I have a question about the surface roughness. The LaB6 powders
that I have seen are
very finely ground and produce very flat, smooth samples. Is the
roughness connected
with the absorption? I'm thinking along the lines that a low
absorbing, rough sample
might not "look" as rough to an x-ray beam as a high absorbing,
smoother sample.
Thanks,
David Lee, Ph.D.
DTLee Scientific, llc
http://www.dtlee.com
614-562-6230
On Jul 23, 2009, at 5:23 AM, Hinrichsen, Bernd wrote:
One intensity correction that is perhaps more realistic is surface
roughness. This does have a vaguely similar angular dependence to
the LP correction. This correction is generally only applied to
highly absorbing samples in Bragg-Brentano (or generally reflection)
setups. This would seem to be the case for the LaB6 measurement
mentioned by Peter.
Greetings
Bernd
-----Ursprüngliche Nachricht-----
Von: Peter Y. Zavalij [mailto:pzava...@umd.edu]
Gesendet: Donnerstag, 23. Juli 2009 05:52
An: rietveld_l@ill.fr
Betreff: RE: LP factor in the Rietveld refinement
Well... the situation with LP is not so simple. Using TOPAS for
refinement
data collected on D8 advance with Ni-filter and LynxEye detector I
observe
the following:
- For all samples LP=0 is OK and gives the best fit as it should be
by the
book.
- HOWEVER for LaB6 standard LP=0 yields very poor fit for several
high angle
reflections (>120 deg. 2theta) while LP=90 gives perfect fit. The
difference
in R factors 12% and 4% cannot be simply ignored...
Can anyone explain this?
Peter Zavalij
X-ray Crystallographic Center
University of Maryland
College Park, MD
Office: (301)405-1861
Lab: (301)405-3230
Fax: (301)314-9121
-----Original Message-----
From: Ross Williams [mailto:ross.willi...@curtin.edu.au]
Sent: Wednesday, July 22, 2009 8:43 PM
To: Jon Wright; xiu...@ualberta.ca
Cc: rietveld_l@ill.fr
Subject: RE: LP factor in the Rietveld refinement
Hi Xiujun,
Jon is correct, but to answer your question fully, the angle is used
in an
equation to scale the peaks as function of 2theta.
If you look in the Technical Reference Manual of TOPAS states that
the LP
factor (for x-rays) is given by
LP = (1 + cos(2th)^2 cos(2th_m)^2) / (cos(theta) sun(theta)^2)
2th_m is the angle mentioned by Jon, ie 26.4° when using Cu Kalpha
with a
graphic monochromator, 0° when using unpolarised beam, and 90° for
full
polarised.
Or in TOPAS macro language : scale_pks = (1 + Cos(CeV(c,v) Deg)^2
Cos(2
Th)^2) /(Sin(Th)^2 Cos(Th));
The Technical Reference has a derivation of the LP equation above and
compares it to parameters used in GSAS and Fullprof.
Kind Regards,
Ross
+++++++++++++++++++++++++
Ross Williams
PhD Student
Centre for Materials Research
Department of Imaging and Applied Physics
Curtin University of Technology
GPO Box U1987 Perth WA 6845
Western Australia
Phone: +61 (0)8 9266 4219
Fax: +61 (0)8 9266 2377
Email: ross.willi...@curtin.edu.au
-----Original Message-----
From: Jon Wright [mailto:wri...@esrf.fr]
Sent: Thursday, 23 July 2009 5:26 AM
To: xiu...@ualberta.ca
Cc: rietveld_l@ill.fr
Subject: Re: LP factor in the Rietveld refinement
Sounds like the parameter is the monochromator angle you would need to
use to convert an unpolarised beam into a beam with the polarisation
state you have (eg, 90 degrees gives 100% polarised). Don't confuse
this
with the actual monochromator angle at the synchrotron, as the bean is
usually polarised before it reaches the monochromator anyway. With
some
packages you can set the monochromator "roll" angle to put the
polarisation in the right plane, depending which way up an area
detector
was mounted.
Good luck,
Jon
xiu...@ualberta.ca wrote:
Hello, everyone,
I have some questions about the refinement in Topas.
When we put the instrument parameters, we always include the LP
factor, and set it to a constant value. I thought LP factor is a
function of theta and not a constant value, so my question is what
exact the constant value means. Why for unpolarized radiation, it is
equal to 0, and for synchrotron radiation it is equal to 90. Sorry to
throw so many questions.
Thank a lot for any help.
Xiujun Li
Master Student
Advanced Materials and Processing Laboratory
Chemical and Materials Engineering
University of Alberta
Edmonton, Alberta, Canada T6G 2G6
Phone: 1-780-492-0701
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