Polymorf - Knowhwere- HCP packing

Cubic closest packed (CCP)

Staggering the two triangulated layers so the spheres of one nestle in the hollows

of the other forms the cubic closest packed (CCP) packing arrangement. It is called

this because this is the densest way that spheres can be packed together to fill 3D

space. When the sphere centers (nuclei) are connected together by lines a lattice is

formed of tetrahedra and octahedra packed together in the space filling ratio of 2:1

respectively.

Things get a little more complicated when a third triangulated layer of spheres

(atoms) is added to the previous two staggered CCP layers. Its spheres can nestle

in the hollows of the second, topmost layer so that they are directly vertical of (i.e.

eclipse) the spheres of the first layer, or so that none of the spheres of the three

layers eclipse each other.

Hexagonal close packed (HCP)

The eclipsed arrangement is called hexagonal closest packed (HCP). The vertical

.	overhead view

		( 24 T, 12 pinges )
		click image to enlarge



Figure 38 - Hexagonal closest packed (HCP) sphere packing
.

arrangement of the spheres is ABA to indicate the eclipsing of the first layer by the

third. Note that the central sphere in this cluster is coordinated with twelve other

spheres - six spheres surrounding it in the same layer and three spheres in both the

lower and upper adjacent layers. This thirteen sphere cluster can be modeled as

a "twist" cuboctahedron to emphasize the symmetry of the coordinated spheres.

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Page 27 - Structure matters - HCP packing