Marek,
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
********************************************************************
Brian H. Toby, Ph.D. Leader, Crystallography Team
[EMAIL PROTECTED] NIST Center for Neutron Research, Stop 8562
voice: 301-975-4297 National Institute of Standards & Technology
FAX: 301-921-9847 Gaithersburg, MD 20899-8562
http://www.ncnr.nist.gov/xtal
********************************************************************
Marek Schmidt wrote:
>
> Dear All,
>
> 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
> is 5-150deg.
>
> Best regards
> M. Schmidt
>
> ======================================================
> Marek W. Schmidt
> Department of Applied Mathematics
> Research School of Physical Sciences and Engineering
> Australian National University
> Canberra ACT 0200, AUSTRALIA
> email: [EMAIL PROTECTED]
> facsimile: 61-2-6249 0732
> telephone: 61-2-6249 0381
> ======================================================
