Close Packed Structures

a) Hexagonal and Cubic Close Packing

The structures of many metals can be described as close packed arrays of spherical atoms. There are two alternative ways to maximize the packing efficiency of a collection of equally sized spheres.

Cubic Close Packing (CCP) - ABCABC Fm` 3m
Hexagonal Close Packing (HCP) - ABABAB P63/mmc

The figure below illustrates the two packing sequences. The spheres of the second layer can be centered over either the sites marked b or c. The symmetry is exactly the same regardless of the choice, however, when a third layer is added it now has a choice of packing over the spheres in the original layer or over the sites marked as c in the diagram of the first layer. The former option leads to the ABAB... stacking characteristic of hexagonal close packing, while the latter option leads to the ABCABC... packing which defines a cubic close packed structure.





To continue clik below


[You must be registered and logged in to see this link.]