Not necessarily! If (some) peaks are super-Lorentzian the effect could be
caused by a broad lognormal distribution of crystallite sizes.
Anyway if the peaks are super-Lorentzians the pattern decomposition by using
pseudo-Voigt is, in principle, incorrect. Indeed, imagine that after
decomposition we wish to clean this profile of instrumental contribution.
Presuming the instrumental profile is Voigt (if x-ray laboratory), the
Fourier transform of such profile is ~exp(-B*y-C*y**2). On the other hand
the Fourier transform of the Lorentzian part of "measured" pseudo-Voigt is
~exp(-A*y), then the Fourier transform of the "deconvoluted" profile for
this term should be ~exp[-(A-B)*y + C*y**2]. Certainly A>B (breadth of
"measured" peak should be larger than of the instrumental), consequently at
small values of y the Fourier transform curve decreases with increasing y,
as expected, but when y = (A-B)/C this curve begins to increase, becoming
infinite when y=infinite, which is absurd. In fact pseudo-Voigt has been
used in pattern decomposition algorithms (including Rietveld) only to mime
and to calculate faster the Voigt profile, then with ETA<1 (in TCH
pseudo-Voigt this condition is explicitly given). In this case there is no
problem with the "deconvolution", that, in fact, is made between two Voigt
functions.
Best wishes,
Nicolae Popa
----- Original Message -----
From: "Jon Wright" <[EMAIL PROTECTED]>
To: <rietveld_l@ill.fr>
Sent: Thursday, November 22, 2007 12:53 PM
Subject: Re: Hyper Lorentzian peak shape
If you can fit via a sum of a broader lorentzian and a narrower one it may
indicate the sample is a "mixture". Are you sure the sample is
microscopically homogenous?
Best,
Jon
Francois Goutenoire schrieb:
Dear Rietveld users,
I am currently working on Williamson-Hall graph in order to check strain
effect on some new phase.
I have some hyper-lorentzian peaks eta>1 from a pseudo-Voigt
decomposition , then I am not able to get BetaL and BetaG from Winplotr
decomposition.
Does anyone have an idea ?
Francois Goutenoire
Laboratoire des Oxydes et des Fluorures, UMR6010.
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