Re: [sdpd] Percentage crystallinity calculation using Powder XRD

[Luca Lutterotti <Luca.Lutterotti...@ing.unitn.it>]

Yes, in fact I wrote also:
"Depends on the amorphous type it may work for you."

And also I specify that he "may" try it if nothing else can be done 
(internal standard or external).

But in reality (apart the reviewer problem that it may be a problem as 
well as not; I didn't find problems in my case), we (I and co-workers) 
used it several times and with other kind of amorphous (TiO2, different 
polymers, pharmaceuticals etc.) and in many cases it was not so 
difficult.
In most cases the amorphous simply has a local structure very similar 
(coincident) with one of its crystalline polymorphs (normally the low 
temperature one, as for the TiO2) and it is easy to see if it works or 
not (Lutterotti et al., Mat. Sci. Forum, 343-346, 657-664, 2000).
I know that some people may not like this 
pseudo-amorphous-nanocrystalline description, but as you demonstrate in 
your paper it works for fitting the spectra and even getting the short 
range atomic arrangement. In our analyses we found that using this 
model/description it works well for the quantitative analysis. As I say 
at one congress, it does not matter too much if the structure model is 
very accurate or not, for the quantitative analysis it is important the 
total scattering and this is proportional to the electron density. If 
the electron density is accurate, and the amorphous fitting is good, 
then the quantitative analysis works.

Obviously is not easy using Rietveld programs that are not meant to do 
it. But in Maud for example, you just play with the crystallite and 
microstrain to get the correct shape for the amorphous and using the 
box to change the size-strain by visually checking simultaneously the 
result on the spectrum is so easy to get the correct amorphous shape.

Obviously all this requires at least to have a pseudo model. So it can 
only be applied if such a model can be found (easily). In the case of 
the silica glass you made it and it was not so easy.

But if there are no other ways...............

				Luca

On Feb 18, 2005, at 9:09, Armel Le Bail wrote:

>
>
>> If you need to do the analysis without internal standard and no
>> calibration curve you may look at:
>> http://www.ing.unitn.it/~luttero/Publications/EPDIC_V/
>> silicate_glass.html
>> reference: Lutterotti et al., Mat. Sci. Forum, 1998, 278, 87-92.
>
> OK Luca, but for that approach you need a very special
> description of the amorphous component by some
> highly defective crystalline model which can then be used
> in a Rietveld refinement with extremely large diffraction
> peaks, allowing quantitative analysis. Such a (controversial)
> model was published for glassy SiO2 in the J. Non-Cryst. Solid
> journal (the model is derived from cristobalite), but you cannot
> find a lot of such models for other amorphous materials...
> So that the method is not generally applicable. And
> the controversial nature of this glassy SiO2 simple model may
> lead to some problems with the reviewers when trying to publish
> a manuscript (though the paper in J. Non-Cryst Solids was
> welcome and really easy to publish - after rejection by
> Nature and Science... in spite of a very good simultaneous
> fit of the neutron and X-ray data of glassy SiO2 by the Rietveld
> method ;-). Many "experts" dislike this approach...
>
> Armel
>
>
>
>
>
> Yahoo! Groups Links
>
>
>
>
>
>
>



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