Polymorf - Knowhwere- BCC packing

Body centered cubic packing (BCC)

Or, the spheres (atoms) can be packed together in the staggered arrangement so

that the spheres of one layer nestle into the depressions formed between the spheres

of adjacent layers. This type of cubic packing is called body centered cubic (BCC).

.	overhead view

click image to enlarge
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Figure - 33 Body centered cubic (BCC) packing of spheres

In the staggered arrangement of the spheres revealed in the above views the "A"

spheres eclipse each other as do the "B" spheres. But the "A" spheres are staggered

relative to the "B" spheres. Hence different letters are used to designate them to

indicate that they are not aligned vertically.

Note that the central void space formed by the cubic cluster of spheres is occupied

by a sphere instead of being empty as is the case with the CP packing. This void

space is not quite large enough to accommodate the size of this central sphere so the

whole packing spreads out slightly to maintain the overall uniformity of the array. As

a result each sphere is in direct contact with eight other spheres for a coordination

number of eight.

The BCC packing and lattice can also be imagined as two interpenetrating CP

packings. In this model the "A" spheres are positioned at the lattice points of one CP

lattice and the "B" spheres are located at the points of the other, interpenetrating

lattice. Thus it can be modeled as two interpenetrating space fillings of cubes.

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	Figure 34 - Polyhedral framework model of the BCC
	packing as two interpenetrating CP packings
	( 66 S, 96 pinges)
	click image to enlarge

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Page 25 - Structure matters - BCC packing