Matthew Henrichsen <[EMAIL PROTECTED]> wrote 20 Jan 1999:
>Why does x-ray powder diffraction fail? Why do the others work?
If you read Rod Hill's intercomparison (J. Appl. Cryst. (1992) 25, 589-610)
you will see that it is not only thermal parameters that are better determined
by single crystal and neutron powder methods :-)
There are simply more systematic errors with X-ray powder diffraction,
and the on-going discussion about preferred orientation illustrates this.
With neutrons you have much larger samples and it is less of a problem.
You also have intensities that don't fall off rapidly with angle due to the
X-ray structure factor (neutron scattering is from the almost point nucleus)
Accurate Debye-Waller factors need good high angle data. Absorption
is also much larger for X-rays, and an error there translates directly into
an error in the overall thermal parameter.
I guess one can debate all these points, but its difficult to argue with the
numbers from the intercomparison.
And hey, if you could do it all with X-rays, we neutron types would be
out of a job !
Alan H.
Alan Hewat, ILL Grenoble, FRANCE <[EMAIL PROTECTED]> tel (33) 4.76.20.72.13
ftp://ftp.ill.fr/pub/dif fax (33) 4.76.48.39.06 http://www.ill.fr/dif/
