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#6 Pierre Villars



Vitznau, 31 December 2003

Dear Working Group members,

Pierre Villars:
Thank you to David for your message #6 (Crystallographic Phase Identifier 
Working Group). I can fully agree with your chapter:  6. Recommendations
Layer 5. State of matter: gas, liquid, crystal etc.
Layer 6. The space group number
Layer 7. Wyckoff Sequence


SOME FURTHER POINTS THAT NEED DISCUSSION

David Brown:
There were two other possible items we considered including in the 
identifier:  the Bravais symbol and the reduced cell.  Are these needed in 
IChI?
Pierre Villars:
No they are not needed.

David Brown:
The reduced cell has proved itself useful in finding the same phase in 
different databases, with the composition, if given, used as a secondary 
key.  Further, if the reduced cell is known, it is most likely that the 
space group is also known.  Are there any occasions when the reduced cell 
would be needed to differentiate two different phases with the same space 
group and Wyckoff sequence, or should the reduced cell be used at a higher 
level?
Pierre Villars:
There exists quite some cases with same composition and same space group 
number and same Wyckoff Sequence (after standardization with STIDY and 
COMPARE). Niggli's Reduced cell is for futher distinction not helpfull for 
such cases, one possibility is to add to each point-set its Atomic 
Environment AET's (Coordination Polyhedra), see e.g. references:
- J.L.C. Daams et al., J Alloys and Compounds, 1997, 252,110-142
- J.L.C. Daams et al., J Alloys and Compounds, 1994, 215,1-34
- J.L.C. Daams et al., J Alloys and Compounds, 1993, 197,177-196
- J.L.C. Daams et al., J Alloys and Compounds, 1993, 197,243-269
- J.L.C. Daams et al., J Alloys and Compounds, 1992, 182,1-33


David Brown:
1. Would IChI, extended to cover crystalline phases as described above, be 
an acceptable identifier for inclusion in all the crystallographic 
databases?
Pierre Villars:
Yes, it is acceptable. The Pauling File has already included this inform  
ation.

David Brown:
2. How can this identifier be implemented, including its incorporation in 
the CCN approved Phase Transition Nomenclature?
The information given in the Phase Transition Nomenclature includes:
1. the common symbol used to identify this phase (e.g., alpha,II, etc.),
2. the temperature (and pressure) range in which it is stable,
3. the Hermann-Mauguin symbol and number of the space group (more than one 
space group may be given, or the Bravais symbol may be given if the space 
group is not known),
4. Z, the number of formula units in the conventional unit cell (though the 
formula unit is not defined within the symbol),
5. the ferroic properties and
6. the structure type.
Pierre Villars:
If the structure type assignment is properly done (after standardization 
with STIDY and COMPARE), and each prototype is defined by a unique 
combination of Space Group Number/Wyckoff Sequence/AET's all is included in 
item 6) the structure type.

Best wishes

Pierre Villars



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