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Body centered cubic packing (BCC) |
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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). |
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. |
. |
overhead view |
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click image to enlarge |
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Figure - 33 Body centered cubic (BCC) packing of spheres |
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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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Knowhere |
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25 - Structure matters - BCC packing |
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