Hi dear,
I am new in the Reitveld analysis of XRD powder for Ferrite nanoparticles. I am using Fullprof software to do the refinement of Cu substituted Ni-Cd ferrites. After refining possible parameters, it gives me large values of Rwp and Rexp, which are more than 60 each, but Chai2 value closer to 1.5.
Could you please help me to provide me step by step refining method to get the best residual with good value of Rwp and Rexp? I need your cordial support. Also mention, I did not find exact .cif file of my material in open database. I downloaded cif file for different materials, whose some elements are close to mine with same space group.
Regards,
Arif
Rexp is "best-expected" value, that depends upon how good your data is. If your spectrum is fuzzy or containing broad peaks, the value of Rexp will be high. In your case, the value of Rexp is high, that means you can try to do a long scan. Although in your case it would be a bit difficult as you have nano-particles which inherently have the characteristics to shows broad and fuzzy spectrum. However, once you have sufficiently good data, Rexp value will go down and hence the ratio of (Rwp/Rexp)^2 (or chi^2) will go high! On this stage, you are good start the refinement. First try to refine, scale factor, zero correction, background, Lattice parameters, UVW and etah (profile in Fullprof) one by one such you reach to the point where you can refine all the parameters simultaneously. After reaching a stability you can refine atomic positions and thermal parameters. This is a standard procedure which might vary, kind of observe and judgment situation.
Thanks
I hope this would be helpful
Khagesh
Rexp is "best-expected" value, that depends upon how good your data is. If your spectrum is fuzzy or containing broad peaks, the value of Rexp will be high. In your case, the value of Rexp is high, that means you can try to do a long scan. Although in your case it would be a bit difficult as you have nano-particles which inherently have the characteristics to shows broad and fuzzy spectrum. However, once you have sufficiently good data, Rexp value will go down and hence the ratio of (Rwp/Rexp)^2 (or chi^2) will go high! On this stage, you are good start the refinement. First try to refine, scale factor, zero correction, background, Lattice parameters, UVW and etah (profile in Fullprof) one by one such you reach to the point where you can refine all the parameters simultaneously. After reaching a stability you can refine atomic positions and thermal parameters. This is a standard procedure which might vary, kind of observe and judgment situation.
Thanks
I hope this would be helpful
Khagesh
These agreement indices are strongly related to the pattern statistic. But...how is the graphical comparison? It's an important thing to verify. From this observation it is possible to understand where are the main fit problems. The cif file of a generic cubic spinel is surely good also for your material. The suggested procedure for the refinement is the standard one that in general well functions. Be careful of a possible spinel inversion that can affect peak intensities, in particular that of the peak at about 18° (Cu wavelenght)
I agree with the previous answers. Without seeing the data/fit it is hard to say how to proceed. Assuming you have the correct cif file, is the issue matching intensities or peak widths? Definitely look at the residual plot, they tell you if you are missing (important) peaks or not. Also plotting sqrt (even log) for the intensity axis helps to show you which peaks are not being fitted adequately.
Refinement of nanomaterials could be difficult. Choice of anisotropic/isotropic peak widths may help (if you have this option in Fullprof), when peak widths are an issue.
Best,
Arman
Try http://www.crystallography.net/cod/ to find the structure .cif file. If it wasn't available you can always create one by clicking on new data and inputting the structure's data on Fullprof. Do not use other materials .cif file. I find these videos helpful for learning Reitveld refinement. You can also learn about parameters involved in Reitveld refinement by watching these videos.
Good luck.
https://www.youtube.com/watch?v=mWNpIIwEcDk
https://www.youtube.com/watch?v=lW5cwmqz_G4
https://www.youtube.com/watch?v=mnxd5ACqR9E
Thank you everyone for your cordial response. For refining thermal parameters, do I need to select as anisotropic instead of isotroic? We synthesized our samples at 550 C. About using cif file, I used Ni0.5Cd0.5Fe2O4 found from open database. Still getting poor residual. How can i measure crystallite size from Fullprof refinement?
Thanks
Arif
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