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


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.

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

page 26 of lesson


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