To increase sensitivity to partial/mixed occupation and also solve the case of 3 or more atoms in the same site (you have an infinite number of solutions that give you the same scattering factor, both x-ray and neutrons) we are developing a combined XRD-XRF analysis. Some preliminary application examples were published and we are preparing a full article on the theory behind. https://www.researchgate.net/publication/288701415_Combined_X-Ray_diffraction_and_fluorescence_analysis_in_the_cultural_heritage_field <https://www.researchgate.net/publication/288701415_Combined_X-Ray_diffraction_and_fluorescence_analysis_in_the_cultural_heritage_field>
Best regards, Luca -----------------------Luca Lutterotti-------------------------------------------- Dipartimento di Ingegneria Industriale, Universita' di Trento, via Sommarive, 9, 38123 Trento, Italy e-mail address : luca.luttero...@unitn.it <mailto:luca.luttero...@ing.unitn.it> Maud page : http://maud.radiographema <http://maud.radiographema/>.com Phone number :+39-0461-28-2414 XRD lab:: +39-0461-282434 Fax : +39-0461-28-1977 ---------------------------------------------------------------------------------------- > On 19 Dec 2016, at 18:23, Vogel, Sven C <s...@lanl.gov> wrote: > > 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 > ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ > 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/ ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
