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contribution to working group

Comments (9th april 2002) on the various questions raised by  David Brown


7.1) Yes the composition should be part of the identifier. This is the first
characteristics by which anyone will refer to the considered phase. As for
variable compositions (e.g. solid solutions ?) , I do not know if they have
a CAS number. They generally have a formula characterized by x,y,… indices
instead of numerical ones.
7.2) To be discussed later. The external identifier is not clear to me yet.
7.3) ?
7.4) Clearly the Pearson symbol is adequate to distinguish phases which
differ in their translational properties (through the centering and the
number of atoms in the unit cell) and also phases which have point groups
belonging to different crystal systems (except for the trigonal and
hexagonal systems which are considered identical in certain cases). Part of
the difficulties pointed out by David Brown for the SiO2 phases (maximal and
minimal symmetries within a crystal system on the one hand leading to
different symbols for the same structure, and, on the other hand, same
symbol for different structures)  arise from the non-specification of the
point-group.
One could think of replacing the indication of the crystal system by a
number associated to one of the 32 point groups, and in keeping the two
other features (centering type and number of atoms). 
This may solve part of the difficulties of transitions between crystalline
phases, but will leave unsolved the question of labelling the phases which
depart more or less from  the crystalline state (which are considered in the
second of the papers on nomenclature) : incommensurate phases, magnetic
phases, quasicrystalline phases , etc… Before submitting proposals, I would
like to know if the working group wishes to consider these systems.

PS I used initially the recommended address (phase-transitions@iucr.org) but
the message came back.
Jean-Claude Toledano
Professeur
Laboratoire des Solides Irradies
et Departement de Physique
Ecole Polytechnique
91128-Palaiseau-cedex
tel.(33)1 69 33 37 39
jean-claude.toledano@polytechnique.fr

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