From:   SMTP%"Mark.Farnworth@ptc.pilkington.co.uk" 14-NOV-2000 09:29:17.16
To:     "'cockcroft@img.cryst.bbk.ac.uk'" <cockcroft@img.cryst.bbk.ac.uk>
CC:
Subj:   RE: PD00: Release 3 on Laboratory Methods

Karl,

I have a question concerning the Transparency section of Release 3. You
write:

"
The latter was collected also in Bragg-Brentano geometry, but as a fine
powder coating on a silicon single-crystal sample holder. The second pattern
has much sharper peaks and provides better data for, say, indexing when the
unit cell is unknown, but it has much less reliable peak intensities due to
preferred orientation as demonstrated by the abnormally intense peak at
approximately 28.5°. In addition, the intensities of the diffracted peaks
are much weaker for the silicon mounted sample due to the fact the volume of
diffracting material is much smaller in this case.
Under these circumstances, maybe the best way to collect data for
multi-purpose use is to exploit transmission geometry, which is ideal for
organic materials with low absorption. "

You say that the peak at 28.5 degrees is abnormally intense and is due to
preferred orientation of the fine powder.Could this peak not be due to the
silicon substrate since my own 'silicon' substrate produces a peak a 28
degrees. If so, the reduced peak intensities from the powder may also be due
to increased sampling of the substrate. Could you please clarify what you
mean by a 'silicon single-crystal sample holder. I know you can get 'zero
background' silicon substrates that are specially cut so as to produce no
reflections. If the holder used for your analysis was one of these then your
observations would certainly stand.

By the way, I am really enjoying the course.

Best Regards
Mark Farnworth
Pilkington European Technical Centre

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