Dear Frank,
minimum crystallite size is no problem with this software (as long as a
minimum of 2 unit cells are stacked coherently). The maximum size is
more or less dependend on your instrumental setup. Dont expect to get
reliable results above of c. 50-60 nm if you perform analysis with a
instrumental broadening in the range of 0.1 2theta. However, if you
improve the diffractometer resolution to c. 0.015 - 0.02 2 theta you may
extend your measurement range well beyond the 100 nm limit.
One shortage of the software is that it uses profile-fitted data and
allows only for a single profile shape function (PSF) for each
reflection. Therefore, the size-distribution you get as "result" is
always a function of the the PSF you used at input. The most crucial
thing is that you always get uni-modal functions, even if your actual
size distribution is bi- or poly-modal.
Software that is capable for treating multimodal size distributions is
(among others) "Mudmaster" from Dennis Eberl or "WinFit". Both are free
and can be downloaded from
http://www.geol.uni-erlangen.de/html/software/winsoft.html
Regarding the "single peak" methods of the software: don�t use this for
separating size and "strain" parameters. The algorithm assumes that the
broadening due to small size causes Cauchy-type reflections and strain
results in Gauss-profiles.
However, many size distributions, e.g. from clay minerals are log-normal
and this results in reflections that have a Pearson parameter of c. 1.4;
in other words, that have a certain Gaussian component. If such strain
free reflections are processed with the above algorithm, the results
will erranously indicate a fictive strain that is not at all present in
the sample. Because strain broadening is asumed to be present, the
resulting crystallite size will be in error too and will yield to high
values.
greetings,
Stefan
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Dr. Stefan Krumm Institut fuer Geologie
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