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MINERALS AND SYNTHETIC COMPOUNDS, AmXn, IN WHICH X
ATOMS ARE |
CLOSE PACKED AND A ATOMS OCCUPY OCTAHEDRAL INTERSTICES |
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Nb3Cl8 |
click here to see a key to this data bar
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octa |
3/8 |
fcc |
layer |
4
edges |
A-
6c; X- 2c, 3c/fcc |
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structure |
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The Nb3Cl8
structure is adopted by Nb3X8 halides in which the X
atoms are FCC |
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and the Nb atoms are present in 75% of the
octahedral voids of the occupied layers. |
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Occupied and unoccupied layers alternate for an
overall occupancy rate of 3/8. |
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Ex. Nb3Cl8 ( Nb - A
, Cl - X ) |
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Crystal lattice |
The FCC lattice model
demonstrates that the octahedrally coordinated AX6 groups |
of atoms in the occupied layers link together to
form hexagonal rings. Compare |
these hexagonal nets with those of
corundum and atacamite shown previously. The |
fact that the coordination octahedra of Nb3Cl8
share four edges with each other while |
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Figure 90 - Nb3Cl8 lattice structure |
click image to enlarge |
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those of corundum share only three results in a
denser concentration of interstitial |
cations and a higher occupancy rate per occupied
layer (75% vs. 66.6% respectively). |
Note how the hexagonal nets of the occupied
layers are staggered relative to each |
other so that the X atoms of the AX6
groups supply all of the close packed atoms |
necessary to build the structure including the
unoccupied layers. |
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Polyhedral framework |
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linking surfaces |
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Figure 91 - Nb3Cl8 polyhedral |
framework model |
( 72 T, 93 pinges ) |
4
edges |
click image to enlarge |
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The above image is
looking down on a section of the occupied layer which is |
parallel to (111).
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Isostructural minerals/compounds |
Nb3X8 halides |
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Page
59 - Structure matters - Nb3Cl8 |
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