The phase behaviour of silver iodide. Constant stress molecular dynamics simulation

The constant stress molecular dynamics (MD) technique using the Parrinello-Rahman Lagrangian is used to investigate the phase behaviour of AgI using a simple rigid-ion model potential. The system of 216 or 256 ions spontaneously transforms from one pure phase to another upon superheating or supercooling and the known phase diagram of AgI is remarkably well mimicked. The high-pressure rocksalt phase has a diffuse fast-ion transition as does the α-phase supercooled below the α-β transition at 300 K, 0 GPa. Some authors have argued that a divergence in defect densities or a diffuse fast-ion transition in the β-phase drives the β-α transition. No evidence is found for this. A heat capacity, compressibility and diffusivity anomaly in the α-phase appears to confirm the order-disorder transition there proposed by Perrott and Fletcher. This perturbs the shape of the rocksalt-α-phase boundary as predicted earlier. Heats of transition are determined and the volume change across the β-α boundary is indeed negative as for real AgI. These results are a remarkable vindication of the MD technique, and should encourage application to further important model systems.