Structure
The structure (or phase) adopted by silver iodide depends on temperature of the solid:
- Up to 420 K (147 °C), AgI is more stable in the β-phase, which has a wurtzite structure. It is known as the mineral iodargyrite. In this motif, the silver and iodide centers are tetrahedrally coordinated.
- Above 420 K (147 °C), the α-phase becomes more stable. This motif is body-centered cubic structure with has the silver centers distributed randomly between 2-, 3-, and 4-coordinate sites. Above 420 K, Ag+ ions can move rapidly through the solid, making it a fast ion conductor. The transition between the β and α forms represents the melting of the silver (cation) sublattice. The entropy of fusion (melting) for α-AgI is approximately half that for sodium chloride (a typical ionic solid). This can be rationalized by noting that the AgI crystalline lattice has essentially already partly melted in the transition between α and β forms.
- A metastable γ-phase also exists below 420 K, which has a zinc blende structure.
Read more about this topic: Silver Iodide
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