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RE: Phase ID Discussion Paper 7

• To: "A Working Group of the IUCr Commission on Crystallographic Nomenclature" <phase-identifiers@iucr.org>, "Steve Stein" <steve.stein@nist.gov>
• Subject: RE: Phase ID Discussion Paper 7
• From: "S. C. Abrahams" <sca@mind.net>
• Date: Thu, 25 Mar 2004 09:24:01 -0800
• Importance: Normal
• In-Reply-To: <40365E26.20201@mcmaster.ca>

Dear David:

Many thanks for your Discussion Paper #7 on phase ID and all the
thought, with which I am in general agreement, that has clearly gone
into it. Specific suggestions for possible incorporation into the draft
Report to be considered by the  Working Group follow:

1.  The IUCr phase nomenclature divides fields by a vertical bar (|),
not a solidus (/) . Since the former symbol is found on all keyboards,
I propose  retaining it in the IChI symbol.

2.  In §2.1, having provided an IChI example you note “The following
is an explanation of the above IChI as far as I can figure it out.” Since
our Report should certainly incorporate an authoritative explanation of
all IChI abbreviations to be used, why not run that explanation by the
IChI group so that possible misunderstandings, including case sensitivity,
may be eliminated.

3.  In §3.2 under COMPOSITION, an example of a formula unit in which
 ‘one or more of the multipliers is non-integral, so that the size of the
formula unit is indeterminate and only the relative multipliers are
meaningful’
should be given for clarity. Similarly for cases in which ‘non-integral
multipliers are encountered’, the ‘tolerance factor to allow for
experimental
uncertainties’ should be defined and a clarifying example given.
     Under PH, I suggest we add each of the crystalline states for which a
specific nomenclature is given in the second IUCr phase nomenclature
Report, namely magnetic, incommensurate, composition-changed
morphotropic, polytype and transient-structural in addition to
quasicrystalline phases. Incidentally, the abbreviation ‘amp’ for
amorphous is not very intuitive; perhaps a five-letter symbol would be
clearer, e.g.

            liqud         liquid
            amphs       amorphous
            solid          solid of unknown form
            cryst          crystal
            liqcr           liquid crystal
	magcr        magnetic crystal
            inccr          incommensurate crystal
	comcr        composition-changed morphotropic crystal
	polcr          polytype crystal
	trncr          transient-structural crystal
	quasi         quasicrystal

4.  Responding to your invitation that we suggest ‘the best’ way in which
the IChI phase identifier could be incorporated into the IUCr phase
transition
nomenclature, I would argue that a union of the  two symbolic systems has
greater value than the sum of the two  used separately, at least when first
introduced. As suggested by the examples below, the united symbol indicates
to the potential user a means of searching the literature for a given
inorganic
or organic crystalline phase for all cases in which that phase can be
appropriately identified.

You also ask for some additional examples under §3.3. Combining the two
requests,  an example taken from each of the two IUCr Phase Nomenclature
Reports is presented. A pair of vertical bars (all bars are in bold type
below,
for emphasis) is used to separate the IChI from the IUCr symbol, as follows:

K2TeBr6|PH:xtl|SG:14:ae4||III|<400K|P21/n(14)|Z=2|Ferroelastic|12 variants

(CH3)3NCH2COO·CaCl2·2H2O|PH:xtl(incom)|SG:33:a28||XVI|<50K, 4GPa,<180K|
Pn21a(33)|Z=4|Ferroelectric|Nonmodulated ferroelectric polarization along b

A pdf file of the above material, with full formatting, is attached. In case
the
file fails to reach a member because of our Web-based reply system, I would
be glad to send a copy directly on request.

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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