Crystallography Online: the website of the International Union of Crystallography

Dear David:



Many thanks for your April 16 draft. My comments follow:



1.   I have noted and am in general agreement with the new additions made
since the last draft. I do, however, have a number of suggested corrections
(in red) as follows.



2.   “1. INTRODUCTION”



Modify first paragraph, third sentence, to read “As the only such convention
already approved by the CCN on phase transition nomenclature (Tolédano et
al, 1998, 2001) is not readily adaptable for electronic use, the CCN
established the Working Group on Crystalline Phase Identifiers to make
recommendations that could be of use to the IChI working group.”



Change the second paragraph, final sentence, to read: “Our recommendations
are therefore cast in the form of additions to IChI, in the knowledge that
the resulting identifier can be as readily used by the crystallographic as
by the chemical community.”



3.  In “2. TERMS OF REFERENCE OF THE WORKING GROUP”,



Change the first sentence to read: “The Working Group was charged with
recommending to the IUCr Commission on Crystallographic Nomenclature:”



Change the second sentence charge to read “2. The best way in which this
identifier can be implemented, including its incorporation in the CCN
recommended phase transition nomenclature.”



Change “working group” in the final sentence of section “2” to “Working
Group”



4.   In “5.1 Construction of the IUPAC chemical identifier”



Change the second sentence, fifth paragraph, to read “These are not part of
the identifier proper and are not shown here.”



5.  In “6.1 Layer 5: State of Matter”



Change the second sentence in the paragraph after the first enumeration list
to read “A crystal is defined as a phase for which, in principle, it is
possible to assign a space group, even if that space group is only related
to the full space group (as in the case of aperiodic structures).”



6.  In “6.2 Layer 6: Space Group”



Change the second sentence in the first paragraph to read “It consists of a
number between 1 and 230 that uniquely identifies the space group type
except for aperiodic crystals, cf. Jansen et al.(2002).



Change the final sentence in the second paragraph to read “Quasicrystals
cannot be assigned a conventional space group and are best treated as a
different state of matter (see Section 6.1 above).”



7.  In “6.3 Layer 7. Wyckoff Sequence”



It would be more appropriate here to use the same Wyckoff site order as used
in ITA, i.e. number, followed by letter. It would also be helpful if the
example contained at least two sites with multiplicity other than 1, so that
it becomes, for example, ‘1a 2c 3f 6g’ or ‘a2c3f6g’, to avoid possible
confusion with the dropped ‘1’ (which I agree should be dropped).



Note that §6.4 is missing, so “Possible further layers” should be labelled
§6.4, not §6.5



8.  In “7.2.1 Composition”, it is appropriate to provide a clear definition
of ‘tolerance factor’ if it is to be required.



9.  In “7.2.4 WS:  The Wyckoff sequence”, the second sentence should use ITA
order, see item 7 above.



Change the fifth sentence to read  “The letters are listed in alphabetic
order but before determining the Wyckoff sequence, the structure must be
normalized according to the algorithm used in the program STRUCTURE TIDY,
see Parthé & Gelato (1984, 1985), Gelato & Parthé (1987).”



[Note: all references should be relocated under REFERENCES  at the end of
the Report].



10.  In “8.1 Description of the IUCr-CCN phase transition symbol.”



Change the first sentence in the first full paragraph to read “Recently the
Commission on Crystallographic Nomenclature of the International Union of
Crystallography adopted a Phase Transition Nomenclature (Tolédano et al.,
1998, 2001) described on the IUCr web page: http://www.iucr.org - look for
Commissions/Commission on Crystallographic Nomenclature/CNOM
Onlineinformation/ /Structural Phase Transition Nomenclature.”



Change the third sentence in the third paragraph to read “The intent of this
symbol is to include the maximum identification information density for the
phase, whereas the philosophy of IChI is to include only the minimum
information needed for phase identification.”



Change fifth and sixth (last) sentences to read “The combined IChI and CCN
Phase Transition Nomenclature identifier symbol provides, in principle, all
the information necessary to access the appropriate database entries
available on a given material.  A method for combining the two is described
in Section 8.3.



In “8.2.2 Iron (Fe) (Donohue, 1974)”



Change the third sentence, first full paragraph after the five phase entries
to read “The correct space group of untwinned alpha (ferromagnetic) iron
cannot be isotropic Im(bar)3m (229) but the deviation from higher symmetry
is very small  and the correct space group is not yet determined.”*

_________________________________



*  The point group is most likely Shubnikov group 4/mm¢m¢ but this
information is not necessary  in the Report.

_________________________________



11. At the end of the Report, add:



REFERENCES



Abrahams, S. C., Ihringer, J., Marsh, P.  & Nassau, K.  (1984). J. Chem.
Phys. 81, 2082-2087.



Donohue, J. (1974). The Structure of the Elements. New York: John Wiley &
Sons.



Gelato, L.M., Parthé, E. (1987). J. Appl. Cryst. 20, 139-143.



Ihringer, J. & Abrahams, S. C. (1984) Phys. Rev.  B30, 6540-6548 (1984).



Janssen, T. , Birman, J. L., Dénoyer, F., Koptsik, V. A., Verger-Gaugry, J.
L., Weigel, D.,

Yamamoto, A., Abrahams, S. C. & Kopsky, V.(2002). Acta Cryst., A58, 605-621.



Parthé, E., Gelato, L.M. (1984). Acta Cryst. A40, 169-183,



Parthé, E., Gelato, L.M. (1985). Acta Cryst. A41, 142-151.



Tolédano, J.-C., Glazer, A. M., Hahn, Th., Parthe, E.,  Roth, R. S. Berry,g
R. S., Metselaar, R. &

Abrahams, S. C. (1998). Acta Cryst.  (1998). A54, 1028-1033.



Tolédano, J.-C., Berry, R. S.,  Brown, P. J.,  Glazer, A. M.,  Metselaar,
R., Pandey, D., Perez-

Mato, J. M., Roth,  R. S. & Abrahams, S. C. (2001). Acta Cryst. (2001). A57,
614-626.



I look forward to seeing the comments of our other members.



With 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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