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Tutorials and Examples

Peak Fitting using Xfit-Koalariet (Coelho and Cheary) for Win95/NT

Fixing Peak Shape to be 100% Gaussian

The CCP14 Homepage is at http://www.ccp14.ac.uk

[Tutorials page] | [XFIT Tutorial Index]

[The reference to use for XFIT or FOURYA in any resulting publications is: Cheary, R. W. & Coelho, A. A. (1996). Programs XFIT and FOURYA, deposited in CCP14 Powder Diffraction Library, Engineering and Physical Sciences Research Council, Daresbury Laboratory, Warrington, England. (http://www.ccp14.ac.uk/tutorial/xfit-95/xfit.htm)]

The Problem

When fitting peaks that have a large background component, it is possible that the peaks will develop large lorentzian tails that start merging into the background. XFIT cannot tell what is a peak and what is background, and will thus happily do you bidding - crunching numbers.

Following is a "trick", when followed intelligently, may help, by fixing the shape to be 100% gaussian. In theory(?), peak shapes in Energy Dispersive diffraction are supposed to be pure gaussian. Though people who blindly believe these peaks much be gaussian should refer to Figure 5 (page 818) and Conclusion VIII (page 842) in the "International Union of Crystallography commission on Powder Diffraction Rietveld Round Robin. II. Analysis of Monclinic ZrO2" By R. J. Hill and L. M. D. Cranswick, J. Appl. Cryst. (1994) 27, 802-844. This shows the effects of the Neutron Powder Diffraction community having the same view with respect to their profiles.


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