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
