Since Matthew Henrichsen put the question to the Rietveld list two days ago
there have been many different suggestions of possible reasons why
"thermal" parameters are so often not reliably refined from powder X-ray
diffraction data. I suggest that when 'Bragg-Brentano' geometry
(reflection) is used, the most commonly encountered reason may be surface
roughness. It is much more important than we gave it credit for being for
many years. The eye opener was the work by Cullie Sparks, et al. now
published in Advances in X-ray analysis, Vol. 35A, pp. 57-62 (1991).
Both GSAS and DBWS now contain refinable models for the surface roughness
effect. If you try to use that feature in either program you will note that
you are given no sage guidance about how best to use it. The reason is
simple: we don't know (or, didn't several months ago. Maybe Bob has since
had some better ideas.) The correlations are ferocious.
Ray Young
Prof. R. A. Young
School of Physics
Georgia Institute of Technology
Atlanta, GA 30332 USA
Tel: 404-894-5208
FAX 404-894-9958
e-mail [EMAIL PROTECTED]
