Good luck with your project! Cheers, Artem
> Dear Artem, > > > Thank you very much for pointing me to these resources. They have also > been extremely useful. > > Have a wonderful holiday and a happy new year. > > > Kind regards, > > > Othman > > ________________________________ > From: baba...@univ.kiev.ua <baba...@univ.kiev.ua> > Sent: Monday, 19 December 2016 9:31:29 PM > To: Othman Al Bahri > Subject: Re: Stoichiometry and occupancy fractions of solid solutions > > Dear Othman, > > General notes and precaution are concisely described in the > Rietveld refnement guidelines "J. Appl. Cryst. (1999). 32, 36-50 > (paragraph 7). > Then, you may refer to Madsen & Hill's Variable Counting Time concept, > J. Appl. Cryst. (1994). 27, 385-392. Definitely, that's the solution > for reliable refinement of site occupancies, when lab PXRD is utilized. > Those considerations are simplified by condition of chemically averaged > grain compositions, as Uwe has noted. > > Sincerely, > Artem > > >> 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 >> ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ >> Please do NOT attach files to the whole list >> <alan.he...@neutronoptics.com> >> Send commands to <lists...@ill.fr> eg: HELP as the subject with no body >> text >> The Rietveld_L list archive is on >> http://www.mail-archive.com/rietveld_l@ill.fr/ >> ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ >> >> > > >
++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ Please do NOT attach files to the whole list <alan.he...@neutronoptics.com> Send commands to <lists...@ill.fr> eg: HELP as the subject with no body text The Rietveld_L list archive is on http://www.mail-archive.com/rietveld_l@ill.fr/ ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++