If you have multiple atoms/vacancies on the same site, it is also worth considering what resonant scattering can do for you.
Zhang, Y.; Wilkinson, A. P.; Lee, P. L.; Shastri, S. D.; Shu, D.; Chung, D. Y.; Kanatzidis, M. G., Determining metal ion distributions using resonant scattering at very high-energy K-edges: Bi/Pb in Pb5Bi6Se14 J. Appl. Crystallogr. 2005, 38, 433-441. http://dx.doi.org/10.1107/S0021889805005686 Wilkinson, A. P.; Lind, C.; Young, R. A.; Shastri, S. D.; Lee, P. L.; Nolas, G. S., Preparation, Transport Properties and Structure Analysis by Resonant X-ray Scattering of the Type-I Clathrate Cs8Cd4Sn42. Chem. Mater. 2002, 14, 1300-1305. http://dx.doi.org/10.1021/cm0107880 Zhang, Y.; Lee, P. L.; Nolas, G. S.; Wilkinson, A. P., Gallium distribution in the clathrates Sr8Ga16Ge30 and Sr4Eu4Ga16Ge30 by resonant diffraction. Appl. Phys. Lett. 2002, 80, 2931-2933. http://dx.doi.org/10.1063/1.1473236 Zhang, Y.; Wilkinson, A. P.; Nolas, G. S.; Lee, P. L.; Hodges, J. P., Strategies for solving neighboring-element problems: a case study using resonant X-ray diffraction and pulsed neutron diffraction to examine Sr8Ga16Ge30. J. Appl. Cryst. 2003, 36, 1182-1189. http://dx.doi.org/10.1107/S0021889803013955 Best wishes, Angus Angus P. Wilkinson Assoc. Chair and Prof. School of Chemistry and Biochemistry and School of Materials Science and Engineering 901 Atlantic Dr. Georgia Institute of Technology Atlanta, GA 30332-0400 Office: MoSE building 1100J Tel: 404 894 4036 Fax: 404 894 7452 Packages to: A. P. Wilkinson 311 Ferst Dr. Atlanta, GA 30332-0400 ________________________________ From: rietveld_l-requ...@ill.fr <rietveld_l-requ...@ill.fr> on behalf of Luca Lutterotti <luca.luttero...@unitn.it> Sent: Tuesday, December 20, 2016 9:55 AM To: rietveld_l@ill.fr Subject: Re: Stoichiometry and occupancy fractions of solid solutions 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 Combined X-Ray diffraction and fluorescence analysis in ...<https://www.researchgate.net/publication/288701415_Combined_X-Ray_diffraction_and_fluorescence_analysis_in_the_cultural_heritage_field> www.researchgate.net X-Ray diffraction (XRD) and X-Ray fluorescence (XRF) techniques are broadly used for materials characterisation. In some respects, they can be regarded as ... 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<mailto: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<mailto:rietveld_l-requ...@ill.fr> [rietveld_l-requ...@ill.fr<mailto:rietveld_l-requ...@ill.fr>] on behalf of Othman Al Bahri [z3435...@zmail.unsw.edu.au<mailto:z3435...@zmail.unsw.edu.au>] Sent: Monday, December 19, 2016 1:54 AM To: rietveld_l@ill.fr<mailto: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. 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