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Figure 40 - |
Figure 41 - |
HCP
lattice and |
FCC
lattice and |
polyhedral |
polyhedral |
framework |
framework |
|
click image to enlarge |
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. |
Notice the subtle differences between these structures. In the HCP
lattice adjacent |
. |
octahedra are joined together at their faces and edges. Whereas, in
the FCC lattice |
. |
they only share their edges with each other. Also the tetrahedra of
the HCP lattice |
. |
share faces and vertices. But in the FCC lattice they only share
vertices with each |
. |
other. The minor changes in these structures brought about by a slight
shift in the |
. |
staggering of their sphere packings accounts for the existence of some
polymorphs |
. |
of
elements and minerals that exhibit either the HCP or FCC atomic packing
structure |
. |
depending on the external environmental conditions existing at the time they |
became
solid. |
|
Symmetry planes of the FCC lattice |
Systematically slicing the FCC lattice parallel to its (100) planes results
in the |
cubic structure that gives the packing its name. Notice that a sphere
is located in the |
. |
 |
|
( 4 ST, 2 REC, 24 RT,
40 pinges ) |
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click image to enlarge |
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Figure 42 - Symmetry planes of the FCC
packing/lattice |
. |
|
center of each face of the cube. This model is useful in relating the
orientation and |
symmetry of FCC packed atoms to cubic symmetry elements. It can be
seen that the |
atoms have the closest packed triangulated arrangement parallel to the (111)
cubic |
plane (green). They have the close packed squared arrangement parallel
to the |
cube's (100) plane of symmetry (red). And they have a slightly
expanded rectangular |
arrangement parallel to the cube's (110) plane (blue). |
Back to
Knowhere |
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29 - Structure matters - FCC symmetry |
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