Re:[sdpd] size and strain analysis

[leosol...@chat.ru]

>It seems that Rieveld is the better way to obtain 
>non-user-dependent results but it is quite hard to
>teach to non crystallographer .thus, most of the
>questions below deal with a peak by peak analysis

Actually, all the questions you asked should be better
answered after the final analysis of the Size/Strain
round robin results, but it may take a long while.

In addition to the response of Armel Le Bail I would
like to add only one remark.

>pseudo-voigt function used for resolution function
>determination on standard compounds do not allow
>to fit the asymetry observed at low angle. Does it
>matter really if i do not take into account this asymetry ╖? 
>Is one side of the peak more significant than the other ╖? 
>The split pseudo-voigt function gives better results but the
>.splitting . parameters show a tendency to oscillate which
>would make difficult the analytical resolution function expression.

Asymmetry .broadens. the low angle part of peaks and this
is the part of additional instrumental broadening which should
be either subtracted or minimized by using more rigorous
Soller slits (or ESRF BM16 beamline if available).

In my local modification of BDWS I introduced the asymmetry
correction so that in calculating the low angle part of peak the
FWHM parameter is given an angular dependent addition. 
See:
L.A.Solovyov, M.L.Blochina, S.D.Kirik, A.I.Blochin, M.G.Derikova,
Powder Diffraction, 1996, V.11, N.1, p.13-16
Having only one refinable parameter this correction provides tolerable
fitting of asymmetrical peaks and also allows taking into account the 
asymmetrical instrumental broadening. If the split pseudo-voigt 
function you use allows it, you may also try refining only one .splitting.
parameter (left FWHM) in order to avoid the oscillations.

Leonid Solovyov

P.S. 
In the archived message www.mail-archive.com/rietveld_l...@ill.fr/msg01544.html,
referred to by Armel, the formulae Ys = Yb - Yi; Zs = Zb - Zi  may look wrong 
due to, I suspect, a font mix-up.

 

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