MINERALS AND SYNTHETIC COMPOUNDS, AmXn, IN WHICH X ATOMS ARE
          CLOSE PACKED AND A ATOMS OCCUPY TETRAHEDRAL INTERSTICES
 
.

Al2Br6

                  click here to see a key to this data bar                                  

tetra

1/6

hcp

molecular

1 edge

A- 4c, X- fcc

.

  structure

.
. .
     The Al2Br6 structure is a layered molecular crystal comprised of A2X6 dimer .
molecules.  The X atoms pack together HCP.  Only 1/3 of the tetrahedral voids of .
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.
  .
Ex. Al2Br6    ( Al - A , Br - X ) .
.
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.
.      
 

Al2Br6.gif

 
    Figure 69 -  Al2Br6 lattice structure

click image to enlarge

                     
   
 
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Polyhedral framework
.      

linking surfaces

Al2Br6.gif

 

        Figure 70 -   Al2Br6 polyhedral
                 framework model
              

1 edge

                       click image to enlarge
.      
     The polyhedral framework model demonstrates intuitively why Al2Br6 vaporizes to
discrete dimer molecules when heated to its sublimation point.
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