Re: [sdpd] size broadening and diffracting volume

[Armel Le Bail <alb...@cristal.org>]

>Being not an expert on size broadening effects I would like to know why an
>'increasing diffracting volume leads to an increased symmetric broadening'.
>See Luca Lutterotti's remark on <http://www.cristal.org/opinions/>.

Explanation without any formula :
If you consider the Bragg Brentano reflexion geometry, the X-ray
penetrating depth is of the order of microns for highly absorbing
compounds and can easily attain one millimeter for very low-absorbing
compounds. This means that you can really put highly absorbing
compounds in the diffracting plane. But for very-low absorbing compounds
a large sample volume is quite under of the diffracting plane. Now, just
think to what happens if your sample is placed under or above the
diffracting plane : your peaks are displaced either at lower or larger
angles. So the "peaks" are "broadened" in this geometry in case
of low-absorbing coumpounds due to the sample thickness. That
broadening reflects the distribution of sample outside of the diffracting
plane.

This is why Al2O3, in such a reflexion geometry, will never show peaks
as narrow as LaB6, even if perfectly crystallized (SRM 1976  is
certainly very well crystallized and the more compact as possible).

Best,

Armel


 

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