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RE: Towards the definition of a phase identifier

  • To: Multiple recipients of list <phase-identifiers@iucr.org>
  • Subject: RE: Towards the definition of a phase identifier
  • From: "S. C. Abrahams" <sca@mind.net>
  • Date: Thu, 27 Jun 2002 19:30:43 +0100 (BST)
From:

Sidney Abrahams 

Many thanks to David for his most helpful message of June 
10, 2002 summarizing the present progress in settling on a 
set of criteria for constructing unambiguous phase identifiers. 

Presumably, we may all agree that the identifier definition 
should be both unambiguous and unique, also that it appears 
necessary for the identifier to be multi-field with the 
possibility of allowing redundant fields that they may not be 
necessary to use for every phase. In addition, there is clear 
value in setting one field aside that can be occupied by an 
external identifier if one is available. 

I also agree with David's suggestion in his Item 7 to make use 
of a composition formula field. 

In addition, the proposal to handle variable composition phases 
by means of a composition loop is fine and should allow identification 
of individual members of large groups of inorganic phases.

In thinking how we can extend these criteria to incommensurate, 
composition-changed morphotropic, polytype, transient-structural, 
and quasicrystalline phases as well as noncrystalline solid state 
phases, the simplest approach may be to provide an additional 
field that identifies the phase category if different from those 
in which the simpler criteria work. For example, the following 
phases could be flagged by a three letter code such as:

incommensurate phase.......Inc 
composition-changed morphotropic phase.....Com 
polytype phase......Pol
transient-structural phase.......Tra
quasicrystalline phase.....Qua
noncrystalline phase.....Non  

or by an initial letter only, so that identification would be 
restricted to phases of the type as given. In the case of the 
compound SO2(C6H4Cl)2, used as an example by Toledano et al., if 
the term "Inc" were present, it would lead to the stable phase 
below 150 K. If not, it would result in the commensurate phase 
above 150 K. This and many other examples in each of the phase 
categories above are given by Toledano et al. in Acta Cryst. 
(2001). A57, 614-626.

A thorough check of all proposals made is of course essential 
to be sure they are both unambiguous and unique. Before any such 
time consuming task is undertaken by the Working Group, it would 
be strongly preferable for members to examine critically the 
proposals made in order to identify any weaknesses they may detect. 

If any member disagrees with any of the proposals made by David or
that above, it would stimulate discussion to have the reasons stated 
forthwith. 

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

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