I would suggest that you use a powder like LaB6 to measure the
instrumental broadening. The size
of the crystalline domains is large enough, and the strain low
enough, that the material contribution to
broadening is negligible when compared to the instrumental broadening
of a typical powder diffractometer.
Some helpful references in modeling peak broadening are:
Stephens, J. Appl. Crystallogr. V32, pp 281-289. 1999
LW Finger, DE Cox, and AP Jephcoat. J. Appl. Crystallogr. V27 pp
892-900 (1994)
Good Luck,
David Lee, Ph.D.
DTLee Scientific, llc
http://www.dtlee.com
614-562-6230
On May 12, 2009, at 11:51 AM, May, Frank wrote:
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
