At 08:49 14/08/00 -0500, you wrote:
>I recently have tried, for the first time, the fundamental parameters
>approach to fitting powder diffraction data using the XFIT program. To the
>best of my knowledge I am using a reasonably well aligned system (with a
>theta compensating slit in place of a fixed divergence slit), yet when I
>refine on all of the geometrical and instrumental parameters the projected
>x-ray source width (nominally 0.04 mm at about 6 degree takeoff angle) and
>the receiving slit width (0.2 mm) both refine to values near zero while the
>remaining parameters seem reasonable. I would appreciate any insights on
>this problem.
>
>
>Karl M. Unruh Be very, very careful what you put into
>Department of Physics that head, because you will never, ever
> and Astronomy get it out.
>University of Delaware
>Newark, DE 19716
>Cardinal Wolsey
What type of sample are you running and is it a very well defined
standard? If not a well defined standard (annealed Cubic Y2O3 is
excellent), this can result in the above.
You have to be careful of refining too many geometry parameters
at once or this can be the type of effect you can see. To check if
the XRD equipment is kosher, I normally would run annealed Cubic Y2O3
and only refine the Sollers Slits angles and Receiving slit height.
If these do not go close to measured values (and sample and sample prep
are good) it could be telling you there is a problem with the XRD
configuration that needs to be looked at before running further samples.
I have only doing this for real on fixed divergence slit instruments.
Other people may have better suggestions on how far you can get away
with refining on geometry parameters.
Lachlan.
Lachlan M. D. Cranswick
Collaborative Computational Project No 14 (CCP14)
for Single Crystal and Powder Diffraction
Daresbury Laboratory, Warrington, WA4 4AD U.K
Tel: +44-1925-603703 Fax: +44-1925-603124
E-mail: [EMAIL PROTECTED] Ext: 3703 Room C14
http://www.ccp14.ac.uk
