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Simple
hexagonal (HP) structure elements |
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Am,
Bk, C (graphite), Cf, Cm |
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Crystal lattice |
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The crystal lattice of HP elements can be modeled as shown previously for
the HP |
sphere packing where hexagonal prisms stack together in an eclipsed
arrangement. |
Graphite has a special type of HP structure where the layers of hexagonal
prisms |
stack together in a staggered formation, as shown in the following image.
In this |
arrangement the hexagonal rings of carbon atoms in adjacent vertical layers
are |
staggered relative to each other, but
alternating layers eclipse each other. |
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Figure 50 - Graphite lattice structure |
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click image to enlarge |
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Polyhedral framework (graphite) |
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Figure 51 - Polyhedral framework |
model of graphite |
( 40 T, 42 S, 106 pinges ) |
click image to enlarge |
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The polyhedral
framework demonstrates that the graphite structure can be viewed |
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as two interpenetrating lattices (pictured red
and blue here) with each cell of each |
lattice comprised of two hexagonal prisms stacked
on top of each other. The carbon |
atoms are located at the corner vertices of
these elongated HP cells. Notice the |
shorter bond length between atoms of adjacent
layers that eclipse each other |
compared to atoms of alternate layers that are
eclipsed. One half of the bonds |
between atoms in the different layers are these
longer bonds. This causes weakness |
in the bond strength between atoms of different
layers compared to the bond strength |
of atoms within the same layer. As a
result the separate layers to tend to slip past |
each other. This accounts for graphite's
excellent lubricant properties and its softness |
as a marker. |
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Cleavage (graphite) |
(100) - perfect and easy - the result of the relatively weaker inter-layer
bonds |
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Crystal habit (graphite) |
tablets, scaly |
Back to
Knowhere |
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Page
35 - Structure matters - HP elements |
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