I measured a fluconazole drug sample and found 2 phases. Then, for the first time, I tried to apply the rietveld refinement method using Match! and got relatively high values for both R bragg factor (9.5) and Chi-square (20.8). The peaks seem to be fitting the 2 phases well in relation to 2-Theta, but as you can see in the graffic, the intensities on some of them are not fitting too well, but even after trying to solve the preferred orientation problem, those values are the ones I got so far. Can someone give me some advice on how to lower them even more and make the model acceptable?
Concentrate the anlaysis on the low angles first, maybe up to about 18 degrees 2 theta. At higher angles you have a lot of reflexes. A large deviation in intensity is at about 12.6 degrees. Is this a preferred orientation?
There is a reflex at 6.7 which has not been indicated yet, could there be another phase?
The technique of Rietveld refinement considerably accompanies a sequence of refinement in a prescribed order. I don't know, how you went for the refinement, but I generally maintain the following order in FULLPROF:
(1) In case of multiple phases, consider only one phase first.Refine various parameters in the following order:
(a) First refine basic profile parameters intensity scale, a, b, c, strain, u, v, w, in this order.
(b) Then go for other derived profile-parameters.
(c) Next go to atomic positions.
(d) If you have impurities, dopings or defects like vacancies present in the system, you need to refine occupacies as well.
(e) After doing the refinement in the above order, and taking all sorts of permutation-combinations during varying the values and refinement; go for any other parameter which is left.
(2) After one phase is appreciably matched, go for the other. The major phase should always be refined first.
(3) Only after doing the whole job for both phases and cross-refinements; go for preferred orientation. Again do the whole thing start to end.
I hope, it'll help you. Please remember that, the order of refinement; and initial values are really important for fast and efficient refinement.
Götz Schuck Thank you very much for the advice. I'm having a problem trying to fit the model with that peak on 18 degrees, because neither refining scale factor nor preferred orientation parameter is able to correct it's intensity without undoing all the progress with the other peaks.
Yes, there is definitely at least one more phase in it, which I assumed was one of the excipients (sodium laureth sulfate and/or sodium croscarmellose), but I couldn`t find any reference pattern for them to help me find which one It is.
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