On May 8, 2008, at 12:30 AM, May, Frank wrote:
You can check for texture effects (preferred orientation) by
obtaining multiple patterns of the material. It's realistic to
expect some differences, but preferred orientation is manifest by
not being able to replicate the pattern.
Not true,
preferred orientation or texture are perfectly reproducible, provided
you use the same sample orientation. What is not reproducible and
probably what Frank May is referring to is not preferred orientation
but graininess or few big grains that do not guarantee the correct
statistic. So if you need to check for graininess, you just move a
little your sample, so the beam covers a different area on the sample.
If you think you have texturte, to check for it you have to change the
sample orientation to see a change. Beware that in a Bragg-Brentano
instrument turning around the axis normal to the sample surface is not
a valid change in orientation as nothing will change for texture; you
have to change the sample inclination instead (omega or chi).
Best Regards,
Luca Lutterotti
That's the simple test. Let us know what you find.
Another issue for "improper intensities" is when the specimen is not
sufficiently wide enough at low angles (typically below 20-degrees 2-
Theta with copper radiation) and the x-ray beam does not fully
impinge on the specimen. The observed reflections in the low angle
region will be less than calculated by a modelling program.
Frank May
Research Investigator
Department of Chemistry and Biochemistry
University of Missouri - St. Louis
One University Boulevard
St. Louis, Missouri 63121-4499
314-516-5098
________________________________
From: Gerard, Garcia S [mailto:[EMAIL PROTECTED]
Sent: Wed 5/7/2008 8:57 AM
To: rietveld_l@ill.fr
Subject: Preferred orientation?
Dear all,
I have a laboratory Bragg-Brentano X-ray (Cu) pattern that shows
intensity mismatches only at low angles, ie 20-50 2theta or 1.8 to 4
Angstroms.
There are overestimated peaks and also underestimated peaks.I have
tried to discard factors that might cause this problem:
The thermal parameters look sensible. Moreover, the data at high
angle looks ok, so intensity transfer from low angle to high angle
or vice versa does not seem to be the cause.
Atomic positions also look sensible. And again, data at high angle
looks ok. Is the scattering angle dependence of the atomic positions
the same as for the thermal parameters? (I cannot remember that, but
i am pretty sure it is not).
Following the advice published in J. Appl. Cryst. 32, 36 (1999), the
other factor that might cause this problem is preferred orientation:
I have tried to find a hkl dependence in the overestimated and
underestimated peaks but i could not find any. If i try to model
preferred orientation with spherical harmonics the problems
disappears nicely. The problem is how to justify the existence of
preferred orientation. The crystal system is orthorhombic. But i
have no other information that supports the existence of preferred
orientation.
Is there any other problem that I cannot think of?Is the preferred
orientation correction masking any of these other problems I cannot
think of?
Regards
Gerard
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