Re: [sdpd] UPPW-7

[Robin Shirley <R.Shirley...@surrey.ac.uk>]

Dear Armel

Happy new year.

Here is a delayed response for UPP-7 - actually done in November then
set aside to be dealt with in the vacation, but unfortunately I have been
out of email communication since December due to system changes here.

Like your other correspondents, I agree that this alpha-artemether
polymorph is orthorhombic.

The main feature of interest with this dataset is the need for a
substantial 2Theta zero correction of 0.068 +-0.0075 degrees to be added
to 2Theta-obs to obtain the corrected values (i.e. a 2theta zero error
of -0.068 degrees).

Otherwise the problem is quite straightforward and was easily solved by
several programs within Crysfire (FJZN6, ITO12, TAUP, etc.).  It was
then passed to Chekcell in the usual way for refinement and evaluation.

The observed lines beyond the 25th are too sparse to be useful, and were
omitted from the refinement, which gave the following orthorhombic cell 
(with esds in brackets):

a = 11.1323 (0.0084)
b = 13.8259 (0.0082)
c = 10.4829 (0.0068)

V = 1613.461

The volume is lower than most other solutions submitted as a consequence 
of allowing for the 2Theta zero error, and hence I think more reliable.

The above cell indexes all of the first 25 lines with 63 calc lines, if 
the spacegroup is taken to be P212121.

A centrosymmetric spacegroup of Pmmn would only require 58 calc lines, 
but was rejected in favour of P212121 on the assumption that a natural 
product like this would probably be chiral, and P212121 is one of the 
commonest orthorhombic spacegroups for such materials.

With best wishes

Robin

P.S.  My apologies to Crysfire users who have been getting linkfile
errors running Crysfire in 2004.  This seems to be a bizarre year-related
problem, which did not occur in 2002 or 2003!!  I'm looking into this and
will hopefully be able to release a fix soon - in the meanwhile a sort of
workaround is to temporarily set the year back to 2003 for the duration
of the Crysfire run.

--------------------------------------

To:            sdpd...@yahoogroups.com
From:          Armel Le Bail <alb...@cristal.org>
Date:          Fri, 09 Jan 2004 09:47:36 +0100
Subject:       [sdpd] UPPW-7
Reply-to:      sdpd...@yahoogroups.com

A few hours (5) are remaining for sending your results about
the Unindexed Powder Pattern of the Week -7

Armel


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