One benefit arises from the collection of the high angle data with
better-than-normal counting precision - this has the most obvious affect on
X-ray data because of the natural fall-off in intensity due (primarily) to
the form factor. However, it is true that the counting precision of the
weak data can also be improved by merely counting the whole pattern for longer.
The second and less acknowledged benefit accrues from the fact that with
variable counting time, by and large, all of the reflections are collected
with more or less the SAME statistical precision. In other words, there is
a substantial change in the relative precision with which the large and
small d-spacing data (i.e., generally the large and small intensity
reflections) are collected relative to constant counting time methods.
Finally, Bill David has provided another fascinating and more sophisticated
wrinkle on the variable counting time technique by specifically targetting
certain reflection types, or regions of individual peaks, to collect with
different precision so as to draw out specific structural features.
Overall, the results speak for themselves - the data is 'cleaner', the
refinements are more stable, and subtle features appear to be more
accessible - at least for some materials.
Best wishes, Rod
Dr Rod Hill R&D Manager & Deputy Chief, CSIRO Minerals,
Box 312, Clayton South, VIC 3169
rod.hill@minerals.csiro.au Tel: 03-9545-8602 Fax: 03-9562-8919