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MINERALS AND SYNTHETIC COMPOUNDS, AmXn, IN WHICH X
ATOMS ARE |
CLOSE PACKED AND A ATOMS OCCUPY TETRAHEDRAL INTERSTICES |
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Al2Br6 |
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tetra |
1/6 |
hcp |
molecular |
1
edge |
A-
4c, X- fcc |
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structure |
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The Al2Br6
structure is a layered molecular crystal comprised of A2X6
dimer |
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molecules. The X atoms pack together
HCP.
Only 1/3 of the tetrahedral voids of |
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every other layer are occupied by A atoms making
it a very dispersed composition. |
Two AX4 groups are joined at one edge
to form each molecule. |
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Ex. Al2Br6
( Al - A , Br - X ) |
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Crystal lattice |
Coloring the AX4
groups in each occupied layer of the HCP lattice clearly |
reveals the dimer molecules and the 1/3
occupancy rate. A careful examination of |
the lattice structure reveals that, despite the
dispersed arrangement of the AX4 |
groups, they pack together so as to provide all
of the necessary close packed |
bromine atoms necessary to build the structure,
including the unoccupied layers. |
This demonstrates how such molecular crystals
achieve the structural stability |
necessary to fill space. |
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Figure 69 - Al2Br6 lattice structure |
click image to enlarge |
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Polyhedral framework |
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linking surfaces |
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Figure 70 -
Al2Br6 polyhedral |
framework model |
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1
edge |
click image to enlarge |
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The polyhedral
framework model demonstrates intuitively why Al2Br6
vaporizes to |
discrete dimer molecules when heated to its
sublimation point. |
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Page
47 - Structure matters - Al2Br6 |
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