Crystallography Online: the website of the International Union of Crystallography

Dear Sidney,

Thank you for your thoughts on the phase identifier - I will go through 
these and incorporate them as appropriate in the next draft I am 
preparing. I have taken your advice and checked with Steve Stein about 
my interpretation of the IChI symbol and he has provided some clarification.

Your attached pdf did not open, but I don't think I need it. We are 
mostly concerned with a computer readable representation of the symbol 
and the symbol is simple enough that it does not need any special 
formatting for a printed version.

Best wishes

David

S. C. Abrahams wrote:

>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 at mind.net
>------------------------------------------
>
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