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Colored closed packing

الكلية كلية العلوم للبنات     القسم قسم الكيمياء     المرحلة 3
أستاذ المادة محمد هاشم مطلوب الغرابي       26/12/2017 20:35:00
Close-packed structure:
When the atoms of a monatomic solid are imagined to be relatively
incompressible spheres, they might be very tightly packed together
in the most compact arrangement for spheres.
[Note: This is expected since the cohesive energy for metallic and
Van der Waals bonding increases with the number of nearest
neighbors, up to the point where core overlap becomes significant].
Two stacking sequences for close-packed structures are
considered:
1) Face-centered cubic (fcc) structure.
2) Hexagonal close-packed (hcp) structure.
In general, the arrangement of spheres in such structures
implies the followings:
a) Each sphere has six touching neighbors in a close-packed
plane array of spheres, as shown in figure 19. Such plane
arrays with hexagonal symmetry can be stacked together to
make the most compact monoatomic solid in two simple
ways.
b) Each atom has twelve nearest neighbors; these 12
neighbors are distributed as follows: six in the plane as in
figure 19, three in the plane above and three in the plane
below, as shown in figure 23.
Close-packed fcc structure:
When the spheres with radius r for one type of atoms are
considered, for a two-dimensional packing, a hexagonal lattice
with primitive cell edge, a=2r, is formed. The stacking of layers
is as follows:
a) The first layer A of spheres is formed in a plane of arrays of
hexagonal lattice in 2-D, as mentioned above.
b) The second layer B of spheres is located such that sphere
centers are over the points labeled (+) (i.e. center of sphere
B is on a point at the center (hole) of the well formed by three
spheres of A (below)).
c) The third layer C of spheres is placed on top of B and is
displaced so that its atoms fall on top of the spaces in layer B
which were also spaces in layer A (labeled o).

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