Ordering of the silver ions in α-AgI: A mechanism for the α-β phase transition

The disorder of the Ag⁺ ions in the superionic conductor {α}-AgI has been studied with use of computer simulations in which reorganizations of the I^- lattice are inhibited. At high temperatures the ions are evenly distributed over the tetrahedral interstitial sites of the crystal but, as the temperature is lowered, the Ag⁺ ions are found to condense onto particular sublattices. The condensation is associated with a loss of mobility. The possible occurrence of such order-disorder transitions in this system has been suggested by Szab\'o [J. Phys. C 19, 3775 (1986)]. The structure of this phase was identified, using molecular-graphics methods. The Ag⁺ ions are found to be in a locally crystalline (monoclinic) arrrangement, which is not one of the stable phases discussed by Szab\'o. From the geometric disposition of the ions at low temperature it is argued that the crystallites found in the simulated system would be unstable with respect to a hexagonal (wurtzite) structure if the boundary-condition-imposed stresses were to be relaxed. On this basis, it is argued that the ordering tendency of the Ag⁺ ions in the {α} phase can be seen as the driving force for the {α}-to-{β} phase transition in this material.