Hello all,
I’m fairly new at XRD analysis. Recently I tried to perform Rietveld refinement of newly synthetized spinel-based material using FullProf software .
The material that was obtained by me (magnetite analog) was analyzed by XRD. Diffractograms (peak positions and intensity) were quite similar for those I could find in the literature for such materials.
But when I tried to perform the Rietveld refinement on Fullprof using data from the literature, I couldn’t obtain a theoretical diffractogram that could fit into the measured one.
I tried to import CIF file into Fullproff and Vesta and, while Vesta is displaying values of site occupancy factor for all atoms as 1, occupancy factors in Fullprof have fractional values. (attached screenshot)
As far as I see, Fullprof is using Wyckoff positions form CIF to calculate occupancy factors featured in program.
So, I have another question. How I can calculate occupancy factors knowing Wyckoff positions, in a way this program is doing it?
Dear Rafal Lysowski ,
Fullprof will not take into account site multiplicity/wyckoff position and will display directly the values of the occupancy (it is easier to manage the refinement this way).
Vesta and most of the software used to display structure will "normalize" the crystallographic informations using site multiplicity/wyckoff position (here 8a, 16d, 32e). The ratio between these multiplicity are the same as the ratio between your occupancy on Fullprof. (32/16 = 2; 0.16667/0.08333 = 2) (32/8 = 4; 0.16667/0.04167 = 4)
By default on fullprof, the 1.0 occupancy is fixed for the highest "theoretical" possible multiplicity in this space group, which is 192 for your Fd-3m space group. A 32e site have then a (1/192*32 = 0.166667 occupancy on Fullprof).
You can use this website for wyckoff position and multiplicity : https://www.cryst.ehu.es/cryst/get_wp.html
It is recommended to put the site with the highest multiplicity on top on Fullprof (here your oxygen), as you can face some error in conversion sometimes.
Hope it helps,
Dear Bastien Leclercq,
Thank you for your answer,
I expected that occupancy factors corresponds to average quantity of atoms in declared position.
I found also another small problem, but I resolved it already.
When, I’m trying to substitute atom in the same position with other, I have to declare in PCR file, atom with highest Wyckhoff position as first. If I’m trying to declare it in other position site occupancy factors for are multiplicity of atom that is listed fist.
Anyway, I tried to refine my diffractograms using provided tips and everything seems to look fine -output from program meets with expectations.
Dear Rafal Lysowski, why is the thermal factor anisotropic instead of isotropic?
It's been changed to isotropic already.
Now I'm looking for tips how to lower R-factors. I’ve got Chi^2 below 2 with believable isotropic B-factors. I was trying to fit shape and FWHM parameters but still my Rp and Rwp parameters are quite high (Rp above 30)
It is easy to achieve Chi2 close or even less than one if you have poor quality data, as indicated from your Rexp. Probably very low in intensity or higher intensity but higher background due to fluorescence effect.
Can you share your diffractogram?
Dear Maykel Manawan,
I'm working right now quite high quality data. Spinels that I'm synthetizing , have a tendency to precipitate some additional phases that could disturb other tests, I’m doing.
I’m attaching diffractogram and pcr file, I’m currently working on. I was trying to add some asymmetry parameters to achieve better fitting
Atomic positions, I used in pcr file comes from database and they are related to very similar spinel.
Dear Rafal Lysowski. Your higher Rwp and Rp are because of higher background although the intensity is quite high as well. The peak to-background ratio is only 3.3. This is due to the fluorescence effect on Cu-source to Fe and Cu elements in your sample. It can be seen by the increasing background as 2theta.
Fullprof already corrected the background so that you get those high Rwp and Rp. Here I compare your result with others.
Dear Maykel Manawan
Thank you for your answer,
I have another question, is there any way to lower those values in Fullprof?
I'm going to try Maud software, I see it is free for download
"I have another question, is there any way to lower those values in Fullprof?" Yes sure, kinda fine-tuning to the profile, asymmetric shape and preferred orientation, etc.
Hi Rafal Lysowski ski,
to add to Maykel Manawan n replies: careful refinement of the profile parameters etc will results in smaller R-values, however the high background will prevent it from decreasing much further.
If you can/if necessary, try using a powder diffractometer with Mo-source or energy dispersive X-ray detector. Either helps to reduce the background caused by fluorescence and improves the R-values
I saw in your pcr-file that Mg- and Cu-cations are mixed on the same site. I assume you used the reactant ratio as ratio of the occupancies. In case you want to experimentally determine the Mg-to-Cu ratio from your diffraction data, set the code (aka flag) for the Cu occupancy to " X1.00 " and the code for the Mg occupancy to " -X1.00 " (where X is the number of variables in your refinement + 1). This allows FullProf to refine the ratio between the two without changing the sum.
Cheers
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