Colin,
While I do not have very much experience using the Finger-Cox-Jephcoat
model on lab diffractometer data, I can say that it works at least
pretty well on every problem I have tried and usually does OK when the
S/L and H/L values are set at the values matching the instrument design
and are not refined, though focussing often requires an increase in S/L.
Adding lots of extra empirical parameters to fit to data will usually
give a better fit and this is often used as the justification to choose
a non-physical line shape function. A split-Pearson used to model
asymmetry will never properly describe the peak shifts due finite
detector and sample size and will bias your lattice constants and
perhaps even your crystallographic model.
Practically, to use the type 3 or type 4 function in GSAS, set S/L to
your sample length divided by the instrument diameter and H/L to the
detector slit length (along the long direction) divided by the
instrument diameter. You may or may not have good results from refining
the values, but refine them one at a time with lots of damping. Never
refine them starting from zero or both at the same time when the the
values are equal. I usually have better luck refining them in a peak fit
program such as GPLSFT (implemented in CMPR) than in GSAS. Hope this
helps.
As for using TOF functions for CW data in GSAS -- this can't be done.
Brian
>
> I have tried function 3 without any joy, and I'm not able to repeat the
> experiment using longer wavelengths. I am, therefore, still seeking help.
>
> Best wishes,
>
> Colin.
--
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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
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