tris-pitt
Hello!
I am learning to use TOPAS Professional V6 and have encountered a couple issues related to WPPF and Pawley refinements. Any guidance would be greatly appreciated!
First, I am trying to fit PXRD data using whole powder pattern fitting by selecting all peaks, then refining on an isotropic crystallite size parameter. I am refining all peak positions and areas. Under these conditions, the peak positions that I select often travel far from their assigned peaks and can even end up in regions of powder pattern where no peaks are present and only the background is present. I understand that the refined peaks positions may vary from their original placement due to instrument and sample aberrations, but I am concerned that faulty refined peak positions that describe only the background may contribute to errors in the average crystalline domain volume, for example. How do you recommend stabilizing the refinement to prevent peaks from shifting so much?
Second, I am trying to run Pawley refinements on PXRD data and had an odd issue related to the above. I find that using only an hkl Phase to refine the data results in a poorer fit (judging by Rwp, hlk tick positions, difference curve) than using a combination of an hkl Phase and a Peaks Phase. When using both simultaneously, the assigned peaks in the Peaks Phase often move from their original placement like in the above case, yet this improves the calculated curve and the hkl tick positions match with the sample peaks very well. Is it appropriate to use both hkl and peaks phases in a Pawley refinement?
Thank you in advance for your help.
Tristan
rowlesmr
0) Hi Tristan. Welcome to the Topas forum.
1) A good thing to do is to fix peak positions and crystallite size while the area is refining. Then let the position refine, then the crystallite size.
2) Adding more parameters will almost always improve the fit. Whether or not it makes physical sense to add them is another question. Have you got the correct space group? Is you specimen pure? Are you modelling your instrument correctly?
Have fun.
Matthew