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**Up:** Notations Used for Representing Close-Packed Structures
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As pointed out earlier, the actual arrangement of layers in all close-packed
structures can be described in terms of the ABC notation for close-packing of
spheres. Thus the SiC type 6*H* has six Si and six C layers in its
hexagonal unit cell stacked as A B C A C
B where the Roman letters denote positions of layers of Si atoms and the
Greek letters those of C atoms. Since the positions of C atoms are fixed
relative to the positions of Si atoms it is customary to omit the Greek letters
and write the structure as ABCACB. In the case of CdI_{2} structures, where
the Cd atoms lie in the octahedral voids between alternate close-packed iodine
layers, one often retains the Greek letters to denote the positions of Cd
layers. Thus CdI_{2} type 4*H* has a structure AB CB.
While this notation gives a complete description of the structure it does not
reveal the symmetry or lattice type directly and becomes cumbersome for
structures with large repeat periods.

**Copyright © 1981, 1997 International Union of
Crystallography**