Silver iodide
| Silver iodide | |
|---|---|
| Silver iodide | |
| General | |
| Other names | Silver(I) iodide |
| Molecular formula | AgI |
| Molar mass | 234.773 g/mol |
| Appearance | yellow, crystalline solid |
| CAS number | [7783-96-2] |
| Properties | |
| Density and phase | 5.675 g/cm3, solid |
| Solubility in water | 3x10-7g/100mL (20 °C) it is not soluble in water |
| Melting point | 552°C |
| Boiling point | 1506°C |
| Thermochemistry | |
| ΔfH0solid | -62.4 kJ/mol at 1 atm |
| Hazards | |
| EU classification | not listed |
| NFPA 704 | File:Nfpa h2.pngFile:Nfpa f0.pngFile:Nfpa r0.png |
| Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa) Infobox disclaimer and references | |
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:[1]
- 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.
References
- ↑ Binner, J. G. P.; Dimitrakis, G.; Price, D. M.; Reading, M.; Vaidhyanathan, B. (2006). "Hysteresis in the β–α Phase Transition in Silver Iodide" (pdf). Journal of Thermal Analysis and Calorimetry 84: 409–412.
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