Re: Axial divergence
Bob Von Dreele ( vondreele@popler.lansce.lanl.gov )
Thu, 30 Oct 1997 08:14:00 -0700
At 12:15 PM 10/30/97 +0100, you wrote:
>Hello everybody !
>
>I try to refine a powder pattern showing strong peak asymmetry due to
>axial divergence using the approach of Finger, Cox and Jephcoat implemented
>in the GSAS program. The pattern was measured on a conventional
diffractometer
>in Bragg-Brentano geometry with Cu K alpha1 radiation ( primary
monochromator )
>and Soller slits.
>Whenever I try to introduce the S/L and H/L ratios in the calculations the
>refinement becomes unstable and diverges. Has anybody gained experience in
>using this asymmetry correction ? Is there someting special to consider ?
Dear Volker (and others),
My own experience with this function is a bit limited (except for
installing it into GSAS, of course). My observations in testing it showed
that refinement of S/L & H/L were unstable particularly if the data did not
start at a very low two-theta value (say less than 10 deg!). You can
generally do better by arbitrarily fixing one of the asymmetry coefficients
and refining the other. A value like 0.01 does pretty well in most cases.
Bob Von Dreele