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RE: phase identifiers

• To: Multiple recipients of list <phase-identifiers@iucr.org>
• Subject: RE: phase identifiers
• From: "S. C. Abrahams" <sca@mind.net>
• Date: Mon, 30 Jun 2003 17:00:27 +0100 (BST)

Dear David:

Thanks for your discussion paper number 5 with its interesting suggestion
for the identity resolution of organic isomers. Preliminary examination of
several trial isomers confirms that the proposed string of four (in
preference to five) numerical variables indeed performs the required
resolution quite handily. I doubt, however, if there is much to be gained by
dropping the zero designation for the number of carbon atoms without H (in
the zeroth through third position), since the space saved is minor whereas
their presence helps recall the resolution scheme for the user/reader.
Incidentally, use of the word "spectrum" for a 'string or sequence of
numbers' is likely to be misleading since a spectrum is generally associated
with a wavelength or energy.

I, too, am unfamiliar with organic molecules containing several optically
active sites. Does each such isomer have a different optical acivity? If
so, that could provide the necessary information for identity resolution.

The next five keys seem fine, but I am unfamiliar with the Pauling file. The
structure types in the Struktur Berichte indeed have many familiar and
well-used names but is hardly encyclopedic.

The names assigned by the International Mineralogical Association(?) to
natural minerals are very useful and widely used by non-mineralogists. They
should, however, be used for their synthetic analogues only if suitably
modified. For example, synthetic materials with the stillwellite-type
structure form a new family of ferroelectrics; they, as is the case for
other mineral analogs,are generally referred to as stillwellite-types or as
stillwellite-type structures.

Use of "_reduced_crystallographic_cell" with prescribed tolerances
should indeed be useful for identifying identical materials. In view of all
other information in the total key, the prescribed tolerances could probably
be relatively relaxed.

I look forward to the comments of our fellow members.

With all best wishes

Sidney
------------------------------------------
Prof. S. C. Abrahams
Physics Department
Southern Oregon University
Ashland, OR 97520

Fax: (541) 552-6415    Tel. (541) 482-7942
Email: sca@mind.net
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