To All: RE: X-RAY DIFFRACTION PROCEDURES ... SECOND EDITION, Klug & Alexander, Table 9-2, p. 659. According to this reference, the relative breadth of a peak in the powder diffraction pattern is related to that of every other peak. The relationship is according to the shape of the crystal. I have used a Scintag/Seifert diffractometer since 1982 and found its instrumental broadening, b(i), is of the order of 0.05 degrees 2-theta. I used single crystal silicon for that determination. It's reasonable that any peak breadth greater than 0.05 is due to "other factors" - principally "crystallite size." NOTE: The term "crystallite size" is really a misnomer except for the measurement of the externally observable dimensions of the physical particle. What we are really measuring is the "domain size" of the scattering entity. Most of the powder patterns I obtain exhibit characteristic "anisotropic" peak broadening which is distinctly not due to spectral broadening caused by separation of K(alpha-1) and K(alpha-2) with increased angle. How might these patterns be successfully modelled to yield a physically meaningful result? Frank May Research Investigator Department of Chemistry and Biochemistry University of Missouri - St. Louis One University Boulevard St. Louis, Missouri 63121-4499 314-516-5098 - office 314-623-4524 - cell
________________________________ From: Andreas Leineweber [mailto:a.leinewe...@mf.mpg.de] Sent: Mon 5/11/2009 9:03 AM To: Ross H Colman Cc: rietveld_l@ill.fr Subject: Re: Anisotropic strain Dear Ross, something like this can definitely be adapted in the launch mode of TOPAS, it has been done in the past. A few lines for orthorhombic (some minor changes required for trigonal/hexagonal): prm s400 8349.85430 prm s040 1967.90075 prm s004 129.62651 prm s220 2451.94087 prm s202 -921.49437 prm s022 -23.92103 prm eta 0.49540 min 0 max 1 prm mhkl = H^4 s400 + K^4 s040 + L^4 s004 + H^2 K^2 s220 + H^2 L^2 s202 + K^2 L^2 s022; prm pp = D_spacing^2 * Sqrt(Max(mhkl,0)) / 1000; gauss_fwhm = 1.8/3.1415927 pp (1-eta) Tan(Th) + 0.0001; lor_fwhm = 1.8/3.1415927 pp eta Tan(Th) + 0.0001; (this way to set up the "Stephens model" in Topas originates from R. Dinnebier or P.W. Stephens; this is the simplest way to do it if you are not really interested in the meaning of the SHKL parameters) I may give detailed hints in direct exchange. Additionally I would like to indicate that something which looks on the first view like anisotropic strain, may instead be anisotropic crystallite size (is S112 positive or negative?) or stacking faults. In particular the latter is very common for "laminar" structures and may give to quite complex diffraction phenomena. Best regards Andreas Leineweber Ross H Colman wrote: > Dear Rietvelders, > > I am a PhD student working at UCL (UK) and was wondering if anyone out > there could help me with a diffraction related problem: > > I am attempting to refine some neutron diffraction data on a powder > sample that has a very laminar structure. The refinement is acceptable > but close inspection shows that some peaks are modelled poorly compared > to others. The relative intensity of each peak appears to be a good fit > but some peaks are sharper than others whilst some are noticably broadened. > > The instrument responsible suggested using anisotropic strain within > Fullprof to attempt to model the peak shape anisotropies (as an ILL > instrument was used to collect the data). This worked quite well and > when considering the crystallite morphologies it seems physically > reasonable. > > I have also been using TOPAS to refine the some of the data for > comparison and as a new user have not found a way of including this kind > of anisotropic strain into the refinement. > > Is it possible? If so does anybody have an example? > > Many thanks for your help > > Ross Colman > > p.s > > If it helps in the discussion, I am analysing a crystal structure with P > -3 m 1 symmetry and so the refinable parameters within Fullprof are > s_400, s_004 and s_112. From a chemical point of view the c direction is > only weekly hydrogen bonded and so the laminar structure seen in SEM > should be within the a-b plane. > > ________________________________ > > Ross Colman > > G19 Christopher Ingold Laboratories > > University College London > > Department of Chemistry > > 20 Gordon Street > > London > > WC1H 0AJ > > Phone: +44 (0)20 7679 4636 > > Internal: 24636 > > Email: ross.col...@ucl.ac.uk <mailto:ross.col...@ucl.ac.uk> > > > -- Dr. Andreas Leineweber Max-Planck-Institut fuer Metallforschung Heisenbergstrasse 3 70569 Stuttgart Germany Tel. +49 711 689 3365 Fax. +49 711 689 3312 e-mail: a.leinewe...@mf.mpg.de home page of department: http://www.mf.mpg.de/de/abteilungen/mittemeijer/english/index_english.htm
