In Rietveld refinement of pure zeolite-A (by Fullprof package), plot visible agreement is ok, but asymmetry in peaks and k-alpha2 peaks causes deviation in observed and calculated peak intensities. I have tried with different peak functions such as pseudo-Voigt, Pearson-7, split pseudo-Voigt, p-V Axial divergence asymmetry, TCH p-V to solve asymmetry shape in peaks. But still, the problem persists. Switching asymmetry parameters leads to the error message 'asymmetry parameters usage is invalid'. How to achieve a good agreement for this kind of peaks?
axial divergence and other instrumental effects. I am convinced that Fullprof is able to handle this. The point is that you have long-angle data, which is challanging but the problem is well-known. Play with the parameters and do not refine them all simultaneously.
Fullprof also has options to incorporate instrumental broadening from a standard. Here you have the problem, that typical standards like LaB6 do not have reflections at such small diffraction angles.
Dear Mahimakumar M.
Is your Zeolite natural or synthetic?
If natural, have you properly clean the zeolite structure?
It has large channels that retain a significant amount of materials (ions, anions, even small molecules).
As already pointed out about axial divergence, consider the lack of ideal stoichiometry, or include the idea of substituted (doped) zeolite structure, especially if natural one.
It looks like you are considering less scattered atoms them already present, even as very light effects.
Be careful with asymmetry in peak description; it tends to overcome the real effect by describing an unreal peak asymmetry, based on a decrease in Chi2 of you fitting procedure, simple mathematical solution.
Best regards
WNM
This issue seems to related with asymmetry or FWHM or their combined parameters effects as they affect each others. FullProf have provision for such corrections. You may adopt different strategies to optimise profile parameters:
Try structureless (Le Bail) approach, which will help to figure out profile dependence from structural (atomic) issues of Zeolite as suggested by Dr. Wagner Da Nova Mussel . If LeBail fitting is OK then see structure.
For profile related issues try these:
1. If you have a LaB6 or Corundum standard sample XRD data, then try to fix asymmetry values as obtained by refining LaB6 or Corundum data. Then refine asymmetry parameters one by one keeping others fix as suggested by Prof Andreas Leineweber . If some parameter value is far away from true one, fitting may diverge.
or
2. Try to fit data in 2 or 3 parts like first from 20 deg and above to get good fit as per your satisfaction then include lower angles as 15 ded, then upto 10 deg then full pattern. This generally helps in getting optimum parameter values of convergence for good fit.
and
3. Proper asymmetry value might resolve Ka-2 issue, but if still present try to check whether there is any deviation in Ka-1/Ka-2 ratio of instrument from generally taken value of 0.5.
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