Re: X-Fit and SPVII, FWHM?

L. Cranswick ( L.M.D.Cranswick@dl.ac.uk )
Thu, 17 Sep 1998 02:18:51 +0100 (BST)

> when fitting a split PVII to a peak profile,
> one gets the values 2theta, H1 and H2, m1 and m2,
> which are coefficients in the SPVII function. But one
> does not get the FWHM of the peak (especially not
> the FWHM left and right, which one could use for
> describing the asymmetry). Is there a way to
> calculate the FWHM from these coefficients?

Superficially, I would have thought summing the H1 and H2,
you would obtain a Half-Width of sorts(?). If the shape
is doing something wierd - would estimation by eye using
the cursor be fruitful to get the FWHM?

Be wary that the Split Pearson PVII is a very powerful
and flexible function and not too be used lightly.
If you are doing empirical peak fitting - try the
PseudoVoight for as many peaks as feasible before going
PVII.

For an example, refer:
"Be extra careful when using Split Pearson Peaks".
http://www.ccp14.ac.uk/tutorial/xfit-95/sp7care.htm
Mainly refering to "m" shape values going lower
than 0.8.

---

Again, personal bias is that if you want good informationon low angle peaks where Peak Asymmetry is causing damage - Fundamental Parameters can give more joy -providing you can define the geometry of the instrumentin a way the software likes.

For example, in the CRYS2RUN indexing suite tutorial(http://www.ccp14.ac.uk/tutorial/crys/basic.htm),Robin Shirley was quick to point out that relying onthe first (low angle) line-pair for self-calibration(least potential error due to peak overlap)of two-theta offset/zero point correction was not a good idea as the low angle peaks are most likely tobe affected by various abberrations that shift apparantpeak position relative to the rest of the pattern. Theexception being if the peaks were profiled usingFundamental Parameters (which they were in the abovetutorial) as FP provides resulting peak positions as though the sample was run on an "ideal diffractometer" - using an ideal sample with respect to parameters you can model - such as linear absorption.

Lachlan.

PS: Just a reminder that for FP peak fitting in XFIT - when using an Automatic Divergence slit - you enable FSFL (X-ray beam sample irradiation length - in mm) rather than FSFA (Divergence Slit angle). This is enabled/disabled in the Instrument part of the "File, Details" menu.

-- Lachlan M. D. CranswickCollaborative Computational Project No 14 (CCP14)    for Single Crystal and Powder DiffractionDaresbury Laboratory, Warrington, WA4 4AD U.KTel: +44-1925-603703  Fax: +44-1925-603124  E-mail: l.cranswick@dl.ac.uk  Ext: 3703  Room C14NEW CCP14 Web Domain (Under heavy construction):                           http://www.ccp14.ac.uk