I saw many examples, in classes, videos or tutorials, of creating a profile file of an XRD instrument, for Rietveld Refinement. But I did not understand how the standards are used to do this.
I know that the total peak broadening of a sample is the summation of the broadening related to this sample and the one related to the instrument, because the instrument itself provides some broadening, this is clear to me. That's why we measure the pattern from some standard, like LaB6 or Silicon-NIST, to determine how the broadening of the instrument is.
However, I don't see any tutorial or video taking into account the crystalite size and microstrain of these standars. Are they equal to 1, or something like it? I can't understand how they just refine the instrument parameters and "ignore" the characteristics of the standards.
Sincerely,
Ricardo Tadeu.
Dear Ricardo Tadeu Maia ,
please have for example a look at the section ' Information values: ' at page 3 of the following link to a NIST certification for LaB6:
https://11bm.xray.aps.anl.gov/documents/NISTSRM/NIST_SRM_660a_%5BLaB6%5D.pdf
Here it is stated, that
a) the measured 'domain' size (=crystallie size) is 2µm, and
b) that there is no residual micro strain.
2µm is large enough to have no effective size boadening of the XRD peaks.
So you are right: for this standard you can ignore peak broadening due to crystallite size and microstrain...(otherwise a 'standard' will make no sense...)
You may 'google' for certificates for other materials...
In the following link background of the certification procedure is given:
https://nvlpubs.nist.gov/nistpubs/jres/125/jres.125.020.pdf
It is worthwhile to have a look at...
Best regards
G.M.
Dear Gerhard Martens ,
Thank you so much for this excellent explanation. I can understand now why crystallite sizes of standards are never mentioned.
But one more question: I followed you suggestion and searched for another materials certificates, like Silicon. I found this file, in which it says that its particle size is 2 nanometers, which is very small and would make a huge difference in peaks broadening. Did I really get the correct certificate for Silicon?
Best Regards
Ricardo Tadeu.
Dear Ricardo Tadeu Maia ,
thanks for feedback...
According to the title of the 'Silicon' certificate, it is a reference for nanoparticles measured by DLS (light scattering), but not for XRD.
According to the Scherrer equation, crystallite*) sizes of 2nm will cause peak broadening around 4 to 5deg (in 2theta) and will thus not be usefull in determining the XRD instrumental broadening.
*) assuming that the NP is a crystallite
Best regards
G.M.
Dear Gerhard Martens ,
I found this other certificate, this one looks the correct for XRD and Silicon standard. Thanks again.
Best regards,
Ricardo Tadeu.
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