PHASE-TRANSFORMATIONS AND POLYTYPISM IN SILVER-IODIDE - A MOLECULAR-DYNAMICS STUDY

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 almost-equal-to 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 alpha-AgI into the wurtzite phase beta-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.