Hi Othman, Another insightful thing to do might be to simulate diffraction patterns for various scenarios. If you have B on two sites and simulate if the diffraction pattern will look different for different occupancies on those two sites, resulting in a given overall stoichiometry, you can convince yourself whether diffraction is a good tool to determine SOFs. You can also check which diffraction peaks show the biggest change, which may affect your count time or region of interest. If you also simulate for neutrons and see a big change for neutrons, but not X-rays, you have a good case to supplement a neutron beam time proposal.
Good luck, Sven ________________________________ From: rietveld_l-requ...@ill.fr [rietveld_l-requ...@ill.fr] on behalf of Othman Al Bahri [z3435...@zmail.unsw.edu.au] Sent: Monday, December 19, 2016 1:54 AM To: rietveld_l@ill.fr Subject: Stoichiometry and occupancy fractions of solid solutions 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 A2C3O12 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
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