[sdpd] Re: GSAS: Stephens Model

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

--- In sdpd...@yahoogroups.com, apu...@... wrote:
> In this case if I use the refined parameters (of Stephens model)to 
draw the 3D-anisotropic strain field within the sample, does it 
carry any physical meaning!

If I remember well, by definition, the Stephens approach is said to 
be "phenomenological". This means that it improves the fit but that 
you cannot deduce any size and microstrain effect parameters having 
physical sense from the results. 

At least, your fit is improved, so that your structure parameters 
are certainly better than without the use of this approach. 
Moreover, from the 3D-anisotropic strain field plot, you obtain the 
directions in which there are the strongest microstructural effects, 
etc.

Anyway, don't worry about not being able to describe a multitude of 
possible defects by only one number... Experts are themselves unable 
to agree on any standardized method for the characterization of 
microstructural effects by powder diffraction methodology. They have 
just more or less agreed on the way to determine a size effect on a 
cubic microstrain-free sample. See the round robin on CeO2:
http://journals.iucr.org/j/issues/2004/06/00/ks0213/ks0213.pdf
(open access).

Best regards,

Armel
 




 
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