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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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SnI4 |
click here to see a key to this data bar
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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 |
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layers. The A atoms occupy only
twenty-five percent of the interstitial voids of the |
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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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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 |
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Figure 72 -
SnI4 polyhedral |
framework model |
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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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Page
48 - Structure matters - SnI4 |
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