Holger,
I am not sure how to answer your question, but perhaps this will
help. A pseudo-Voigt is the sum of a Lorentz function and a Gaussian
function and simulates a Voigt, which is their convolution.
There are two ways to parameterize a pseudo-Voigt. One is to have the
same peak widths for both the Lorentz and Gaussian functions and then
use a mixing parameter (eta) where 0 is pure Gaussian, 1 is pure
Lorenzian and 0<eta<1 is a mixture. There is a different approach, where
the Lorentz and Gaussian functions each have their own parameterization.
(I think that Bill David wrote something about this a while back). GSAS
uses this latter approach. For CW profiles 2 & 3, the primary terms are
Gaussian: GU, GV & GW; Lorentzian: LX & LY. Do read the GSAS manual,
there are many more terms that have specific uses.
In profile type 4 (not in the manual), there is an appearance of our
friend eta. In this case it is the mixing parameter for the treatment of
anisotropic strain broadening. See Peter Stephens J. Appl. Cryst. paper
for info on that.
Brian
Holger Kohlmann wrote:
>
> Hi,
>
> coming from FullProf and trying a first refinement with GSAS I am a bit
> confused about profile functions for CW data. How can I refine the
> mixing paramter eta in a pseudo-Voigt function with GSAS?
>
> - Holger Kohlmann
--
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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
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