Molecular-Dynamics Simulation of the Phase Behavior of AgI

The phase diagram of silver iodide is simulated by the method of constant-stress molecular dynamics with use of the Parrinello-Rahman Langrangean. The $\beta$, $\alpha$, rocksalt, and liquid phases are reproduced and the boundaries are similar to those observed for real AgI. The supercooled $\alpha$ phase and the equilibrium rocksalt phase both have diffuse fast-ion transitions and the latter cuts the rocksalt-$\alpha$ boundary causing it to cusp into an apparent triple point with an order-disorder transition in the $\alpha$ phase. This is characterized by a heat-capacity anomaly and a rise in the cation diffusion coefficient.