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





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