Hello everyone,
I've bee struggling with Rietveld Refinement of this ZIF-8 MOF XRD diffractogram. The peak correspondent to 110 reflection is very asymmetric and my refinement is not good enough.
How can I proceed with this example? Should I smooth the graph? (I never do it, but this one made me think about it)
What one could do, in XRD analysis or in synthesis, to prevent this asymmetry?
I don't know how the function is approximated in the GSAS-II program. But you can try the FCJAsymm function. (Axial asymmetry peak shape in the Finger, Cox and Jephcoat model, see J. Appl. Cryst. (1994) 27, 892 and J. Appl. Cryst. (2013) 46, 1219.)
It is implemented in the Fityk program.
https://fityk.nieto.pl/
To my experience the asymmetry of reflections in ZIF-8 are visible also in commercially available samples (picture). To some extent it may be reduced by inserting narrow Soller Slits (in the picture 0.03 rad slits were used) to reduce axial divergence. I use FullPorf, but I am pretty sure in GSAS there is possibility of refining parameters of profile asymmetry.
Dear Ricardo.
You can try to refine the sample displacement?. Normally, with sample displacement and microstrain parameters, the asymmetry could be modeled.
In GSAS it was through S/L and H/L:
GSAS offers two ways to treat low-angle asymmetry poor: asym (profile functions #1 & #2) – works only if small or no low-angle data excellent: Finger-Cox-Jephcoat (profile #3 & #4) – two terms: S/L & H/L • refine with care, if at all (I never do) • never refine both together – S = sample height – H = detector height, – L = diffractometer diameter (N.B. FCJ use radius & half-heights, equivalent, but messy)
https://www.aps.anl.gov/sites/default/files/APS-Uploads/XSD/Powder-Diffraction-Crystallography/5ParametersRecipes.pdf
I believe, in GSASII it is SH/L but since they are somehow combined you have poor fit.
From your screenshot, I see a tail on your data's, left side of the peak, which is normal thing for TOF data. If so, normally it is an instrumental artifact and could be well corrected with instrumental file-parameters. In Topas, you have the option to include in the refinement possible tails as well.
similar topic and good suggestions from Ricardo Tadeu Maia
https://www.researchgate.net/post/Can_GSAS-ii_model_a_peak_using_a_pseudo_split-Voight_function_for_asymmetrically_broad_peaks
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