Hi Prof, Thank you so much for this amazing resource! I was just curious as to why when calculating Rwp that there is a bias toward the lower-intensity peaks? Is this done to exaggerate the differences between observed and calculated patterns in the case of one having a peak when the other does not? Apologies if this is a very simple question I am still very much a novice to crystallography.
I have to admit I don't know the history of why that weighting scheme was chosen, but I believe the reason is because the errors are higher when measuring the intensity of the diffracted beam when the intensity of that beam is higher.
Hello, sir. Thank you for your excellent presentation. I have a doubt about what you said about preferred orientation. You said that it is possible to have much higher or much lower peaks than they should be. I am refining samples that I presume that are preferred oriented, because one peak is much lower than it should be. How can I confirm that it is a preferred orientation, and not just a defect in the crystal?
It can be hard to know for sure if preferred orientation is the problem. If you pack the powder differently (ideally make a flat plate reflection measurement and a capillary transmission measurement the preferred orientation will change significantly or possibly go away in the capillary. However, I realize that isn't always possible. If you are doing a Rietveld refinement you can add a preferred orientation correction. If that leads to a much better fit it is usually justified. If it doesn't help much, the issue is probably not preferred orientation.
