Phase transformations and polytypism in silver iodide: A molecular-dynamics study

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Abstract
Molecular-dynamics studies have been carried out to investigate structural phase transformations in AgI as the temperature is lowered below the superionic transition temperature of ≊420 K, while holding the pressure constant at selected values. Upon lowering the temperature for pressures below 0.27 GPa the system always transformed from the superionic phase α-AgI into the wurtzite phase β-AgI. Starting from the superionic phase and lowering the temperature at the pressures of 0.28, 0.29, 0.30, and 0.31 GPa we obtained various polytypes of the same hexagonality as has been reported experimentally. At pressures above 0.32 GPa the structure obtained upon temperature lowering was always the rocksalt form of AgI in agreement with previous simulations and experiments. At pressures between 0.31 and 0.32 GPa the system did not transform but underwent sluggish distortions among unrecognizable structures over the entire time of the simulation. This ‘‘frustration’’ of the system near the polytype-rocksalt phase boundary may be associated with the, so-called, intermediate phase mentioned in the literature. We also present results for nucleation from the liquid phase to the solid phase as a function of pressure in rapid-quench simulations.

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