I am refining my XRD data using in FullProf program. The intensity of the modulated X-ray peaks is almost flat. I tried to adjust the peak intensities but unsuccessful. What is the reason? I guess I am doing some hidden mistakes, but not sure where it is...
Any suggestions would be appreciated!
Thanks in advance!
The X2 value equal to 1 means it is an ideal fitting. The X2 value lower than 1 is impossible it must be above than 1. It means your initial applied model is completely wrong. Therefore many peaks of the applied model are not matched with exp data as evident from the blue line (Yobs - Ycalc).
Dear Jamil Ur Rahman ,
By inspecting the Bragg positions (green lines) of your fitting, they look very far away from the correct or desirable ones, based on the experimental peaks.
Is the model used to start fitting the adequate one? Even the lattice parameters seem not right.
The initial fitting procedure in FullProf needs to be the scale factor; otherwise, all efforts result in no convergence.
Just as a suggestion (there is no general recipe):
And so on, depending on your sample characteristics.
The question on Chiˆ2: the value is compensating the theoretical points calculated (black fitted line) with the experimental one (red dots). Some experimental points appear under the line, others over the line. At the resulting computation of the total chiˆ2, they compensate somehow each other. The resulting value has no physical meaning et al.
A good fit not always means a low Chiˆ2 and R-Bragg values. It means a physically correct and acceptable description of the material. There are several ways to get relatively low Chiˆ2 and Bragg-R factors completely meaningless; it is a widespread mistake.
Please, check this paper; it is an excellent discussion on how good a fit is good enough.
https://www.cambridge.org/core/journals/powder-diffraction/article/r-factors-in-rietveld-analysis-how-good-is-good-enough/17439A1F889B689C495549A234D53682/core-reader
Best regards,
WNM
Thanks, Muhammad Habib for your suggestion. Maybe you are right, but maybe not!
The model that I used is based on TEM analysis. To validate the model, I visualized it in the VEST and the calculated powder diffraction pattern is very close to my XRD's data. So, I guess the chance of claiming MODEL error only is very low. See the attached snap!
Thanks
Dear Jamil Ur Rahman ,
The question in using the TEM analysis as starting model belongs to the sample's constituents. The X-ray beam probes the surface, few fraction of millimeters under the surface. The TEM probes the core as well. If there is, let say, oxidized or no oxidized core material compared to the particles' surface for some unexpected reason, the TEM will give some mixed guess. Naturally, not fully reacted reactants will contaminate the data as well.
Nevertheless, I agree that TEM would be a good start, if from a for the sure pure sample.
Check the starting space group for a more symmetric one; the TEM predicts unrelated Bragg-reflections, your material probably presents a more symmetric space group not a general one.
The X-ray experiment looks good; the observed inconsistency probably is related to the scale factor.
Best regards,
WNM
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