Constant-stress molecular dynamics: The phase diagram of silver iodide

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Abstract
The method of constant-stress molecular dynamics using the Parrinello-Rahman Lagrangian is described, particularly as applied to Coulomb systems. The method is used to simulate the phase behavior of silver iodide. The β, α, rocksalt, and liquid phases are reproduced and the boundaries are similar to those observed for real AgI. Evidence is found for the order-disorder transition in the α phase proposed by Perrott and Fletcher. This is characterized by a heat-capacity anomaly and a rise in the cation diffusion coefficient. The supercooled α phase and the equilibrium rocksalt phase both have diffuse fast-ion transitions and the latter cuts the rocksalt–α-phase boundary, causing it to cusp in to an apparent triple point with the α-phase order-disorder transition.