Dear Gerard,
The low-angle intensity problem might come from disordered
regions, like water or solvent molecules occupying some voids.
Also, poor modeling of weakly scattering atoms, like hydrogen,
may lead to the similar problem.
Best regards,
Yaroslav
http://filinchuk.com
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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
--
Heriot-Watt University is a Scottish charity
registered under charity number SC000278.
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