#6 Pierre Villars
Dr. Pierre Villars villars at mpds.chWed Dec 31 15:12:17 GMT 2003
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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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