It was recalled that Ag and Cu halides usually exhibited a sudden (first-order) transition to a superionic state. On the other hand, both powder diffraction studies and molecular dynamics simulations of AgI under hydrostatic pressure indicated that a continuous superionic transition should occur on heating. The gradual onset of the highly conducting state was associated with an increasing fraction of dynamic Frenkel defects, with a peak in the specific heat and an anomalous behavior of the lattice expansion. Similar methods were used to investigate a proposed continuous superionic transition between the 2 ambient-pressure face-centered cubic phases of CuI. This was difficult to investigate experimentally because the hexagonal β-phase existed over a narrow temperature range between the γ (cation-ordered) and α (cation-disordered) phases. Molecular dynamics simulations, performed with the simulation box constrained so as to remain cubic at all temperatures, showed that although limited Cu+ Frenkel disorder occurred within γ-CuI, the material underwent an abrupt superionic transition, at 670K, to give the superionic α phase. This result was supported by powder neutron diffraction studies of CuI which was lightly doped with Cs+ so as to prevent stabilization of the β phase.

Nature of the Superionic Transition in Ag+ and Cu+ Halides. D.A.Keen, S.Hull, A.C.Barnes, P.Berastegui, W.A.Crichton, P.A.Madden, M.G.Tucker, M.Wilson: Physical Review B, 2003, 68[1], 014117 (11pp)