In a message dated 8/14/00 6:56:35 AM Pacific Daylight Time, 
[EMAIL PROTECTED] writes:

<< a) what is/are the difference(s) between this 'size' compared to the 
 size measured by other, mostly optical, methods? (such as laser-
 size analyser?) Is the difference orginated from the direction of the 
 measurement of size, in which the direction from size analysis by 
 line broadening is fixed to be parallel to diffraction vector; while the 
 size analysers only take the longest dimension in a random 
 manner? >>

one should differentiate between 'crystallite' size and 'particle' size 
(measured by laser scattering and other means); 'particle' size is always 
reported as a distribution of sizes and rarely has a correlation with the 
'crystallite' size which is reported as a length in a crystallographic 
direction [hkl] (hence, for an amorphous powder a 'particle' size can be 
measured but no 'crystallite' size);
the concept of coherently diffracting domains (='crystallite' size) which are 
parallel to the diffraction vector was developed over 50 years ago and 
conveniently adapted to theta/2theta diffractometers (where the diffraction 
vector is always parallel to the surface normal); 
 
<< b) if the size effect in the line broadening of the whole powder 
 diffraction pattern is taken as isotropic, then which direction is the 
 <D> taken for (normal to which diffraction vector) in the analysis?>>

the broadening of each reflection hkl in a powder pattern should yield the 
same crystallite size in an isotropic case, i.e. all column lengths 
regardless of direction [hkl] are identical (rare but possible); 

I am not sure if it is a good idea to derive average coherent domain lengths 
from Rietveld analysis programs. I am not aware of any package currently 
available which is set up to follow the strict analytical procedures 
necessary (= profile deconvolution, Fourier analysis of line profile etc). 
Exact size cannot be determined reliably from simple profile width.

L. Keller    

 

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