Methods, Problems and Solutions
GSAS (General Structure Analysis System) Rietveld powder diffraction and Single Crystal software
Lp correction in GSAS
The CCP14 Homepage is at http://www.ccp14.ac.uk
[The reference to use for GSAS in any resulting publications is:
A.C. Larson and R.B. Von Dreele, "General Structure Analysis System
(GSAS)", Los Alamos National Laboratory Report LAUR 86-748 (1994).]
Rietveld Mailing List
Date: Sun, 9 Jul 2000 14:51:58 +1000 (EST)
From: Marek Schmidt
I have two questions concerning the polarization factor in GSAS. I am
analyzing synchrotron x-ray diffraction patterns and to work out the
diffractometer constants I refined a silicon pattern. My questions are:
1.What is the formula for the RBVD Lp correction? Is it the third formula on
page 125 of the manual?
2. The pattern was refined using the RBVD correction. The refined
polarization fraction POLA reaches a value of 1.14 (the initial value was
set to 0.95) is it possible?
The experiment was carried out in Debye-Scherrer geometry with an incident
beam monochromator and the patterns were collected using imaging plates.
The camera radius is 573mm, the sample was placed in 0.3mm glass capillary.
The refinement includes absorption correction and corrections to the silicon
scattering factor (f' and f"). Wavelength= 1.796A, the angular range
Marek W. Schmidt
Department of Applied Mathematics
Research School of Physical Sciences and Engineering
Australian National University
Canberra ACT 0200, AUSTRALIA
Date: Mon, 10 Jul 2000 16:28:35 -0400
From: "Brian H. Toby" [Brian.Toby@nist.gov]
Organization: NIST Center for Neutron Research
To: Marek Schmidt , Rietveld_l@ill.fr
Subject: Re: Lp correction in GSAS
There are three polarization functions listed in the manual, but only 2
when I try to select the function in EXPEDT. I seem to remember that the
1st two functions are equivalent, other than scaling, but I am not going
to work this mystery today.
As I recall, with IPOLA=0 (RBVD Lp correction) POLA sets the relative
amount of in-plane vs out-of-plane polarization coming off the incident
monochromator. Thus for you, POLA should be slightly less than 1.0
(synchrotrons tend to be highly polarized, but the actual amount depends
on many factors). I have used 0.95 to 0.99 for my work at X7A. The bad
(or good) issue here is that the P part of the LP correction is
virtually indistinguishable from the Debye-Waller 2theta curve, which
means if you refine it, the displacement parameters (aka "thermal"
factors) couple to POLA and you can refine to an unrealistic value, like
>1. If you fix POLA to a wrong value, your displacement parameters shift
to compensate. For that matter, absorption corrections have ~ the same form,
too, particularly since you have data over such a small Q range, so that will
also interact with POLA and displacement parameters.
Pick a value of POLA and fix it. Someone at the beamline you used must
have a rough idea of the right value. If you are really adventurous,
pick a few values and compare the results of the different refinements.
That will give you a feeling for the impact of POLA on your results.
Brian H. Toby, Ph.D. Leader, Crystallography Team
Brian.Toby@NIST.gov NIST Center for Neutron Research, Stop 8562
voice: 301-975-4297 National Institute of Standards & Technology
FAX: 301-921-9847 Gaithersburg, MD 20899-8562
Date: Tue, 11 Jul 2000 11:44:20 +0100 (BST)
From: Jon Wright [firstname.lastname@example.org]
Subject: Re: Lp correction in GSAS
On Mon, 10 Jul 2000, Brian H. Toby wrote:
> There are three polarization functions listed in the manual, but only 2
> when I try to select the function in EXPEDT. I seem to remember that the
> 1st two functions are equivalent, other than scaling, but I am not going
> to work this mystery today.
My apologies for such a public display of ignorance, I have a related
mystery to ask about. Having had a quick look at the manual, all three
corrections are for parallel geometry, I'm guessing "no monochromator",
then "incident beam mono." and then "diffracted beam mono.". OK.
The question is what about flat plate transmission (STOE) type data versus
Bragg-Brentano versus Debye-Sherrer? Doesn't a factor of cos(theta) at
least turns up in the transmission data. This is where I start to feel
rather stupid, but how can you specify these cases? It doesn't appear to
come into the instrument parameter file or the lorentz corrections
Any hints or advice would be appreciated. Probably Bob has the best idea
of what GSAS is doing, and how it's doing it. I've heard rumours of
manual "corrections" being applied to data before presenting it for
refinement, which sounds ugly. Fullprof has a flag for transmission
geometry, hence my question.
Thanks in advance,
PS : If someone could refer me to simple derivations of the LP
corrections for all the various geometries, I'd be very grateful.
Dept. of Chemistry, Lensfield Road, Cambridge, CB2 1EW, UK
Date: Tue, 11 Jul 2000 08:50:25 -0600
To: Jon Wright [email@example.com]
From: firstname.lastname@example.org (Bob Von Dreele)
Subject: Re: Lp correction in GSAS
Dear Jon (and others),
The "standard" reference for X-ray Lp corrections is the paper by L.
Azaroff, Acta Cryst., 8,701-704 (1955). It applies equally well for both
incident beam and diffracted beam monochromator cases and is independent of
choice of Debye-Scherrer, Bragg-Brentano or for that matter single crystal
diffraction (hence also applies to transmission geometry - but there may be
a separate "foot print" effect that depends on the optics. See below.). The
L (Lorentz) part is applied to CW neutron data (usually Debye-Scherrer)
since there is no neutron polarization effect. There are three "options" in
GSAS for x-ray Lp corrections. The first two are equivalent except for
scaling and the third is no correction at all; some folks wanted this as
they apply the correction beforehand. However, they must do proper error
propagation for this correction and use the "ESD" or "ALT" form for the raw
data input to GSAS. The third equation in the GSAS Manual was incorrect, it
was deleted sometime ago.
The crucial thing to establish for transmission geometry is whether the
illuminated (and seen!) sample volume is independent of scattering angle.
If it is independent then either GSAS Lp correction will work just fine. If
not, then an additional correction must be applied for the change in
sampling volume - this correction is not in GSAS as I don't know what it is.
It should also be noted that the sampling volume error can occur in
Bragg-Brentano geometry when the sample isn't "infinitely thick" or doesn't
cover completely the beam foot print at all measured scattering angles.
This is a particular problem for diffraction from organic solids with low
absorption or for thin "smears" of sample on a glass slide or samples that
are too small for complete beam coverage at low angles. GSAS doesn't have
anything to deal with these elementary experimental errors.
Bob Von Dreele