Structural Evidence for a Fast-Ion Transition in the High-Pressure Rocksalt Phase of Silver Iodide

This Letter describes neutron diffraction measurements of the rocksalt structured phase of AgI at high pressure and temperature and the structural disorder which accompanies its high conductivity. In contrast to the first-order structural phase transition which results in fast-ionic α-AgI at ambient pressure, the fast-ionic behavior in rocksalt structured AgI occurs above a diffuse transition with a small anomaly in lattice parameter and a continuous increase in occupation of interstitial tetrahedral sites with increasing temperature. There are approximately 10 times more defects in the fast-ionic phase of rocksalt AgI than in isostructural AgBr at ambient pressure, 1 K below melting.