>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
>
That is right.
Moreover, in powder structure refinement in the same way as in single crystal
refinement "thermal" parameters absorb all types of error and other effects that
either were not included in the refinement or were refined incorrectly, etc.
Of course, in the single crystal regiment these uncounted or miscounted
parameters are negligible at least in routine experiment (not in one for electron
density distribution). BUT in powder data they are not and affected more or less
by following errors (given in arbitrary order):
- surface roughness;
- background (it correlated with thermal parms very much when peaks are heavily
overlapped);
- sample size (diameter) for Bragg-Brentano geometry (it cuts intensity of low
angle peaks, when beam is wider than sample;
- preferred orientation;
- extinction;
- PSF;
- sample transparency;
- alpha1/alpha2 ration;
- etc, etc.
- and of course almost everything that affects single crystal data too...
Dr. Peter Y. Zavalij University Crystallographer
Materials Research Center, SUNY at Binghamton
Phone: (607)777-4298 Fax:(607)777-4623
E-mail:[EMAIL PROTECTED]
http://imr.chem.binghamton.edu/zavalij/zavalij.html
