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 .
 

overhead view

 

ccp.gif

ccp.gif

 
  Figure 37 -  Cubic closest
  packing (CCP) of spheres
 
             click image to enlarge
.    
.
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

   
 

hcp.gif

hcp.gif

 
  ( 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

page 28 of lesson

 

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