First, I want to say that I just started learning XRD refinement. If I asked any stupid questions, please forgive me.
I have XRD measurements for nano materials. The structures are refined by fullRMC. The total agreement is pretty good (see Fig 1 and 2 which are errors for sample #1 and #2, respectively ). However, I noticed that there are sharp peaks at low r (~2 Å). In the figure of sample #1, the sharp peak is contributed by Al-O, while the sharp one is contributed by Ti-O in the figure of sample #2. I adjusted the related cutoffs to larger or smaller, however, the peak cannot be eliminated.
I do not understand which factor(s) results such errors. Any suggestion and help would be highly appreciated.
I do not use fullrmc, but any of the commonly used programs for Nanoparticle refinement (PDFgui by the Billinge group, RMCprofile (ISIS/OakRidge) and my own DISCUS program). Thus I'll have to guess a bit.
A sharp peak like the Al-O distance in a PDF/RDF signals that in the underlying structure model, all interatomic distances for this pair are fixed. fullrmc seems to have an option to constrain bond distances, please check that parameter in your refinement. In sample 1 you seem to have used a cutoff at 1.6(?) Angstrom. This bears the risk that further distances may be present that give too short of an interatomic distance.
A few general hints. As you are modelling fairly complex oxides (Al, Ba, Cs, Ti)O a RMC algorithm may end up with a nice fit that does not bear any physical meaning. There are simply many parameters that you fit compared to the observed data. There seem to be about 17 peaks in the distance range up to 11 Angstroem which give you ~ 51 peaces of information and you are moving around many atoms, with 30 interatomic distances. The refinement is highly underdetermined. Make sure that the calculated PDF does fit the range at larger distances as well and that the original powder diffraction pattern is reproduces as well. Also check if the local coordination polyhedra do make sense. Is all Al in an octahedral coordination for example, Ti probably should be within an octahedron as well.
Finally do you really need an RMC algorithm? The RDF does not look like that of an amorphous material. Thus a refinement starting from the nominal bulk structure might be a better strategy.
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