I have synthesized strontium based hydroxy-apatite by co-precipitation process. i have done the rietveld refinement of it. The problem is its chi-square is not less than 2, even though I have refined them properly and zooming into each peaks after every refinement.
What can be the possible cause for this? Is it because nano-particles have been formed as a result we cant do its refinement as I read it somewhere else or is there something wrong which I have not taken care of. I have uploaded the pcr file and the image of final refinement.
You might theoretically attain chi^2 = 1 if the difference is determined by counting statistics and not by systematic errors. But virtually every X-ray powder diffraction pattern has been measured with such a accuracy that you have systematic problems e.g. with the description of the profile shape (as it is the case in your case).
-you might decrease you chi^2 by improving the description of your profile
-You get very nice Rwp or chi^2 values especially if you have high background. This is typically what you do not like to have. But such an example tells you not to believe (to much) into the residual parameters.
See e.g.
https://www.cambridge.org/core/journals/powder-diffraction/article/r-factors-in-rietveld-analysis-how-good-is-good-enough/17439A1F889B689C495549A234D53682
or
Book Rietveld Refinement Practical Powder Diffraction Pattern Ana...
Murat Sertkol , what kind of backgrounds should I choose in the software, and can you tell the possible reason to try different background to get good chi square?
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