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Proposed modification of the W.G.Report on crystal phase identifiers



Dear David:

I fully agree that use of the Parthé-Gelato convention is essential
for the universal derivation of a single unique set of Wyckoff symbols 
for each phase studied. 

I also agree with moving the Gelato & Parthé (1987) reference to the
footnote but question the omission of "lower-case" before "letters of the
alphabet" as in the November 2004 version. Certainly ITA uses lower-case
letters only for Wyckoff positions but the report should not present the
option of upper-case letters. 

The suggested explanatory footnote may be useful to many, hence  I am in
full agreement with its insertion.

Let us hope we will all hear soon from each member on these small proposed
changes!

With kindest wishes

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

Tel.  (541) 482-7942
Fax.  (541) 552-6415
---------------------------

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Subject: phase-identifiers Digest, Vol 10, Issue 1

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Today's Topics:

   1. Crystalline Phase Identifier final report (David Brown)


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

Message: 1
Date: Thu, 24 Feb 2005 16:29:25 -0500
From: David Brown <idbrown@mcmaster.ca>
Subject: Crystalline Phase Identifier final report
To: phase-identifiers@iucr.org
Message-ID: <421E4735.1090308@mcmaster.ca>
Content-Type: text/plain; charset="iso-8859-1"

Dear Colleagues,

Since circulating the 'final' draft of the report on the Crystalline 
Phase Identifier I have now heard back from all of you.  Most of you 
approved the report but a couple of people were concerned about two 
points.  I discuss these below and I have modified Section 7.2.4 of the 
report as noted.  The revised wording is attached to this email.  
Section 7.2.4, dealing with the calculation of the Wyckoff sequence, has 
been significantly changed and I am afraid I have to ask you to check it 
and let me know if you approve.  I need to hear from each one of you 
before the report can be submitted.

John Faber raised a question about the rules concerning the composition 
given in Section 7.2.1. The reports suggests that in cases where the 
composition involves non-integral multipliers, it is only the relative 
composition that is important, and that when compositions are being 
compared they should be normalized, e.g., by setting the largest 
multiplier to 1.0.  John was concerned that this would result in the 
loss of significant information about whether the compound was anion 
deficient or had a cation excess.  It is true that such information 
would be lost, but that is not the purpose of the composition field 
which is merely to use the relative amounts of the different elements 
present as the first step in differentiating between different 
crystalline phases.

Both John and Vicky Karen were concerned about the recommendation that 
we use the Parthe-Gelato convention for normalizing the structures 
before determining the Wyckoff sequence.  Their concern was that the P-G 
choice of the unit cell and space group setting conflicted with other 
conventions already in use.  However, we do not really have a choice 
because there is only one algorithm described in the literature that 
ensures a unique Wyckoff sequence.  (We could invent our own algorithm 
and ensure that it adheres to a more traditional convention - but which 
convention?)  In any case the conventions used in this algorithm are 
invisible to anyone wanting only to calculate a Wyckoff sequence.  The 
P-G normalization is the only one that guarantees a unique Wyckoff 
sequence because it is the only one that normalizes the atomic 
coordinates as well as the cell and space group.  There is no conflict 
with conventions that might be used in a database for other purposes.  
Neither the space group setting nor the unit cell basis that are used by 
the P-G algorithm are part of the proposed identifier.  The new 
(attached) text now puts the emphasis on the algorithm rather than on 
the program, and it provides an explanation for those who might be 
concerned about the non-standard conventions used.

Let me know if you need a copy of the whole report.

I look forward to receiving your approval of the attached wording.

David
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