Dear Md Saiful Alam ,

A wild answer to this question takes some considerations.

Nevertheless, there is no simple way to distinguish a proper good fit from one that is just a set of wrong based on R factors or other discrepancies.

Many Rietveld indices (R-factors) have been proposed; there is no way, to pick up one that can be used as an absolute measure of refinement quality.

The reasons need to be precise; a good fit, not always means a low Bragg-R factor. It means a physically correct and acceptable description of the material. There are several ways to get relatively low Bragg-R factors completely meaningless; it is a widespread mistake.

Diffraction data are a set of intensities measured at a set of specific momentum transfer (Q) values, usually expressed as 2θ scanning.

For substitutional samples, stacking, the stoichiometry deviation, and solid-solutions with several members with slightly different compositions must be evaluated.

An attempt with a profile using divergent axial symmetry is worth to try, as this asymmetric modeling helps better fit the above phenomenological occurrence in the sample. 

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

Just in time, the relative high Rwp may reflect the necessity of a profile using divergent axial symmetry.

WNM

Thank you Wagner Da Nova Mussel for your suggestions. Actually, I am just learning the refinement. Whatever, in my refinement data I found that one of the occ values and two of the B value is negative. What are the meanings of these values. Thank you in advance. If you allow I can share my data with you.

Dear Md Saiful Alam ,

The negative value is a fitting mathematical artifact; by adopting negative occ the Rw or chi2 drops, there is no physical meaning in negative occupancy.

Sometimes, a specific atom's suggestion is made in a particular site with some occupancy describing substitutional or interstitial occurrence, and the fitting result gives zero or negative near-zero value. That means the suggestion is unacceptable or at least unprovable. However, with known atoms distribution in already symmetric described sites, the negative value has no physical meaning for a regular structure description.

Some software parameter descriptors are polynomial functions. Eventually, one of them may assume negative values. Check the software documentation description for that particular parameter descriptor, whatever it is. Always be sure the negative value is not a mathematical artifact.

Best regards,

WNM