Silver+iodide

Silver iodide is
an inorganic compound with the formula AgI. The compound is a bright yellow solid, but samples almost always contain impurities of metallic silver that give a gray coloration. The silver contamination arises because AgI is highly photosensitive. This property is exploited in silver-based photography. Silver iodide is also used as an antiseptic and in cloud seeding.

Structure
The structure (or phase) adopted by silver iodide depends on temperature of the solid:[2]

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

Preparation and properties
Silver iodide is prepared by reaction of an iodide solution (e.g., potassium iodide) with a solution of silver ions (e.g., silver nitrate). A yellowish solid quickly precipitates. The solid is a mixture of the two principal phases. Dissolution of the AgI in hydroiodic acid, followed by dilution with water precipitates β-AgI. Alternatively, dissolution of AgI in a solution of concentrated silver nitrate followed by dilution affords α-AgI.[3] If the preparation is not conducted in the absence of sunlight, the solid darkens rapidly. The photosensitivity varies with sample purity.