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Next: (c) Zhdanov notation Up: Notations Used for Representing Close-Packed Structures Previous: (a) Ramsdell`s notation

(b) The classical ABC notation

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 6H has six Si and six C layers in its hexagonal unit cell stacked as A$\alpha$ B$\beta$ C$\gamma$ A$\alpha$ C$\gamma$ B$\beta$ 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 CdI2 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 CdI2 type 4H has a structure A$\gamma$B C$\alpha$B. 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.


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Next: (c) Zhdanov notation Up: Notations Used for Representing Close-Packed Structures Previous: (a) Ramsdell`s notation

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