Polymorf - Knowhwere- SnI4

MINERALS AND SYNTHETIC COMPOUNDS, A_mX_n, IN WHICH X ATOMS ARE

CLOSE PACKED AND A ATOMS OCCUPY TETRAHEDRAL INTERSTICES

SnI₄

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tetra	1/8	fcc	molecular	no links	A- 4c, X- fcc

structure

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.

Ex. SnI₄ ( Sn - A , I - X )

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
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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.

Polyhedral framework

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.

Isostructural minerals/compounds

GeI₄, SnCl₄, TiBr₄, TiI₄, ZnBr₄, ZrI₄

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Page 48 - Structure matters - SnI₄