribution of the solute. > > > I've constrained the sum of the occupancy fractions for each relevant > site to equal 1. At low concentrations of the solute, I initially set the > solute's occupancy fractions to 0 and keep the solvent's occupancy at 1 > then refine the fractions (after following the usual Rietveld refinement > steps). This seems to give reasonable occupancy fraction values (no big > numbers or negative values) but the stoichiometry is way off. This is > probably because each site has different Wykoff multiplicities so > constraining the sum of each site's fractions to 1 is insufficient. > > > Let's assume that I knew the stoichiometry from Mass Spectroscopy or XPS - > is there a way to constrain the stiochiometry in a Rietveld refinement? I'm > using GSAS-II and comfortable with FullProf but feel free to give advice > for any other open-source software. > > > I've seen a few papers where the authors mention, typically in the > supplementary info, that their refinements' stoichiometry was off and that > it should be ignored. However I'm not comfortable with this approach and > would appreciate your advice. > > > This is my first time working with solid solutions so please feel free to > offer any general advice on what I should be careful with. I've tested for > phase mixtures (insolubility) by visually comparing my XRD patterns with > the sum of simulated XRD patterns of molar mixtures and through Rietveld > refinements > with two phases. The system I'm working with has been reported but the > original authors didn't do Rietveld refinements - they were interested in > physical property measurements. > > > Kind Regards, > > > Othman > > ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ > Please do NOT attach files to the whole list <[email protected] > > > Send commands to <[email protected]> eg: HELP as the subject with no body > text > The Rietveld_L list archive is on http://www.mail-archive.com/ > [email protected]/ > ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ > > > -- Dr. Leopoldo Suescun Prof. Agr (Assoc. Prof.) de Física Tel: (+598) 29290705/29249859 Cryssmat-Lab./Cátedra de Fisica/DETEMA Fax: (+598) 29241906* Facultad de Quimica, Universidad de la Republica. Montevideo, Uruguay Ahora la cristalografía importa más (www.iucr.org) Crystallography Matters more.
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