>Why does x-ray powder diffraction fail?
Overlapping.
>Why do the others work?
No overlapping.
Overlapping leads to difficulties in locating the background
at high diffracting angle. If the background is underestimated,
the B values will be too low, if not negative, and vice versa.
Possibly, a wrong profile shape may be responsible of errors
in locating the background. For instance, using Lorentzian with
long tails instead of Gaussian, may lead to underestimate the
background. Selecting a too low profile tails contributing range
is also a cause of error on B values with the Rietveld method.
There are experimental causes as well : sample not intercepting
the whole X-ray beam at low angle. Etc, etc. But I do not think
that not using the fundamental parameter approach would lead
systematically to wrong B values.
Armel Le Bail - Universit� du Maine, Laboratoire des Fluorures,
CNRS ESA 6010, Av. O. Messiaen, 72085 Le Mans Cedex 9, France
http://www.cristal.org/
