Difference between revisions of "Z and Z'"
From Online Dictionary of Crystallography
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== Definitions == | == Definitions == | ||
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== See also == | == See also == | ||
| − | + | Looking deeper into molecular structures with Z > 1, Chapuis,G., ''Acta Cryst.'' (2020). B76, 510–511 | |
Revision as of 07:19, 14 June 2024
Definitions
In the description of a crystal structure, the letter Z characterises the number of (chemical) formula units contained in the unit cell. For 3D-periodic structures, this number is always an integer which is at least 1.
When dealing with molecular structures, it is also convenient to define Z'. Z' indicates the number of formula units in the asymmetric unit. In other words, Z' is the value of Z divided by the multiplicity of the general position of the corresponding space group.
Examples
The cubic structure of NaCl belongs to space group [math]Fm\overline{3}m[/math]. The unit cell contains 4 Na and 4 Cl atoms. Consequently Z=4. for inorganic structures, Z' might be a fraction. For NaCl, Z' would be 4/196 or 1/48 indicating that each atom lies on highly symmetric special positions with site symmetry [math]m\overline{3}m[/math].
In molecular structures, Z' is very often equal to one. However, when Z' is an integer larger than 1 this gives some interesting information about the stereochemistry of the molecular compound.
See also
Looking deeper into molecular structures with Z > 1, Chapuis,G., Acta Cryst. (2020). B76, 510–511