Dear Paulina, you can find plenty of names of experts on electron powder diffraction at this announcement of the ePDF workshop, https://www.uni-ulm.de/einrichtungen/hrem/epdf/epdf/, organised by Tatiana Gorelik.
They can probably answer your question Best regards, Tim On Thu, 12 Nov 2020 08:40:32 +0100 Paulina Dominiak <pdo...@chem.uw.edu.pl> wrote: > Dear Kamil and Dimitris, > > I wonder what kind of scattering factors are mentioned below programs > using for Rietfveld refinement? Are these from IT for electron > scattering? > > Best, > > Paulina > > > W dniu 12.11.2020 o 07:35, Dimitris Triandafillidis pisze: > > Dear Kamil, > > > > Since you have an initial model that is presumably close to what > > you expect to find in your sample, then Rietveld refinement is the > > right choice. In cases where no initial model is available, other > > types of refinement are suitable, such as Pawley or Le Bail, which > > refine lattice and peak shape parameters only. > > > > Regarding the order in which parameters are introduced to a fit, a > > good starting point would be: > > - lattice parameters and zero shift > > - peak shape parameters and scale factor > > - background terms > > - atomic positions > > Depending on the quality of your data and the nature of your > > sample, more parameters could be refined (i.e. atomic displacement > > parameters, preferred orientation, absorption etc.). > > > > My suggestion would be to get a better understanding of the effect > > of each parameter to the fit, in order to troubleshoot your > > refinement more effectively. For example, peak positions depend on > > lattice parameters (and space group setting), therefore refining > > e.g. the scale factor would not help much if expected peak > > positions are in the wrong place. The following article might be > > helpful to you. McCusker, L.B., Von Dreele, R.B., Cox, D.E., Louër, > > D., Scardi, P., 1999. Rietveld refinement guidelines. Journal of > > Applied Crystallography 32, 36–50. > > https://dx.doi.org/10.1107/S0021889898009856 > > > > As for the single peak not being explained by the fit, it could be > > due to deviations in lattice parameters, but in that case more than > > one peak would not match. If every other peak fits nicely and only > > one does not, I would think that it is due to coexistance of a > > second, trace, crystalline phase in the sample. In most refinement > > software you have the option of excluding a peak from refinement. > > Refining atomic positions would not help, as these change the > > relative peak intensities, not peak positions. However, if you > > could share a few images of the fit, it would really help to better > > understand what's going on. > > > > Finally, with regards to software, you could also try GSAS > > <https://subversion.xray.aps.anl.gov/trac/pyGSAS>. It is one of the > > most widely used software for powder diffraction and it is free. > > There is also a mailing list > > <https://mailman.aps.anl.gov/mailman/listinfo/gsas-ii>, consisting > > mainly of powder diffractionists, and a handful collection of > > tutorials > > <https://subversion.xray.aps.anl.gov/pyGSAS/trunk/help/Tutorials.html>. > > > > Best wishes, > > Dimitris > > > > > > On Thu, Nov 12, 2020 at 12:18 AM Kamil Krawczyk > > <kamil.krawczyk7...@gmail.com > > <mailto:kamil.krawczyk7...@gmail.com>> wrote: > > > > Hello! > > > > First, apologies if the scope of the question is beyond that of > > the listserv - I was recommended this discussion page through a > > colleague. > > > > I am incredibly new to refinement, and have been trying to > > employ the use of Rietveld refinement to better determine the > > crystal structure of my lead sulfide (PbS) quantum dots. Data was > > acquired as images on a home-built tabletop time-resolved electron > > diffraction setup, and converted to diffractogram through a > > combination of center finding and background subtraction. > > However, since these are small (~ 5 nm), and we are certainly > > sampling the ensemble average (as our focused beam is about 120 um > > in diameter), our pattern is that of a polycrystalline sample. > > > > >From what I've encountered in my research, Rietveld is best > > suited for this case. I have taken an initial structure as > > inferred from literature/the bulk and refined that (relatively > > successfully to a Rwp value of ~2 - 6%). I do have a few > > questions regarding this process as I believe I am not being as > > rigorous as possible: > > > > - is there a 'best practices' order of refinement operations? > > e.g. refine the lattice parameters first, then peak profiles, etc. > > - there is a single peak that is not matching (likely due to a > > slight deviation in atomic positions from the expected simple > > cubic structure) - is refinement capable of moving individual > > atoms or clusters of atoms to better match a small, unaccounted > > for reflection? > > > > For what it's worth, I have been using Reflex in Materials > > Studio; if there is a more appropriate software for this > > application, I would be happy to hear! > > > > If any additional clarification is need, I'll fill it in! > > > > Best, > > Kamil > > > > ######################################################################## > > > > To unsubscribe from the CCP4BB list, click the following link: > > https://www.jiscmail.ac.uk/cgi-bin/WA-JISC.exe?SUBED1=CCP4BB&A=1 > > > > This message was issued to members of www.jiscmail.ac.uk/CCP4BB > > <http://www.jiscmail.ac.uk/CCP4BB>, a mailing list hosted by > > www.jiscmail.ac.uk <http://www.jiscmail.ac.uk>, terms & > > conditions are available at > > https://www.jiscmail.ac.uk/policyandsecurity/ > > > > > > ------------------------------------------------------------------------ > > > > To unsubscribe from the CCP4BB list, click the following link: > > https://www.jiscmail.ac.uk/cgi-bin/WA-JISC.exe?SUBED1=CCP4BB&A=1 > > -- -- Tim Gruene Head of the Centre for X-ray Structure Analysis Faculty of Chemistry University of Vienna Phone: +43-1-4277-70202 GPG Key ID = A46BEE1A ######################################################################## To unsubscribe from the CCP4BB list, click the following link: https://www.jiscmail.ac.uk/cgi-bin/WA-JISC.exe?SUBED1=CCP4BB&A=1 This message was issued to members of www.jiscmail.ac.uk/CCP4BB, a mailing list hosted by www.jiscmail.ac.uk, terms & conditions are available at https://www.jiscmail.ac.uk/policyandsecurity/
pgpydqmlEDoWo.pgp
Description: OpenPGP digital signature
