Silver iodide

Silver iodide (AgI) is a chemical compound used in photography and as an antiseptic in medicine. Silver iodide is highly insoluble in water and has a crystalline structure similar to that of ice, allowing it to induce freezing (heterogeneous nucleation) in cloud seeding for the purpose of rainmaking.

The crystalline structure adopted by silver iodide changes with temperature. The following phases are known:
 * Up to 420K (147 °C), AgI exists in the β-phase, which has a wurtzite structure.
 * Above 420K (147 °C), AgI undergoes a transition to the α-phase, which has a body-centered cubic structure and has the silver ions distributed randomly between 2-, 3-, and 4-coordinate sites.
 * A metastable γ-phase also exists below 420K, which has a zinc blende structure.

Silver iodide as 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 rationalised by noting that the AgI crystalline lattice has essentially already partly melted in the transition between α and β forms. Adding the entropy of transition from α-AgI to β-AgI to the entropy of fusion gives a value that is much closer to the entropy of fusion for sodium chloride.