Just want to add confusion :-).
Last pattern I rietveldized with FULLPROF had considerable
asymmetric profile shape. With pseudo-Voigt and asymmetry
parameters, the zero point was 0.07, Rp=13.6 %. With split
pseudo-Voigt, the zero point was 0.01, Rp=10.2 %.....
I am tempted to write that a bad fit of considerable peak asymmetry
may lead to quite wrong zero point.
Now if you have transparency effect + large asymmetry (which
is likely to occur with some diffractometer geometry), I don't
know what could happen if the software has not adequate
asymmetry correction.
Armel Le Bail - Universite du Maine, Laboratoire des Fluorures,
CNRS ESA 6010, Av. O. Messiaen, 72085 Le Mans cedex 9, France
http://fluo.univ-lemans.fr:8001/
http://www.cybercable.tm.fr/~cristal/welcome.html