MINERALS AND SYNTHETIC COMPOUNDS, AmXn, IN WHICH X ATOMS ARE
          CLOSE PACKED AND A ATOMS OCCUPY TETRAHEDRAL INTERSTICES
 
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SnI4

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tetra

1/8

fcc

molecular

no links

A- 4c, X- fcc

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  structure

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     In the SnI4 structure layers of discrete AX4 molecules alternate with unoccupied .
layers.  The A atoms occupy only twenty-five percent of the interstitial voids of the .
FCC packed X atoms in the occupied layers for an overall occupancy rate of 1/8.
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Ex. SnI4          ( Sn - A , I - X ) .
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Crystal lattice
     The crystal structure can be modeled as a simple FCC lattice with the occupied
AX4 coordination groups colored differently than the unoccupied tetrahedral groups
of close packed iodine atoms (blue).  The lack of any linkage between discrete AX4
groups is readily apparent as is the occupancy rate.
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SnI4.gif

 
    Figure 71 -  SnI4 lattice structure

click image to enlarge

                    
   
 
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     Despite the very dispersed distribution of the molecules they still pack together
so as to provide all of the necessary X atoms to build the structure.
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Polyhedral framework
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linking surfaces

SnI4.gif

 

   Figure 72 -   SnI4 polyhedral
            framework model
            

none

                 click image to enlarge
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     Only one molecular layer is shown to demonstrate the dispersed pattern of the
molecules in this molecular crystal.  This model demonstrates why the crystal
vaporizes to discrete AX4 molecules when it is heated to the sublimation point.
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Isostructural minerals/compounds
GeI4, SnCl4, TiBr4, TiI4, ZnBr4, ZrI4
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