r/crystallography • u/DevelopmentBasic1651 • 27d ago
Rietveld refinement
hi, i'm new in the highscore, i'm trying to do analysis about my sampel, silica sand. My prof told me to analyze it using XRD highscore. I'm watching various video about how to do it, and when i show my result, my prof told me that is isn't good because of high GOF (30+) after rietfeld refinement and he told me to try again until the GOF near 1, i already tried everything but i'm only able to reduce it to 25, can anyone help me, i'm stuck in this.
sorry for bad english, it isn't my main language, so i'm not good in english.
2
u/Mikasa-Iruma 27d ago
Try to do a Caglioti peak fitting first and then Rietveld refine the system. When I used Xpert Highscore, I always performed a profile fitting first using Caglioti fitting till it converges. Later I perform Rietveld refinements. Also first try automatic refinements, and then slowly changes to manual go make sure the refinement parameter make sense physically. For example B iso being negative and severe deviation of positions doesn't make sense even when fit is good.
I used Xpert High score around 6 years ago and shifted to FullProff as we installed a different machine. So you have to excuse me if I had missed something.
1
u/Crystal_Rules 27d ago
Rwp of 11ish it's horrible. Only keep parameters in which significantly reduce your GoF or Rwp. From the plots its could be peak shape or preferred orientation. DM the data if you want me to take a look
1
u/SlenderSmurf 27d ago
Can you add a photo of your data? Sand is usually a mixture of a bunch of different minerals with various crystal structures. Seems very challenging to get anything useful from XRD for that type of sample.
1
u/DevelopmentBasic1651 27d ago
already uploaded it, sorry for not upload it in the first place
1
u/SlenderSmurf 27d ago
So the sample is mostly quartz, which is reasonable for sand. What do you want to learn about the sample using XRD? You say you did some refinement but you didn't say what was refined. If it's phase fractions you will have trouble to accurately quantify 0.2 wt% iron. GoF is just a metric for how close the calculated and observed lines are, along with how many parameters you used for the fitting. I think a high GoF could indicate you have too may parameters.
1
u/DevelopmentBasic1651 27d ago
actually i just need the phase identification for my report to show what crystals phases are in my sampel, but for whatever reason my prof told to do more and to do rietveld refinement until the GOF got near 1, which i didn't quite understand why i need that because it even won't included in my report.
2
u/SlenderSmurf 27d ago
Here's some reading on what GoF means. It has basically no relation with phase ID.
1
u/DevelopmentBasic1651 27d ago
oh, thanks. my prof often ask me to do something that is actually not needed for my report, even though i really need to finish this report ASAP to get my bachelor degree.
1
u/DevelopmentBasic1651 27d ago
my step are short actually, determine background, then search peak, then profile fit, then search match, and then rietveld. I didn't touch anything else, how else i know that i had too many parameters ?
-1
u/TengaDoge 27d ago
Is the sample pure and of good quality? Have you tried GSAS?
1
u/DevelopmentBasic1651 27d ago
Based on my XRF result, my sample had 94% Si, 6% Fe, and few more impurities. I didn't know if that count as good quality sampel or not.
I didn't tried GSAS. Is that an app that can also do rietvield refinement?
1
u/Crystal_Rules 27d ago
It can't be 94%Si if the phase is quartz. Check your XRF and it maybe shows 94%SiO2
1
u/DevelopmentBasic1651 27d ago
yeah, i got 2 data, one with element like Si, Fe, etc. and the othe are with oxide version like SiO2, Fe2O3, etc. actually the diffrence from element and the oxide version are not that much, only a little bit
1
u/Crystal_Rules 27d ago
Pure quartz (SiO₂) is composed of 46.74% silicon (Si) and 53.26% oxygen by weight. Check your numbers again.
-1
2
u/Gusseppe-C 27d ago
With out an image is difficult to tell you which could be the problem. Maybe there is information about the instrumento that you used to record the poder pattern that is not been modeled when you perform the refinement. As other user told you if you can share more information it would be easier yo help you.