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                                  

octa

3/8

fcc

layer

4 edges

A- 6c; X- 2c, 3c/fcc

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  structure

.
. .
     The Nb3Cl8 structure is adopted by Nb3X8 halides in which the X atoms are FCC .
and the Nb atoms are present in 75% of the octahedral voids of the occupied layers. .
Occupied and unoccupied layers alternate for an overall occupancy rate of 3/8.
. .
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
.      
 

Nb3Cl8.gif

 
    Figure 90 -  Nb3Cl8 lattice structure

click image to enlarge

                    
   
 
.      
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

Nb3Cl8.gif

 

         Figure 91 -  Nb3Cl8 polyhedral
                   framework model
                   ( 72 T,  93 pinges )

4 edges

                          click image to enlarge
.      
     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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