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I was trying to refine a crystal structure using Rietveld refinement yesterday. The crystal structure is already known for a related compound and the powder pattern seems to show a phase pure compound. An automatic peak indexing and refinement gave a possible cell that matches the literature values. Using the Pseudo-Voigt function however did not lead to any good results as the function did not match the asymetric peak profile.

The profile itself shows something I'd call 'fronting', so there is a slope at the front of the peak but then the measured curve just falls off directly after the peak. I was looking for that in literature but I could not find the term 'fronting' here. There was something similar called 'transparent' or 'transparency' but I wasn't sure whether this is referring to the profile or thr setup.

Does somebody know what this effect is called and what type of refinement is used here instead of Pseudo-Voigt?

Picture of the peak shape from the diffraction pattern

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  • $\begingroup$ Could it be that each of the visible peaks is actually composed of 2+ peaks? What's the radiation source? $\endgroup$
    – marcin
    Jul 11, 2018 at 19:40
  • $\begingroup$ Pretty sure it is K-alpha lines of Cu which is typical for x-ray diffraction. $\endgroup$
    – MaxW
    Jul 11, 2018 at 23:33
  • $\begingroup$ I think the term you are looking for "asymmetric peak." I'm guessing low angles are more asymmetric. One cause is instrument geometry. See second paragraph, second column on page 896 of paper Finger et al "A Correction for Powder Diffraction Peak Asymmetry due to Axial Divergence " where it discuses zeolite peaks. Constants H and S are discussed on lower right of first page of the paper. $\endgroup$
    – MaxW
    Jul 12, 2018 at 0:31
  • $\begingroup$ It is a molybdenum source due to the presence of Europium in the sample. That links seems to summarize the problem but is there a different polynomial that solves the issue besides the Pseudo-Voigt one? $\endgroup$
    – Justanotherchemist
    Jul 12, 2018 at 7:53

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