What sequence one should follow for refining the parameters to get the best fit using Rietveld analysis? I have bring down the value of chi2 to 12 (still too large). Though its reported in literature that for nano particle cases the chi2 is expected to be more. How can I reduce the chi2 further? Moreover the position of the fitted peak was shifted in compare to experimental data. Am I following the right sequence of refinement? Or the high chi2 may be due to the fact that I have not given any information of dopant in the input file?
Dear Rastogi,
sorry for late, but I don't have much time to concentrate on web, however I've refined your pattern quickly (see the attached file), I've did the rietveld refinement for the "A" sample, you can try to fit it more if you like, but it is good for now, I've corrected the parameters you input and some shape parameters, note that your sample has a noticeable preferred orientation , you should grind it better than that and be very accurate when preparing the XRD plate.
Your phase data
Formula sum: Na2 La2 W4 O16
Formula weight: 1315.18 g/mol
Crystal system: tetragonal
Space-group: I 41/a (88)
Cell parameters: a=5.3456 Å c=11.6290 Å
Cell ratio: a/b=1.0000 b/c=0.4597 c/a=2.1754
Cell volume: 332.30 Å3
Calc. density: 6.57163 g/cm3
Pearson code: tI24
Formula type: ABX4
Wyckoff sequence: fba
Atomic parameters
Atom Ox. Wyck. Site sof x/a y/b z/c
Na +1 4b -4.. 0.5 1/2 -1/4 1/8
La +3 4b -4.. 0.5 1/2 -1/4 1/8
W +6 4a -4.. 1 0 1/4 1/8
O -2 16f 1 1 0.26690 0.11026 0.03729
Bragg R-factor: 2.79 Rf-factor= 2.22
Rp: 14.9 Rwp: 12.0 Rexp: 9.70
DW-Stat.: 0.9458 Chi2: 1.52
With respect,
Dear Rastogi,
The refinement sequence is very depending on the XRD pattern profile you have and on how much accurate information about your pattern do you have. However, in general you are performing a very good sequence.
if you did not input or refine the dopancy, that would affect the peaks relative intensity to move away of your observed XRD pattern, you should take that into account.
you could attach your refinement with the data so I could check it for you.
In fact a chi2 of 12 is bad. But what about the experimental conditions that you used to collect the pattern? The sequence of parameters variation is not entirely correct. In general the background and the different correction should be varied first, after the scale factor, but I don't think that this is the problem in your refinement. If you post the graphical comparison I think that it could be more simple for us to try to give you some suggestion.
Dear Yazan & Marcella,
Thanks for your kind suggestions. I am attaching .rar file containing the .pcr as well as graphics for two cases. For case "A" i got chi2 ~ 15 and for case "N" I have reduced it to ~ 7. However, it is still to high. The differences in chi2 may be due to the differences in the intensities of the peaks in these two different cases. Please suggest how can I reduce the chi2 to its minimum.
Please post your data in some universal format for the rest of us turtles to be in the race :-)
Keep on sharing the knowledge of non-intrusive in situ XRD!
BTW I just got trained on the Reitveld method by the expert @ MIT recently and would like to enhance my understanding. Excellent question!
My response:
These items need to be addresses prior to any Reitveld application, isn't that so?
Dear Rastogi,
sorry for late, but I don't have much time to concentrate on web, however I've refined your pattern quickly (see the attached file), I've did the rietveld refinement for the "A" sample, you can try to fit it more if you like, but it is good for now, I've corrected the parameters you input and some shape parameters, note that your sample has a noticeable preferred orientation , you should grind it better than that and be very accurate when preparing the XRD plate.
Your phase data
Formula sum: Na2 La2 W4 O16
Formula weight: 1315.18 g/mol
Crystal system: tetragonal
Space-group: I 41/a (88)
Cell parameters: a=5.3456 Å c=11.6290 Å
Cell ratio: a/b=1.0000 b/c=0.4597 c/a=2.1754
Cell volume: 332.30 Å3
Calc. density: 6.57163 g/cm3
Pearson code: tI24
Formula type: ABX4
Wyckoff sequence: fba
Atomic parameters
Atom Ox. Wyck. Site sof x/a y/b z/c
Na +1 4b -4.. 0.5 1/2 -1/4 1/8
La +3 4b -4.. 0.5 1/2 -1/4 1/8
W +6 4a -4.. 1 0 1/4 1/8
O -2 16f 1 1 0.26690 0.11026 0.03729
Bragg R-factor: 2.79 Rf-factor= 2.22
Rp: 14.9 Rwp: 12.0 Rexp: 9.70
DW-Stat.: 0.9458 Chi2: 1.52
With respect,
Dear Yazan,
Thanks a lot. Great effort. I am really very thankful to you.
The same way, I have fitted the XRD pattern of sample N and have reduced the chi2 to ~3.8. For this case (Sample N), there is change in the relative intensities of peaks (graphic file attached comparing with sample A). Can you please suggest in such cases where the relative intensities of peaks changes, which parameter specifically should be refined? I have seen the pcr file that you have done for case A. You have used linear interpolation method (LIM). Can you please suggest while using LIM how many number of background points one should use and what should be the 2thetha and corresponding count value?
The attached file contains (i) graphics showing the differences in peak intensities for the case A and N (ii) rar files containing .pcr and supporting files
Thanking you with regards
Chandresh
Dear Rastogi,
the deference in the relative intensities between these two patterns is coming from the preferred orientation as I told you previously, check it.
the attached picture shows the calculated XRD pattern for your structural, I reversed the process by calculating the XRD pattern from the atomic structure, assuming that your material particles are oriented in all directions equally.
but in your case, the sample "A" has a preferred orientation, that means: the particles have a coherently stacking over each other in specific direction, destroying the statistical equilibrium of the particles orientation in all space direction to have some preferred reflection planes, where the intensity in its direction is higher, relative to the others directions in normal patterns.
the next sample "N" has a very small quantity of these coherently staking particles so the preferred orientation is more close to the zero, (see the fitted attached file), and it is more close to the calculated picture I've attached.
the N.dat file you provided contain an modified x-ray observed data, it is not the original, so I normalized it to fit the pattern correctly, and why am using the linear interpolation method for background, simply because your sample background is noisy some how, so the 12 poly or any other background fitting style will not be useful where they build to deal with the high intensity XRD patterns with a negligible background fluctuations.
is it clear now my friend ?
and by the way, I did not take a look at your fitting data n.rar I have no time sorry, but if you have a specific question about it just ask me again and specify the place and the parameters
Dear Yazan,
Thank you for such a explicit explanation and nice fitting. I understood your explanation. Your expertise in Rietveld has helped me a lot.
Thanking you with regards
Chandresh
Dear Yazan, I read your explanation, regarding Rietveld refinement, I am doing the same for MgH2 but my chi2 is in the range 4-10, in any case it is not going below 4. Can you please help me in that case or suggest the refinement steps if possible.
Regards,
Apurva
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